Introduction to Transparent Conductive Films

Graphene for Transparent Conductors Qingbin Zheng • Jang-Kyo Kim

Graphene for Transparent Conductors

Synthesis, Properties, and Applications

1 3 Qingbin Zheng Jang-Kyo Kim Leibniz Institute of Polymer Research Department of Mechanical and Aerospace Dresden Engineering Dresden The Hong Kong University of Science and Germany Technology Kowloon Hong Kong SAR

ISBN 978-1-4939-2768-5 ISBN 978-1-4939-2769-2 (eBook) DOI 10.1007/978-1-4939-2769-2

Library of Congress Control Number: 2015938323

Springer New York Heidelberg Dordrecht London © Springer Science+Business Media New York 2015 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made.

Printed on acid-free paper

Springer Science+Business Media LLC New York is part of Springer Science+Business Media (www. springer.com) Preface

Transparent conductors (TCs) have been used in a wide variety of optoelectronic and photovoltaic devices, such as liquid crystal displays (LCDs), solar cells, optical communication devices, and solid-state lighting. Thin films made from indium tin oxide (ITO) have been the dominant source of TCs, and demand of indium from the explosive growth of LCD computer monitors, television sets, and smart phones has risen rapidly in recent years, which now account for 50 % of indium consumption. In 2002, the price was US$ 94 per kg, and it rose to over US$ 1000 per kg recently, a 1000 % increase in 10 years. Graphene, a two-dimensional monolayer of sp2- bonded carbon atoms, has attracted significant interests recently because of the unique transport properties. Due to the high optical transmittance and electrical conductivity, thin film electrodes made from graphene have been considered an ideal candidate to replace the currently used expensive ITO films. Compared with the ITO films, graphene films have high mechanical strength, flexibility, chemical stability, and are much cheaper to produce. A key to success in such applications is to develop methods to produce large- size graphene sheets with high yields and deposit them onto a substrate layer-by- layer in an orderly manner. The graphene sheets in current study for the fabrication of TCs are very small, mostly with an area of hundreds of square micrometers. The small graphene sheets result in high intersheet contact resistance due to a large amount of intersheet junctions. To reduce the number of intersheet tunneling barriers, production of inherently large-size graphene sheets are highly desirable. Although mechanical cleavage of graphite was shown to prepare high quality graphene with a millimeter size, the yield of this method is extremely low, being unsuitable for mass production. Alternatively, graphitization of Si-terminated SiC (0001) in an argon atmosphere could produce monolayer graphene films with a domain size of several tens of micrometers. However, the graphene obtained thereby was difficult to transfer to other substrates and the yield was very low. The chemical vapor deposition (CVD) technique has been extensively explored to grow extremely large-area graphene on Ni films or Cu foils. This technique usually requires specific substrate materials that have to be removed chemically after the growth of graphene. The high cost of single crystal substrates and the ultrahigh vacuum conditions necessary to maintain for the CVD growth significantly limit the use of the CVD method for large-scale applications. In spite of the significant v vi Preface progress for CVD-grown graphene achieved so far, these important challenges must be overcome before the industry applications. Owing to the scalability of production and the convenience in processing, graphene oxide (GO) has been considered an important precursor for the fabrication of TCs. GO sheets are hydrophilic and can produce stable and homogeneous colloidal suspensions in aqueous and various polar organic solvents due to the electrostatic repulsion between the negatively charged GO sheets. These GO dispersions are easy to be processed to produce TCs on a substrate. Transparent conducting films (TCFs) containing GO or chemically reduced GO sheets have been deposited via several well-established techniques, including spin or spray coating, transfer printing, dip coating, electrophoretic deposition, and the Langmuir–Blodgett (L–B) assembly, followed by chemical reduction and/or thermal annealing. While there are some books that specialize in fabrication processes and properties of graphene and GO, very few books are available specifically dealing with the following topics for their application in transparent conductors: (i) how to produce TCs by using CVD grown graphene, (ii) how to synthesize GO with different size and control their surface functionalities to enhance the electrical conductivity, (iii) how to incorporate these nanostructured materials into thin films with layered structure, and (iv) how to improve the conductivity and transparency. In light of the authors’ experiences on graphene fabrication and application for TCs in the past few years, this book is aimed to provide a comprehensive overview of traditional and novel techniques in producing and functionalizing graphene for highly conductive transparent thin films. It will offer a systematic presentation of the principles, theories, and technical practices behind the structure–property relationship of the thin films, which we believe to be the key for the development of high-performance TCs. The book is intended primarily for an audience of graduate students, research scientists, and professors in the area of carbon materials, transparent conductors, and related fields, as well as to professionals from the electronic and chemical manufacturing industries. Nanotechnology, as an emerging new subject, has been established as a major in postgraduate level in many universities and research institutes. This book would be well suited as a textbook for an intermediate level class in nanotechnology and/or materials science and engineering as part of such a program or as a stand-alone course. It will be accessible equally to readers with either science or engineering background. At the same time, the unique perspectives provided in the applications of graphene as TCs will serve as a useful guide for design and fabrication of these thin film materials for specific applications. The authors are grateful for the assistance, discussion, and encouragement offered during the preparation of this book by past and current colleagues and friends, including Dr. B Zhang, Prof. QZ Xue, Prof. ZG Li, Prof. PC Ma, Prof. JH Yang, Dr. J Li, Dr. ZD Huang, Dr. Y Geng, Dr. X Shen, Dr. XY Lin, as well as the research group members who produced the research outputs quoted in this book. Dr. Zheng was partly supported by the Hong Kong University of Science & Technology (HKUST) Postgraduate Studentships (PGS), Finetex-HKUST R & D Center at HKUST, Research Grant Council (RGC) of Hong Kong, Shanghai Pujiang Preface vii

Talent Project, and the Alexander von Humboldt Foundation during the course of completing this book. The authors specially thank Prof. E. Mäder, Dr. SL Gao, Dr. HS Qi, Dr. C Scheffler, and Dr. U Gohs at Leibniz Institute of Polymer Research Dresden (IPF) for stimulating discussion on this book. The authors are also grateful to Drs. David Packer, Ho Ying Fan, and Kanchan Kumari at Springer for their kind reviewing and processing of our manuscript.

Clear Water Bay, Dr. Qingbin Zheng, Hong Kong Prof. Jang-Kyo Kim January, 2014 Contents

1 Introduction to Transparent Conductive Films �� 1 1.1 Applications of Transparent Conductive Films (TCFs) �� 1 1.2 Transparent conducting oxides (TCOs) �� 5 1.2.1 ITO �� 5 1.2.2 ITO Substitutes �� 7 1.3 Transparent Conducting Polymers �� 9 1.3.1 Polythiophene (PT) �� 9 1.3.2 Poly(para-phenylene vinylene) (PPV) �� 10 1.3.3 Polypyrrole (PPy) �� 14 1.3.4 Polyaniline (PANI) �� 14 1.3.5 Poly(3,4-ethylenedioxythiophene) (PEDOT) �� 15 1.4 Transparent Conducting Metals �� 17 1.4.1 Metal Nanogrids �� 17 1.4.2 Metal Nanowires �� 17 1.5 Transparent Conducting Carbon �� 18 1.5.1 Carbon Nanotubes (CNTs) �� 18 1.5.2 Graphene �� 19 References �� 21

2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide �� 29 2.1 Introduction �� 29 2.2 Synthesis Methods of Graphene and Graphene Oxide �� 31 2.2.1 Mechanical Cleavage �� 31 2.2.2 Epitaxial Growth �� 32 2.2.3 Chemical Vapor Deposition (CVD) �� 32 2.2.4 Total Organic Synthesis �� 36 2.2.5 Chemical Methods �� 38 2.3 Preparation of Large-Size GO �� 43 2.3.1 Mild Oxidation and Sonication �� 43 2.3.2 Edge-Selective Functionalization of Graphite (EFG) �� 45

ix x Contents

2.3.3 Elimination of Sonication �� 45 2.3.4 Size Separation Methods �� 47 2.4 Structures of Graphene and GO �� 51 2.5 Properties of Graphene and GO �� 55 2.5.1 Electrical/Electronic Properties �� 55 2.5.2 Thermal Properties �� 63 2.5.3 Optical Properties �� 67 2.5.4 Mechanical Properties �� 68 2.6 Common Tools for Characterization of Graphene and Its Derivatives �� 73 2.6.1 Atomic Force Microscopy �� 73 2.6.2 Scanning Electron Microscopy (SEM) �� 74 2.6.3 Transmission Electron Microscopy (TEM) �� 74 2.6.4 Scanning Tunneling Microscope (STM) �� 76 2.6.5 Raman Spectroscopy �� 79 References �� 81

3 Fabrication of Graphene-Based Transparent Conducting Thin Films �� 95 3.1 Introduction �� 95 3.2 CVD-Grown Graphene-Based TCs �� 95 3.2.1 Etching Method �� 95 3.2.2 Stamping Method �� 97 3.2.3 Thermal Release Method �� 101 3.2.4 Photoresist Method �� 101 3.2.5 Roll-to-Roll Transfer Method �� 103 3.2.6 Challenges of Transferred CVD-Grown Graphene for TCs �� 105 3.3 Fabrication of GO-based TCs �� 105 3.3.1 Electrophoretic Deposition �� 109 3.3.2 Spin Coating �� 110 3.3.3 Spray Coating �� 111 3.3.4 Dip Coating �� 112 3.3.5 Transfer Printing of GO Films �� 114 3.3.6 Langmuir–Blodgett Method �� 114 3.3.7 Rod Coating �� 117 3.3.8 Inkjet Printing �� 117 References �� 119

4 Improvement of Electrical Conductivity and Transparency �� 123 4.1 Introduction �� 123 4.2 Chemical Doping �� 124 4.2.1 Chemical Doping of Carbon Materials �� 124 4.2.2 Chemical Doping of Graphene �� 127 4.2.3 Stability of Doped Graphene Films �� 136 Contents xi

4.3 Hybridization �� 138 4.3.1 Hybridization with CNTs �� 138 4.3.2 Hybridization with Metal Wires �� 151 4.3.3 Hybridization with Metal Grids �� 157 4.4 Using UL-GO �� 158 4.4.1 Solution Casting of UL-GO �� 158 4.4.2 Dip Coating of UL-GO �� 159 4.4.3 L–B Assembly of UL-GO �� 161 References �� 173

5 Application of Graphene-Based Transparent Conductors (TCs) �� 179 5.1 Introduction �� 179 5.2 Touch Screen �� 179 5.3 Displays �� 181 5.3.1 Liquid Crystal Displays �� 181 5.3.2 Light-Emitting Diodes �� 181 5.4 Solar Cells �� 184 5.5 Transistors �� 184 5.6 Other Applications �� 187 5.6.1 Electromagnetic Interference (EMI) Shielding �� 187 5.6.2 Functional Glasses �� 190 5.6.3 Transparent Loudspeakers �� 192 5.6.4 Transparent Heaters �� 192 5.6.5 Transparent Actuators �� 194 5.6.6 Transparent Sensors �� 196 5.6.7 Transparent Supercapacitors �� 198 References �� 200

6 Conclusions and Perspectives �� 205 References �� 210

Index �� 215 Acronyms and Symbols

Acronyms

AFM Atomic force microscopy AR-XPS Angle-resolved X-ray photoelectron spectroscopy AZO Aluminum-doped zinc oxide BA Benzylamine BDM Bubble deposition method BHJ Bulk-heterojunction BSE Backscattered electrons CB Carbonaceous byproducts CCFT Cold cathode fluorescent lamp CGOWs Concentrated graphene oxide wrinkles CMOS Complementary metal–oxide–semiconductor CNS Carbon nanoscroll CNT Carbon nanotube CRT Cathode ray tubes C-rUL-GO Chemically doped, reduced ultra-large graphene oxide CSA Chlorosulfonic acid CVD Chemical vapor deposition DC Direct current DCB o-dichlorobenzene DCE Dichloroethane DEG Diethylene glycol DGU Density gradient ultracentrifugation DI Deionized DMA Dimethylacetamide DMF N, N-Dimethylform DMSO Dimethyl sufoxide DSLR Digital single lens reflex EBA Ethylbenzoic acid ED Electron diffraction EMI Electromagnetic interference

xiii xiv Acronyms and Symbols

EPD Electrophoretic deposition EFG Edge selective functionalization of graphite FDP Flat panel display FLG Few-layer-graphene FTIR Fourier transform infrared FTO Fluorine tin oxide FTS Fluoroalkyl trichlorosilane FWHM Full width at half maximum GIC Graphite intercalation compounds GNP Graphite nanoplatelet GNR Graphene nano ribbons GO Graphene oxide GOP Graphene oxide paper GOW Graphene oxide wrinkles GP Graphene paper HF Hydrogen floride HI Hydrogen iodine

HNO3 Nitric acid HOPG Highly ordered pyrolytic graphite HRTEM High-resolution transmission electron microscopy ICO Indium-doped cadmium-oxide IPA Isopropanol ITO Indium tin oxide L–B Langmuir–Blodgett LbL Layer-by-layer LCDs Liquid crystal displays LED Light-emitting diode L-GO Large graphene oxide MS Magnetron sputtering MSA Methanesulfonic acid MSD Magnetron sputtering deposition MDs Molecular dynamics MLG Multilayer graphene MMs Molecular mechanics MWCNT Multi-walled carbon nanotube MQW Multiple quantum wells NGO Nanosized graphene oxide

(NH4)2S2O8 Ammonium persulfate NMP N-methyl-2-pyrrolidone NPs Nanoparticles NWs Nanowires OLEDs Organic light emitting diodes OPVs Organic photovoltaics PAHs Polyacrylic hydrocarbons Acronyms and Symbols xv

PANI Polyaniline PDMS Polydimethylsiloxane PDP Plasma display panel PEDOT/PSS Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) PEI Polyethyleneimine PEG Polyethylene glycol PET Polyethylene terephthalate PLD Pulsed laser deposition PMMA Poly(methyl methacrylate) PmPV Poly(m-phenylene vinylene-co-2,5-dioctyloxy-pphenylene vinylene)

P2O5 Phosphorus pentoxide POPT Poly(2,5-dioctyloxy-1,4-phenylene-alt-2,5-thienylene) PPA Polyphosphoric acid PPV Poly(para-phenylene vinylene) PPy Polypyrrole PTs Polythiophenes PVDF Polyvinylidene fluoride PVP Poly(4-vinylphenol) RAM Radar absorbing materials rGO Reduced graphene oxide RF Radio frequency RP Rear-projection SAED Selected area electron diffraction SE Secondary electrons SEM Scanning electron microscope SDS Sodium dodecyl sulfate S-GO Small graphene oxide SRL Self-release layer SPL Sound pressure level STM Scanning tunneling microscope SWCNT Single-walled carbon nanotube TCs Transparent conductors TCFs Transparent conductive films TCOs Transparent conducting oxides THD Harmonic distortion ToF-SIMS Time-of-flight secondary ion mass spectrometry UL-GO Ultralarge graphene oxide UHV Ultra-high vacuum UV-Vis Ultraviolet–visible spectroscopy VAPE Vacuum arc plasma deposition VL-GO Very large graphene oxide WHM Width-at-half-maximum XPS X-ray photoelectron spectroscopy XRD X-ray diffraction xvi Acronyms and Symbols

Symbols

Rs Sheet resistance (Ω/sq) t Thickness (nm) T Transparency (%) U Strain energy (kcal/mol) Z Impedance of free space (= 377 Ω) ε Strain (%) σσDC/ Op DC to optical conductivity ratio Chapter 1 Introduction to Transparent Conductive Films

1.1 Applications of Transparent Conductive Films (TCFs)

TCFs, which are optically transparent and electrically conductive thin layers, are necessary components in many modern devices [1]. TCFs have been used in a wide variety of optoelectronic devices, such as touch screens, liquid crystal displays (LCDs), organic light-emitting diodes (OLEDs), solar cells, sensors, etc. [2, 3]. It is reported that the market for TCFs in 2012 was ~ US$ 3.9 billion and it will grow to nearly US$ 10 billion by 2019 (Fig. 1.1a) [4]. LCD is by far the largest user of TCF materials (Fig. 1.1b). It is estimated that ~ 290 million displays will be produced in 2014 and the demand for TCF materials will continuously increase [5]. Different applications may require differing TCF materials because each application has its own set of optoelectronic parameters and requirements [6, 7]. Table 1.1 shows representative examples of applications and the corresponding TCFs chosen for them. Flat-panel displays and touch screens need TCFs as the front electrode. Except low electrical resistance and high transparency, etchability is another very important consideration to form patterns in transparent conductor (TC) electrodes. By virtue of its easy etchability, indium tin oxide (ITO) is favored over other TC oxide materials. The low temperature required for ITO films is another vital advantage for color displays where the TC is deposited on thermally sensitive organic dyes [7]. Another good example is the low-emissivity glass window that can improve the energy efficiency of buildings. Because the free electrons reflect infrared radiation for wavelengths longer than the plasma wavelength, it should be long enough (~ 2 µm) in cold weather so that most of the solar spectrum can be transmitted. Due to the suitable plasma wavelength of > 1.6 µm with excellent durability and low cost, fluorine-doped tin oxide (FTO) is the best material for this purpose [7]. How- ever, a short plasma wavelength (≤ 1 µm) is needed in hot climates so that the near- infrared portion of the incident sunlight can be reflected out of the building. Silver and titanium nitride are widely used for this application due to their short plasma wavelengths of 0.4 and 0.7 µm, respectively [7]. TCFs serve as the front electrodes in thin-film solar cells. Thermal stability and low cost are the primary selection criteria. For thermally sensitive solar cells, TCFs need to be deposited on flexible

Fig. 1.1 Transparent conductive film markets (US$ millions). a Forecast of TCF markets [4]. b Flat-panel display and transparent conductor markets. ITO indium tin oxide, TCO transparent conducting oxides, OLED organic light-emitting diodes, RP rear-projection, PDP plasma display panel, LCD liquid crystal displays, CCFL cold cathode fluorescent lamp, CRT cathode ray tube, FPD flat-panel display [11] steel or plastic substrates. Due to the low temperature deposition requirement, ITO or ZnO is chosen for this purpose [7]. Electrochromic mirrors and windows have been widely used in automobiles and smart windows with electrically controllable transmission. The main considerations are chemical inertness, high transparency, 1.1 Applications of Transparent Conductive Films (TCFs) 3 cost – – Low Low Low – – – – Toxicity Low Low Low Low Low Low – – – – – – Low 200̊C)

Deposition temperature – Low (≤ – – – – – – – Chemical stability – – Good – – – Mechanical stability Good Good Low Good Good Low – – Good Good Good – – 7 ] Thermal stability – Good Good Good – – Good Good Good – – – Good Good Good wavelength Long when cold and short when hot Transparency Plasma High – High – High – High – High – High – Sheet resistance Low Low – High Low – High – Low – High – – High – Low Low – High Selection criteria for various transparent conductors (TCs) [

Applications Flat-panel displays Touch screens Touch Low-emissivity windows Solar cell Electrochromic mirrors and windows Defrosting windows Oven window Static dissipation Electromagnetic shielding Invisible security circuits Protection layer for glass Table 1.1 Table 4 1 Introduction to Transparent Conductive Films and low cost, making tin oxide an ideal choice for this purpose [8]. Defrosting windows are required in many areas, such as the display windows in supermarkets and windshields or windows in airplanes. Low cost and durability are the main factors and tin oxides are widely used for this application [7]. Laboratory ovens are often constructed entirely of TCF-coated glass because they need to be transparent while possessing high temperature stability, chemical and mechanical durability, and low cost. Static dissipation TCFs are used to dissipate static charges that often grow excessively on cathode ray tubes (CRT), computer monitors, xerographic copiers, and television tubes. The main concern here is the mechanical and chemical durability, although the sheet resistance can be relatively high [7]. In order to prevent the eavesdropping on computers and communications, electromagnetic shielding TCFs are used to block the stray signals by detecting electromagnetic signals pass- ing through windows [9]. Invisible security circuits can be used for both military and consumer applications [10]. Very high transparency is the most important factor for this special purpose. In order to improve the durability of glass, TCFs with good abrasion resistance are needed. Hence, different sets of parameters are desired for a wide variety of applications based on different TCFs [6, 7]. Especially, there are rapidly growing markets for flexible TCFs such as flexible displays [12], flexible touch screens [13], printable electronics [14, 15], OLEDs [16], and thin-film photovoltaics [17, 18]. Figure 1.2 shows examples of flexible organic optoelectronic devices like OLEDs and organic photovoltaics (OPVs). OLEDs consume power to emit light, while OPVs absorb light to store energy for future power output. OLEDs can be used to create digital displays, such as TV screens, computer monitors, and portable systems [19]. Their unique flexible and lightweight nature makes them an excellent candidate for use in emerging technologies, such as roll-up displays. OPV devices convert solar energy to electrical energy and are promising candidates for meeting the increasing future energy demands [18]. Flexible and lightweight OPVs can be incorporated into ev- eryday products, such as clothing, backpacks, and other wearable apparel. ITO has been the dominant material for TCF applications. It has a huge global market worth US$ 3 billion in 2010 with a 20 % growth rate annually [1]. The limited supply of indium and ever-increasing demands, however, have pushed the price of ITO up continuously [20]. Due to the brittleness of ITO, to apply it in flexible devices is prohibitively difficult [6]. Several types of new TCF materials, including conductive polymers [21, 22], metallic nanowires [23, 24], carbon nanotubes (CNTs) [2], and graphene films [25] have been developed as alternative transparent electrode materials. Graphene, a rapidly rising star on the horizon of materials science [26], is considered the most interesting material to replace ITO benefited from its exceptional mechanical, optical, and electrical properties [6]. 1.2 Transparent Conducting Oxides (TCOs) 5

Fig. 1.2 Examples of flexible organic optoelectronic devices. a Example of an organic light- emitting diode (OLED) display (LG Display’s 5″ plastic-based flexible OLED prototype panel) [27]. b Example of an organic photovoltaic (OPV) device [28]

1.2 Transparent Conducting Oxides (TCOs)

1.2.1 ITO

TCOs, which belong to a unique class of materials that exhibit both high transparency and electrical conductivity, have been well studied for decades [29]. ITO is a solid solution of indium(III) oxide (In2O3) and tin(IV) oxide (SnO2), and has 6 1 Introduction to Transparent Conductive Films

Fig. 1.3 Transparent conducting ITO thin films grown by a pulsed laser deposition method. a, b AFM images of ITO films with thicknesses of 40 nm a and 200 nm b, respectively. c, d Effect of film thickness on sheet resistance c and transparency d [40] been the dominant material for producing TCFs owing to their excellent electrical conductivity and optical transparency [1]. Several fabrication methods, including magnetron sputtering [30], radio frequency sputtering [31], molecular beam epitaxy [32], screen printing [33], pulsed laser deposition (PLD) [34], sol-gel techniques [35], spray pyrolysis [36], and electron beam evaporation [37], have been developed to produce ITO films. Additional high-temperature annealing is usually needed for the solution-based methods to achieve certain transmittance ( T) and resistance standard. ITO nanowires [38] or ITO nanoparticles [39] have been deposited via solution-based deposition techniques. Figure 1.3a–b shows the typical atomic force microscopy (AFM) images of ITO films grown by the PLD method. The root- mean-square roughness of the ITO films with thicknesses of 40 and 200 nm were 2.8 and 0.5 nm, respectively. The higher surface roughness for the thinner film was attributed to the formation of islands. It is also noticed that the roughness increased again when the films were thicker than 200 nm due to the crystallinity [40]. It is seen from Fig. 1.3c that the sheet resistance initially decreased with an increase in film thickness and remained almost constant for films up to 870 nm thick. Figure 1.3d shows the variation of optical T and reflectance as a function of film thickness. The optical T was above 80 % for films with thickness from 40 to 300 nm in the visible range (~ 400–700 nm), while the reflectance was relatively low [40], which is why ITO has been the dominant TCF material. 1.2 Transparent Conducting Oxides (TCOs) 7

Table 1.2 TCO materials for ITO substitutes [41] Binary/ternary Dopant Resistivity Toxicity ZnO Al, Ga, B, In, Y, Sc, V, Si, Ge, Ti, Very good – Zr, Hf//F CdO In, Sn Very good Very high

In2O3 Sn, Ge, Mo, Ti, Zr, Hf, Nb, Ta, W, Very good High Te//F

Ga2O3 Sn Average –

SnO2 Sb, As, Nb, Ta//F Good –

TiO2 Nb, Ta Average –

MgIn2O4 – Average –

GaInO3, (Ga, In)2O3 Sn, Ge Average –

CdSb2O6 Y Average High

SrTiO3 Nb, La Bad –

Zn2In2O5, Zn3In2O6 ZnO–In2O3 system Good –

In4Sn3O12 In2O3–SnO2 system Good –

CdIn2O4 CdO–In2O3 system Good High

Cd2SnO4, CdSnO3 CdO–SnO2 system Good High

Zn2SnO4, ZnSnO3 ZnO–SnO2 system Average –

Zn2SnO4, ZnSnO3 ZnO–In2O3–SnO2 system Good –

Zn2SnO4, ZnSnO3 CdO–In2O3–SnO2 system Good High

Zn2SnO4, ZnSnO3 ZnO–CdO–In2O3–SnO2 system Good High

1.2.2 ITO Substitutes

Due to the limited nature of world indium reserves, it is widely believed that a severe shortage of indium may occur in the near future [41]. This means that a long-term stable supply of ITO might be difficult to satisfy the expanding TCF markets. Hence, developing other TCO alternatives to ITO becomes critically important. A number of low-cost alternatives have been proposed recently [1]. TCOs that contain a reduced amount or no indium, such as Al- and Ga-doped ZnO (AZO and GZO) and ZnO–In2O3–SnO2 (or Zn–In–Sn–O) multicomponent oxides, have attracted much attention [41]. Table 1.2 shows typical TCO materials developed as ITO substitutes. For example, ITO has been prepared on amorphous substrates at a temperature below 200 °C by direct current (DC) magnetron sputtering deposition (MSD) for LCD applications. ITO TCFs possess low resistivity in the order of 10−4 Ω cm and thicknesses of approximately 15–100 nm. Due to the toxicity of cadmium and the required high temperature heat treatment, it is practically difficult to use cadmium oxide-based and titanium oxide-based TCO materials under such circumstances [42]. For titanium oxide and titanium oxide-based TCO TCFs, a high temperature procedure (> 300 °C) is required to reach a low resistivity [43].

Three different kinds of binary compounds (ZnO, In2O3, and SnO2) and multicomponent oxides composed of any combination of these binary compounds may pos- 8 1 Introduction to Transparent Conductive Films

Fig. 1.4 Stability of resistivity of Al-doped ZnO (AZO) in a humid environment. a Resistivity as a function of exposure time for AZO films prepared by pulsed laser deposition ( PLD). b Normalized resistivity as a function of exposure time for AZO thin films prepared by (radio frequency ( rf) and direct current ( dc)) magnetron sputtering deposition ( MSD) and PLD. (Reprinted with permission from [45]. Copyright (2008) by Elsevier) sibly be employed as TCFs in LCDs. The usage of indium could also be reduced by synthesizing multicomponent oxides that contain less indium, e.g., ZnO–In2O3, In2O3–SnO2, and Zn–In–Sn–O [44]. The required optoelectrical properties could be achieved by magnetron sputtering (MS) and vacuum arc plasma deposition (VAPE) under optimized deposition conditions [44]. Thus, these multicomponent oxides with an appropriate composition are potential candidates for TCFs in LCDs [41]. The amount of indium used can be reduced to approximately half.

Indium-free oxides, such as ZnO, SnO2, and ZnO–SnO2 multicomponent oxides, are another solution to avoid the use of indium [41]. However, due to the difficulty of producing low resistivity TCFs at a low temperature and the patterning problems, impurity-doped SnO2 and SnO2-based materials are unsuitable for use in LCDs. The current indium-free candidate is impurity-doped ZnO, such as AZO and GZO [41]. In addition, AZO possesses superior electrical stability upon exposure to humid environments [1, 45]. Figure 1.4a shows the electrical stability of the AZO films prepared with different thicknesses and Al contents, revealing the importance of these factors. The stability of resistivity was improved as the thickness increased while the resistivity was dependent on the deposition method, which influenced the crystallinity of deposited AZO films. It is also worth noting that the stability was relatively independent of substrate deposition temperatures ranging from 68 to 200 °C. Figure 1.4b shows normalized resistivity (ρ/ρI, referenced to the initial resistivity) as a function of exposure time for 200-nm-thick AZO films. It is found that the AZO films were sufficiently stable for use in practical TCF applications, irrespective of the deposition method [45]. However, there are still several critical weaknesses in these TCOs, namely (i) ever-increasing material cost due to the limited availability of the elements needed, such 1.3 Transparent Conducting Polymers 9 as In, Zn, and Sn, on the earth, (ii) TCO being inherently unable to be etched, patterned, or processed at high temperatures, (iii) TCOs being unable to satisfy property requirements for emerging applications, such as flexible devices (e.g., flexible LCDs and organic solar cells) due to its brittle nature, (iv) susceptibility to ion diffusion into polymer layers, (v) low transparency in the near-infrared region, and (vi) current leakage caused by structural defects [46–50]. In addition to light weight, high flexibility, and low cost, the TCFs required for the next generation optoelectronic devices should be compatible with the available large-scale manufacturing processes [51].

1.3 Transparent Conducting Polymers

In response to the aforementioned limitations of traditional TCOs, there have been significant efforts in search of alternative materials that possess comparable or even better characteristics, including high electrical conductivity, excellent transparency, and good mechanical properties. The most promising materials among a myriad of alternatives are conducting polymers, metal nanogrids, and nanowires (NWs), carbon nanotube (CNT) and graphene [1, 6, 52]. Conducting polymers are organic materials exhibiting excellent electrical conductivities and mechanical flexibilities and have been widely explored for the development of TCF materials [53]. They have many advantages compared with other materials, such as low cost, light weight, mechanical flexibility, and excellent compatibility with plastic substrates [54–56]. Polythiophene (PT), poly(para-phenylene vinylene) (PPV), polyaniline (PANI), polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT) are currently the most popular conducting polymers. Figure 1.5 shows the molecular structures of these polymers [53] and their electrical conductivities are compared in Table 1.3.

1.3.1 Polythiophene (PT)

Owing to the good electrical properties and environmental stabilities, PT has received much attention from the scientific communities [57]. PTs are conjugated polymers that can be used for a variety of applications, such as electrical conductors, nonlinear optical devices, polymer LEDs, and smart windows [72]. New types of PTs with enhanced properties were developed for certain devices by devising new design strategies. For example, the electronic properties of solution-processable conjugated PTs were modified using self-assembled silane molecules [73]. The electrical conductivities of ultrathin PT films increased by up to six orders of magnitude by doping with hydrolized fluoroalkyl trichlorosilane (FTS). Because the interband optical absorption of the polymers in the doped state was drastically reduced, the doped PT films were highly transparent in the visible range (Fig. 1.6). The PT films were very stable in vacuum and nonpolar environments because the dopants within the porous polymer matrix were partially cross-linked via a silane self-polymerization mechanism [73]. The interaction of the silanol groups with po- 10 1 Introduction to Transparent Conductive Films

Fig. 1.5 Molecular structures of PT, PPV, PANI, PPy, and PEDOT. PT polythiophene, PPV poly(para-phenylene vinylene), PANI polyaniline, PPy polypyrrole, PEDOT poly(3,4-ethylenedioxythiophene). (Reprinted with permission from [53, 57]. Copyright (2013, 1998) by Elsevier) lar components led to a drastic and reversible change in conductivity in response to the ambient polar molecules.

1.3.2 Poly(para-phenylene vinylene) (PPV)

A conducting polymer of the rigid-rod polymer family with high levels of crystallinity, PPV has been used in many TCF applications, such as LEDs and photovoltaic devices, owing to its excellent stability, processability, and optoelectrical properties. The polymer can be easily doped to form electrically conductive materials, and thus its electronic and physical properties are easily modified by adding functional groups [74]. PPV was firstly used as the emissive layer in polymer-based LEDs in 1989 [75]. PPV synthesized using a solution-processable precursor polymer (Fig. 1.7) had advantages of easy processing and reduced tendency for crystalliza- tion [75]. Since then, a large number of PPV derivatives have been synthesized for TCF applications [76–78]. 1.3 Transparent Conducting Polymers 11 65 ] ] 58 ] ] ] ] ] al. [

63 63 60 60 61 ] ] al. [

67 59 ] ] al. [ al. [ al. [ al. [ al. [ ]

62 66 al. [ al. [ 64

al. [ al. [

al. [

vlyanov et Alemu et Xia et Cirpan et McCullough et Avlyanov et Avlyanov et Avlyanov Lim et A Hohnholz et Winther-Jensen et Winther-Jensen Hohnholz et Ha et 1362 1325 0.001 Conductivity (S/cm) References 1000 0.1 200 850 2–6 1000 0.6–1.8 900

3 VO

M - 4

as the oxidiz- as the oxidiz- 3 3 polymerizing solution of aniline using NH 4 PO 3 , PPy, PANI, and PEDOT prepared by various processing conditions [ 53 ] and PEDOT PANI, , PPy, M H

Coating of an aqueous PEDOT/PSS product on glass substrates, followed by immersion in methanol Coating of an aqueous PEDOT/PSS product on glass substrates, followed by treatment with hexafluoroacetone Prepared electrochemically from p-xylene-bis(diethylsulphonium chloride) in a solvent-electrolyte couple Deposition in a polymerizing solution of pyrrole using FeCl Processing conditions Solid-state macromolecular self-assembly ing agent on PET substrates ing agent on PET ing agent, with sodium anthraquinone-2-sulfonate and 5-sulfosalicylic acid substrates as the additives, on PET Deposition in a polymerizing solution of pyrrole using FeCl Deposition in a 4 Casting of camphorsulfonic acid-doped PANI solutions mixed with Casting of camphorsulfonic acid-doped PANI substrates crystallinity-promoting additives on PET HCl substrates Coating of a commercial aqueous PEDOT/PSS product on PET substratesCoating of a commercial aqueous PEDOT/PSS product on PET 0.01–0.06 phase polymerization using ferric tosylate as the oxidizing agent Vapor with the addition of a weak base, pyridine, in oxidant solutions on PET as the oxidizing agent on glass substrates, followed by washing with 1 Coating of a commercial aqueous PEDOT/PSS product mixed with EG as substrates the additive on PET Casting of polymerizing solution methanol-substituted 3,4-ethylene dioxythiophene using ferric tosylate as the oxidizing agent and various alcohol solvents containing a weak base, imidazole, on glass substrates Electrical conductivities of PT, PPV Electrical conductivities of PT,

PEDOT PEDOT PPV PPy Conducting polymers PT PPy PANI PANI PEDOT PEDOT PEDOT PEDOT Table 1.3 Table 12 1 Introduction to Transparent Conductive Films

] 21 ] al. [

] 71 ] ]; Fabretto 22 ]; ] 69 68 al. [

70 al. [ al. al. [

]; Na et

al. [

al. [ 71

al. [

Badre et Kim et Xia et Fabretto et Chang et Chang et 2084 Conductivity (S/cm) References 1418 3065 3400 3300–6740 PPy polypyrrole, PEDOT poly(3,4-ethylenedioxythiophene) polyaniline, PANI 4 SO 2 Coating of an aqueous PEDOT/PSS product mixed with ionic liquid, 1-ethyl-3-methylimidazolium tetracyanoborate, as the additive on plastic and glass substrates Coating of an aqueous PEDOT/PSS product on glass substrates, followed by treatment with H Processing conditions Coating of an aqueous PEDOT/PSS product mixed with ethylene glycol as Substrates the additive on glass and PET Vacuum vapor phase polymerization using ferric tosylate as the oxidiz- Vacuum ing agent with the addition of block copolymers based on poly(ethylene glycol–propylene glycol–ethylene glycol) in oxidant solutions on glass substrates Commercial products PPV poly(para-phenylene vinylene), (continued)

PEDOT PEDOT Conducting polymers PEDOT PEDOT ITO Table 1.3 Table PT polythiophene, 1.3 Transparent Conducting Polymers 13

Fig. 1.6 Photographs of a doped PT film. a As-spun insulating film, b doped with hydrolyzed FTS for saturation (highly conductive), and c after restoration (de-doping) in air under ambient illumination for 16 h (insulating). (Reprinted with permission from [73]. Copyright (2009) by Wiley) 14 1 Introduction to Transparent Conductive Films

Fig. 1.7 Synthetic route for synthesis of PPV. (Reprinted with permission from [75]. Copyright (1990) by Nature Publishing Group)

1.3.3 Polypyrrole (PPy)

PPy is a Nobel Prize-winning organic polymer (in Chemistry, 2000) formed by polymerization of pyrrole [79]. PPy films darken in air due to oxidation, while doped ones are blue or black depending on the degree of polymerization and film thickness. As seen from Table 1.4, the electrical conductivity of PPy thin films increased by three orders of magnitude when the dopant anion was varied from chloride to anthraquinone-2-sulfonate [60]. This finding is attributed mainly to the dopant anions containing fused aromatic rings, enabling the PPy films to have a higher conductivity than those doped with smaller or bulky dopant anions. Hydrophobic surface was found to be able to enhance the conductivity of PPy films when a chloride dopant anion was used [80]. The conductivity was also sensitive to the type of solvents used for PPy. For example, the PPy thin films cast from PPy dissolved in bulkier alcohol (e.g., oleyl alcohol) had a higher conductivity than that dissolved in alcohol, such as methanol [81].

1.3.4 Polyaniline (PANI)

PANI is a conducting polymer of the semiflexible rod polymer family, which has been extensively studied over the past 50 years, focusing mainly on improving its electrical conductivity [82]. The dependence of color and electrical conductivity on different oxidation states or doping levels makes PANI a suitable candidate for sensors and electrochromic devices [83, 84]. Camphorsulfonic acid-doped PANI 1.3 Transparent Conducting Polymers 15 thin films casted from the PANI’s m-cresol solution showed a higher conductivity than those casted from the PANI’s chloroform solution [85]. M-cresol facilitated the extension of the dissolved PANI polymer chains, while chloroform caused the dissolved PANI polymer chains to coil [86]. The extended conformation of PANI also improved the mobility of the charge carriers [87], whereas the vapor-phase secondary doping, such as m-cresol and o-chlorophenol, enhanced the conductivity of PANI [88]. PANI also has significant advantages over inorganic silicon- and metal-oxide-based memory materials for the production of both volatile and non- volatile memory devices [89]. Their dimensions as well as electrical properties can be easily tailored by controlling the chemical synthesis procedure, producing novel materials with electrical memory capabilities [90]. Moreover, due to its excellent electrical conductivity and mechanical stability, PANI finds emerging technological applications, including rechargeable batteries, solar cells, corrosion devices, and OLEDs [89].

1.3.5 Poly(3,4-ethylenedioxythiophene) (PEDOT)

PEDOT is a conducting polymer based on 3,4-ethylenedioxythiophene monomer and is highly transparent in its conducting state [91]. Because of the poor solubility of PEDOT, PEDOT:polystyrene sulfonate (PSS) is often used to overcome this disadvantage [92]. Commercially available PEDOT solutions or dispersions show electrical conductivities ranging from 0.05 to 10 S/cm [92–94]. Its conductivity can be enhanced by mixing with one or more additives, such as methanol, ethanol, isopropanol (IPA), ethylene glycol (EG), glycerol, diethylene glycol (DEG), sorbitol, dimethyl sufoxide (DMSO), N-methyl-2-pyrrolidinone (NMP), N,N-di- methylformamide (DMF), N,N-dimethylacetamide, zwitterions, and ionic liquids [53]. More approaches to improve the conductivity of PEDOT thin films were reported recently (Table 1.3). For example, oxygen-plasma enhanced the conductivity of PEDOT thin films prepared by vapor phase deposition [95]. PEDOT-based thin films with an excellent conductivity of 900 S/cm were synthesized using methanol-substituted 3,4-ethylenedioxythiophene [64]. Immersion treatment in EG further enhanced their conductivities to 1418 S/cm [68]. Mixing with an ionic liquid, 1-ethyl-3-methylimidazolium tetracyanoborate, showed a remarkable conductivity of up to 2084 S/cm [69]. Sulfuric acid post treatment also presented an enhanced conductivity (up to 3065 S/cm) [70]. Ferric tosylate was used as the oxidizing agent and poly(ethylene glycol–propylene glycol–ethylene glycol) as the additive, producing PEDOT-based thin films with a remarkable conductivity up to 3400 S/cm. 16 1 Introduction to Transparent Conductive Films ] 0 ] ] ] ] ] 11 ] 19 109 2 ] 106 103 107 ] 108 al. [ 11

al. [ 111 al. [ al. [

al. [

Ref. CBs carbonaceous by / DC Op

σσ 13.9 Jackson et (Ω/sq) s R 160115 16.3 Zhang et 90.9 60 64.1 Hecht et 80 340 4.7 Song et 8087 858580 71 18.8 8077.682 et Yim 59 27.8 56 76 17.1 Geng et 24.9 23.8 Jo et et Wang Liu et T (%) T 2 2 TN-PEG terminated poly(ethylene glycol), + SOCl + SOCl 2 2 3 3 3 3 3 SOCl HNO Doping agent HNO – HNO – HNO HNO Transfer printing Transfer Spin coating method Transfer printingTransfer SOBr Transfer printing Transfer Transfer printing Transfer Spin coating Dip or spray coating Transfer printing Transfer Spin coating SDS sodium dodecyl sulfate, SDS DCE Solvents or surfactant Fabrication Triton X-100 Triton CBs SCA SDS DCE SDS Oligothiophene-TN-PEG TCs prepared using different approaches [ 101 ] TCs prepared using different SCA superacid chlorosulfonic acid, Fabrication of SWNT Fabrication of SWNT

Non-covalent functionalization Dispersion method Using specific solvents products Table 1.4 Table DCE dichloroethane, 1.4 Transparent Conducting Metals 17

1.4 Transparent Conducting Metals

1.4.1 Metal Nanogrids

The unique nanostructure of metals offers opportunities to control photons and electrons, which is not feasible for ITO electrodes. Nanoscale grids consisting of metal lines in a vertically aligned manner have been explored to use as TCFs [30]. Figure 1.8 presents typical examples of nanogrids made from metal lines. The prototype metal lines were much thicker than metal films, achieving much reduced electron scattering arising from the roughness and strain boundaries of the substrate. Thus, the metal nanogrid exhibits excellent optoelectrical properties, with ~ 70 % T and sheet resistance of ~ 10 Ω/sq (Fig. 1.8b). The synthesis of metal nanogrids, however, is very costly, making it difficult to employ large-scale production.

1.4.2 Metal Nanowires

As noted above, the fabrication of metal nanogrids is very costly [24]. Thus, randomly distributed metal NWs made of especially copper and silver nanowires (Cu and Ag NWs) [97, 98] have been investigated as another candidate for TCFs. Cu NWs were synthesized in gram quantity in aqueous solution and they were assembled into flexible films [101]. The scanning electron microscope (SEM) image in Fig. 1.9a shows long wires of 90 ± 10 nm in diameter and 10 ± 3 μm in length. The as-produced Cu NW films exhibited a very low sheet resistance ( Rs) of 15 Ω/sq at a T of 65 %, along with good stability upon exposure to air for 1 month or after bending for 1000 cycles. The films consisting of Ag NWs with higher aspect ratios and uniform dimensions (Fig. 1.9b) [98] significantly improved the optoelectrical properties, i.e., 20 Ω/sq at ~ 80 % specular T and 8 Ω/sq at 80 % diffusive T in the visible spectral range. Analogous to metal nanogrids discussed above, their high

Fig. 1.8 Metal nanogrids as TCs. a Scanning electron microscope (SEM) image of Au nanogrids synthesized via a nano-imprinting technique. b Average transmittance plotted as a function of sheet resistance ( Rs) of Au nanogrids. (Reprinted with permission from [96]. Copyright (2007) by Wiley) 18 1 Introduction to Transparent Conductive Films

Fig. 1.9 Randomly distributed metal nanowires and carbon nanotubes as TCs. a SEM image of Cu NWs 90 ± 10 nm in diameter and 10 ± 3 μm in length (with inset showing Cu nanowires with spherical copper particles attached at one end (scale bar = 200 nm)) [97]. b SEM image of Ag NW films [98]; and c SEM and d AFM images of single-walled carbon nanotube (SWNT) thin film deposited on glass [99]. (Reprinted with permission from [97–99]. Copyright (2010, 2009) by Wiley and ACS) manufacturing and material costs are the main challenges for metal NWs to replace ITO-based TCFs [24].

1.5 Transparent Conducting Carbon

1.5.1 Carbon Nanotubes (CNTs)

CNTs have been regarded as the ultimate candidate for high-quality TCFs for more than two decades due to their unique mechanical, electrical, optical, and electrochemical properties along with extremely high aspect ratios (~ 1000) [19, 99, 100]. Figure 1.9c and d present the SEM and AFM images of single-walled carbon nanotube (SWNT) films uniformly deposited on a glass surface. After treatment with fuming sulfuric acid, the TCFs yielded Rs values of 100 and 300 Ω/sq at transparencies of 70 and 90 %, respectively. One of the main challenges of producing CNT- based TCs through a solution process is poor dispersion caused by the high aspect ratio, large specific area, and strong van der Waals attraction [101]. Three major strategies have been performed to disperse CNTs in liquid media, i.e., (i) using specific solvents, (ii) non-covalent functionalization by surfactants, and (iii) covalent 1.5 Transparent Conducting Carbon 19 functionalization of CNTs. Since covalent functionalization locally destroys the sp2 structure of SWNTs with an accompanying reduction in electrical conductivity, the techniques devised for producing CNT-based TCs have been focused on using specific solvents and non-covalent functionalization. The optoelectrical properties of SWNT films produced by solution processes are compared in Table 1.4. In order to have a direct comparison, the DC to optical conductivity ratio, σσDC/ Op , is used to characterize the relative performance of transparency ( T) and sheet resistance ( Rs) between TCs with different thicknesses and those prepared using different synthesis routes and materials [102]. The rela- σσ/ tionship between T and Rs is controlled by the “conductivity ratio,” DC Op . A high σσDC/ Op ratio represents a high T and a low sheet resistance. Although the SWNT films produced by coating the SWNT dispersed in neat solvents can reach a high σσDC/ Op ratio of ~ 64, the high cost of the special solvent greatly limits industry applications. To reduce the cost, surfactants including sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS), sodium cholate (SC), and commercial Triton X-100 have been widely used to disperse SWNTs in common liquid media, such as water, which is low cost, safe, and environment-friendly [101]. However, the insulating surfactants are usually hard to wash out after forming films, leading to low electrical conductivities. To further remove the remaining surfactants and also increase the carrier densities of the SWNT films, significant research efforts have been directed towards improving the electrical conductivities of TCFs made from CNTs based on several different chemical treatments (as compared in Table 1.4), such as immersion of CNT films in HNO3 [103], SOCl2 [104], HNO3 followed by SOCl2 [105], or SOBr2 [106]. Nevertheless, the CNT-based TCFs have generally underperformed ITO films.

1.5.2 Graphene

A most lately emerging candidate to replace ITO is graphene, a two-dimensional monolayer of sp2-bonded carbon atoms. The peculiar atomic structure of graphene allows it to possess unique mechanical, electrical, thermal, and optical properties that are different from those of CNTs or fullerenes [113]. A few techniques have been developed to synthesize graphene, such as mechanical peel-off [114], epitaxial growth [115], and chemical vapor deposition (CVD) [116]. Graphene has attracted significant interests as the transparent conductive electrodes because of the exceptional electrical transport properties with high optical T. Graphene thin films are produced by transferring graphene sheets onto transparent substrates through physical contact printing [117] and chemical etching processes [118]. The details of these techniques for synthesis and transferring are discussed in Chap. 2. Here, the upper and lower bound estimates of the T–R curves of graphene were predicted based on the relation between T and Rs [119]:

ZG− T =(1 + 00 )2 , (1.1) 2RSDσ 2 20 1 Introduction to Transparent Conductive Films

Fig. 1.10 Outstanding optoelectrical properties of graphene [119]. a Transmittance for different TCs (graphene [121], single-walled carbon nanotubes (SWNTs) [103], indium tin oxide (ITO)

[122], ZnO/Ag/ZnO [123], and TiO2/Ag/TiO2 [124]). b Transmittance ( T) versus sheet resistance ( Rs) for different TCs. Blue rhombuses: roll-to-roll graphene TCFs based on chemical vapor deposition (CVD)-grown graphene [121]; red line: ITO [122]; grey dots: metal nanowires [122]; green triangles: SWNTs [103]. (Reprinted with permission from [119]. Copyright (2010) by Nature Publishing Group)

where Zc00=1/ε = 377 Ω is the free-space impedance, ε0 is the free space electric constant, and c is the speed of light. For graphene sheet, σµ2D = ne, where n is the number of charge carriers and μ is the mobility. For an ideal single layer graphene, the transparency can reach up to 97.7 % with a sheet resistance of ~ 6 kΩ/ sq. The sheet resistance can be further reduced to ~ 400 Ω/sq without sacrificing the transparency by a chemical doping treatment [120]. In additional, as compared in Fig. 1.10a, graphene shows a higher transparency over a wider wavelength range References 21 than ITO, SWNTs, and thin metallic films [119]. Figure 1.10b compares the T and

Rs for different types of TCF materials reported in the literature, including ITO, SWNTs, Ag NW mesh, CVD grown graphene. In addition to higher transparency, graphene films have higher mechanical strength, flexibility, and chemical stability than traditional electrodes made from ITO or FTO [25].

References

1. Hecht, D. S., Hu, L. B., & Irvin, G. (2011). Emerging transparent electrodes based on thin films of carbon nanotubes, graphene, and metallic nanostructures. Advanced Materials, 23(13), 1482–1513. 2. Wu, Z. C., Chen, Z. H., Du, X., Logan, J. M., Sippel, J., Nikolou, M., Kamaras, K., Reynolds, J. R., Tanner, D. B., Hebard, A. F., & Rinzler, A. G. (2004). Transparent, conductive carbon nanotube films. Science, 305(5688), 1273–1276. 3. Wang, X., Zhi, L., Tsao, N., Tomovic, Z., Li, J., & Muellen, K. (2008). Transparent carbon films as electrodes in organic solar cells. Angewandte Chemie-International Edition, 47(16), 2990–2992. 4. NanoMarkets (2012). Accessed August 1, 2013 from: www.NanoMarkets.net. 5. Display-search (2014). Accessed July 15, 2014 from: http://www.displaysearch.com/cps/rde/ xchg/displaysearch/hs.xsl/news.asp. 6. Wassei, J. K., Kaner, R. B. (2010). Graphene, a promising transparent conductor. Materials Today, 13(3), 52–59. 7. Gordon, R. G. (2000). Criteria for choosing transparent conductors. MRS Bulletin, 25(8), 52–57. 8. Kim, S., & Taya, M. (2012). Electrochromic windows based on V2O5-TiO2 and poly (3,3-dimethyl-3,4-dihydro-2 H-thieno[3,4-b][1,4]dioxepine) coatings. Solar Energy Materials and Solar Cells, 107, 225–229. 9. Hu, M. J., Gao, J. F., Dong, Y. C., Li, K., Shan, G. C., Yang, S. L., & Li, R. K. Y. (2012). Flexible transparent PES/Silver nanowires/PET sandwich-structured film for high-efficiency electromagnetic interference shielding. Langmuir, 28(18), 7101–7106. 10. Thomas, G. (1997). Materials science—Invisible circuits. Nature, 389(6654), 907–908. 11. Display-search. Quarterly advanced global TV shipment and forecast report. 12. Schindler, A., Schau, P., & Fruehauf, N. (2009). Active-matrix and flexible liquid-crystal displays with carbon-nanotube pixel electrodes. Journal of the Society for Information Display, 17(10), 853–860. 13. Wang, J., Liang, M. H., Fang, Y., Qiu, T. F., Zhang, J., & Zhi, L. J. (2012). Rod-coating: Towards large-area fabrication of uniform reduced graphene oxide films for flexible touch screens. Advanced Materials, 24(21), 2874–2878. 14. Luechinger, N. A., Athanassiou, E. K., Stark, W. J. (2008). Graphene-stabilized copper nanoparticles as an air-stable substitute for silver and gold in low-cost ink-jet printable electronics. Nanotechnology, 19(44), 445201. 15. Wang, S., Ang, P. K., Wang, Z. Q., Tang, A. L. L., Thong, J. T. L., & Loh, K. P. (2010). High mobility, printable, and solution-processed graphene electronics. Nano Letters, 10(1), 92–98. 16. Zhu, X. Z., Han, Y. Y., Liu, Y., Ruan, K. Q., Xu, M. F., Wang, Z. K., Jie, J. S., & Liao, L. S. (2013). The application of single-layer graphene modified with solution-processed TiOx and PEDOT:PSS as a transparent conductive anode in organic light-emitting diodes. Organic Electronics, 14(12), 3348–3354. 17. Giangregorio, M. M., Losurdo, M., Bianco, G. V., Dilonardo, E., Capezzuto, P., & Bruno, G. (2013). Synthesis and characterization of plasmon resonant gold nanoparticles and graphene for photovoltaics. Materials Science and Engineering B-Advanced Functional Solid-State Materials, 178(9), 559–567. 22 1 Introduction to Transparent Conductive Films

18. Park, H., Brown, P. R., Buloyic, V., & Kong, J. (1996). Graphene as transparent conducting electrodes in organic photovoltaics: Studies in graphene morphology, hole transporting layers, and counter electrodes. Nano Letters, 12(1), 133–140. 19. Zhang, D. H., Ryu, K., Liu, X. L., Polikarpov, E., Ly, J., Tompson, M. E., Zhou, C. W. (2006). Transparent, conductive, and flexible carbon nanotube films and their application in organic light-emitting diodes. Nano Letters, 6(9), 1880–1886. 20. Forrest, S. R. (2004). The path to ubiquitous and low-cost organic electronic appliances on plastic. Nature, 428(6986), 911–918. 21. Na, S. I., Kim, S. S., Jo, J., & Kim, D. Y. (2008). Efficient and flexible ito-free organic solar cells using highly conductive polymer anodes. Advanced Materials, 20(21), 4061–4067. 22. Chang, Y. M., Wang, L., & Su, W. F. (2008). Polymer solar cells with poly(3,4-ethylenedioxythiophene) as transparent anode. Organic Electronics, 9(6), 968–973. 23. Azulai, D., Belenkova, T., Gilon, H., Barkay, Z., & Markovich, G. (2009). Transparent metal nanowire thin films prepared in mesostructured templates. Nano Letters, 9(12), 4246–4249. 24. Hu, L., Wu, H., & Cui, Y. (2011). Metal nanogrids, nanowires, and nanofibers for transparent electrodes. MRS Bulletin, 36, 760–765. 25. Zheng, Q. B., Ip, W. H., Lin, X. Y., Yousefi, N., Yeung, K. K., Li, Z. G., & Kim, J. K. (2011). Transparent conductive films consisting of ultra large graphene sheets produced by Langmuir-Blodgett assembly. Acs Nano, 5(7), 6039–6051. 26. Geim, A. K., & Novoselov, K. S. (2007). The rise of graphene. Nature Materials, 6(3), 183–191. 27. OLED-Info (2014). Accessed May 1, 2014 from: http://www.oled-info.com/flexible-oled. 28. Holst-Centre (2014). Accessed July 5, 2014 from: http://www.holstcentre.com/Home/Part- neringinResearch/SharedPrograms/TechnologyIntegration/OPV.aspx. 29. Ginley, D. S. (2010). Handbook of Transparent Conductors. Springer New York Heidelberg Dordrecht London: Springer. 30. Lin, H., Yu, J. S., Wang, N. N., Huang, C. H., & Jiang, Y. D. (2010). Fabrication and characterization of photo cathode materials for transparent organic light-emitting diodes. 5th international symposium on advanced optical manufacturing and testing technologies: optoelectronic materials and devices for detector, imager, display, and energy conversion technology, 7658. 31. Sberveglieri, G., Groppelli, S., & Coccoli, G. (1988). Radio-frequency magnetron sputtering

growth and characterization of indium tin oxide (ITO) thin-films for NO2 gas sensors. Sensor Actuator, 15(3), 235–242. 32. O’Dwyer, C., Szachowicz, M., Visimberga, G., Lavayen, V., Newcomb, S. B., & Torres, C. M. S. (2009). Bottom-up growth of fully transparent contact layers of indium tin oxide nanowires for light-emitting devices. Nature Nanotechnology, 4(4), 239–244. 33. Mayer, G. (2006). New classes of tough composite materials—Lessons from natural rigid biological systems. Materials Science and Engineering C-Bio S, 26(8), 1261–1268. 34. Zuev, D. A., Lotin, A. A., Novodvorsky, O. A., Lebedev, F. V., Khramova, O. D., Petuhov, I. A., Putilin, P. N., Shatohin, A. N., Rumyanzeva, M. N., & Gaskov, A. M. (2012). Pulsed laser deposition of ITO thin films and their characteristics. Semiconductors, 46(3), 410–413. 35. Korosi, L., Papp, S., Beke, S., Pecz, B., Horvath, R., Petrik, P., Agocs, E., & Dekany, I. (2012). Highly transparent ITO thin films on photosensitive glass: sol-gel synthesis, structure, morphology and optical properties. Applied Physics a-Materials Science & Processing, 107(2), 385–392. 36. Wu, G. M., Zhou, Y., Ding, Y., & Yin, T. L. (2013). Preparation of ITO Thin Films by In- jection Ultrasound Spray Pyrolysis and its Physical Properties. Integrated Ferroelectrics, 144(1), 161–168. 37. Senthilkumar, V., Vickraman, P., Jayachandran, M., & Sanjeeviraja, C. (2010). Structural and optical properties of indium tin oxide (ITO) thin films with different compositions prepared by electron beam evaporation. Vacuum, 84(6), 864–869. 38. Wan, Q., Dattoli, E. N., Fung, W. Y., Guo, W., Chen, Y. B., Pan, X. Q., & Lu, W. (2006). High-performance transparent conducting oxide nanowires. Nano Letters, 6(12), 2909–2915. References 23

39. Heusing, S., de Oliveira, P. W., Kraker, E., Haase, A., Palfinger, C., & Veith, M. (2009). Wet chemical deposited ITO coatings on flexible substrates for organic photodiodes. Thin Solid Films, 518(4), 1164–1169. 40. Kim, H., Horwitz, J. S., Kushto, G., Pique, A., Kafafi, Z. H., Gilmore, C. M., & Chrisey, D. B. (2000). Effect of film thickness on the properties of indium tin oxide thin films. Journal of Applied Physics, 88(10), 6021–6025. 41. Minami, T. (2008). Present status of transparent conducting oxide ing oxide thin-film development for Indium-Tin-Oxide (ITO) substitutes. Thin Solid Films, 516(17), 5822–5828. 42. Coutts, T. J., Young, D. L., & Li, X. N. (2000). Characterization of transparent conducting oxides. MRS Bulletin, 25(8), 58–65. 43. Furubayashi, Y., Hitosugi, T., Yamamoto, Y., Inaba, K., Kinoda, G., Hirose, Y., Shimada, T., & Hasegawa, T. (2005). A transparent metal: Nb-doped anatase TiO2. Applied Physics Let- ters, 86(25), 252101. 44. Minami, T. (2005). Transparent conducting oxide semiconductors for transparent electrodes. Semiconductor Science and Technology, 20(4), S35–S44. 45. Minami, T. (2008). Substitution of transparent conducting oxide thin films for indium tin oxide transparent electrode applications. Thin Solid Films, 516(7), 1314–1321. 46. Wang, X., Zhi, L. J., & Mullen, K. (2008). Transparent, conductive graphene electrodes for dye-sensitized solar cells. Nano Letters, 8(1), 323–327. 47. Schlatmann, A. R., Wilms Floet, D., Hilberer, A., Garten, F., Smulders, P. J. M., Klapwijk, T. M., & Hadziioannou, G. (1996). Indium contamination from the indium–tin–oxide electrode in polymer light-emitting diodes. Applied Physics Letter, 69, 1764. 48. Kaufman, J. H., Brock, P. J., DiPietro, R., Salem, J., & Goitia, J. A. (1996). Degradation and failure of MEH-PPV light-emitting diodes. Journal of Applied Physics, 79, 2745–2751 49. Andersson, A., Johansson, N., Broms, P., Yu, N., Lupo, D., & Salaneck, W. R. (1998). Fluo- rine tin oxide as an alternative to indium tin oxide in polymer LEDs. Advanced Materials, 10(11), 859–863. 50. Wang, L., Yang, Y., Marks, T. J., Liu, Z. F., & Ho, S. T. (2005). Near-infrared transparent electrodes for precision Teng-Man electro-optic measurements: In2O3 thin-film electrodes with tunable near-infrared transparency. Applied Physics Letters, 87(16), 161107. 51. Kumar, A., & Zhou, C. W. (2010). The race to replace tin-doped indium oxide: which material will win? Acs Nano, 4(1), 11–14. 52. Jo, G., Choe, M., Lee, S., Park, W., Kahng, Y. H., & Lee, T. (2012). The application of graphene as electrodes in electrical and optical devices. Nanotechnology, 23(11), 112001. 53. Wang, P. C., Liu, L. H., Mengistie, D. A., Li, K. H., Wen, B. J., Liu, T. S., & Chu, C. W. (2013). Transparent electrodes based on conducting polymers for display applications. Dis- plays, 34(4), 301–314. 54. Wang, P. C., & MacDiarmid, A. G. (2007). Integration of polymer-dispersed liquid crystal composites with conducting polymer thin films toward the fabrication of flexible display devices. Displays, 28(3), 101–104. 55. Wang, Y., Liu, S. H., Dang, F. Y., Li, Y., Yin, Y. M., Liu, J., Xu, K., Piao, X. C., & Xie, W. F. (2012). An efficient flexible white organic light-emitting device with a screen-printed conducting polymer anode. Journal of Physics D: Applied Physics, 45(40), 402002. 56. Lee, H. J., Park, T. H., Choi, J. H., Song, E. H., Shin, S. J., Kim, H., Choi, K. C., Park, Y. W., & Ju, B. K. (2013). Negative mold transfer patterned conductive polymer electrode for flexible organic light-emitting diodes. Organic Electronics, 14(1), 416–422. 57. Stenger-Smith, J. D. (1998). Intrinsically electrically conducting polymers. Synthesis, characterization, and their applications. Progress in Polymer Science, 23(1), 57–79. 58. Mccullough, R. D., Tristramnagle, S., Williams, S. P., Lowe, R. D., & Jayaraman, M. (1993). Self-orienting head-to-tail poly(3-alkylthiophenes)—new insights on structure-property relationships in conducting polymers. Journal of the American Chemical Society, 115(11), 4910–4911. 59. Cirpan, A., Kucukyavuz, Z., & Kucukyavuz, S. (2003). Synthesis, characterization and electrical conductivity of poly(p-phenylene vinylene). Turkish Journal of Chemistry, 27(2), 135–143. 24 1 Introduction to Transparent Conductive Films

60. Avlyanov, J. K., Kuhn, H. H., Josefowicz, J. Y., & MacDiarmid, A. G. (1997). In-situ deposited thin films of polypyrrole: Conformational changes induced by variation of dopant and substrate surface. Synthetic Metals, 84(1–3), 153–154. 61. Avlyanov, J. K., Josefowicz, J. Y., & Macdiarmid, A. G. (1995). Atomic-force microscopy surface-morphology studies of in-situ deposited polyaniline thin-films. Synthetic Metals, 73(3), 205–208. 62. Lim, T. H., Oh, K. W., & Kim, S. H. (2012). Self-assembly supramolecules to enhance electrical conductivity of polyaniline for a flexible organic solar cells anode. Solar Energy Mate- rials and Solar Cells, 101, 232–240. 63. Hohnholz, D., Okuzaki, H., & MacDiarmid, A. G. (2005). Plastic electronic devices through line patterning of conducting polymers. Advanced Functional Materials, 15(1), 51–56. 64. Ha, Y. H., Nikolov, N., Pollack, S. K., Mastrangelo, J., & Martin, B. D., Shashidhar, R. (2004). Towards a transparent, highly conductive poly(3,4-ethylenedioxythiophene). Ad- vanced Functional Materials, 14(6), 615–622. 65. Winther-Jensen, B., & West, K. (2004). Vapor-phase polymerization of 3,4-ethylenedioxythiophene: A route to highly conducting polymer surface layers. Macromolecules, 37(12), 4538–4543. 66. Xia, Y. J., Sun, K., & Ouyang, J. Y. (2012). Highly conductive poly(3,4-ethylenedioxythioph ene):poly(styrene sulfonate) films treated with an amphiphilic fluoro compound as the transparent electrode of polymer solar cells. Energy & Environmental Science, 5(1), 5325–5332. 67. Alemu, D., Wei, H. Y., Ho, K. C., & Chu, C. W. (2012). Highly conductive PEDOT:PSS electrode by simple film treatment with methanol for ITO-free polymer solar cells. Energy & Environmental Science, 5(11), 9662–9671. 68. Kim, Y. H., Sachse, C., Machala, M. L., May, C., Muller-Meskamp, L., & Leo, K. (2011). Highly conductive PEDOT:PSS electrode with optimized solvent and thermal post-treatment for ito-free organic solar cells. Advanced Functional Materials, 21(6), 1076–1081. 69. Badre, C., Marquant, L., Alsayed, A. M., & Hough, L. A. (2012). Highly conductive poly (3,4-ethylenedioxythiophene): poly (styrenesulfonate) films using 1-ethyl-3-methylimidazolium tetracyanoborate ionic liquid. Advanced Functional Materials, 22(13), 2723–2727. 70. Xia, Y. J., Sun, K., & Ouyang, J. Y. (2012). Solution-processed metallic conducting polymer films as transparent electrode of optoelectronic devices. Advanced Materials, 24(18), 2436–2440. 71. Fabretto, M. V., Evans, D. R., Mueller, M., Zuber, K., Hojati-Talemi, P., Short, R. D., Wal- lace, G. G., & Murphy, P. J. (2012). Polymeric material with metal-like conductivity for next generation organic electronic devices. Chemistry of Materials, 24(20), 3998–4003. 72. McCullough, R. D. (1998). The chemistry of conducting polythiophenes. Advanced Materi- als, 10(2), 93–116. 73. Kao, C. Y., Lee, B., Wielunski, L. S., Heeney, M., McCulloch, I., Garfunkel, E., Feldman, L. C., & Podzorov, V. (2009). Doping of conjugated polythiophenes with alky silanes. Advanced Functional Materials, 19(12), 1906–1911. 74. De Carvalho, L. C., Dos Santos, C. N., Alves, H. W. L., & Alves, J. L. A. (2003). Theoretical studies of poly (para-phenylene vinylene) (PPV) and poly (para-phenylene) (PPP). Micro- electronics Journal, 34(5–8), 623–625. 75. Burroughes, J. H., Bradley, D. D. C., Brown, A. R., Marks, R. N., Mackay, K., Friend, R. H., Burns, P. L., & Holmes, A. B. (1990). Light-emitting-diodes based on conjugated polymers. Nature, 347(6293), 539–541. 76. Sariciftci, N. S., Braun, D., Zhang, C., Srdanov, V. I., Heeger, A. J., Stucky, G., & Wudl, F. (1993). Semiconducting polymer-buckminsterfullerene heterojunctions—diodes, photodiodes, and photovoltaic cells. Applied Physics Letters, 62(6), 585–587. 77. Arias, A. C., Roman, L. S., Kugler, T., Toniolo, R., Meruvia, M. S., & Hummelgen, I. A. (2000). The use of tin oxide thin films as a transparent electrode in PPV based light-emitting diodes. Thin Solid Films, 371(1–2), 201–206. 78. Soylu, M. (2012). Fabrication and characterization of transparent MEH-PPV/n-GaN (0001) heterojunction devices. Optical Materials, 34(5), 878–883. References 25

79. MacDiarmid, A. G. (2001). “Synthetic metals”: A novel role for organic polymers (Nobel lecture). Angewandte Chemie-International Edition, 40(14), 2581–2590. 80. Wang, P. C., & MacDiarmid, A. G. (2001). Dependency of properties of in situ deposited polypyrrole films on dopant anion and substrate surface. Synthetic Metals, 119(1–3), 367–368. 81. Jang, K. S., Han, S. S., Suh, J. S., & Oh, E. J. (2001). Synthesis and characterization of alcohol soluble polypyrrole. Synthetic Metals, 119(1–3), 107–108. 82. Stejskal, J., & Gilbert, R. G. (2002). Polyaniline. Preparation of a conducting polymer (IUPAC technical report). Pure and Applied Chemistry, 74(5), 857–867. 83. Huang, L. M., Chen, C. H., & Wen, T. C. (2006). Development and characterization of flexible electrochromic devices based on polyaniline and poly (3,4-ethylenedioxythiophene)- poly(styrene sulfonic acid). Electrochimica Acta, 51(26), 5858–5863. 84. Virji, S., Huang, J. X., Kaner, R. B., & Weiller, B. H. (2004). Polyaniline nanofiber gas sensors: Examination of response mechanisms. Nano Letters, 4(3), 491–496. 85. Macdiarmid, A. G., & Epstein, A. J. (1994). The concept of secondary doping as applied to polyaniline. Synthetic Metals, 65(2–3), 103–116. 86. Avlyanov, J. K., Min, Y. G., Macdiarmid, A. G., & Epstein, A. J. (1995). Polyaniline—conformational-changes induced in solution by variation of solvent and doping level. Synthetic Metals, 72(1), 65–71. 87. Xia, Y. N., Macdiarmid, A. G., & Epstein, A. J. (1994). Camphorsulfonic acid fully doped polyaniline emeraldine salt—in-situ observation of electronic and conformational-changes induced by organic vapors by an ultraviolet-visible near-infrared spectroscopic method. Mac- romolecules, 27(24), 7212–7214. 88. Bruk, L., Fedorov, V., Sherban, D., Simashkevich, A., Usatii, I., Bobeico, E., & Morvillo, P. (2009). Isotype bifacial silicon solar cells obtained by ITO spray pyrolysis. Materials Science and Engineering B-Advanced Functional Solid-State Materials, 159–60, 282–285. 89. Jaymand, M. (2013). Recent progress in chemical modification of polyaniline. Progress in Polymer Science, 38(9), 1287–1306. 90. Park, S., Lee, T. J., Kim, D. M., Kim, J. C., Kim, K., Kwon, W., Ko, Y. G., Choi, H., Chang, T., & Ree, M. (2010). Electrical Memory Characteristics of a Nondoped pi-Conjugated Poly- mer Bearing Carbazole Moieties. Journal of Physical Chemistry B, 114(32), 10294–10301. 91. Bello, A., Giannetto, M., Mori, G., Seeber, R., Terzi, F., & Zanardi, C. (2007). Optimization of the DPV potential waveform for determination of ascorbic acid on PEDOT-modified electrodes. Sensor Actuat B-Chem, 121(2), 430–435. 92. Groenendaal, B. L., Jonas, F., Freitag, D., Pielartzik, H., & Reynolds, J. R. (2000). Poly(3,4- ethylenedioxythiophene) and its derivatives: Past, present, and future. Advanced Materials, 12(7), 481–494. 93. Pettersson, L. A. A., Ghosh, S., & Inganas, O. (2002). Optical anisotropy in thin films of poly(3,4-ethylenedioxythiophene)-poly(4-styrenesulfonate). Organic Electronics, 3(3–4), 143–148. 94. Jonsson, S. K. M., Birgerson, J., Crispin, X., Greczynski, G., Osikowicz, W., van der Gon, A. W. D., Salaneck, W. R., & Fahlman, M. (2003). The effects of solvents on the morphology and sheet resistance in poly (3,4-ethylenedioxythiophene)-polystyrenesulfonic acid (PEDOT-PSS) films. Synthetic Metals, 139(1), 1–10. 95. Kim, Y. B., Park, S., & Hong, J. W. (2009). Fabrication of flexible polymer dispersed liquid crystal films using conducting polymer thin films as the driving electrodes. Thin Solid Films, 517(10), 3066–3069. 96. Kang, M. G., & Guo, L. J. (2007). Nanoimprinted semitransparent metal electrodes and their application in organic light-emitting diodes. Advanced Materials, 19(10), 1391–1396. 97. Rathmell, A. R., Bergin, S. M., Hua, Y. L., Li, Z. Y., & Wiley, B. J. (2010). The growth mechanism of copper nanowires and their properties in flexible, transparent conducting films. Advanced Materials, 22(32), 3558–3563. 98. Hu, L. B., Kim, H. S., Lee, J. Y., Peumans, P., & Cui, Y. (2010). Scalable coating and properties of transparent, flexible, silver nanowire electrodes. Acs Nano, 4(5), 2955–2963. 26 1 Introduction to Transparent Conductive Films

99. Dan, B., Irvin, G. C., & Pasquali, M. (2009). Continuous and scalable fabrication of transparent conducting carbon nanotube films. Acs Nano, 3(4), 835–843. 100. Li, J., Hu, L., Wang, L., Zhou, Y., Gruner, G., & Marks, T. J. (2006). Organic light-emitting diodes having carbon nanotube anodes. Nano Letters, 6(11), 2472–2477. 101. Du, J. H., Pei, S. F., Ma, L. P., & Cheng, H. M. (2014).5th anniversary article: Carbon nanotube- and graphene-based transparent conductive films for optoelectronic devices. Ad- vanced Materials, 26(13), 1958–1991. 102. De, S., & Coleman, J. N. (2010). Are there fundamental limitations on the sheet resistance and transmittance of thin graphene films? Acs Nano, 4(5), 2713–2720. 103. Geng, H. Z., Kim, K. K., So, K. P., Lee, Y. S., Chang, Y., & Lee, Y. H. (2007). Effect of acid treatment on carbon nanotube-based flexible transparent conducting films. Journal of the American Chemical Society, 129(25), 7758–7759. 104. Dettlaff-Weglikowska, U., Skakalova, V., Graupner, R., Jhang, S. H., Kim, B. H., Lee, H. J., Ley, L., Park, Y. W., Berber, S., Tomanek, D., & Roth, S. (2005). Effect of SOCl2 treatment on electrical and mechanical properties of single-wall carbon nanotube networks. Journal of the American Chemical Society, 127(14), 5125–5131. 105. Parekh, B. B., Fanchini, G., Eda, G., & Chhowalla, M. (2007). Improved conductivity of transparent single-wall carbon nanotube thin films via stable postdeposition functionalization. Applied Physics Letters, 90(12), 121913. 106. Wang, Y., Di, C. A., Liu, Y. Q., Kajiura, H., Ye, S. H., Cao, L. C., Wei, D. C., Zhang, H. L., Li, Y. M., & Noda, K. (2008). Optimizing single-walled carbon nanotube films for applications in electroluminescent devices. Advanced Materials, 20(23), 4442–4449. 107. Song, Y. I., Yang, C. M., Kim, D. Y., Kanoh, H., & Kaneko, K. (2008). Flexible transparent conducting single-wall carbon nanotube film with network bridging method. Journal of Colloid and Interface Science, 318(2), 365–371. 108. Yim, J. H., Kim, Y. S., Koh, K. H., & Lee, S. (2008). Fabrication of transparent single wall carbon nanotube films with low sheet resistance. Journal of Vacuum Science & Technology B, 26(2), 851–855. 109. Hecht, D. S., Heintz, A. M., Lee, R., Hu, L. B., Moore, B., Cucksey, C., & Risser, S. (2011). High conductivity transparent carbon nanotube films deposited from superacid (vol 22, 075201, 2011). Nanotechnology, 22(16), 075201. 110. Jackson, R., Domercq, B., Jain, R., Kippelen, B., & Graham, S. (2008). Stability of doped transparent carbon nanotube electrodes. Advanced Functional Materials, 18(17), 2548–2554. 111. Jo, J. W., Jung, J. W., Lee, J. U., & Jo, W. H. (2010). Fabrication of highly conductive and transparent thin films from single-walled carbon nanotubes using a new non-ionic surfactant via spin coating. Acs Nano, 4(9), 5382–5388. 112. Liu, W. B., Pei, S. F., Du, J. H., Liu, B. L., Gao, L. B., Su, Y., Liu, C., & Cheng, H. M. (2011). Additive-free dispersion of single-walled carbon nanotubes and its application for transparent conductive films. Advanced Functional Materials, 21(12), 2330–2337. 113. Novoselov, K. S., Jiang, D., Schedin, F., Booth, T. J., Khotkevich, V. V., Morozov, S. V., & Geim, A. K. (2005). Two-dimensional atomic crystals. Proceedings of the National Acad- emy of Sciences of the United States of America, 102(30), 10451–10453. 114. Novoselov, K. S., Geim, A. K., Morozov, S. V., Jiang, D., Zhang, Y., Dubonos, S. V., Grig- orieva, I. V., & Firsov, A. A. (2004). Electric field effect in atomically thin carbon films. Science, 306(5696), 666–669. 115. Berger, C., Song, Z. M., Li, X. B., Wu, X. S., Brown, N., Naud, C., Mayou, D., Li, T. B., Hass, J., Marchenkov, A. N., Conrad, E. H., First, P. N., & de Heer, W. A. (2006). Electronic confinement and coherence in patterned epitaxial graphene. Science, 312(5777), 1191–1196. 116. Reina, A., Jia, X. T., Ho, J., Nezich, D., Son, H. B., Bulovic, V., Dresselhaus, M. S., & Kong, J. (2009). Layer area, few-layer graphene films on arbitrary substrates by chemical vapor deposition. Nano Letters, 9(1), 30–35. 117. Li, D. S., Windl, W., & Padture, N. P. (2009). Toward site-specific stamping of graphene. Advanced Materials, 21(12), 1243–1246. References 27

118. Kim, K. S., Zhao, Y., Jang, H., Lee, S. Y., Kim, J. M., Kim, K. S., Ahn, J. H., Kim, P., Choi, J. Y., & Hong, B. H. (2009). Large-scale pattern growth of graphene films for stretchable transparent electrodes. Nature, 457(7230), 706–710. 119. Bonaccorso, F., Sun, Z., Hasan, T., & Ferrari, A. C. (2010). Graphene photonics and optoelectronics. Nature Photonics, 4, 611–622. 120. Blake, P., Brimicombe, P. D., Nair, R. R., Booth, T. J., Jiang, D., Schedin, F., Ponomarenko, L. A., Morozov, S. V., Gleeson, H. F., Hill, E. W., Geim, A. K., & Novoselov, K. S. (2008). Graphene-based liquid crystal device. Nano Letters, 8(6), 1704–1708. 121. Bae, S., Kim, H., Lee, Y., Xu, X. F., Park, J. S., Zheng, Y., Balakrishnan, J., Lei, T., Kim, H. R., Song, Y. I., Kim, Y. J., Kim, K. S., Ozyilmaz, B., Ahn, J. H., Hong, B. H., & Iijima, S. (2010). Roll-to-roll production of 30-inch graphene films for transparent electrodes. Nature Nanotechnology, 5(8), 574–578. 122. Lee, J. Y., Connor, S. T., Cui, Y., & Peumans, P. (2008). Solution-processed metal nanowire mesh transparent electrodes. Nano Letters, 8(2), 689–692. 123. Sahu, D. R., Lin, S. Y., & Huang, J. L. (2006). ZnO/Ag/ZnO multilayer films for the application of a very low resistance transparent electrode. Applied Surface Science, 252(20), 7509–7514. 124. Hamberg, I., & Granqvist, C. G. (1986). Evaporated Sn-Doped In2o3 films—basic optical-properties and applications to energy-efficient windows. Journal of Applied Physics, 60(11), R123–R159. Chapter 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

2.1 Introduction

To develop large-area graphene-based TCFs, one of the foremost challenges is to produce sufficient amounts of high-quality graphene sheets [1]. The techniques developed for synthesizing graphene can be grouped into six major methods, i.e., mechanical cleavage, epitaxial growth, chemical vapor deposition (CVD), total organic synthesis, and chemical method. The first attempt to produce graphene can go back to 1960, when the electron microscopist, Fernandez-Moran, was looking for a robust, electron-beam transparent, and uniform support membrane [2]. Millimeter-sized graphene sheets as thin as 5 nm (~ 15 layers of graphene) were produced by micromechanical exfoliation from graphite [2]. Single layers and bilayers of colloidal graphite oxide were observed by electron microscopy by Boehm et al. in 1962 [3]. Chemical intercalation and exfoliation of oxidized graphite were extensively investigated in the next decade [4]. Since the discovery of fullerenes and nanotubes in the early 1990s, great interests were attracted to study all kinds of carbon materials including graphene [4]. Nanoscale origami-like structures of one-graphene thickness were observed by atomic force microscopy (AFM) manipulation of freshly cleaved pyrolytic graphite [5]. Sub-10 nm stacks of graphite were obtained by rubbing micro fabricated graphite pillars on a substrate in 1999 [6], suggesting a possibility to produce single layer using this technique [7]. In 2004, Geim’s group successfully extracted monolayer graphene sheets by repeatedly cleaving a graphite crystal with an adhesive tape to its limit [8]. The success in mechanical cleavage led to the synthesis of graphene using other techniques that had reputedly failed in the past [4]. Among others, epitaxial growth [9] and CVD [10] were shown to produce high-quality graphene. New methods have emerged to transfer CVD-grown graphene to other substrates for applications in devices [11, 12]. In order to produce large quantities of graphene for industry applications, developing large-scale and mass production methods became necessary [13]. Among those feasible for large-scale production include the

© Springer Science+Business Media New York 2015 29 Q. Zheng, J.-K. Kim, Graphene for Transparent Conductors, DOI 10.1007/978-1-4939-2769-2_2 30 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

Table 2.1 Synthesis of graphene. (Reprinted with permission from [15]. Copyright (2014) by Elsevier) Method Figure/illustration Images Advantages Disadvantages References

Neither scalable nor Novoselov et Mechanical Less defects capable for mass al.8, 16; cleavage 14 production Singh et al.

No defects for Epitaxial every single Discontinuous Sutter et al. 17 growth graphene island

Compatible with High cost and CVD the current CMOS complex transfer Kim et al. 18 technologies process

Potentially suitable Total organic for mass Many defects Yang et al. 19 synthesis production

Low cost and The majority of Chemical Stankovich et suitable for mass defects can be method al. 20 production removed

total organic synthesis of polyaromatic hydrocarbons (PAH) [14] and the chemical route to produce reduced graphene oxide (rGO) sheets. At present, the chemical method has emerged to be a viable route to afford graphene-based single sheets in considerable quantities [14]. Table 2.1 summarizes the relative advantages and disadvantages of the above synthesis methods in term of the feasibility to scale-up the process for mass production, materials and production costs, and the presence of defects. The detailed fabrication processes of these methods are described in Sect. 2.2. 2.2 Synthesis Methods of Graphene and Graphene Oxide 31

2.2 Synthesis Methods of Graphene and Graphene Oxide

2.2.1 Mechanical Cleavage

Mechanical cleavage is the method developed to isolate graphene by peeling it off from graphite flakes using a Scotch tape. As shown in Fig. 2.1, the presence of graphene was optically identified by transferring it to a silicon dioxide layer on Si [4, 8]. Since the interlayer van der Waals force in graphite is very weak with interaction energy of ~ 2 eV/nm [2], graphite can be easily exfoliated using an adhesive tape [21]. The graphene and graphite pieces can be transferred onto the cleaned substrate by a gentle press of the tape after checking for smooth and thin fragments on the tape with optical microscopy [8, 16]. The method involves manual search- ing for single graphene sheets among a myriad of multilayer flakes, and after likely specimens are identified with an optical microscope, conclusive evidence of their thicknesses must be provided by performing AFM or Raman techniques [16]. As such, the yields of this approach are extremely low due to the manual operation. The choice of substrate is critically important and the apparent contrast of graphene monolayer on a SiO2/Si substrate (with an oxide thickness of either 300 or 90 nm) was maximized at about 12 % at 550 nm. This observation was explained by considering a Fabry–Perot multilayer cavity in which the optical path added by graphene to the interference of the SiO2/Si system became maximum for specific oxide thicknesses [22–24]. Thicker graphite flakes deposited on a 300 nm SiO2 appeared yellow to bluish as the thickness decreased (Fig. 2.1a), while few- or one-layer graphene appeared darker to lighter shades of purple (Fig. 2.1b) [4]. It should be noticed that the tape technique can leave glue residues on the substrate

Fig. 2.1 Micromechanically exfoliated graphene: optical images of a thin graphite, and b few- layer graphene and single-layer graphene ( lighter purple contrast) on a ~ 300 nm SiO2 layer. Yellow color indicates thicker samples whereas bluish and lighter contrast indicates thinner samples. (Reprinted with permission from [4]. Copyright (2010) by Elsevier) 32 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide surface, which may limit the carrier mobility [25]. This technique is neither scalable nor capable of mass production, even though the samples of graphene thereby obtained could be useful for fundamental studies to characterize their chemistries and properties.

2.2.2 Epitaxial Growth

Epitaxial growth is a substrate-based method, where isolated monolayer of graphene is grown on a single-crystal silicon carbide (SiC) by vacuum graphitization. Since the thermal treatment of silicon carbide at ~ 1300 °C under vacuum results in sublimation of the silicon atoms while the carbon-enriched surface undergoes reorganization and graphitization, graphene islands over the entire surface of SiC wafers were obtained through careful control of the sublimation (Table 2.2) [4, 26– 29]. The thickness of graphene layers was controlled mainly by annealing temperature and time, and the uniformity of thickness was improved by vapor-phase annealing [30, 31]. A major advantage is that epitaxial-grown graphene can be patterned using standard lithography methods, a useful property for nanoelectronics. The physical properties of graphene varied significantly between those grown epitaxially and exfoliated mechanically, due mainly to the influence of interfacial effects in epitaxial graphene. Similar approach was applied to other metallic substrates, such as ruthenium (Ru), to produce graphene layers [17, 32]. It is found that the (0001) faces of Ru crystals were able to grow epitaxial graphene layers, where a very sparse graphene nucleated at high temperatures. The first graphene layer coupled strongly to the Ru substrate and the second layer was free of the substrate interaction, which had an electronic structure similar to freestanding graphene. However, several hurdles must be overcome before real applications are found [14]. First of all, it is very hard to control the thickness of graphene in the routine production. Second, unusual rotational graphene stacking were observed in multilayer graphene due to the different epitaxial growth patterns on different SiC polar faces, which had profound effects on the physical and electronic properties of epitaxial graphene [33]. The growth mechanisms need to be further investigated. Third, the relationship between the structure and electronic properties of the interface layer between graphene and the substrate needs to be clearly understood.

2.2.3 Chemical Vapor Deposition (CVD)

In CVD methods, graphene is grown directly on a transition metal substrate via saturation of carbon upon exposure to a hydrocarbon gas at a high temperature [12, 45–50]. Ni or Cu films are typically used as the substrate with methane as the precursor gas. When the substrate is cooled, the solubility of carbon on the substrate decreases and the carbon precipitates to form mono- to multilayer graphene sheets 2.2 Synthesis Methods of Graphene and Graphene Oxide 33

Table 2.2 Epitaxial growth of graphene on SiC substrates [34] SiC substrates Fabrication method Characterization References Si-face 6 H-SiC CVD reactor, Ar atmosphere Thickness between 0.25 VanMil et al. (1500−1600 ºC, 90 min) and 1 nm having a mobil- [35] ity of 860 cm2/(V·s) for an electron concentration of 1.13 × 1013 cm2 C-face 6 H-SiC SiC sample covered with Large, homogeneous, mono- Camara et al. a graphite cap. RF-heated layer or bilayer graphene [36] furnace under high vacuum ribbons (5 × 600 μm) (1700 ºC, 15 min) C-face 6 H-SiC AlN mask on the substrate. A few-layer graphene (FLG) Camara et al. RF under high vacuum [37] (~ 1.33 × 10−4 Pa, 1550 ºC, 5 min) 6 H-SiC (0001) Inductively heated furnace, Homogeneous large-area Virojanadara 2000 ºC at an ambient argon graphene layers et al. [38] pressure of 1.013 × 105 Pa. 6 H-SiC (0001) UHV chamber Bilayer graphene Unarunotai (1.33 × 10−8 Pa), 1550 ºC et al. [39] C-face 6 H-SiC UHV MBE chamber Non-Bernal rotated graphene Moreau et al. (0001) ((1030−1050 ºC), (10−60) planes, single-layer or few- [40] min) layer graphene 4 H-SiC(0001) LEEM instrument, Bilayer and few-layer Hibino et al. 1300−1500ºC graphene [41] C-face Heated for 10 min to tempera- A mesh-like network of ridges Prakash 4 H-SiC(0001) ture T > 1350 ºC in vacuum with high curvature that et al. [42] bound atomically flat, tile-like facets of few-layer graphene Si-face 4 H-SiC UHV(pressure < 10−6 Pa) Single-layer or few-layer Jernigan (1200−1600 ºC, 10−40 min) graphene et al. [43] 3 C-SiC(111)/ Resistively heated hot wall Continuity on terraces and Ouerghi Si(111) reactor (1250−1350 ºC, step edges suggesting the et al. [44] 10 min) possibility of growing large- scale graphene suitable for industrial applications CVD chemical vapor deposition, UHV ultrahigh vacuum on the substrate. One of the major advantages of the epitaxial and CVD growth techniques is their high compatibility with the current complementary metal–oxide–semiconductor (CMOS) technology. A typical weakness is that controlling the film thickness is difficult and secondary crystals are easily formed [51], although some progress has been made to grow uniform graphene layers using the CVD method [52]. Another important disadvantage is the need of expensive substrate materials for graphene growth, considerably limiting its applications for large-scale production. Nevertheless, the CVD approach has emerged as an important method for mass production of graphene with less structural and electronic disorder or defects, making it an excellent potential for TC applications. 34 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

Graphene Grown on Ni

The formation of FLG on transition metal surfaces has been known for nearly a half century [53, 54]. The typical process for CVD of graphene onto Ni film is shown in Fig. 2.2a. In general, the polycrystalline Ni films deposited on SiO2/Si are first annealed at 900–1000 °C under H2 and Ar flow to grow a smooth surface. The Ni films are then exposed to a H2 and CH4 mixture so that the carbon atoms can dissolve into the Ni film to form a solid solution. Finally, the substrate is cooled down in an Ar atmosphere. As the solubility of carbon atoms decreases as the temperature goes down, the carbon atoms diffuse out from the Ni–C solid solution during cooling and precipitate onto the Ni surface to form graphene films.

Fig. 2.2 Graphene grown on a Ni substrate. a Illustration of graphene growth in three different stages [56]. b Schematic of full-wafer scale deposition of graphene layers on polycrystalline Ni by chemical vapor deposition (CVD). c E-beam-evaporated Ni film of thickness 100 nm on a

10 cm diameter Si/SiO2 wafer. d Atomic force microscopy (AFM) image of a Ni film after CVD of graphene layers [55]. (Reprinted with permission from [56, 58]. Copyright (2009, 2013) by Springer and ACS) 2.2 Synthesis Methods of Graphene and Graphene Oxide 35

Figure 2.2b shows the wafer-scale graphene synthesis on evaporated Ni films [55].

About 10 cm diameter Si/SiO2 wafers were used as the substrate to deposit 100-nm- thick Ni films (Fig. 2.2c). It is found that using diluted methane was key to the growth of single-layer graphene (Fig. 2.2d), whereas concentrated methane led to the growth of multilayer graphene.

Graphene Grown on Pd

Graphene islands can also be in situ grown on another metal surface, such as Pd(111), using the CVD method [57]. Figure 2.3a, b shows the scanning tunneling microscope (STM) images of graphene islands grown on a Pd(111) surface using ethylene at 968 K. The size of the graphene islands largely varied between 200 and 2000 Å, which are common, especially near the Pd step edges or spanning across the multiple terraces. The STM image in Fig. 2.3c presents the ordered honeycomb structure in the graphene island formed by the precipitation of carbon atoms. A periodicity of ~ 20 Å was observed from the surface height profile (Fig. 2.3d), which was measured along the white line in Fig. 2.3c. Due to the superposition of the

Fig. 2.3 a–c Scanning tunneling microscope (STM) images of graphene on Pd(111) acquired in situ during growth. d Surface height profile along the white line shown in c; and e atomic model showing the orientation of graphene. (Reprinted with permission from [57]. Copyright (2009) by ACS) 36 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide honeycomb lattice of graphene and the hexagonal lattice of Pd (111), these superstructures are of Moiré patterns [57]. Figure 2.3e shows the atomic model of such a commensurate Moiré superstructure and the determined epitaxial relationship between the monolayer graphene and Pd(111).The method of growing graphene islands on a Pd surface opened up a new avenue of preparing metal-semiconducting graphene structures and metal-doped graphene-based devices.

Graphene Grown on Cu

Due to the grain boundaries of Ni or Pd surface, the grown graphene films usually contain both monolayer and few-layer graphene [58]. Because carbon has extremely low solubility in Cu, Cu has become a potentially outstanding substrate for the growth of monolayer graphene [54]. Ruoff’s group at the University of Texas at Austin first reported the growth of high-quality monolayer graphene on polycrystalline Cu foils, which attracted great attention due to the advantages of good control of graphene layers, low cost, and ability to transfer [10]. The Cu foil was annealed at 1000 °C in a hydrogen atmosphere. A mixture of H2/CH4 was then introduced into the system to initiate graphene growth on the Cu foil. The system was cooled down to room temperature after a continuous graphene layer was formed [10]. The SEM image in Fig. 2.4a clearly shows Cu grains with color contrast. The high-resolution SEM in Fig. 2.4b indicates that these Cu surface steps were formed during thermal annealing. The darker flakes indicate multilayer graphene, while the wrinkles originated from the different thermal expansion coefficients between graphene and Cu. The wrinkles went across the Cu grain boundaries, confirming that the graphene film was continuous. The inset of Fig. 2.4b shows TEM images of single and bilayer graphene. The grown graphene films can be transferred to another substrate using various transfer methods. Figure 2.4c, d shows the transferred graphene film on a SiO2/Si or glass substrate. It is seen that the graphene film on the glass substrate was optically uniform. The quality and uniformity were evaluated by Raman spectroscopy and Fig. 2.4e–g shows the SEM and optical images with the corresponding Raman spectra. The Raman spectra (Fig. 2.4g) indicated by a red circle has a G-to-2D intensity ratio (IG/I2D) of ~ 0.5 and a symmetric 2D band centered at ~ 2680 cm−1 with a full width at half maximum (FWHM) of ~ 33 cm−1, confirming the monolayer graphene [10]. The blue circle and green arrow represent bilayer- and trilayer-graphene sheets, respectively.

2.2.4 Total Organic Synthesis

The graphene-like polyacrylic hydrocarbons (PAHs) are another alternative route to produce graphene. PAHs are known to have an intermediate structure between the molecular and micromolecular phases; they are highly versatile and can be substituted with a range of aliphatic chains to modify the solubility. Yang et al. [19] 2.2 Synthesis Methods of Graphene and Graphene Oxide 37

Fig. 2.4 CVD-grown graphene on Cu foil [10]. a SEM image of graphene grown for 30 min. b High- resolution SEM image showing a Cu grain boundary and steps, one- and two-layer graphene, and graphene wrinkles (Inset shows TEM images of folded graphene edges). c, d Graphene films transferred onto a SiO2/Si substrate c and a glass plate d, respectively. e SEM image of graphene transferred onto SiO2/Si showing wrinkles, as well as one-, two- and three-layer regions. f Optical microscope image of the same regions as in e. g Raman spectra for the marked spots with corresponding colored circles in e and f showing the presence of one, two, and three layers of graphene [10]. CVD chemical vapor deposition, SEM scanning electron microscope, TEM transmission electron microscope 38 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide reported the synthesis of nanoribbon-like PAHs up to 12 nm in length. Although the electrical properties of these graphene nanoribbons (GNRs) are unknown, they may indeed exhibit graphene-like properties (Table 2.1). If their size can be further extended, PAHs may offer a new route for graphene synthesis. However, it may be very difficult to totally eliminate the defects present along the boundaries between the molecules. Another bottom-up method was reported to fabricate GNRs on gold surfaces using 10,10′-dibromo-9,9′-bianthryl precursor monomers [59]. The monomers were thermally deposited onto a gold surface to remove the halogen substituents from the precursors and provide the molecular building blocks for the targeted graphene ribbons, as shown in Fig. 2.5a. The biradical species diffused across the surface and underwent radical addition reactions to form linear polymer chains during the first thermal activation step. Then, a surface-assisted cyclodehydrogenation created an extended fully aromatic system during the second thermal activation step (Fig. 2.5b–e) [59].

2.2.5 Chemical Methods

The most common source of graphite used for oxidation is flake graphite, which can be produced by removing heteroatomic contaminations from naturally occurring graphite [60]. Due to the spaces between graphene layers in graphite, the intercalating agents are able to reside between the graphene layers under chemical reactions, forming graphite intercalation compound (GIC) [61]. The experiments to investigate the insertion of additional chemical species between the basal planes have been performed extensively since the successful formation of the first intercalation compound using potassium [51]. GICs have layered compounds with different stages, which are defined as the number of graphitic layers in between adjacent planes of intercalant [51]. Figure 2.6a–d shows the structures of second- to fifth-stage bromide GICs, which correspond to approximate compositions of C16Br2, C24Br2, C32Br2, and C40Br2 [62]. The interlayer spacing of GIC can increase from 0.34 nm to more than 1 nm depending on the intercalant, leading to a significant reduction in the van der Waals forces between adjacent sheets [61]. The weak van der Waals forces make GICs much easier to be further exfoliated, offering a possible route to fabricate single layer graphene or graphene oxide (GO). The interlayer spacing in GICs can be expanded by thermal shock (~ 1000 °C) to produce expanded graphite (EG). As shown in Fig. 2.6e, the halogen intercalants, such as iodine chloride (ICl) (to form Stage-2 ionic GIC) and iodine bromide (IBr) (to form Stage-3 ionic GIC), can be introduced into the host material of highly ordered pyrolytic graphite (HOPG) and sequentially to form layered structures [63]. Upon high-temperature annealing, the volume of the obtained Stage-2 and Stage-3 GICs (Fig. 2.6f, g) increases rapidly due to the volatilization of the IBr or ICl intercalants between the graphene layers [63]. Graphite oxide was first prepared almost 150 years ago by Brodie, who treated graphite repeatedly with potassium chlorate and nitric acid [64]. The oxidizing agent 2.2 Synthesis Methods of Graphene and Graphene Oxide 39

Fig. 2.5 a Bottom-up fabrication of atomically precise graphene nanoribbons (GNRs). Basic steps for surface- supported GNR synthesis illustrated with a ball-and- stick model of the example of 10,10′-dibromo-9,9′- bianthryl monomers ( 1). Top, dehalogenation during adsorption of dihalogen- functionalized precursor monomers. Middle, formation of linear polymers by covalent interlinking of dehalogenated intermediates. Bottom, formation of fully aromatic GNRs by cyclodehydrogenation. b, c STM images of straight GNRs from bianthryl monomers [59]. d, e STM images of Chevron-type GNRs from tetraphenyl- triphenylene monomers. STM scanning tunneling microcscope. (Reprinted with permission from [59]. Copyright (2010) by Nature Publishing Group)

was changed to a mixture of sulfuric acid, nitric acid, and potassium chlorate [64]. A less hazardous and more efficient method for graphite oxidation was later devised by Hummers and Offeman [65], who employed a mixture of sodium nitrate, potassium permanganate, and concentrated sulfuric acid. The latter two modified methods are at present most widely employed [66]. Graphite oxide in water hydrolyzes to form 40 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

Fig. 2.6 Interlayer ordering in graphite intercalation compounds (GICs) with different stages and the expanded graphite (EG) [61–63]. a–d Second to fifth stage structure of bromine-intercalated GICs. The dashed lines indicate bromine intercalate layers; the solid lines indicate carbon layers [61, 62]. e 3D computer-generated molecular models of highly ordered pyrolytic graphite (HOPG) ( top), iodine bromide (IBr) Stage-3 GIC ( middle), and chloride (ICl) Stage-2 GIC ( bottom). f Photographs of HOPG ( top), Stage-3 GIC ( middle), and Stage-3 EG ( bottom). g Photographs of HOPG ( top), Stage-2 GIC ( middle), and Stage-2 EG ( bottom) [63]. (Reprinted with permission from [61, 63]. Copyright (2002, 2011) by Springer and Nature Publishing Group) negatively charged thin platelets that consist of single- to multilayer carbon. Mono- layer graphite oxide is now widely acknowledged and recognized as GO [20, 67]. The term “sheets” usually indicate monolayer to several layers, while “platelets” is often used to describe thicker multilayer GO or rGO [68]. Figure 2.7 shows the 2.2 Synthesis Methods of Graphene and Graphene Oxide 41

Fig. 2.7 Typical scheme for production of screened graphite oxide ( GO) [15, 76]. GIC graphite intercalation compound, EG expanded graphite, FGS funtionalized graphite sheets. (Reprinted with permission from [15]. Copyright (2014) by Elsevier)

process involved in the exfoliation of graphite oxide into individual GO sheets. The exfoliation is facilitated by rapid heating [69] or ultrasonic agitation [70, 71], while excessive ultrasonication often results in breakage or fragmentation along with a significant reduction in lateral dimensions of GO sheets [72, 73]. Individual GO sheets can be viewed as graphene decorated with oxygenated functional groups on the basal plane and around the edges [74]. Due to the ionization of carboxyl groups present at the edges, GO can be electrostatically stable to form a colloidal suspension [70] in water, alcohols, and certain organic solvents [75] without surfactants. To restore the inherent electrical conductivity of graphene, GO should be reduced either in solutions [20, 71, 77–79] or after films are formed on a substrate (Fig. 2.8a). Many different reducing agents have been identified, including hydrazine [20, 80, 81], dimethylhydrazine [67], hydroquinone [82], hydrogen iodine (HI)

[83, 84], and NaBH4 [85, 86]. The reduced GO sheets become less hydrophilic and instantly aggregate in the solution because of the removal of oxygenated groups (Fig. 2.8b–d). However, charge-stabilized colloidal dispersions were obtained by raising the pH value during reduction, even for deoxygenated sheets [87]. More recently, the reduction step was further improved by preparing dispersions directly in anhydrous hydrazine [80, 81, 88]. It should be noted, however, that anhydrous hydrazine is highly toxic and potentially explosive, thus a great caution should be exercised when using it. The solution-based exfoliation method involves direct exfoliation of natural graphite (NG) flakes in organic solvents, such as methanesulfonic acid (MSA) [90], ionic liquids [91], benzylamine (BA), N-methyl-2-pyrrolidone (NMP), and N,N-dimethylacetamide (DMA) with the addition of NaOH [92]. Disper- sion and exfoliation of graphite are simultaneously achieved with the aid of sonication and the sonication energy required for exfoliation is balanced by the 42 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

Fig. 2.8 Reduction of graphene oxide (GO). a Oxidation of graphite to GO and reduction to reduced graphene oxide (rGO) [14]. b–d Aberration-corrected TEM images of the following materials. Scale bar, denoting 2 nm, is valid for all images. b Single suspended sheet of graphene. c Single suspended sheet of GO. d Suspended monolayer of rGO [89]. (Reprinted with permission from [14, 89]. Copyright (2011, 2010) by Elsevier and Wiley) solvent–graphene interaction for solvents whose surface energies match that of graphene [93]. High-quality monolayer graphene sheets were stably suspended in organic solvents through the continuous exfoliation–reintercalation–expansion processes [94]. Green and Hersam [95] demonstrated that graphene sheets with controlled thicknesses were obtained using sodium cholate as a surfactant to exfoliate graphite (Fig. 2.9). The exfoliation of graphite often yields a dispersion consisting of both monolayer and few-layer graphene [95]. Although the library of available exfoliating solvents has been greatly expanded [96–100], the issues as to how the exfoliation efficiency is improved and the number of layer of exfoliated flakes is controlled still remain challenging. Another disadvantage of this process is that the high boiling point of the solvent makes the deposition of graphene sheets in the downstream process rather difficult [14]. It can be said that the approach to produce TCFs from GO colloidal suspensions is not only scalable for high-volume production, but also facile to process at low costs, making it among the most extensively studied for practical use. In particular, 2.3 Preparation of Large-Size GO 43

Fig. 2.9 Schematic illustration of graphene exfoliation process. (Reprinted with permission from [95]. Copyright (2009) by ACS)

the possibility of directly depositing GO or rGO sheets on a flexible substrate using the well-established techniques offers unparalleled benefits over the other graphene synthesis routes. Therefore, GO sheets fully exfoliated in an aqueous dispersion are considered the most versatile precursor material for large-scale production of TCFs at low costs.

2.3 Preparation of Large-Size GO

2.3.1 Mild Oxidation and Sonication

Due to the presence of a large number of intersheet junctions, small-size GO sheets may result in high intersheet contact resistance in the reduced GO films [101]. Because of the needs to synthesize large-size graphene sheets with uniform size distributions, many studies have been directed towards optimizing the parameters employed in various steps of the chemical method. Table 2.3 summarizes the representative methods of producing large-size GO by controlling the oxidation and sonication parameters. 44 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

Table 2.3 Preparation of large-size GO Starting material Oxidation Exfoliation GO size References method method Natural flake-like Hummers’ Gentle shaking Up to ~ 200 µm Zhou et al. [102] graphite of 50 method mesh size NG flake with an Mild oxidation Mild sonication Up to Zhao et al. [103] average size of ~ 40,000 µm2 in 500–600 µm area NG flakes with Modified Hum- By stirring Approximately Su et al. [104] average size of mers’ method millimeter 3–5 mm with a bath sonication Process NG flakes with Modified Hum- Short sonication Up to 50 µm Dong et al. [105] lateral size of mer’s Method 2–5 mm Graphite powder Edge-selective By stirring ~ 20 µm Bae et al. [106] functionalization NG flakes lateral Thermal expan- Gentle shaking Up to ~ 200 µm Zheng et al. size of 1–5 mm sion and modi- [107] Aboutalebi fied Hummer’s et al. [108] Jia method et al. [109] GO graphene oxide, NG natural graphite

To produce large-size GO, an attractive and necessary strategy is to control the oxidation conditions to maintain mild processes for intercalation and exfoliation of graphite. Zhou et al. [102] reported a modified solution-phase processing route to synthesize large-size GO and graphene by simple and gentle shaking, instead of using conventional ultrasonication. The sheets prepared thereby had a lateral size in the order of 100 µm, and some of them were as large as 200 µm, which are approximately comparable to those of the pristine graphite flakes. Zhao et al. [103] proposed a modified chemical exfoliation method, including mild oxidation at 0 °C and a mild sonication at 80 W for 5 min, to produce large-area GO sheets. GO sheets with an area up to ~ 40,000 µm2 were obtained (Fig. 2.10a). It is found that the C–O content in the graphite oxide greatly influenced the size of the resultant GO sheets, which in turn enabled size-controlled synthesis of GO sheets. GO sheets with average areas of 100–300, 1000–3000, ~ 7000 µm2 were selectively obtained by simply changing the oxidation conditions, such as temperature (0–90 °C) and the amount of

KMnO4 used (12–24 g KMnO4 for 2 g NG). Su et al. [104] also obtained GO sheets up to a millimeter in lateral size by replacing the aggressive oxidation process with a short sonication step in H2SO4 solutions. By adjusting the sonication time from 1 to 6 h at 300 W, the lateral size of the GO sheets reached up to ~ 3 mm (Fig. 2.10b). Dong et al. [105] also found that sonication time was crucial for GO size. The results showed that the shorter the sonication time, the larger the GO sheets. By adjusting the sonication time, reduced ultralarge graphene oxide (rUL-GO) sheets with size up to ~ 50 µm were obtained after reduction by hydrazine in the presence of aromatic tetrasodium 1,3,6,8-pyrenetetrasulfonic acid (TPA), which efficiently dispersed the resulting rUL-GO sheets in aqueous solutions. 2.3 Preparation of Large-Size GO 45

Fig. 2.10 SEM images of ultralarge graphene oxide (UL-GO) sheets. a Large graphene oxide (GO) sheets obtained via a mild oxidation and sonication method [103]. b UL-GO sheets obtained by avoiding sonication process [104]. (Reprinted with permission from [103, 104]. Copyright (2010, 2009) by ACS)

2.3.2 Edge-Selective Functionalization of Graphite (EFG)

EFG is an effective approach that was developed to minimize the breakage of graphite during the chemical synthesis of GO sheets [106]. As shown in Fig. 2.11a, the covalently linked edge groups provide steric repulsion to open pristine graphite edges and the EFG in solid state should consist of many stacks of graphene layers, allowing further exfoliation of EFG into FLG platelets and graphene in solution. The EFG technique involves a simple one-pot reaction between graphite and 4-ethylbenzoic acid (EBA) in a viscous polyphosphoric acid (PPA)/phosphorus pentoxide (P2O5) solution under mild conditions. Its main advantage is that no damage would be created on the basal plane. Figure 2.11b shows the EFG platelets with many wrinkles on a SiO2 surface over a large area. Although the as-cast films had high sheet resistance of ∼108 kΩ/sq due to the edge groups present at the boundaries between platelets, uniformly welded large-area graphene films with a low-sheet resistance of 0.52–3.11 kΩ/sq were obtained after thermal annealing at 600 °C. This is because the edge EB moieties acted as an in situ feedstock for carbon to “weld” the graphene platelets to form large-area graphene films (Fig. 2.11c).

2.3.3 Elimination of Sonication

Efficient and highly reproducible chemical methods have been developed which involve intercalation, thermal exfoliation, and chemical oxidation to produce gram- scale large-size GO sheets, up to ~ 50–200 µm in lateral size with a yield exceeding 50 % [107, 108]. The UL-GO stable suspension was obtained through oxidation and washing processes without any sonication step. It is found that the GO size produced through chemical method mainly depended on three factors: namely, (i) to control the degree of oxidation; (ii) to completely eliminate damaging ultrasonication process; 46 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

Fig. 2.11 Transparent graphene films produced via a simple solution casting of exfoliated platelets from edge-selectively functionalized graphite ( EFG). a Schematic presentation of pristine graphite, EFG in solid state and EFG in dispersed solution. b As-cast EFG film. c Heat-treated

EFG ( HEFG) film on a SiO2 surface. (Reprinted with permission from [106]. Copyright (2011) by ACS) and (iii) to use the inherently large NG flakes as the precursor material [109]. Be- cause the use of large graphite flakes as the precursor material does not necessarily guarantee large-size GO sheets, the pre-exfoliation process and the use of exfoliated graphite are necessary to avoid the damaging ultrasonication process, and thus en- sure minimum breakage of the GO sheets during fabrication. As shown in Fig. 2.12a, the fabrication process includes three steps: (i) intercalation of acid molecules into NG to obtain GICs, (ii) expansion of the interlayer space by thermal shock at a high temperature (~ 1000 °C), and (iii) chemical oxidation by modified Hummers’ method. Graphite was sufficiently exfoliated before oxidation, thus individual GO sheets were easily obtained by gentle shaking. Figure 2.12b, c shows that the sizes of the as-prepared GO sheets varied from several to hundreds of micrometers. To further understand the effects of ultrasonication, thermal shock expansion, degree of oxidation, and precursor NG flake size on final GO size, well-planned parallel experiments were performed [109]. As shown in Fig. 2.13, different 2.3 Preparation of Large-Size GO 47

Fig. 2.12 a Flow chart for the synthesis of ultralarge graphene oxide ( UL-GO) [110]. b–c SEM images of as-prepared graphene oxide (GO) sheets deposited on a Si substrate at b low and c high magnifications [107]. GIC graphite intercalation compound. (Reprinted with permission from [107, 110]. Copyright (2011, 2012) by ACS and RSC) precursor materials and processing conditions were used in three different approaches. The results show that the pre-exfoliation of NG flakes by thermal expansion was better than ultrasonication as the exfoliation step in terms of both the maximum and mean sizes of GO. Because the distorted sp3-hybridized structure caused by oxidation is much weaker than the original sp2-hybridized structure, the degree of oxidation significantly influenced the size of GO sheets. It is also worth noting that the use of large precursor NG flakes did not necessarily produce large-size GO sheets, and pre-exfoliation and the degree of oxidation were more important than the precursor size [109].

2.3.4 Size Separation Methods

The as-prepared GO sheets with polydispersity in size need to be further processed to collect those with large areas for efficient use. Three major techniques, including high-speed centrifugation [76], density gradient separation [73, 111], and pH- assisted selective sedimentation [112], have been employed to group GO sheets with different sizes, in the so-called size fractionation process. 48 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

Fig. 2.13 Effects of processing and material parameters on the size of final graphene oxide (GO) sheets. The flowchart shows the preparation of GO sheets from nano-graphene ( NG) flakes based on three different approaches. (Reprinted with permission from [109]. Copyright (2014) by Elsevier)

High-Speed Centrifugation

Several different size groups 4 of GO sheets were obtained after sequential high- speed centrifugation of GO polydispersion, and Fig. 2.14 shows the flow chart for the sequential centrifugation at gradually slower speeds. Typically, the unsorted, as-prepared GO dispersion was centrifugated initially at 8000 rpm for 40 min, di- viding GO into supernatant and precipitate, and the supernatant was collected to label as “small GO” (S-GO). The precipitate was re-dispersed for the second run of centrifugation at 6000 rpm for 25 min, producing supernatant and precipitate. The supernatant obtained in this run was designated as “large GO” (L-GO). The precipitate was re-dispersed in water for the third run of centrifugation at 4000 rpm for 25 min, producing “very large GO” (VL-GO) (supernatant) and “ultralarge GO” (UL-GO) (precipitate). Figure 2.15 presents the area distributions of the different 2.3 Preparation of Large-Size GO 49

Fig. 2.14 Flow chart for high-speed centrifugation method. (Reprinted with permission from [110]. Copyright (2012) by RSC) groups obtained, namely, S-GO, L-GO, VL-GO, and UL-GO, which were measured by counting more than 200 sheets for each group.

Density Gradient Separation

GO Dispersions containing monodispersed GO sheets can also be separated by density gradient centrifugation [73, 111, 113]. The separation method is based on the isopycnic density gradient ultracentrifugation and involves balancing the density of colloids of the material with that of a supporting medium. The technique was successfully employed to separate graphene sheets by the number of layers [95, 113] and carbon nanotubes (CNTs) by the diameter/chirality/wall thickness [114, 115]. As illustrated in Figure 2.16a, GO or rGO can be separated into a specific size range, which has particular properties for different applications by optimizing the separation parameters [73, 111]. According to the Stokes’ Law [111], larger sheets have a higher sedimentation rate after balancing the centrifugal force against buoyancy and viscous drag. The density and viscosity increased at the gradient boundaries, causing smaller GO sheets to move slowly, allowing larger and heavier sheets to pass through. Therefore, GO sheets with different lateral sizes and surface chemistries were captured along the centrifuge tube at appropriate positions when the centrifugal force was removed. Figure 2.16b shows the centrifugation tubes after separation using different conditions, and the horizontal dark bands/lines correspond to the gradient boundaries fixed before centrifugation. Figure 2.16c shows the images of an ultracentrifuge tube after density gradient separation of functionalized GO, confirming that the separation method offers an effective way to obtain GO or rGO sheets with suitable sizes in bulk quantities. 50 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

Fig. 2.15 Size distributions of a UL-GO, b VL-GO, c L-GO, and d S-GO; and e average areas of graphene oxide (GO) sheets. UL-GO ultralarge graphene oxide, VL-GO very large graphene oxide, L-GO large graphene oxide, S-GO small graphene oxide. (Reprinted with permission from [107, 110]. Copyright (2011, 2012) by ACS and RSC)

pH-Assisted Selective Sedimentation

Another useful method was established using the pH-dependent amphiphilicity of GO sheets. It is proven that the electrostatic repulsion forces existed between the GO sheets arising from the ionized carboxyl groups, which in turn effectively prevented GO sheets from aggregation in aqueous media [87]. Smaller GO sheets tend to have higher solubility than larger GO with the same pH value because the edge- to-area ratio of a GO sheet increases and so does the density of ionized –COOH 2.4 Structures of Graphene and GO 51

Fig. 2.16 Density gradient separation method: schematic a and picture b of separation of reduced graphene oxide (rGO) sheets into different sizes via density gradient centrifugation [111]; and c area sorting of polyethyleneglycol-functionalized graphene oxide (GO) after density gradient separation [73]. (Reprinted with permission from [111, 73]. Copyright (2010, 2008) by ACS and Springer) groups with decreasing GO size [112]. Thus, the technique involves selective precipitation of GO sheets with sizes larger than 40 μm2 at a pH of 4.0 which contain larger hydrophobic planes and fewer hydrophilic oxygenated groups. The zeta potentials of GO dispersions decreased with increasing pH because of the ionization of –COOH functional groups, see Fig. 2.17a. GO dispersions became stable when their zeta potential values were lower than about − 30 mV. It is worth noting that there was a “pH window” between 3.34 and 4.24, where the zeta potential of the large GO was higher than − 30 mV, and that of the small GO was lower than this value. By controlling the pH value of a GO dispersion, one can realize the fractionation of GO sheets (Fig. 2.17b).

2.4 Structures of Graphene and GO

Graphene is an atomic-scale honeycomb lattice of carbon atoms. The 2D material is the building block of all graphitic materials: it can be wrapped up in 0 D buckyballs, rolled into 1D nanotubes or stacked into 3D graphite, as schematically shown in Fig. 2.18. The graphene honeycomb lattice is composed of two equivalent sublat- tices of carbon atoms bonded together with σ bonds, where a π orbital contributes to a delocalized network of electrons for each carbon atom in the lattice, as shown in Fig. 2.19a–c [116]. It is also equally important to determine the graphene chirality for both fundamental and application view of points [117]. The intensity of disorder-induced Raman feature is correlated to the edge chirality (Figs. 2.19d–g); thus, identifying the edge chirality can be a reliable and practical method to determine the crystal orientation [117]. The chirality of graphene 52 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

a /DUJHVL]H*2 6PDOOVL]H*2

Fig. 2.17 pH-assisted selective sedimentation method. a Zeta potentials of large and small graphene oxide (GO) aqueous dispersions as a function of pH value. b SEM images of unsorted GO before fractionation, large and small GO monolayers after fractionation. (Reprinted with permission from [112]. Copyright (2011) by ACS) edge can also be directly determined by TEM, which is capable of producing mov- ies of the dynamics of carbon atoms at the edge of a hole in a suspended graphene sheets (Figs. 2.19h, i) [118]. As the first truly 2D crystal, it is particularly important to understand the mechanisms of the stability of graphene. By means of atomistic Monte Carlo simulations based on a very accurate many-body interatomic potential for carbon [119], it is found that that ripples spontaneously appear arising from thermal fluctuations with a size distribution peaked at about 80 Å (Figs. 2.19j, k), a reflection of the multiplicity of chemical bonding in carbon [120]. The structure of GO is largely different from that of pristine graphene because of the attachment of oxygenated functional groups. The nonstoichiometric chemical composition of GO makes it a challenging task to determine the structure of GO [64]. One of the most credible and well-known models of GO was proposed by Lerf [74] and Klinowski [65, 122], who focused on a nonstoichiometric and amorphous structure. As schematically shown in Fig. 2.20a, b, three major features including graphitic regions (with an original honeycomb structure), disordered regions 2.4 Structures of Graphene and GO 53

Fig. 2.18 2D building material for other dimension carbon materials. (Reprinted with permission from [121]. Copyright (2007) by Nature Publishing Group)

(with high oxidation), and defect regions (with holes) are presented on GO sheets. The oxygenated functional groups attached on GO sheets have been studied using different characterization techniques, such as high-resolution TEM, STM, XRD, XPS, and Fourier transform infrared spectra analyzer (FT-IR) [64]. It is found that partial oxidation was thermodynamically favored over complete oxidation according to both experimental observations and density functional calculations [123]. The variations in the degree of oxidation caused by the difference in graphite source or oxidation procedure may cause considerable modifications in the structure and properties of GO. In addition, the precise nature and distribution of these oxygenated groups were determined by the level of coverage and GO size [65, 124]. A low O/C ratio is normally associated with a large GO area, suggesting that a smaller GO contains relatively more oxygenated groups given the GO surface area [124]. It is also found that the epoxide to alcohol ratio increased with more oxidation [123]. Characterizing the degree of oxidation is necessary to understand the structure–property relationships because the optical and electrical properties of GO are controlled mainly by π-electrons from the sp2 carbon atoms [125]. To reveal further structural information, the surface of GO was examined [90, 91] under an STM. Although the hexagonal lattice of the sheets was partially preserved, the GO sheets were distinguishable from pristine graphene by the appearance of 54 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

Fig. 2.19 Structure of graphene. a–c Schematics of crystal structure a, Brillouin zone b and dispersion spectrum (c) of graphene [116]. d–g Raman imaging results from edges with angles d 30°, e 60° ( zigzag), f 90°, and d 60° ( armchair) [117]. h, i Aberration-corrected TEM image of h an armchair, and i zigzag configurations of carbon atoms at the edge of a hole in graphene [118]. j, k A representative configuration of graphene j and the bond length distribution k [120] 2.5 Properties of Graphene and GO 55

Fig. 2.20 Structure of GO sheets. a Structural model [128]. b 3D view [124] of graphene oxide (GO) showing hydroxyl and epoxy groups on the basal plane and carboxylic acid groups mainly at the edges. c STM image of a GO monolayer on a highly oriented pyrolytic graphite (HOPG) substrate, taken under ambient conditions (oxidized regions are marked by the line contours) [129]. d AFM image of a GO monolayer deposited on a SiO2 substrate, showing a backfolded edge [129]. e AFM section profiles along the three different lines in d, showing a mono-, bi-, and trilayer structure [129]. STM scanning tunneling microscope, AFM atomic force microscopy. (Reprinted with permission from [124, 128, 129]. Copyright (2012, 2009, 2007) by ACS) bright regions that correspond to the destruction of ordered lattice features due to the presence of oxygenated functional groups (Fig. 2.20c). Because of the oxygenated functional groups, the thickness of a monolayer GO sheet is much thicker than ~ 0.34 nm of an ideal monolayer of graphene. The thickness of a monolayer GO sheet is shown ~ 1 nm, while that of double layer GO is ~ 2 nm (Fig. 2.19d, e). However, the thickness of GO sheets may largely vary depending on the degree of oxidation [20, 126, 127].

2.5 Properties of Graphene and GO

2.5.1 Electrical/Electronic Properties

Due to the 2D single-atom-thick honeycomb lattices, pristine graphene has unique electronic properties, including the exceptionally high-charge carrier mobility, ambipolar field effect, anomalous quantum hall effect, ballistic transport, Klein 56 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

Table 2.4 Comparison of electrical properties of pristine graphene, GO, and rGO Type of graphene or Carrier mobility (cm2 Sheet resistance (Ω/ References GO v−1 s−1) sq) Graphene 2000–200,000 30–1000 [8, 45, 121, 132, 133, 143 ], GO Insulator >1012 [141] rGO 1–200 103–107 [104, 128, 141, 144–146] paradox, and weak antilocation [130, 131]. The carrier mobility of graphene can reach up to 200,000 cm2 v−1 s−1 when graphene is suspended on a clean surface, which can be extremely useful for ultrafast electronics and optoelectronics [132]. In reality, however, its mobility is largely limited to 2000–15,000 cm2 v−1 s−1 because of the presence of microscopic ripples, scattering, and defects [8, 121, 133]. Since graphene is a zero gap semimetal, many efforts have been directed towards opening a band gap using several methods, such as narrowing 2D graphene to 1D nanoribbon, forming nanomesh and hydrogen patterning [134–137]. An on/off ratio of over 104 was observed in very narrow GNRs [138]. Due to the high charge mobility, the quantum Hall effect was observed even at room temperature [139]. In sharp contrast, however, the electrical properties of GO sheets are significantly different from those of the pristine graphene. The long-range conjugated network of the graphitic lattice is the main reason for the excellent conductivity of pristine graphene sheets, while the functional groups and defects on GO sheets break the conjugated structure and localized π-electrons [140]. As compared in Table 2.4, owing to the much reduced carrier mobility, GO film is insulating with Rs values typically higher than about 1012 Ω/sq [141]. The reduction of GO is the key to restore the excellent electrical conductivities of graphene. Chemical and thermal reduction methods are known as the two main strategies to reduce GO [142]. The relation between sheet resistance ( Rs) (unit: Ω/sq) and bulk conductivity (σ) (unit: S/m) can be described by the following equation:

1 R = (2.1) s σt where t is the sample thickness.

Chemical Reduction

Chemical reduction is based on the chemical reactions with GO and the requirement for equipments is not critical, making the method easy and cheap to reduce GO for mass production [142]. Chemical reduction involves the exposure of GO sheets to reducing agents. The commonly used reducing agents are summarized in Table 2.5. Being a common antioxidant, hydrazine is known to be an effective reducing agent because it scavenges oxygen while it is chemically broken down to nitrogen and 2.5 Properties of Graphene and GO 57

Table 2.5 Representative chemical reducing agents for GO Agents Conditions C/O ratio Conductivity References (S/m) Hydrazine 100 °C, 24 h 10.3 2420 Stankovich et al. [20]

Hydrazine DMF/H2O, 80 °C, 11 1700 Park et al. [150] 12 h Phenylhydrazine RT, 24h 9.5 4700 Pham et al. [147]

NaBH4 80 °C, 1 h 4.8 82 Gao et al. [148].

NaBH4 RT, 2 h 8.6 45 Shin et al. [149] Al/HCl RT, 30 min 18.6 2100 Fan et al. [151] Fe/HCl RT, 6 h 7.9 2300 Fan et al. [152] Zn/HCl RT, 1 min 33.5 15,000 Mei et al. [153]

Zn/H2SO4 RT, 2 h 21.2 3416 Dey et al. [154] Sn(II)/HCl RT, 7 h 7.6 – Kumar et al. [155] Al foil/HCl RT, 20 min 21.1 12,530 Pham et al. [156]. Mg/HCl RT, 5 min 3.9 10 Barman et al. [157]

Zn/NH3 RT, 10 min 7.6 – Liu et al. [158] Zn/NaOH RT, 6 h 17.9 7540 Pham et al. [156] Al foil/NaOH RT, 20 min 5.3 1120 Pham et al. [156] HI/AcOH 40 °C, 40 h 11.5 30,400 Moon et al. [84] HI 100 °C, 1 h 12 29,800 Pei et al. [83] water. Although hydrazine has been one of the most widely used agents in industry, it is highly toxic and dangerous, especially the anhydrous hydrazine, thus extreme care has to be taken in using it. Figure 2.21a shows the fabrication process of rGO by hydrazine. The direct dispersion of hydrophobic graphite or graphene sheets in water without the assistance of dispersing agents has generally been considered to be an insurmountable challenge. To avoid the serious aggregation of GO after reduction, soluble polymer surfactants [71] or ammonia [87, 126, 144] have been employed to retain the colloidal state in water. Li et al. [87] discovered that it is easy to produce stable aqueous rGO dispersions by adding ammonia to the reaction solution to increase the pH value (Fig. 2.21b). This method is a facile approach to large-scale production of aqueous rGO dispersions without the need of polymeric or surfactant stabilizers, making it possible to process graphene materials using low-cost solution processing techniques [87]. The derivatives of hydrazine, such as dimethylhydrazine [67] and phenylhydrazine [147], were also shown effective reducing agents for GO. Metal hydride, such as NaBH4, was shown comparable with hydrazine as a reducing agent [148, 149]. It is also found that an additional dehydration process using concentrated sulfuric acid at 180 °C after reduction by NaBH4 further improved the reduction efficiency of GO [148]. However, these chemicals are not suitable for the reduction of GO films, especially for those needing high flexibility for applications in flexible devices, because of the stiffening effect and disintegration of the films during reduction. Figure 2.22a shows optical photographs of the reduction process by immersing GO films into 58 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

Fig. 2.21 Chemical reduction by hydrazine. a Scheme showing the chemical route to the synthesis of aqueous graphene dispersions. 1 Oxidation of graphite ( black blocks) to graphite oxide ( lighter colored blocks) with greater interlayer distance. 2 Exfoliation of graphite oxide in water by sonication to obtain graphene oxide (GO) colloids stabilized by electrostatic repulsion. 3 Controlled conversion of GO colloids to conducting graphene colloids through deoxygenation by hydrazine reduction. b Effect of ammonia on dispersion state of CCG sheets, characterized by measuring average particle sizes over a long period of time. The photographs shown in the inset were taken 2 days after the reduction reaction was complete with ( left) and without ( right) ammonia [87]

different reducing agents (like NaBH4, N2H4, and HI) for different durations [83]. It is observed that the bubbles appeared when GO films were immersed into NaBH4 and N2H4 solution, indicating that they reacted with GO to produce gases. The GO films were broken down to small graphene debris after 16 h reaction. However, immersion of GO films in HI acid solution hardly generated bubbles around them. The films in the HI acid solution maintained their integrity very well even after a long reaction time. Due to the advantage of HI reduction, Moon et al. [84] produced HI-reduced GO films on a PET substrate (Fig. 2.22b). 2.5 Properties of Graphene and GO 59

Fig. 2.22 a Optical photographs of reduction process by immersing a graphene oxide (GO) film into different reducing agents for different durations at room temperature [83]. b Flexible GO film ( left) and HI-reduced GO thin films on a polyethylene terephthalate (PET) substrate [84]. (Reprinted with permission from [83, 84]. Copyright (2010) by Elsevier and Nature Publishing Group)

Thanks to the extremely fast and efficient reduction abilities, the use of metal and acid mixture for reduction of GO has gained much attention recently [151, 154]. The reduction is achieved by fast electron transfer between the metal and GO, and the evolution of nascent hydrogen as the active reducing agent [159]. For example, a mixture of aluminum powder (10 mm) and hydrochloric acid produced rGO with a C/O ratio of 18.6 and an electrical conductivity of 2100 S/m [151]. The following study shows that iron powder (10 mm) in the presence of hydrochloric acid (Fe/ HCl) [152], zinc powder in hydrochloric acid (Zn/HCl) [153], solid zinc filings and sulfuric acid (Zn/H2SO4) [154], tin(II) chloride in hydrochloric acid (Sn(II)/HCl) [155], aluminum foil and hydrochloric acid (Al foil/HCl) [156], and magnesium in hydrochloric acid (Mg/HCl) [157] were all effective reducing agents of GO, which 60 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide offered one of the shortest reaction times to obtain rGO. Metal–alkaline, such as aluminum or zinc metals in alkaline environments, was also shown to reduce GO [156, 158]. For example, zinc powder in the presence of ammonia solution successfully reduced GO in 10 min at room temperature [158]. Similarly, the reduction of GO was also achieved with aluminum foil and zinc powder in sodium hydroxide (Al foil/NaOH, Zn/NaOH) solution [156]. Apart from chemical reduction using various reducing agents, several other strategies, including photocatalyst reduction, electrochemical reduction, and solvothermal reduction, have been explored recently [142]. Photocatalyst reduction is based on the photochemical reactions with the assistance of a photocatalyst such as

TiO2 [160, 161]. It is proposed that charge separation occurs on the surface of TiO2 particles upon UV-irradiation. Because the holes are scavenged to produce ethoxy radicals, the electrons accumulate within the TiO2 particles and interact with GO sheets to reduce functional groups [162]. Electrochemical reduction is usually carried out in a normal electrochemical cell using an aqueous buffer solution at room temperature without special chemical agents [163, 164]. The reduction of GO is achieved mainly by the electron exchange between GO and electrodes, thus the use of toxic reducing agents like hydrazine and producing byproducts can be avoided. Solvothermal reduction is normally performed in a sealed container so that the temperature is maintained above solvent’s boiling point [165, 166]. Particularly in the hydrothermal process, the overheated supercritical water plays the role of reducing agent and the physiochemical properties can be controlled by pressure and temperature, offering a green chemistry alternative to organic solvents [165].

Thermal Reduction

Significant research has also been conducted for thermal reduction of GO into a more pure form of graphene to restore the sp2 carbon structure and thus increase the electrical conductivity [167]. It is found that the carriers traveling across rGO thin films are scattered or trapped by sp3 carbon sites, defects, sheet junctions, and other structural imperfections and impurities [64]. The large variation of electrical conductivities can be explained by percolation transport. Mattevi et al. [167] investigated the role of residual oxygen and sp2 carbon fraction on electrical conductivities of thermally reduced GO. Figure 2.23a shows the evolution of carbon bonds in GO thin films as a function of annealing temperature in ultrahigh vacuum (UHV). It is suggested that GO undergoes structural changes due to the loss of oxygen and the carbon atoms in the basal plane may also rearrange. Figure 2.23b shows the plot of conductivities of rGO films as a function of sp2- carbon fraction as well as the data for 100 % sp2-bonded materials like graphene and polycrystalline graphite for comparison. A high conductivity 1.25 × 103 S cm−1 of polycrystalline graphite at a sp2 fraction of ~ 0.87 in reduced GO was estimated by extrapolating the experimental data [64]. Even a minimum conductivity ~ 6 × 103 S cm−1 was also suggested for a monolayer graphene [168], if the sp2 fraction were to increase above 0.9. The inset of Fig. 2.23b shows the structural model 2.5 Properties of Graphene and GO 61

Fig. 2.23 a The atomic per- centages of different carbon bonds identified by X-ray photoelectron spectroscopy (XPS) as a function of annealing temperature. The sp2 carbon and the corresponding oxygen concentration are plotted as a function of annealing temperature in the inset [167]. b Conduc- tivity of thermally reduced graphene oxide (GO) as a function of sp2 carbon fraction obtained from XPS [64, 167]. (Reprinted with permission from [167]. Copyright (2009) by Wiley)

for the essential features of transport through an rGO sheet at different stages of reduction. Since the sp2 clusters are isolated by oxygen atoms (indicated by dots), the GO film is insulating prior to reduction. As reduction restores sp2 carbon in GO, the transport barrier between the clusters narrows, allowing small fraction of carriers to hop or tunnel among sp2 sites. Upon further reduction of GO, better connectivity is formed among the original sp2 domains by forming new, smaller sp2 clusters along with concurrent formation of structural defects. Percolation among the sp2 clusters dominates the transport at higher sp2 fractions. Percolation is found to occur at sp2 fraction of 0.6 from the fit and it is in reasonable agreement with the theoretical threshold values for conduction among 2D disks [169]. Based on the above discussion, it is obvious that the heating temperature significantly affects the reduction of GO [126, 170, 171]. Table 2.6 presents thermal reduction of GO carried out under different processing conditions, such as annealing temperature and atmosphere. It is shown that the C/O ratio was no more than 9 if the temperature was less than 500 °C, while the C/O ratio could be higher than 62 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

Table 2.6 Thermal reduction of GO under different conditions Sample Temperature Atmosphere C/O ratio Conductivity References (°C) (S/m) GO 220 Argon – 80 Chen et al. [180] GO 500 Argon – 5900 Chen et al. [180] Hydrazine rGO 220 Argon – 11,800 Chen et al. [180] Hydrazine rGO 500 Argon – 35,100 Chen et al. [180] Solvothermal 1000 Helium 6.03 57,300 Dubin et al. [166] rGO Hydrazine rGO 1000 Helium 6.36 66,700 Dubin et al. [166] GO 500 Argon 6.8 – Yang et al. [79] GO 1000 Argon 11.36 – Yang et al. [79] GO 500 Argon and 7.3 – Yang et al. [79] hydrogen GO 1000 Argon and 12.4 – Yang et al. [79] hydrogen GO 500 Ultrahigh 8.9 – Yang et al. [79] vacuum GO 1050 Argon 10 – Schniepp et al. [69] GO 1100 Argon and – 55,000 Mattevi et al. [167] Hydrogen Hydrazine rGO 400 + 1100 Argon – 649 Wang et al. [144] GO 1050 Hydrogen – 2000 Wu et al. [177] GO 1050 Argon – 200 Wu et al. [177] GO 1050 Vacuum – 80 Wu et al. [177] rGO 800 Ethylene – 350 Lopez et al. [178] Aromatic 1000 Ultrahigh – 1314 Su et al. [179] molecules- vacuum functionalized rGO GO graphene oxide, rGO reduced graphene oxide

10 if the reduction temperature increased beyond 1000 °C. In addition to annealing temperature, annealing atmosphere is also found important for GO reduction. To avoid oxygen during thermal reduction, which is reactive with GO at high temperatures, annealing is normally carried out in vacuum [141], an inert [76, 110, 126, 144, 172] or reducing atmosphere [172–175]. For example, it is found that a quality vacuum (< 10−5 Torr) is the key to the recovery of GO, otherwise the GO films may be quickly lost through decomposition with residual oxygen in the system [141]. Similarly, inert gases such as argon [76, 176] and helium [166] are shown to avoid the reaction with oxygen. By adding gases such as H2 that has an excellent reduction capability, the reduction of GO was found more efficient than argon or vacuum alone (see Table 2.6) [177]. Li et al. [175] found that reduction and simultaneous nitrogen doping of GO were realized by annealing GO in low- pressure ammonia (2 Torr NH3/Ar (10 % NH3)). Due to the doping effect of N, the GO annealed in NH3 exhibited a higher conductivity than that annealed in H2. To 2.5 Properties of Graphene and GO 63 repair the defects in rGO sheets, Lopez et al. [178] exposed rGO to a carbon source, such as ethylene, at an elevated temperature of 800 °C. The results showed that the vacancies were partially “repaired” and the conductivity of the individual rGO sheet reached up to 350 S/m. Su et al. [179] discovered that rGO sheets functionalized by aromatic molecules produced a highly graphitic material with a conductivity as high as 1314 S/cm after thermal annealing at 1000 °C, which is attributed to a similar defect-healing effect. Although reduction of GO at high annealing temperatures is effective, there are also obvious drawbacks, such as large energy consumption, critical treatment conditions, and inability to apply for substrates with a low melting point, e.g., glass and polymer [142].

2.5.2 Thermal Properties

The properties of phonons—quanta of the crystal lattice vibrations—in graphene have attracted much attention in the physics and engineering communities. It was shown both theoretically and experimentally that transport properties of phonons, i.e., energy dispersion and scattering rates, were substantially different in the quasi- 2D system, such as graphene, compared to basal planes in graphite or 3D bulk crystals. Because the carrier density of pristine graphene is relatively low, the electronic contribution to thermal conductivity is negligible according to the Wiedemann– Franz law [181]. The thermal conductivity of graphene is dominated by transport properties of phonon [182]. Acoustic phonons are the main heat carriers in graphene near room temperature, whereas optical phonons are used for counting the number of atomic planes in Raman experiments with FLG [183, 184]. Table 2.7 compares the thermal conductivities of graphene in various forms and CNTs. The theoretical thermal conductivity of suspended monolayer graphene is about 6000 Wm−1K−1 at room temperature according to the MD simulations based on the Green–Kubo approach, which is much higher than that of graphitic carbon [185]. The thermal conductivities of graphene were measured using an optothermal Raman technique [186, 187], where a laser light was focused on a suspended graphene sheet over a 3-μm-wide trench, which was connected to heat sinks at its ends to provide heat sources (Fig. 2.24a, b). Thermal conductivities were determined from the dependence of Raman G-peak frequency on the excitation laser power and the independently measured G-peak temperature coefficient. The thermal conductivity of monolayer graphene obtained thereby ranged from 4.84 × 103 to 5.30 × 103 W/mK at room temperature. Similar to the mechanical properties and electrical conductivities, the thermal conductivities of GO or rGO are much lower than that of pristine graphene because of the disorders arising from the residual oxygenated groups and the presence of defects [199, 201]. The thermal conductivities of dielectrophoretically deposited rGO sheets were measured using electrical four-point measurement method [158], giving a range of very low values (0.14–2.87 Wm−1 K−1). Indeed, these values are in similar orders of magnitude estimated by the MD simulations: the thermal conductivity of GO with an O/C ratio of 0.5 was 8.9 W/mK, and it increased with 64 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

Fig. 2.24 Measurements of a DŝĐƌŽͲZĂŵĂŶƐƉĞĐƚƌŽŵĞƚĞƌ thermal conductivities of graphene and rGO. a Schematic of experimental setup showing the excitation laser light focused on a graphene layer &ŽĐƵƐĞĚůĂƐĞƌůŝŐŚƚ ^ƵƐƉĞŶĚĞĚŐƌĂƉŚĞŶĞ suspended across a trench [187]. b An SEM image of ,ĞĂƚƐŝŶŬ the suspended single- and few-layer graphene across a trench in a Si/SiO2 substrate [186]. rGO reduced graphene oxide, SEM scanning electron microscope. (Reprinted with permission from [186, 187]. KƉƚŽƚŚĞƌŵĂůZĂŵĂŶ Copyright (2008, 2011) by ŵĞĂƐƵƌĞŵĞŶƚƚĞĐŚŶŝƋƵĞ ACS and Nature Publishing Group) b

increasing the degree of reduction, reaching about 42 W/mK when the O/C ratio was 0.05, see Table 2.7. The inverse relationship between the thermal conductivity and the O/C ratio is attributed to the increase in phonon scattering with increasing oxygenation [201]. It is also worth noting that GO showed an increasing thermal conductivity in response to an external tensile stress, an entirely opposite trend to those shown by other nanostructured materials, including pristine graphene. The thermal conductivities of multilayer graphene and GO sheets were measured using the thermal flash technique [159]. It was shown that the multilayer graphene comprising 30–45 layers had a thermal conductivity similar to bulk graphite, whereas that of 3-layer GO was higher than that of bulk graphite. The enhancement in thermal conductivity of multilayer GO than graphite is attributed to the intercalating oxygen atoms that introduced covalent bridges between the interlayers for interaction [200]. Due to the intrinsic properties of the individual graphene sheets and the highly aligned structure of GO papers (GOPs) and graphene papers (GPs), these flexible materials also show unique thermal properties [204]. Thermal conductivities of GOPs and GOPs modified by divalent ions, Mg2+ and Ca2+, were measured using a laser flash method. Their thermal conductivities were 3.91, 32.05, and 61.38 W/mK, respectively [205] (see Table 2.7), indicating that the modification of GOP with these metal ions gave rise to significant enhancements in thermal 2.5 Properties of Graphene and GO 65

Table 2.7 Thermal conductivities of graphene, GO, rGO, graphene, and GO papers in comparison with CNTs at room temperature Material Method Thermal conductivity References (W/mK) Single-layer graphene Confocal micro- 4840–5300 Balandin et al. [186] Raman spectroscopy Suspended graphene Confocal micro- 4100–4800 Ghosh et al. [188] Raman spectroscopy Suspended graphene Thermal measurement 3000–5000 Seol et al. [189] method

Graphene on SiO2 Thermal measurement 600 Seol et al. [189] method Few-layer graphene Raman optothermal ~ 1300–2800 Ghosh et al. [190] (2–4 layer) Suspended graphene Raman optothermal ~ 2500 Cai et al. [46] Suspended graphene Raman optothermal 1500–5000 Jauregui et al. [191] Suspended graphene Raman optothermal 600 Faugeras et al. [192] Suspended graphene Raman optothermal 1100 Murali et al. [193] Graphene Theory: valence force 2000–5000 Nika et al. [194] field, Boltzmann transport equation Graphene Theory: relaxation- 1000–5000 Nika et al. [195] time approximation Graphene Theory: molecular 8000–10,000 Evans et al. [196] dynamics, Tersoff Graphene Theory: Boltzmann 1400–2400 Lindsay et al. [197] transport equation Graphene Theory: ballistic ~ 4000 Munoz et al. [198] rGO Electrical four-point 0.14–2.87 Schwamb et al. [199] measurement rGO Thermal flash 2180–2275 Mahanta et al. [200] technique GO Thermal flash 18–776 Mahanta et al. [200] technique Graphene Molecular dynamics 2188 Shen et al. [201] simulation rGO Molecular dynamics 42a Shen et al. [201] simulation GO Molecular dynamics 8.9b Shen et al. [201] simulation Graphene paper Laser flash method 313 Wu et al. [202] Graphene paper Light flash system 178 Xiang et al. [203] Graphene paper Self-heating method 1434 Xin et al. [204] GO paper Laser flash method 3.91 Yu et al. [205] Mg2+ modified GO Laser flash method 32.05 Yu et al. [205] paper Ca2+ modified GO Laser flash method 61.38 Yu et al. [205] paper 66 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

Table 2.7 (continued) Material Method Thermal conductivity References (W/mK) Individual MWCNT Electrical, ~ 3000 Kim et al. [206] micro-heater Individual SWCNT Electrical, self-heating ~ 3500 Pop et al. [207] SWCNT bundles Thermocouples 1750–5800 Hone et al. [208] Individual SWCNT Electrical, 3000–7000 Yu et al. [182] micro-heater Individual CNT Electrical, 1100 Chang et al. [209] micro-heater Individual CNT Electrical ~ 1500–2900 Fujii et al. [210] CNT Theory: molecular ~ 6600 Berber et al. [185] dynamics CNT Theory: molecular ~ 3000 Che et al. [211] dynamics SWCNT Theory: Boltzmann ~ 2500 Lindsay et al. [197] transport equation SWCNT Theory: molecu- ~ 7000 Donadio et al. [212] lar dynamics and Boltzmann transport equation rGO reduced graphene oxide, GO graphene oxide, CNT carbon nanotubes, MWCNT multiwalled, SWCNT single-walled CNT a O/C ratio = 0.05 b O/C ratio = 0.5 conductivity. Several ameliorating mechanisms, including the intercalation of thermally conducting metal ions into the gallery and cross-linking between GO sheets and metal ions thermally connecting the GO basal planes, were mainly responsible for the reduction in thermal resistance along the intersheet galleries, and thus the increase in thermal conductivity. GPs fabricated by vacuum filtration and high- temperature annealing showed a thermal conductivity of 313 W/mK [202], which is much high than GOPs. Xin et al. [204] demonstrated a facile approach to produce large area freestanding GPs through direct electrospray deposition of graphene films and simple water exfoliation from highly hydrophilic aluminum substrates. After mechanical press and high-temperature annealing, the highly aligned and defect-free GPs displayed superior thermal properties with thermal conductivities up to 1434 W/mK. Table 2.7 also includes both the experimental and theoretical thermal conductivities for single-walled CNTs (SWCNTs) and multiwalled CNTs (MWCNTs). The reported thermal conductivities of CNTs measured at room temperature are largely scattered, ranging from ~ 1100 [209] to ~ 7000 W/mK [182], with typical values of MWCNTs and SWCNTs are ~ 3000 [206] and ~ 3500 W/mK [207], respectively. Although CNTs possess similar thermal conductivities as graphene sheets, the unique 2D structure makes graphene serve as an interconnect in 3D electronics where graphene can simultaneously function as lateral heat spreaders [187, 213]. 2.5 Properties of Graphene and GO 67

2.5.3 Optical Properties

The high-frequency conductivity of Dirac fermions in graphene in the infrared to visible range of the spectrum is constant and given by [214, 215]:

πeh2 /2 . (2.2)

For normal incidence light, the optical transmittance ( T) and reflectance ( R) are given as follows:

1 T =(1 + πα )− 2 , (2.3) 2

1 RT= πα22, (2.4) 4 where

απ=2e2 / hc ≈ 1/137. (2.5) e is the electron charge, c the light speed, and h is the Planck’s constant. Combining Eqs. (2.3) and (2.5) yields the opacity of graphene:

(1−≈T )πα ≈ 2.3%. (2.6)

According to Fig. 2.25a, b, the transparency of single layer graphene is ~ 97.7 %, and the opacity is linearly proportional to the number of layers with one layer equivalent to 2.3 % [216]. These findings were further confirmed for CVD-grown graphene that the optical transmittance was reduced by 2.2–2.3 % for an additional layer (Fig. 2.25c) [45] Therefore, the thickness of n-layer (n < 10) graphene was determined using white light illumination on samples supported on a SiO2/Si substrate, as shown in Fig. 2.25d [217]. It is also noted that the color of aqueous GO dispersion varied from light yellow to dark brown depending on the GO concentration (Fig. 2.26a). After chemical reduction, the electronic structure was modified, and so was the rGO dispersion color: it turned black (Fig. 2.26b). Figure 2.26c presents the UV−Vis spectra of GO and rGO dispersions, showing an upshift of the absorption peak from 231 to 270 nm after reduction by hydrazine [218], a reflection of partial restoration of the electronic conjugation in the graphene sheets [172].This result is comple- mented by a uniform surge in absorption over the whole spectral region. The peak at 227–231 nm determines the remaining conjugation, i.e., π–π* transition, whereas the shoulder at 300 nm is attributed to the n–π* transition of carbonyl groups [69, 219]. The transparency of rGO films is poorer than that of as-prepared GO films because of the partial restoration of the π-electron system [141]. Figure 2.26d con- firms this finding in that the GO films became darker after reduction at a high 68 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

Fig. 2.25 Optical properties of graphene. a Optical image of a 50 µm aperture partially covered by monolayer and bilayer graphene sheets [216]. b Transmittance spectrum of single-layer graphene ( open circles). (Inset) Transmittance of white light as a function of number of graphene layers [216]. c UV–vis spectra of roll-to-roll layer-by-layer transferred graphene films on quartz substrates [45]. d Optical image of graphene with one, two, three, and four layers [217] temperature. A thicker film normally has a lower transparency because of the higher degree of absorption of light (Fig. 2.26e). The adsorption of small particles on the quartz substrate surface may also be partly responsible for the darkening of rGO films after the thermal treatment.

2.5.4 Mechanical Properties

The mechanical properties of pristine graphene have been determined by nanoindentation under an AFM (Figs. 2.27a, b), showing that the defect-free graphene had a Young’s modulus of ~ 1.0 TPa and a fracture strength of 130 GPa [220]. Table 2.8 presents the Young’s moduli of pristine graphene determined from a number of theoretical and experimental studies, which range from 0.9 to 1.1 TPa [221]. There are 2.5 Properties of Graphene and GO 69

abc

e ϵŶŵ;ŶŽŶͲƌĞĚƵĐĞĚͿ ϲŶŵ

dƌ ϴŶŵ ĂŶ Ɛ ŵŝ Ϯϳ Ŷŵ Ʃ ĂŶ ϰϭ Ŷŵ ĐĞ ;й Ϳ tĂǀĞůĞŶŐƚŚ;ŶŵͿ Fig. 2.26 Characterization of optical properties of GO solution and GO films: digital images of a GO and b rGO dispersions [171]; c UV−Vis spectra of GO and rGO dispersions [218]; d photograph of rGO films with different thicknesses ( black scale bar is 1 cm) obtained after thermal treatment at 1100°C [141]; and e optical transmittance spectra of the films in d with their thicknesses indicated [141]. GO graphene oxide, rGO reduced graphene oxide. (Reprinted with permission from [141, 171, 218]. Copyright (2008, 2010, 2011) by ACS, Elsevier and Wiley) 70 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

a b

Zigzag Chiral vector Armchair d c

ŝŐǌĂŐ e ƌŵĐŚĂŝƌ f сϭ͘ϬϴdWĂ сϭ͘ϬϮdWĂ

Fig. 2.27 Mechanical properties of graphene sheets, graphene oxide (GO), and GO papers. a SEM image of a graphene sheet spanning an array of circular holes (scale bar = 3 mm). b Schematic of nanoindentation on suspended graphene [220]. c Graphene molecular model with chiral angles between 0 and 30° [222]. d Young’s modulus of graphene with different chiral angles [222]. 2.5 Properties of Graphene and GO 71 many parameters that affect the Young’s modulus of graphene, such as the chirality [222], the presence of defects and wrinkles [223], and functionalization [226, 231]. Figure 2.27c shows the molecular model for graphene with chiral angles varying from 0 (zigzag) to 30º (armchair), showing that the Young’s modulus varied in a narrow range from 1.086 TPa for the zigzag configuration to 1.050 TPa for the armchair configuration (Figs. 2.27d). This observation agreed with the previous proposal in that the elastic properties of graphene with a hexagonal structure should be relatively independent of the loading direction and thus the Young’s modulus should not be greatly affected by the chirality [222]. Mechanical properties of one to three layers of GO sheets were investigated by AFM [234]. The measured Young’s modulus of a monolayer GO sheet was 207.6 GPa, which is much lower than the pristine graphene, due to the oxygenated functional groups which adversely alter the “perfect” 2D structure of monolayer graphene [220, 234]. To better understand the effects of defects and functionalization on mechanical properties of graphene sheets, molecular dynamics (MDs) and molecular mechanics (MMs) simulations were performed to evaluate the Young’s modulus, shear modulus, and wrinkling properties of pristine graphene and GO sheets [223, 229, 235]. The simulation results show that the Young’s modulus was dependent on the presence of Stone–Wales defects, degree of functionalization, and the molecular structure of functional groups (Fig. 2.27e). The presence of Stone– Wales defects caused the graphene sheet to wrinkle which affected only the initial part of the stress–strain curves due to straightening of wrinkles, but a small number of defects had only a negligible influence on Young’s modulus of graphene. The shear modulus and critical wrinkling strain was also reduced once oxygen-containing functional groups were introduced (Fig. 2.27f). It is found that the change in the molecular structure of graphene sheet was associated with the binding energy between the functional groups and GO. The altered structure of graphene sheets caused their instability. Hydrogen [236] or methyl [237] groups showed similar reduction in mechanical properties of graphene sheets. GO paper, which are a “paper-like” material made by flow directed or self-assembled GO sheets, is another useful GO-based material [238]. GO papers have many potential applications, such as protective layers, chemical filters, supercapacitors, adhesive layers, electronic or optoelectronic components, and molecular storage [180, 238–241]. The mechanical properties of GO papers have also been extensively studied, for example, tightly packed GO papers fabricated by vacuum filtration showed an average elastic modulus of ~32 GPa [238]. Compare with those prepared by vacuum filtration, the GO papers obtained at a liquid/air interface by evaporating the hydrosol of GO showed a similar tensile strength but a slightly lower modulus [241]. Chemical cross-linking by using divalent ions [239] or polyallylamine [240] e Stress–strain curve of graphene sheets containing a row of Stone–Wales defects along with morphological changes with increasing strain [223]. f Shear stress–strain curves for pristine graphene and GO sheets functionalized with carboxyl and hydroxyl groups [222]. g Tearing toughness of GO papers with different size groups [224]. h Cross-sectional SEM images and schematic of self-assembly process of GO sheets with different sizes (S-GO in the left panel and UL-GO in the right panel) [124]. SEM scanning electron microscope, S-GO small graphene oxide, UL-GO ultralarge graphene oxide. (Reprinted with permission from [124, 220, 222, 223, 224]. Copyright (2012, 2008, 2010, 2014) by ACS,Science Publishing Group and Elsevier) 72 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

Table 2.8 Young’s modulus of pristine graphene from representative experimental and theoretical investigations. (Reprinted with permission from [221]. Copyright (2013) by RSC) Method E (TPa)a Nanoindentation measurements [220] 1.02 Phonon dispersion measurements [225] 1.01 Ab initio computations [226] 1.05 Density functional theory [227] 1.05 First principles calculations [228] 1.01 MD and MM simulation [229] 1.05–1.09 Empirical force constant calculations [230] 1.13 Atomistic Monte Carlo simulations [231] 1.04 Continuum elasticity theory and tight-binding atomistic simulations [232] 0.92 Energetic model [233] 0.90 a Because the Young’s modulus for a 2D sample is just force/length, elastic properties of graphene in some references are reported in N/m, which are converted to TPa by taking effective thickness as 0.34 nm. MD molecular dynamics, MM molecular mechanics was found to be an effective approach to improve the mechanical properties of GO papers. The mechanical properties of GO papers depended on the size of the precursor GO sheets [125, 224, 242]. The GO papers made from large-size GO sheets had higher Young’s modulus, fracture toughness, and tearing strength than those made from smaller GO (Figs. 2.27g). The dependence of mechanical properties of GO papers on GO sheets’ size is attributed to two interrelated characteristics, namely, the compactness of GO papers and the presence of defects in GO sheets [224]. The larger sheets tended to form a more compact and aligned GO papers (see Fig. 2.27h) and had fewer defects for a given area than smaller sheets [125]. Molecular dynamic (MD) simulations were performed to investigate the effect of GO sheets on mechanical properties of GO papers [124]. According to the Lerf– Klinowski model [243], the molecular model were built consisting of two sheets of monolayer GO (Figs. 2.28a–d) and water moisture was also taken into account by randomly adding water molecules to the models to simulate the real structure (Figs. 2.28c–d). It is found that the Young’s moduli of GO papers increased consistently as the GO sheet size increased from 1.1 to 5.4 nm, and that the GO papers with water always had higher Young’s moduli than their counterparts without water (Fig. 2.28e). To estimate the dependence of Young’s modulus on GO size on the micrometer scale, the results were extrapolated using a logarithmic fitting equation. Although the predictions were approximately an order of magnitude higher than the experimental results, the general trend was correctly predicted. Figure 2.28f plots the contributions of two different components to the total deformation of GO papers as a function of GO size, showing that more than 90 % of the deformation originated from the intersheet deformation regardless of the GO size on the nanoscale while the deformation arising from the extension of GO sheets themselves were relatively small. This observation implies that for improved mechanical properties of GO papers, the intersheet shear and tensile deformation should be reduced by cross-linking them using metal ions or functionalization. 2.6 Common Tools for Characterization of Graphene and Its Derivatives 73

ĚŐĞͲƚŽͲĞĚŐĞŝŶƚĞƌĂĐƟŽŶ ab

&ĂĐĞͲƚŽͲĨĂĐĞŝŶƚĞƌĂĐƟŽŶ 'KƐŚĞĞƚƐ cdĚŐĞͲƚŽͲĞĚŐĞŝŶƚĞƌĂĐƟŽŶ

&ĂĐĞͲƚŽͲĨĂĐĞŝŶƚĞƌĂĐƟŽŶ 'KƐŚĞĞƚƐ

e f

Fig. 2.28 3D model snapshots of graphene oxide ( GO) sheets a without water and c with intercalated water molecules; and 2D schematics of GO paper models with functional groups bonded between adjacent GO sheets through hydrogen bonds b directly and d mediated by water molecules. e extrapolation of modulus values; and f plots of two different components of deformation as a function of GO length. (Reprinted with permission from [124]. Copyright (2012) by ACS)

2.6 Common Tools for Characterization of Graphene and Its Derivatives

Various techniques, such as AFM, SEM, high-resolution TEM, STM, Raman spectroscopy, XRD analysis, FTRI spectroscopy, and UV-vis spectroscopy, have been successfully used for the characterization of graphene and its derivatives. Two or more techniques are usually combined together to map the complete picture of their morphologies, structures, and properties. Some of these techniques are discussed in the following sections.

2.6.1 Atomic Force Microscopy

AFM was the first technique used to establish that optically identified graphene (Fig. 2.29a) was indeed one-atom thick monolayer (Fig. 2.29b). A typically 0.4-nm- thick monolayer graphene was observed on crystalline graphite using an intermittent 74 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

Fig. 2.29 AFM images of graphene, GO, and rGO. a Single layer graphene was first observed by Geim et al. [8]. A few layer flake is shown, with optical contrast enhanced by an interference effect at a carefully chosen thickness of oxide [8]. b AFM image of single-layer graphene. Notice the folded part of the film near the bottom, exhibiting a differential height of ~ 0.4 nm [8]. c–f Height c and e and corresponding phase d and f tapping mode AFM images of unreduced c and d and chemically reduced e and f GO nanosheets deposited from aqueous dispersions onto 2.6 Common Tools for Characterization of Graphene and Its Derivatives 75

Fig. 2.30 SEM images of a graphene [246] and b graphene oxide (GO) [107] sheets on a SiO2/Si substrate. (Reprinted with permission from [107, 246]. Copyright (2011, 2010) by ACS and RSC)

contact AFM mode [8]. The measured thicknesses of GO and rGO were 1.0 and 0.6 nm, respectively (Fig. 2.29c–f) [244]. Besides imaging and thickness detection, AFM was also used for the study of mechanical, frictional, electrical, magnetic, and even elastic properties of graphene sheets and flakes [220]. Although the AFM technique can successfully determine the surface morphology on a nanometer scale, it is cumbersome for imaging large area graphene [14]. In addition, the samples freshly cleaved HOPG. The images were recorded in the attractive regime of tip sample interaction. Superimposed onto each image is a line profile taken along the marked red line [244]. AFM atomic force microscopy, GO graphene oxide, rGO reduced graphene oxide, HOPG highly ordered pyrolytic graphite 76 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

Fig. 2.31 a, b High-resolution transmission electron microscopy (TEM) images of solution cast a monolayer and b bilayer graphene (scale bar 500 nm). c Electron diffraction pattern of the sheet in a, with the peaks labeled by Miller–Bravais indices. d, e Electron diffraction patterns taken from the positions of the black d and white spots e, respectively, of the sheet shown in b, using the same labels as in c. The graphene is clearly one layer thick in d and two layers thick in e. f–h Diffracted 2.6 Common Tools for Characterization of Graphene and Its Derivatives 77 used for AFM characterization need to be deposited on a very flat substrate, such as Si wafer, freshly cleaved mica, and quartz [245].

2.6.2 Scanning Electron Microscopy (SEM)

SEM is routinely used to image nanomaterials and has become a standard, easy- to-use instrument capable of clearly describe the morphologies of graphene or GO layers [245]. A variety of imaging modes, including backscattered electrons (BSE), secondary electrons (SE), auger electrons, X-ray, and cathodoluminescence, are produced when an electron beam impinges on a sample. These signals can be selectively collected and combined to generate images. The BSE signal is sensitive to atomic composition of the specimen, while the SE signal can generate high- resolution images but is sensitive to surface charging. It is found that SEM images with good contrast was difficult to obtain for graphene or GO, while a relatively low accelerating voltage delivered the best result on a 300 nm thick SiO2/Si substrate (Fig. 2.30) [107]. The main drawbacks of SEM imaging are that it works only for samples deposited on electrically conducting substrates and the graphene/GO samples are easily damaged by acceleration voltages [245].

2.6.3 Transmission Electron Microscopy (TEM)

TEM is capable of measuring the thickness of a graphene sample, and provides an accurate means to measure the number of layers at multiple locations on the film [93]. Figure 2.31a, b shows monolayer and bilayer of graphene. It is found that a more definitive identification of graphene can be made using electron diffraction patterns [247]. As shown in Fig. 2.31c–h, the selected area electron diffraction (SAED) patterns of monolayer and bilayer graphene sheets had different features. Typical sixfold graphene symmetry was observed in the SAED pattern of monolayer graphene [93]. The use of traditional TEMs is limited by their resolution at low operating voltages and the operation at high voltages damages the monolayer. Therefore, aberration-corrected TEM in combination with a monochromator was to provide 1 Å resolution at an acceleration voltage of only 80 kV [118, 248]. Figure 2.31i shows the aberration-corrected, high-resolution image of the graphene lattice depicting every single carbon atom arranged in the hexagonal framework [249]. The high-resolution TEM image of a GO sheet (Fig. 2.31j) and the corresponding SAED pattern (inset of Fig. 2.31j) confirmed that the monolayer GO partially collapsed into an amorphous structure [126]. intensity taken along the 1–210 to − 2110 axis for the patterns shown in c–e, respectively [93]. i Aberration-corrected, high-resolution TEM image of graphene sheet [249]. j High-resolution TEM image of a GO sheet with corresponding selected area electron diffraction pattern (SAED) in the inset [126]. (Reprinted with permission from [93, 126, 249]. Copyright (2008, 2011, 2008) by Nature Publishing Group, Elsevier and ACS) 78 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

2.6.4 Scanning Tunneling Microscope (STM)

STM was developed in 1981 by Gerd Binnig and Heinrich Rohrer at the Interna- tional Business Machines Corporation (IBM) [250]. The inventors earned the Nobel Prize in Physics in 1986 [251]. Since STM is able to image surfaces with resolution up to 0.1 nm in lateral and 0.01 nm in depth, individual atoms within materials are routinely imaged and manipulated [252]. Recently, STM is also used to image or manipulate single-layer graphene sheets [253–256]. As shown in Fig. 2.32, in situ atomic-resolution STM is able to reveal the atomic structure of the graphene nanoribbon. The graphene nanoribbon is ~ 15 -nm wide and its axis has a crystal- lographic orientation close to the zigzag direction (Fig. 2.32a). The STM image also reveals an atomically flat and defect-free structure in the central region (Fig. 2.32b) and position-dependent superstructures near the edges (Fig. 2.32c). Except for imaging graphene, STM is also able to manipulate free-standing graphene sheets [256]. As illustrated in Fig. 2.32d, the Z-V spectroscopy mode STM can control the deformation of free-standing graphene’s nanoripples. The nanoripples were controlled by the means of attractive electrostatic force, repulsive interatomic force, and restoring force of graphene itself.

Fig. 2.32 Image or manipulate single-layer graphene sheets by scanning tunneling microscope (STM). a STM image of a 15-nm-wide graphene nanoribbon displaying an atomically flat and defect- free structure. b–c Magnified images of the defect-free lattice taken at the center of the ribbon b and position- dependent superstructures near the edges c [253]. d Illustration of pulling and pushing nanoripples by controlling the STM tip–graphene interaction forces [256] 2.6 Common Tools for Characterization of Graphene and Its Derivatives 79

2.6.5 Raman Spectroscopy

The Major features of the Raman spectra for graphite and graphene are the G band at ~ 1584 cm−1 and the G’band at ~ 2700 cm−1 [257]. The positions and relative peak heights of the G and G’bands indicate the number of graphene layers. As shown in Fig. 2.33a–c, the location of the G peak for a single layer graphene is 3–5 cm−1 higher than that of bulk graphite, while the G’peak shows a significant change in both shape and intensity as the number of layers decreases. For bulk graphite, the G’ band is comprised of two components. The low and high shift are roughly 1/4 and 1/2 of the G peak intensities, respectively. For a single-layer graphene, the G’band shows a single sharp peak and the intensity is roughly four times the G peak. The Raman spectroscopy is also an important tool to investigate the surface chemistries of GO and rGO [76]. Because of the presence of isolated double bonds that resonate at frequencies higher than that of the G-band of graphite, the G-band peak of UL-GO was upshifted from 1581 to 1607 cm−1, see Fig. 2.33d, f [258, 259]. The corresponding G-band after the thermal reduction (rUL-GO) occurred at 1590 cm−1, a reflection of the recovery of the hexagonal network of carbon atoms containing defects. Due to the possibility of charge transfer reaction between the (host) carbon in graphene and the (guest) chloride [260], the G-band of chemically reduced C-rUL-GO at 1587 cm−1 was marginally downshifted. The high electronegativity of chloride species encouraged carbon–chloride interactions that in turn triggered charge transfer reactions and created holes in graphene. The G band is Raman active for sp2-hybridized carbon-based material, while the D band is activated only if the defects participate in the double resonance Raman scattering near

K point of Brillouin zone [261]. Hence, the intensity ratio of ID/IG is often used for 2 estimating the sp domain size of graphite-based materials. The ID/IG ratio of rUL- GO was notably lower than UL-GO (Figs. 2.33e), indicating that the thermal reduction process removed the functional groups and recovered the graphitic structure.

However, the ID/IG ratio increased after the chemical treatments that again altered the graphene structure. Besides the G and D bands, there are two other Raman bands, called 2D and D + G at 2600–3000 cm−1 (Fig. 2.33f), which are often ignored due to their weak intensities compared to the D and G bands. The 2D band is Raman active for crystalline graphitic materials and is sensitive to the  band in the graphitic electronic structure, while the combination mode of D + G band is induced by disorder [261]. These two bands were employed to distinguish the electronic conjugation of UL-GO obtained at different stages of reduction and treatments. The intensity ratio

I2D/ID + G shown in Fig. 2.33g was more sensitive to the change in electronic conjugation from UL-GO to rUL-GO than the ID/IG ratio (Figs. 2.33e), as a reflection of the recovery of graphitic electronic conjugation. The reduction of the I2D/ID + G ratio corresponding to the modification from rUL-GO to C-rUL-GO indicates that the newly doped functional groups, such as –Cl, –SOCl, and –COOH, introduced disorder again [76]. 80 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

Fig. 2.33 Raman spectra of graphite, graphene, GO, and rGO. a Comparison of Raman spectra at 514 nm for bulk graphite and graphene. Evolution of the spectra b at 514 nm and c 633 nm with the number of layers [257]. d, f Raman spectra for NG, UL-GO, reduced rUL-GO, and chemically doped, reduced C-rUL-GO. e, g The corresponding D/G and 2D/(D + G) intensity ratios [76]. GO graphene oxide, rGO reduced graphene oxide, NG nano-graphene, UL-GO ultralarge graphene, rUL-GO reduced ultralarge graphene oxide, C-rUL-GO carbon reduced ultralarge graphene oxide. (Reprinted with permission from [76, 257]. Copyright (2011, 2006) by ACS and APS) References 81

References

1. Konatham, D., & Striolo, A. (2008). Molecular design of stable graphene nanosheets dispersions. Nano Letters, 8, 4630–4641. 2. Fernandez-Moran, H. (1960). Single crystals of graphite and mica as specimen support for electron microscopy. Journal of Applied Physics, 31, 1840. 3. Boehm, H. P., Clauss., A., Hofmann, U, & Fischer, G. O. (1962). Dunnste, Kohlenstoff- Folien. Zeitschrift Fur Naturforschung Part B—Chemie Biochemie Biophysik Biologie Und Verwandten Gebiete, B17, 150. 4. Soldano, C., Mahmood, A., & Dujardin, E. (2010). Production, properties and potential of graphene. Carbon, 48, 2127–2150. 5. Ebbesen, T. W., & Hiura, H. (1995). Graphene in 3-dimensions—towards graphite origami. Advanced Materials, 7, 582–586. 6. Lu, X. K., Huang, H., Nemchuk, N., & Ruoff, R. S. (1999). Patterning of highly oriented pyrolytic graphite by oxygen plasma etching. Applied Physics Letters, 75, 193–195. 7. Lu, X. K., Yu, M. F., Huang, H., & Ruoff, R. S. (1999). Tailoring graphite with the goal of achieving single sheets. Nanotechnology, 10, 269–272. 8. Novoselov, K. S., Geim, A. K., Morozov, S. V., Jiang, D., Zhang, Y., Dubonos, S. V., Grig- orieva, I. V., & Firsov, A. A. (2004). Electric field effect in atomically thin carbon films. Science, 306, 666–669. 9. Berger, C., Song, Z. M., Li, T. B., Li, X. B., Ogbazghi, A. Y., Feng, R., Dai, Z. T., Marchen- kov, A. N., Conrad, E. H., First, P. N., & de Heer, W. A. (2004). Ultrathin epitaxial graphite: 2D electron gas properties and a route toward graphene-based nanoelectronics. Journal of Physical Chemistry B, 108, 19912–19916. 10. Li, X. S., Cai, W. W., An, J. H., Kim, S., Nah, J., Yang, D. X., Piner, R., Velamakanni, A., Jung, I., Tutuc, E., Banerjee, S. K., Colombo, L., & Ruoff, R. S. (2009). Large-area synthesis of high-quality and uniform graphene films on copper foils. Science, 324, 1312–1314. 11. Meitl, M. A., Zhu, Z. T., Kumar, V., Lee, K. J., Feng, X., Huang, Y. Y., Adesida, I., Nuzzo, R. G., Rogers, J. A. (2006). Transfer printing by kinetic control of adhesion to an elastomeric stamp. Nature Materials, 5, 33–38. 12. Reina, A., Jia, X. T., Ho, J., Nezich, D., Son, H. B., Bulovic, V., Dresselhaus, M. S., Kong, J. (2009). Layer area, few-layer graphene films on arbitrary substrates by chemical vapor deposition. Nano Letters, 9, 30–35. 13. Ruoff, R. (2008). Graphene: Calling all chemists. Nature Nanotechnology, 3, 10–11. 14. Singh, V., Joung, D., Zhai, L., Das, S., Khondaker, S. I., & Seal, S. (2011). Graphene based materials: Past, present and future. Progress in Materials Science, 56, 1178–1271. 15. Zheng, Q., Li, Z., Yang, J., & Kim, J.-K. (2014). Graphene oxide based transparent conductive films. Progress in Materials Science, 64, 200–247. 16. Novoselov, K. S., Jiang, D., Schedin, F., Booth, T. J., Khotkevich, V. V., Morozov, S. V., & Geim, A. K. (2005). Two-dimensional atomic crystals. Proceedings of the National Academy of Sciences of the United States of America, 102, 10451–10453. 17. Sutter, P. W., Flege, J. I., Sutter, E. A. (2008). Epitaxial graphene on ruthenium. Nature Ma- terials, 7, 406–411. 18. Kim, K. S., Zhao, Y., Jang, H., Lee, S. Y., Kim, J. M., Kim, K. S., Ahn, J. H., Kim, P., Choi, J. Y., & Hong, B. H. (2009). Large-scale pattern growth of graphene films for stretchable transparent electrodes. Nature, 457, 706–710. 19. Yang, X. Y., Dou, X., Rouhanipour, A., Zhi, L. J., Rader, H. J., & Mullen, K. (2008). Two- dimensional graphene nanoribbons. Journal of the American Chemical Society, 130, 4216– 4217. 20. Stankovich, S., Dikin, D. A., Piner, R. D., Kohlhaas, K. A., Kleinhammes, A., Jia, Y., Wu, Y., Nguyen, S. T., & Ruoff, R. S. (2007). Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon, 45, 1558–1565. 82 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

21. Zhang, Y. B., Small, J. P., Pontius, W. V., & Kim, P. (2005). Fabrication and electric-field- dependent transport measurements of mesoscopic graphite devices. Applied Physics Letters, 86, 073104. 22. Blake, P., Hill, E. W., Neto, A. H. C., Novoselov, K. S., Jiang, D., Yang, R., Booth, T. J., & Geim, A. K. (2007). Making graphene visible. Applied Physics Letters, 91, 063124. 23. Gao, L. B., Ren, W. C., Li, F., & Cheng, H. M. (2008). Total color difference for rapid and accurate identification of graphene. ACS Nano, 2, 1625–1633. 24. Jung, I., Pelton, M., Piner, R., Dikin, D. A., Stankovich, S., Watcharotone, S., Hausner, M., & Ruoff, R. S. (2007). Simple approach for high-contrast optical imaging and characterization of graphene-based sheets. Nano Letters, 7, 3569–3575. 25. Chen, J. H., Jang, C., Xiao, S. D., Ishigami, M., & Fuhrer, M. S. (2008). Intrinsic and extrin- sic performance limits of graphene devices on SiO2. Nature Nanotechnology, 3, 206–209. 26. Berger, C., Song, Z. M., Li, X. B., Wu, X. S., Brown, N., Naud, C., Mayou, D., Li, T. B., Hass, J., Marchenkov, A. N., Conrad, E. H., First, P. N., de Heer, W. A. (2006). Electronic confinement and coherence in patterned epitaxial graphene. Science, 312, 1191–1196. 27. de Heer, W. A., Berger, C., Wu, X. S., First, P. N., Conrad, E. H., Li, X. B., Li, T. B., Sprinkle, M., Hass, J., Sadowski, M. L., Potemski, M., & Martinez, G. (2007). Epitaxial graphene. Solid State Communications, 143, 92–100. 28. Hass, J., de Heer, W. A., & Conrad, E. H. (2008). The growth and morphology of epitaxial multilayer graphene. Journal of Physics-Condensed Matter, 20, 323202. 29. Kedzierski, J., Hsu, P. L., Healey, P., Wyatt, P. W., Keast, C. L., Sprinkle, M., Berger, C., & de Heer, W. A. (2008). Epitaxial graphene transistors on SIC substrates. IEEE Transactions on Electron Devices, 55, 2078–2085. 30. Tedesco, J. L., Jernigan, G. G., Culbertson, J. C., Hite, J. K., Yang, Y., Daniels, K. M., Myers- Ward, R. L., Eddy, C. R., Robinson, J. A., Trumbull, K. A., Wetherington, M. T., Campbell, P. M., & Gaskill, D. K. (2010). Morphology characterization of argon-mediated epitaxial graphene on C-face SiC. Applied Physics Letters, 96, 222103. 31. Emtsev, K. V., Bostwick, A., Horn, K., Jobst, J., Kellogg, G. L., Ley, L., McChesney, J. L., Ohta, T., Reshanov, S. A., Rohrl, J., Rotenberg, E., Schmid, A. K., Waldmann, D., Weber, H. B., & Seyller, T. (2009). Towards wafer-size graphene layers by atmospheric pressure graphitization of silicon carbide. Nature Materials, 8, 203–207. 32. de Parga, A. L. V., Calleja, F., Borca, B., Passeggi, M. C. G., Hinarejos, J. J., Guinea, F., & Miranda, R. (2008). Periodically rippled graphene: Growth and spatially resolved electronic structure. Physical Review Letters, 100, 056807. 33. Sprinkle, M., Siegel, D., Hu, Y., Hicks, J., Tejeda, A., Taleb-Ibrahimi, A., Le Fevre, P., Ber- tran, F., Vizzini, S., Enriquez, H., Chiang, S., Soukiassian, P., Berger, C., de Heer, W. A., Lanzara, A., & Conrad, E. H. (2009). First direct observation of a nearly ideal graphene band structure. Physical Review Letters, 103, 226803. 34. Wang, L., Tian, L. H., Wei, G. D., Gao, F. M., Zheng, J. J., & Yang, W. Y. (2011). Epitaxial growth of graphene and their applications in devices. Journal of Inorganic Materials, 26, 1009–1019. 35. VanMil, B. L., Myers-Ward, R. L., Tedesco, J. L., Eddy, C. R., Jernigan, G. G., Culbertson, J. C., Campbell, P. M., McCrate, J. M., Kitt, S. A., & Gaskill, D. K. (2009). Graphene formation on SiC substrates. Silicon Carbide and Related Materials 2008, 615–617, 211–214. 36. Camara, N., Huntzinger, J. R., Rius, G., Tiberj, A., Mestres, N., Perez-Murano, F., Godignon, P., & Camassel, J. (2009). Anisotropic growth of long isolated graphene ribbons on the C face of graphite-capped 6 H-SiC. Physical Review B, 80, 125410. 37. Camara, N., Rius, G., Huntzinger, J. R., Tiberj, A., Mestres, N., Godignon, P., & Camassel, J. (2008). Selective epitaxial growth of graphene on SiC. Applied Physics Letters, 93, 123503. 38. Virojanadara, C., Syvajarvi, M., Yakimova, R., Johansson, L. I., Zakharov, A. A., & Balasu- bramanian, T. (2008). Homogeneous large-area graphene layer growth on 6 H-SiC(0001). Physical Review B, 78, 245403 References 83

39. Unarunotai, S., Murata, Y., Chialvo, C. E., Kim, H. S., MacLaren, S., Mason, N., Petrov, I., & Rogers, J. A. (2009). Transfer of graphene layers grown on SiC wafers to other substrates and their integration into field effect transistors. Applied Physics Letters, 95, 202101. 40. Moreau, E., Godey, S., Ferrer, F. J., Vignaud, D., Wallart, X., Avila, J., Asensio, M. C., Bour- nel, F., & Gallet, J. J. (2010). Graphene growth by molecular beam epitaxy on the carbon-face of SiC. Applied Physics Letters, 97, 241907 41. Hibino, H., Mizuno, S., Kageshima, H., Nagase, M., & Yamaguchi, H. (2009). Stacking domains of epitaxial few-layer graphene on SiC(0001). Physical Review B, 80, 085406. 42. Prakash, G., Capano, M. A., Bolen, M. L., Zemlyanou, D., & Reifenberger, R. G. (2010). AFM study of ridges in few-layer epitaxial graphene grown on the carbon-face of 4 H- SiC(000(1)over-bar). Carbon, 48, 2383–2393. 43. Jernigan, G. G., VanMil, B. L., Tedesco, J. L., Tischler, J. G., Glaser, E. R., Davidson, A., Campbell, P. M., & Gaskill, D. K. (2009). Comparison of epitaxial graphene on Si-face and C-face 4 H SiC formed by ultrahigh vacuum and RF furnace production. Nano Letters, 9, 2605–2609. 44. Ouerghi, A., Belkhou, R., Marangolo, M., Silly, M. G., El Moussaoui, S., Eddrief, M., Largeau, L., Portail, M., & Sirotti, F. (2010). Structural coherency of epitaxial graphene on 3 C-SiC(111) epilayers on Si(111). Applied Physics Letters, 97, 161905. 45. Bae, S., Kim, H., Lee, Y., Xu, X. F., Park, J. S., Zheng, Y., Balakrishnan, J., Lei, T., Kim, H. R., Song, Y. I., Kim, Y. J., Kim, K. S., Ozyilmaz, B., Ahn, J. H., Hong, B. H., & Iijima, S. (2010). Roll-to-roll production of 30-inch graphene films for transparent electrodes. Nature Nanotechnology, 5, 574–578. 46. Cai, W. W., Moore, A. L., Zhu, Y. W., Li, X. S., Chen, S. S., Shi, L., & Ruoff, R. S. (2010). Thermal transport in suspended and supported monolayer graphene grown by chemical vapor deposition. Nano Letters, 10, 1645–1651. 47. Cai, W. W., Piner, R. D., Zhu, Y. W., Li, X. S., Tan, Z. B., Floresca, H. C., Yang, C. L., Lu, L., Kim, M. J., & Ruoff, R. S. (2009). Synthesis of isotopically-labeled graphite films by cold- wall chemical vapor deposition and electronic properties of graphene obtained from such films. Nano Research, 2, 851–856. 48. Fallahazad, B., Hao, Y. F., Lee, K., Kim, S., Ruoff, R. S., & Tutuc, E. (2012). Quantum hall effect in bernal stacked and twisted bilayer graphene grown on Cu by chemical vapor deposition. Physical Review B, 85, 201408. 49. Li, X. S., Magnuson, C. W., Venugopal, A., Tromp, R. M., Hannon, J. B., Vogel, E. M., Co- lombo, L., & Ruoff, R. S. (2011). Large-area graphene single crystals grown by low-pressure chemical vapor deposition of methane on copper. Journal of the American Chemical Society, 133, 2816–2819. 50. Suk, J. W., Kitt, A., Magnuson, C. W., Hao, Y. F., Ahmed, S., An, J. H., Swan, A. K., Gold- berg, B. B., & Ruoff, R. S. (2011). Transfer of CVD-grown monolayer graphene onto arbitrary substrates. ACS Nano, 5, 6916–6924. 51. Allen, M. J., Tung, V. C., & Kaner, R. B. (2010). Honeycomb carbon: A review of graphene. Chemical Reviews, 110, 132–145. 52. Chen, Z. P., Ren, W. C., Gao, L. B., Liu, B. L., Pei, S. F., & Cheng, H. M. (2011). Three- dimensional flexible and conductive interconnected graphene networks grown by chemical vapour deposition. Nature Materials, 10, 424–428. 53. Banerjee B. C., Hirt, T. J., & Walker, P. L. (1961). Pyrolytic carbon formation from carbon suboxide. Nature, 192, 450–451. 54. Mattevi, C., Kim, H., & Chhowalla, M. (2011). A review of chemical vapour deposition of graphene on copper. Journal of Materials Chemistry, 21, 3324–3334. 55. De Arco, L. G., Zhang, Y., Kumar, A., & Zhou, C. W. (2009). Synthesis, transfer, and devices of single- and few-layer graphene by chemical vapor deposition. IEEE T Nanotechnol, 8, 135–138. 56. Reina, A., Thiele, S., Jia, X. T., Bhaviripudi, S., Dresselhaus, M. S., Schaefer, J. A., & Kong, J. (2009). Growth of large-area single- and Bi-layer graphene by controlled carbon precipitation on polycrystalline Ni surfaces. Nano Research, 2, 509–516. 84 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

57. Kwon, S. Y., Ciobanu, C. V., Petrova, V., Shenoy, V. B., Bareno, J., Gambin, V., Petrov, I., & Kodambaka, S. (2009). Growth of semiconducting graphene on palladium. Nano Letters, 9, 3985–3990. 58. Zhang, Y., Zhang, L. Y., & Zhou, C. W. (2013). Review of chemical vapor deposition of graphene and related applications. Accounts of Chemical Research, 46, 2329–2339. 59. Cai, J. M., Ruffieux, P., Jaafar, R., Bieri, M., Braun, T., Blankenburg, S., Muoth, M., Seit- sonen, A. P., Saleh, M., Feng, X. L., Mullen, K., & Fasel, R. (2010). Atomically precise bottom-up fabrication of graphene nanoribbons. Nature, 466, 470–473. 60. Wissler, M. (2006). Graphite and carbon powders for electrochemical applications. Journal of Power Sources, 156, 142–150. 61. Chung, D. D. L. (2002). Review graphite. Journal of Materials Science, 37, 1475–1489. 62. Sasa, T., Takahash, Y., Mukaibo, T. (1971). Crystal structure of graphite bromine lamellar compounds. Carbon, 9, 407–416. 63. Shih, C. J., Vijayaraghavan, A., Krishnan, R., Sharma, R., Han, J. H., Ham, M. H., Jin, Z., Lin, S. C., Paulus, G. L. C., Reuel, N. F., Wang, Q. H., Blankschtein, D., & Strano, M. S. (2011). Bi- and trilayer graphene solutions. Nature Nanotechnology, 6, 439–445. 64. Eda, G., & Chhowalla, M. (2010). Chemically derived graphene oxide: Towards large-area thin-film electronics and optoelectronics. Advanced Materials, 22, 2392–2415. 65. Dreyer, D. R., Park, S., Bielawski, C. W., & Ruoff, R. S. (2010). The chemistry of graphene oxide. Chemical Society Reviews, 39, 228–240. 66. Niyogi, S., Bekyarova, E., Itkis, M. E., McWilliams, J. L., Hamon, M. A., & Haddon, R. C. (2006). Solution properties of graphite and graphene. Journal of the American Chemical Society, 128, 7720–7721. 67. Stankovich, S., Dikin, D. A., Dommett, G. H. B., Kohlhaas, K. M., Zimney, E. J., Stach, E. A., Piner, R. D., Nguyen, S. T., & Ruoff, R. S. (2006). Graphene-based composite materials. Nature, 442, 282–286. 68. Stankovich, S., Piner, R. D., Nguyen, S. T., & Ruoff, R. S. (2006). Synthesis and exfoliation of isocyanate-treated graphene oxide nanoplatelets. Carbon, 44, 3342–3347. 69. Schniepp, H. C., Li, J. L., McAllister, M. J., Sai, H., Herrera-Alonso, M., Adamson, D. H., Prud’homme, R. K., Car, R., Saville, D. A., & Aksay, I. A. (2006). Functionalized single graphene sheets derived from splitting graphite oxide. Journal of Physical Chemistry B, 110, 8535–8539. 70. Geng, Y., Wang, S. J., & Kim, J. K. (2009). Preparation of graphite nanoplatelets and graphene sheets. Journal of Colloid and Interface Science, 336, 592–598. 71. Stankovich, S., Piner, R. D., Chen, X. Q., Wu, N. Q., Nguyen, S. T., & Ruoff, R. S. (2006). Stable aqueous dispersions of graphitic nanoplatelets via the reduction of exfoliated graphite oxide in the presence of poly(sodium 4-styrenesulfonate). Journal of Materials Chemistry, 16, 155–158. 72. Eda, G., & Chhowalla, M. (2009). Graphene-based composite thin films for electronics. Nano Letters, 9, 814–818. 73. Sun, X. M., Liu, Z., Welsher, K., Robinson, J. T., Goodwin, A., Zaric, S., & Dai, H. J. (2008). Nano-graphene oxide for cellular imaging and drug delivery. Nano Research, 1, 203–212. 74. Lerf, A., He, H. Y., Forster, M., & Klinowski, J. (1998). Structure of graphite oxide revisited. Journal of Physical Chemistry B, 102, 4477–4482. 75. Paredes, J. I., Villar-Rodil, S., Martinez-Alonso, A., & Tascon, J. M. D. (2008). Graphene oxide dispersions in organic solvents. Langmuir, 24, 10560–10564. 76. Zheng, Q. B., Ip, W. H., Lin, X. Y., Yousefi, N., Yeung, K. K., Li, Z. G., & Kim, J. K. (2011). Transparent conductive films consisting of ultra large graphene sheets produced by Langmuir-Blodgett assembly. ACS Nano, 5, 6039–6051. 77. Jung, I., Dikin, D., Park, S., Cai, W., Mielke, S. L., & Ruoff, R. S. (2008). Effect of water vapor on electrical properties of individual reduced graphene oxide sheets. Journal of Physi- cal Chemistry C, 112, 20264–20268. 78. Jung, I., Dikin, D. A., Piner, R. D., & Ruoff, R. S. (2008). Tunable electrical conductivity of individual graphene oxide sheets reduced at “low” temperatures. Nano Letters, 8, 4283– 4287. References 85

79. Yang, D., Velamakanni, A., Bozoklu, G., Park, S., Stoller, M., Piner, R. D., Stankovich, S., Jung, I., Field, D. A., Ventrice, C. A., & Ruoff, R. S. (2009). Chemical analysis of graphene oxide films after heat and chemical treatments by X-ray photoelectron and Micro-Raman spectroscopy. Carbon, 47, 145–152. 80. Tung, V. C., Allen, M. J., Yang, Y., & Kaner, R. B. (2009). High-throughput solution processing of large-scale graphene. Nature Nanotechnology, 4, 25–29. 81. Tung, V. C., Chen, L. M., Allen, M. J., Wassei, J. K., Nelson, K., Kaner, R. B., & Yang, Y. (2009). Low-temperature solution processing of graphene-carbon nanotube hybrid materials for high-performance transparent conductors. Nano Letters, 9, 1949–1955. 82. Wang, G. X., Yang, J., Park, J., Gou, X. L., Wang, B., Liu, H., & Yao, J. (2008). Facile synthesis and characterization of graphene nanosheets. Journal of Physical Chemistry C, 112, 8192–8195. 83. Pei, S. F., Zhao, J. P., Du, J. H., Ren, W. C., & Cheng, H. M. (2010). Direct reduction of graphene oxide films into highly conductive and flexible graphene films by hydrohalic acids. Carbon, 48, 4466–4474. 84. Moon, I. K., Lee, J., Ruoff, R. S., & Lee, H. (2010). Reduced graphene oxide by chemical graphitization. Nature Communications, 1, 73. 85. Muszynski, R., Seger, B., & Kamat, P. V. (2008). Decorating graphene sheets with gold nanoparticles. Journal of Physical Chemistry C, 112, 5263–5266. 86. Si, Y., & Samulski, E. T. (2008). Synthesis of water soluble graphene. Nano Letters, 8, 1679–1682. 87. Li, D., Muller, M. B., Gilje, S., Kaner, R. B., & Wallace, G. G. (2008). Processable aqueous dispersions of graphene nanosheets. Nature Nanotechnology, 3, 101–105. 88. Fowler, J. D., Allen, M. J., Tung, V. C., Yang, Y., Kaner, R. B., & Weiller, B. H. (2009). Prac- tical chemical sensors from chemically derived graphene. ACS Nano, 3, 301–306. 89. Erickson, K., Erni, R., Lee, Z., Alem, N., Gannett, W., & Zettl, A. (2010). Determination of the local chemical structure of graphene oxide and reduced graphene oxide. Advanced Mate- rials, 22, 4467–4472. 90. Wang, Y., Shi, Z. X., Fang, J. H., Xu, H. J., Ma, X. D., & Yin, J. (2011). Direct exfoliation of graphene in methanesulfonic acid and facile synthesis of graphene/polybenzimidazole nanocomposites. Journal of Materials Chemistry, 21, 505–512. 91. Wang, X. Q., Fulvio, P. F., Baker, G. A., Veith, G. M., Unocic, R. R., Mahurin, S. M., Chi, M. F., & Dai, S. (2010). Direct exfoliation of natural graphite into micrometre size few layers graphene sheets using ionic liquids. Chemical Communications, 46, 4487–4489. 92. Liu, W. W., & Wang, J. N. (2011). Direct exfoliation of graphene in organic solvents with addition of NaOH. Chemical Communications, 47, 6888–6890. 93. Hernandez, Y., Nicolosi, V., Lotya, M., Blighe, F. M., Sun, Z. Y., De, S., McGovern, I. T., Holland, B., Byrne, M., Gun'ko, Y. K., Boland, J. J., Niraj, P., Duesberg, G., Krishnamurthy, S., Goodhue, R., Hutchison, J., Scardaci, V., Ferrari, A. C., & Coleman, J. N. (2008). High- yield production of graphene by liquid-phase exfoliation of graphite. Nature Nanotechnol- ogy, 3, 563–568. 94. Li, X. L., Zhang, G. Y., Bai, X. D., Sun, X. M., Wang, X. R., Wang, E., & Dai, H. J. (2008). Highly conducting graphene sheets and Langmuir-Blodgett films. Nature Nanotechnology, 3, 538–542. 95. Green, A. A., & Hersam, M. C. (2009). Solution phase production of graphene with controlled thickness via density differentiation. Nano Letters, 9, 4031–4036. 96. Lee, J. H., Shin, D. W., Makotchenko, V. G., Nazarov, A. S., Fedorov, V. E., Kim, Y. H., Choi, J. Y., Kim, J. M., & Yoo, J. B. (2009). One-step exfoliation synthesis of easily soluble graphite and transparent conducting graphene sheets. Advanced Materials, 21, 4383–4387. 97. Lotya, M., Hernandez, Y., King, P. J., Smith, R. J., Nicolosi, V., Karlsson, L. S., Blighe, F. M., De, S., Wang, Z. M., McGovern, I. T., Duesberg, G. S., & Coleman, J. N. (2009). Liquid phase production of graphene by exfoliation of graphite in surfactant/water solutions. Journal of the American Chemical Society, 131, 3611–3620. 86 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

98. Coleman, J. N. (2009). Liquid-phase exfoliation of nanotubes and graphene. Advanced Functional Materials, 19, 3680–3695. 99. Khan, U., O’Neill, A., Lotya, M., De, S., & Coleman, J. N. (2010). High-concentration solvent exfoliation of graphene. Small, 6, 864–871. 100. Valles, C., Drummond, C., Saadaoui, H., Furtado, C. A., He, M., Roubeau, O., Ortolani, L., Monthioux, M., & Penicaud, A. (2008). Solutions of negatively charged graphene sheets and ribbons. Journal of the American Chemical Society, 130, 15802–15804. 101. Eda, G., Lin, Y. Y., Mattevi, C., Yamaguchi, H., Chen, H. A., Chen, I. S., Chen, C. W., & Chhowalla, M. (2010). Blue photoluminescence from chemically derived graphene oxide. Advanced Materials, 22, 505–509. 102. Zhou, X. F., & Liu, Z. P. (2010). A scalable, solution-phase processing route to graphene oxide and graphene ultralarge sheets. Chemical Communications, 46, 2611–2613. 103. Zhao, J. P., Pei, S. F., Ren, W. C., Gao, L. B., & Cheng, H. M. (2010). Efficient preparation of large-area graphene oxide sheets for transparent conductive films. ACS Nano, 4, 5245–5252. 104. Su, C. Y., Xu, Y. P., Zhang, W. J., Zhao, J. W., Tang, X. H., Tsai, C. H., & Li, L. J. (2009). Electrical and spectroscopic characterizations of ultra-large reduced graphene oxide monolayers. Chemistry of Materials, 21, 5674–5680. 105. Dong, X. C., Su, C. Y., Zhang, W. J., Zhao, J. W., Ling, Q. D., Huang, W., Chen, P., & Li, L. J. (2010). Ultra-large single-layer graphene obtained from solution chemical reduction and its electrical properties. Physical Chemistry Chemical Physics, 12, 2164–2169. 106. Bae, S. Y., Jeon, I. Y., Yang, J., Park, N., Shin, H. S., Park, S., Ruoff, R. S., Dai, L. M., & Baek, J. B. (2011). Large-area graphene films by simple solution casting of edge-selectively functionalized graphite. ACS Nano, 5, 4974–4980. 107. Zheng, Q., Ip, W. H., Lin, X., Yousefi, N., Yeung, K. K., Li, Z., & Kim, J.-K. (2011). Transparent conductive films consisting of ultra large graphene sheets produced by Lang- muir-Blodgett Assembly. ACS Nano, 5, 6039–6051. 108. Aboutalebi, S. H., Gudarzi, M. M., Zheng, Q. B., & Kim, J.-K. (2011). Spontaneous formation of liquid crystals in ultralarge graphene oxide dispersions. Advanced Functional Materials, 21, 2978–2988. 109. Jia, J. J., Kan, C., Lin, X. M., Shen, X., & Kim, J. K. (2014). Effects of processing and material parameters on synthesis of monolayer ultralarge graphene oxide sheets. Carbon, 77, 244–254. 110. Zheng, Q., Zhang, B., Lin, X., Shen, X., Yousefi, N., Huang, Z.-D., Li, Z., & Kim, J.-K. (2012). Highly transparent and conducting ultralarge graphene oxide/single-walled carbon nanotube hybrid films produced by Langmuir-Blodgett assembly. Journal of Materials Chemistry, 22, 25072–25082. 111. Sun, X. M., Luo, D. C., Liu, J. F., & Evans, D. G. (2010). Monodisperse chemically modified graphene obtained by density gradient ultracentrifugal rate separation. ACS Nano, 4, 3381–3389. 112. Wang, X. L., Bai, H., & Shi, G. Q. (2011). Size fractionation of graphene oxide sheets by ph-assisted selective sedimentation. Journal of the American Chemical Society, 133, 6338–6342. 113. Green, A. A., & Hersam, M. C. (2010). Emerging methods for producing monodisperse graphene dispersions. Journal of Physical Chemistry Letters, 1, 544–549. 114. Arnold, M. S., Stupp, S. I., & Hersam, M. C. (2005). Enrichment of single-walled carbon nanotubes by diameter in density gradients. Nano Letters, 5, 713–718. 115. Haroz, E. H., Rice, W. D., Lu, B. Y., Ghosh, S., Hauge, R. H., Weisman, R. B., Doorn, S. K., & Kono, J. (2010). Enrichment of armchair carbon nanotubes via density gradient ultracentrifugation: Raman spectroscopy evidence. ACS Nano, 4, 1955–1962. 116. Zhu, Y. W., Murali, S., Cai, W. W., Li, X. S., Suk, J. W., Potts, J. R., & Ruoff, R. S. (2010). Graphene and graphene oxide: Synthesis, properties, and applications. Advanced Materi- als, 22, 3906–3924. References 87

117. You, Y. M., Ni, Z. H., Yu, T., & Shen, Z. X. (2008). Edge chirality determination of graphene by Raman spectroscopy. Applied Physics Letters, 93, 163112. 118. Girit, C. O., Meyer, J. C., Erni, R., Rossell, M. D., Kisielowski, C., Yang, L., Park, C. H., Crommie, M. F., Cohen, M. L., Louie, S. G., & Zettl, A. (2009). Graphene at the edge: Stability and dynamics. Science, 323, 1705–1708. 119. Los, J. H., Ghiringhelli, L. M., Meijer, E. J., & Fasolino, A. (2005). Improved long-range reactive bond-order potential for carbon. I. Construction. Physical Review B, 72, 214102. 120. Fasolino, A., Los, J. H., & Katsnelson, M. I. (2007). Intrinsic ripples in graphene. Nature Materials, 6, 858–861. 121. Geim, A. K., & Novoselov, K. S. (2007). The rise of graphene. Nature Materials, 6, 183–191. 122. He, H. Y., Klinowski, J., Forster, M., & Lerf, A. (1998). A new structural model for graphite oxide. Chemical Physics Letters, 287, 53–56. 123. Boukhvalov, D. W., & Katsnelson, M. I. (2008). Modeling of graphite oxide. Journal of the American Chemical Society, 130, 10697–10701. 124. Lin, X., Shen, X., Zheng, Q., Yousefi, N., Ye, L., Mai, Y.-W., & Kim, J.-K. (2012). Fabrica- tion of highly-aligned, conductive, and strong graphene papers using ultralarge graphene oxide sheets. ACS Nano, 6, 10708–10719. 125. Kopidakis, G., Remediakis, I. N., Fyta, M. G., & Kelires, P. C. (2007). Atomic and electronic structure of crystalline-amorphous carbon interfaces. Diamond and Related Materials, 16, 1875–1881. 126. Zheng, Q. B., Gudarzi, M. M., Wang, S. J., Geng, Y., Li, Z. G., & Kim, J. K. (2011). Im- proved electrical and optical characteristics of transparent graphene thin films produced by acid and doping treatments. Carbon, 49, 2905–2916. 127. Jung, I., Vaupel, M., Pelton, M., Piner, R., Dikin, D. A., Stankovich, S., An, J., & Ruoff, R. S. (2008). Characterization of thermally reduced graphene oxide by imaging ellipsometry. Journal of Physical Chemistry C, 112, 8499–8506. 128. Cote, L. J., Kim, F., & Huang, J. X. (2009). Langmuir-Blodgett assembly of graphite oxide single layers. Journal of the American Chemical Society, 131, 1043–1049. 129. Gomez-Navarro, C., Weitz, R. T., Bittner, A. M., Scolari, M., Mews, A., Burghard, M., & Kern, K. (2007). Electronic transport properties of individual chemically reduced graphene oxide sheets. Nano Letters, 7, 3499–3503. 130. Chang, H. X., & Wu, H. K. (2013). Graphene-based nanomaterials: Synthesis, properties, and optical and optoelectronic applications. Advanced Functional Materials, 23, 1984–1997. 131. Katsnelson, M. I. (2007). Graphene: Carbon in two dimensions. Materials Today, 10, 20–27. 132. Du, X., Skachko, I., Barker, A., & Andrei, E. Y. (2008). Approaching ballistic transport in suspended graphene. Nature Nanotechnology, 3, 491–495. 133. Geim, A. K. (2009). Graphene: Status and prospects. Science, 324, 1530–1534. 134. Li, X. L., Wang, X. R., Zhang, L., Lee, S. W., & Dai, H. J. (2008). Chemically derived, ultrasmooth graphene nanoribbon semiconductors. Science, 319, 1229–1232. 135. Kosynkin, D. V., Higginbotham, A. L., Sinitskii, A., Lomeda, J. R., Dimiev, A., Price, B. K., & Tour, J. M. (2009). Longitudinal unzipping of carbon nanotubes to form graphene nanoribbons. Nature, 458, 872–876. 136. Bai, J. W., Zhong, X., Jiang, S., Huang, Y., & Duan, X. F. (2010). Graphene nanomesh. Nature Nanotechnology, 5, 190–194. 137. Balog, R., Jorgensen, B., Nilsson, L., Andersen, M., Rienks, E., Bianchi, M., Fanetti, M., Laegsgaard, E., Baraldi, A., Lizzit, S., Sljivancanin, Z., Besenbacher, F., Hammer, B., Ped- ersen, T. G., Hofmann, P., & Hornekaer, L. (2010). Bandgap opening in graphene induced by patterned hydrogen adsorption. Nature Materials, 9, 315–319. 138. Wang, X. R., & Dai, H. J. (2010). Etching and narrowing of graphene from the edges. Na- ture Chemistry, 2, 661–665. 88 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

139. Novoselov, K. S., Jiang, Z., Zhang, Y., Morozov, S. V., Stormer, H. L., Zeitler, U., Maan, J. C., Boebinger, G. S., Kim, P., & Geim, A. K. (2007). Room-temperature quantum hall effect in graphene. Science, 315, 1379–1379. 140. Kopelevich, Y., & Esquinazi, P. (2007). Graphene physics in graphite. Advanced Materials, 19, 4559–4563. 141. Becerril, H. A., Mao, J., Liu, Z., Stoltenberg, R. M., Bao, Z., & Chen, Y. (2008). Evaluation of solution-processed reduced graphene oxide films as transparent conductors. ACS Nano, 2, 463–470. 142. Pei, S. F., & Cheng, H. M. (2012). The reduction of graphene oxide. Carbon, 50, 3210– 3228. 143. Li, X. S., Zhu, Y. W., Cai, W. W., Borysiak, M., Han, B. Y., Chen, D., Piner, R. D., Colom- bo, L., & Ruoff, R. S. (2009). Transfer of large-area graphene films for high-performance transparent conductive electrodes. Nano Letters, 9, 4359–4363. 144. Wang, S. J., Geng, Y., Zheng, Q., & Kim, J.-K. (2010). Fabrication of highly conducting and transparent graphene films. Carbon, 48, 1815–1823. 145. Eda, G., Fanchini, G., & Chhowalla, M. (2008). Large-area ultrathin films of reduced graphene oxide as a transparent and flexible electronic material. Nature Nanotechnology, 3, 270–274. 146. Feng, H. B., Cheng, R., Zhao, X., Duan, X. F., & Li, J. H. (2013). A low-temperature method to produce highly reduced graphene oxide Nature Communications, 4, 1539. 147. Viet, H. P., Tran, V. C., Nguyen-Phan, T. D., Hai, D. P., Kim, E. J., Hur, S. H., Shin, E. W., Kim, S., & Chung, J. S. (2010). One-step synthesis of superior dispersion of chemically converted graphene in organic solvents. Chemical Communications, 46, 4375–4377. 148. Gao, W., Alemany, L. B., Ci, L. J., & Ajayan, P. M. (2009). New insights into the structure and reduction of graphite oxide. Nature Chemistry, 1, 403–408. 149. Shin, H. J., Kim, K. K., Benayad, A., Yoon, S. M., Park, H. K., Jung, I. S., Jin, M. H., Jeong, H. K., Kim, J. M., Choi, J. Y., & Lee, Y. H. (2009). Efficient reduction of graphite oxide by sodium borohydrilde and its effect on electrical conductance. Advanced Functional Materi- als, 19, 1987–1992. 150. Park, S., An, J. H., Jung, I. W., Piner, R. D., An, S. J., Li, X. S., Velamakanni, A., & Ruoff, R. S. (2009). Colloidal suspensions of highly reduced graphene oxide in a wide variety of organic solvents. Nano Letters, 9, 1593–1597. 151. Fan, Z. J., Wang, K., Wei, T., Yan, J., Song, L. P., & Shao, B. (2010). An environmentally friendly and efficient route for the reduction of graphene oxide by aluminum powder. Car- bon, 48, 1686–1689. 152. Fan, Z. J., Kai, W., Yan, J., Wei, T., Zhi, L. J., Feng, J., Ren, Y. M., Song, L. P., & Wei, F. (2011). Facile synthesis of graphene nanosheets via Fe reduction of exfoliated graphite oxide. ACS Nano, 5, 191–198. 153. Mei, X. G., & Ouyang, J. Y. (2011). Ultrasonication-assisted ultrafast reduction of graphene oxide by zinc powder at room temperature. Carbon, 49, 5389–5397. 154. Dey, R. S., Hajra, S., Sahu, R. K., Raj, C. R., & Panigrahi, M. K. (2012). A rapid room temperature chemical route for the synthesis of graphene: Metal-mediated reduction of graphene oxide. Chemical Communications, 48, 1787–1789. 155. Kumar, N. A., Gambarelli, S., Duclairoir, F., Bidan, G., & Dubois, L. (2013). Synthesis of high quality reduced graphene oxide nanosheets free of paramagnetic metallic impurities. Journal of Materials Chemistry A, 1, 2789–2794. 156. Pham, V. H., Pham, H. D., Dang, T. T., Hur, S. H., Kim, E. J., Kong, B. S., Kim, S., & Chung, J. S. (2012). Chemical reduction of an aqueous suspension of graphene oxide by nascent hydrogen. Journal of Materials Chemistry, 22, 10530–10536. 157. Barman, B. K., Mahanandia, P., & Nanda, K. K. (2013). Instantaneous reduction of graphene oxide at room temperature. RSC Advances, 3, 12621–12624. 158. Liu, Y. Z., Li, Y. F., Zhong, M., Yang, Y. G., Wen, Y. F., & Wang, M. Z. (2011). A green and ultrafast approach to the synthesis of scalable graphene nanosheets with Zn powder for electrochemical energy storage. Journal of Materials Chemistry, 21, 15449–15455. References 89

159. Chua, C. K., & Pumera, M. (2014). Chemical reduction of graphene oxide: A synthetic chemistry viewpoint. Chemical Society Reviews, 43, 291–312. 160. Williams, G., Seger, B., & Kamat, P. V. (2008). TiO2-graphene nanocomposites. UV-assisted photocatalytic reduction of graphene oxide. ACS Nano, 2, 1487–1491. 161. Liu, X. J., Pan, L. K., Zhao, Q. F., Lv, T., Zhu, G., Chen, T. Q., Lu, T., Sun, Z., & Sun, C. Q. (2012). UV-assisted photocatalytic synthesis of ZnO-reduced graphene oxide composites with enhanced photocatalytic activity in reduction of Cr(VI). Chem Eng J, 183, 238–243. 162. Kamat, P. V. (1993). Photochemistry on nonreactive and reactive (semiconductor) surfaces. Chemical Reviews, 93, 267–300. 163. Zhou, M., Wang, Y. L., Zhai, Y. M., Zhai, J. F., Ren, W., Wang, F. A., & Dong, S. J. (2009). Controlled synthesis of large-area and patterned electrochemically reduced graphene oxide films. Chemistry-a European Journal, 15, 6116–6120. 164. An, S. J., Zhu, Y. W., Lee, S. H., Stoller, M. D., Emilsson, T., Park, S., Velamakanni, A., An, J. H., Ruoff, R. S. (2010). Thin film fabrication and simultaneous anodic reduction of deposited graphene oxide platelets by electrophoretic deposition. Journal of Physical Chemistry Letters, 1, 1259–1263. 165. Zhou, Y., Bao, Q. L., Tang, L. A. L., Zhong, Y. L., & Loh, K. P. (2009). Hydrothermal dehydration for the “green” reduction of exfoliated graphene oxide to graphene and demonstra- tion of tunable optical limiting properties. Chemistry of Materials, 21, 2950–2956. 166. Dubin, S., Gilje, S., Wang, K., Tung, V. C., Cha, K., Hall, A. S., Farrar, J., Varshneya, R., Yang, Y., & Kaner, R. B. (2010). A one-step, solvothermal reduction method for producing reduced graphene oxide dispersions in organic solvents. ACS Nano, 4, 3845–3852. 167. Mattevi, C., Eda, G., Agnoli, S., Miller, S., Mkhoyan, K. A., Celik, O., Mostrogiovanni, D., Granozzi, G., Garfunkel, E., & Chhowalla, M. (2009). Evolution of electrical, chemical, and structural properties of transparent and conducting chemically derived graphene thin films. Advanced Functional Materials, 19, 2577–2583. 168. Novoselov, K. S., Geim, A. K., Morozov, S. V., Jiang, D., Katsnelson, M. I., Grigorieva, I. V., Dubonos, S. V., & Firsov, A. A. (2005). Two-dimensional gas of massless Dirac fermions in graphene. Nature, 438, 197–200. 169. Pike, G. E., & Seager, C. H. (1974). Percolation and conductivity: A computer study. I. Phys Rev B, 10, 1421–1434. 170. Kang, H., Kulkarni, A., Stankovich, S., Ruoff, R. S., & Baik, S. (2009). Restoring electrical conductivity of dielectrophoretically assembled graphite oxide sheets by thermal and chemical reduction techniques. Carbon, 47, 1520–1525. 171. Wang, S. J., Geng, Y., Zheng, Q. B., & Kim, J. K. (2010). Fabrication of highly conducting and transparent graphene films. Carbon, 48, 1815–1823. 172. Wang, X., Zhi, L. J., & Mullen, K. (2008). Transparent, conductive graphene electrodes for dye-sensitized solar cells. Nano Letters, 8, 323–327. 173. Wu, Z. S., Ren, W. C., Gao, L. B., Liu, B. L., Jiang, C. B., & Cheng, H. M. (2009). Synthe- sis of high-quality graphene with a pre-determined number of layers. Carbon, 47, 493–499. 174. Lu, Y., Pich, A., & Adler, H. J. P. (2004). Synthesis and characterization of polypyrrole dispersions prepared with different dopants. Macromolecular Symposia, 210, 411–417. 175. Li, X. L., Wang, H. L., Robinson, J. T., Sanchez, H., Diankov, G., & Dai, H. J. (2009). Simultaneous nitrogen doping and reduction of graphene oxide. Journal of the American Chemical Society, 131, 15939–15944. 176. Zheng, Q. B., Zhang, B., Lin, X. Y., Shen, X., Yousefi, N., Huang, Z. D., Li, Z. G., & Kim, J. K. (2012). Highly transparent and conducting ultralarge graphene oxide/single-walled carbon nanotube hybrid films produced by Langmuir-Blodgett assembly. Journal of Mate- rials Chemistry, 22, 25072–25082. 177. Wu, Z. S., Ren, W. C., Gao, L. B., Zhao, J. P., Chen, Z. P., Liu, B. L., Tang, D. M., Yu, B., Jiang, C. B., & Cheng, H. M. (2009). Synthesis of graphene sheets with high electrical conductivity and good thermal stability by hydrogen arc discharge exfoliation. ACS Nano, 3, 411–417. 90 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

178. Lopez, V., Sundaram, R. S., Gomez-Navarro, C., Olea, D., Burghard, M., Gomez-Herrero, J., Zamora, F., & Kern, K. (2009). Chemical capor deposition repair of graphene oxide: a route to highly conductive graphene monolayers. Advanced Materials, 21, 4683–4686. 179. Su, Q., Pang, S. P., Alijani, V., Li, C., Feng, X. L., & Mullen, K. (2009). Composites of graphene with large aromatic molecules. Advanced Materials, 21, 3191–3195. 180. Chen, H., Muller, M. B., Gilmore, K. J., Wallace, G. G., & Li, D. (2008). Mechanically strong, electrically conductive, and biocompatible graphene paper. Advanced Materials, 20, 3557–3561. 181. Vlassiouk, I., Smirnov, S., Ivanov, I., Fulvio, P. F., Dai, S., Meyer, H., Chi, M. F., Hensley, D., Datskos, P., & Lavrik, N. V. (2011). Electrical and thermal conductivity of low temperature CVD graphene: The effect of disorder. Nanotechnology, 22, 275716. 182. Yu, C. H., Shi, L., Yao, Z., Li, D. Y., & Majumdar, A. (2005). Thermal conductance and thermopower of an individual single-wall carbon nanotube. Nano Letters, 5, 1842–1846. 183. Nika, D. L., & Balandin, A. A. (2012). Two-dimensional phonon transport in graphene. Journal of Physics-Condensed Matter, 24, 233203. 184. Nika, D. L., Pokatilov, E. P., & Balandin, A. A. (2011). Theoretical description of thermal transport in graphene: The issues of phonon cut-off frequencies and polarization branches. Physica Status Solidi B-Basic Solid State Physics, 248, 2609–2614. 185. Berber, S., Kwon, Y. K., & Tomanek, D. (2000). Unusually high thermal conductivity of carbon nanotubes. Physical Review Letters, 84, 4613–4616. 186. Balandin, A. A., Ghosh, S., Bao, W. Z., Calizo, I., Teweldebrhan, D., Miao, F., & Lau, C. N. (2008). Superior thermal conductivity of single-layer graphene. Nano Letters, 8, 902–907. 187. Balandin, A. A. (2011). Thermal properties of graphene and nanostructured carbon materials. Nature Materials, 10, 569–581. 188. Ghosh, S., Calizo, I., Teweldebrhan, D., Pokatilov, E. P., Nika, D. L., Balandin, A. A., Bao, W., Miao, F., & Lau, C. N. (2008). Extremely high thermal conductivity of graphene: Prospects for thermal management applications in nanoelectronic circuits. Applied Physics Letters, 92, 151911. 189. Seol, J. H., Jo, I., Moore, A. L., Lindsay, L., Aitken, Z. H., Pettes, M. T., Li, X. S., Yao, Z., Huang, R., Broido, D., Mingo, N., Ruoff, R. S., & Shi, L. (2010). Two-dimensional phonon transport in supported graphene. Science, 328, 213–216. 190. Ghosh, S., Bao, W. Z., Nika, D. L., Subrina, S., Pokatilov, E. P., Lau, C. N., & Balandin, A. A. (2010). Dimensional crossover of thermal transport in few-layer graphene. Nature Materials, 9, 555–558. 191. Jauregui, L. A., Yue, Y. N., Sidorov, A. N., Hu, J. N., Yu, Q. K., Lopez, G., Jalilian, R., Benjamin, D. K., Delk, D. A., Wu, W., Liu, Z. H., Wang, X. W., Jiang, Z. G., Ruan, X. L., Bao, J. M., Pei, S. S., & Chen, Y. P. (2010). Thermal transport in graphene nanostructures: experiments and simulations. Graphene, Ge/Iii-V, and Emerging Materials for Post-Cmos Applications 2, 28, 73–83. 192. Faugeras, C., Faugeras, B., Orlita, M., Potemski, M., Nair, R. R., & Geim, A. K. (2010). Thermal conductivity of graphene in corbino membrane geometry. ACS Nano, 4, 1889–1892. 193. Murali, R., Yang, Y. X., Brenner, K., Beck, T., & Meindl, J. D. (2009). Breakdown current density of graphene nanoribbons. Applied Physics Letters, 94, 243114. 194. Nika, D. L., Pokatilov, E. P., Askerov, A. S., & Balandin, A. A. (2009). Phonon thermal conduction in graphene: Role of Umklapp and edge roughness scattering. Physical Review B, 79, 155413. 195. Nika, D. L., Ghosh, S., Pokatilov, E. P., & Balandin, A. A. (2009). Lattice thermal conductivity of graphene flakes: Comparison with bulk graphite. Applied Physics Letters, 94, 203103. 196. Evans, W. J., Hu, L., & Keblinski, P. (2010). Thermal conductivity of graphene ribbons from equilibrium molecular dynamics: Effect of ribbon width, edge roughness, and hydrogen termination. Applied Physics Letters, 96, 203112. References 91

197. Lindsay, L., Broido, D. A., Mingo, N. (2010). Diameter dependence of carbon nanotube thermal conductivity and extension to the graphene limit. Physical Review B, 82, 161402(R). 198. Munoz, E., Lu, J. X., & Yakobson, B. I. (2010). Ballistic thermal conductance of graphene ribbons. Nano Letters, 10, 1652–1656. 199. Schwamb, T., Burg, B. R., Schirmer, N. C., & Poulikakos, D. (2009). An electrical method for the measurement of the thermal and electrical conductivity of reduced graphene oxide nanostructures. Nanotechnology, 20, 405704. 200. Mahanta, N. K., & Abramson, A. R. (2012). Thermal conductivity of graphene and graphene oxide nanoplatelets. 13th IEEE ITHERM Conference, 1–6. 201. Shen, X., Lin, X. Y., Jia, J. J., Wang, Z. Y., Li, Z. G., & Kim, J. K. (2014). Tunable thermal conductivities of graphene oxide by functionalization and tensile loading. Carbon, 80, 235–245. 202. Wu, H., & Drzal, L. T. (2012). Graphene nanoplatelet paper as a light-weight composite with excellent electrical and thermal conductivity and good gas barrier properties. Carbon, 50, 1135–1145. 203. Xiang, J. L., & Drzal, L. T. (2011). Thermal conductivity of exfoliated graphite nanoplatelet paper. Carbon, 49, 773–778. 204. Xin, G. Q., Sun, H. T., Hu, T., Fard, H. R., Sun, X., Koratkar, N., Borca-Tasciuc, T., & Lian, J. (2014). Large-area freestanding graphene paper for superior thermal management. Advanced Materials, 26, 4521–4526. 205. Yu, W., Xie, H. Q., Li, F. X., Zhao, J. C., & Zhang, Z. H. (2013). Significant thermal conductivity enhancement in graphene oxide papers modified with alkaline earth metal ions. Applied Physics Letters, 103, 141913. 206. P. Kim, L. Shi, A., Majumdar, & McEuen, P. L. (2001). Thermal transport measurements of individual multiwalled nanotubes. Physical Review Letters, 87, 215502. 207. Pop, E., Mann, D., Wang, Q., Goodson, K. E., & Dai, H. J. (2006). Thermal conductance of an individual single-wall carbon nanotube above room temperature. Nano Letters, 6, 96–100. 208. Hone, J., Whitney, M., Piskoti, C., & Zettl, A. (1999). Thermal conductivity of single- walled carbon nanotubes. Physical Review B, 59, R2514–R2516. 209. Chang, C. W., Fennimore, A. M., Afanasiev, A., Okawa, D., Ikuno, T., Garcia, H., Li, D. Y., Majumdar, A., & Zettl, A. (2006). Isotope effect on the thermal conductivity of boron nitride nanotubes. Physical Review Letters, 97, 085901. 210. Fujii, M., Zhang, X., Xie, H. Q., Ago, H., Takahashi, K., Ikuta, T., Abe, H., & Shimizu, T. (2005). Measuring the thermal conductivity of a single carbon nanotube. Physical Review Letters, 95, 065502. 211. Che, J. W., Cagin, T., & Goddard, W. A. (2000). Thermal conductivity of carbon nanotubes. Nanotechnology, 11, 65–69. 212. Donadio, D., & Galli, G. (2007). Thermal conductivity of isolated and interacting carbon nanotubes: Comparing results from molecular dynamics and the Boltzmann transport equation. Physical Review Letters, 99, 255502. 213. Subrina, S., Kotchetkov, D., & Balandin, A. A. (2009). Heat removal in silicon-on-insulator integrated circuits with graphene lateral heat spreaders. IEEE Electr Device Letters, 30, 1281–1283. 214. Peres, N. M. R., Guinea, F., & Neto, A. H. C. (2006). Electronic properties of disordered two-dimensional carbon. Physical Review B, 73, 125411. 215. Gusynin, V. P., Sharapov, S. G., & Carbotte, J. P. (2006). Unusual microwave response of Dirac quasiparticles in graphene. Physical Review Letters, 96, 256802. 216. Nair, R. R., Blake, P., Grigorenko, A. N., Novoselov, K. S., Booth, T. J., Stauber, T., Peres, N. M. R., & Geim, A. K. (2008). Fine structure constant defines visual transparency of graphene. Science, 320, 1308–1308. 217. Ni, Z. H., Wang, H. M., Kasim, J., Fan, H. M., Yu, T., Wu, Y. H., Feng, Y. P., & Shen, Z. X. (2007). Graphene thickness determination using reflection and contrast spectroscopy. Nano Letters, 7, 2758–2763. 92 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

218. Aboutalebi, S. H., Gudarzi, M. M., Zheng, Q. B., Kim, J. K. (2011). Spontaneous formation of liquid crystals in ultralarge graphene oxide dispersions. Advanced Functional Materials, 21, 2978–2988. 219. Marcano, D. C., Kosynkin, D. V., Berlin, J. M., Sinitskii, A., Sun, Z. Z., Slesarev, A., Ale- many, L. B., Lu, W., & Tour, J. M. (2010). Improved synthesis of graphene oxide. ACS Nano, 4, 4806–4814. 220. Lee, C., Wei, X. D., Kysar, J. W., & Hone, J. (2008). Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science, 321, 385–388. 221. Zheng, Q., Li, Z., & Yang, J. (2013). Effects of N doping and NH2 grafting on the mechanical and wrinkling properties of graphene sheets. Rsc Advances, 3, 923–929. 222. Zheng, Q., Geng, Y., Wang, S., Li, Z., & Kim, J.-K. (2010). Effects of functional groups on the mechanical and wrinkling properties of graphene sheets. Carbon, 48, 4315–4322. 223. Shen, X., Lin, X. Y., Yousefi, N., Jia, J. J., & Kim, J. K. (2014). Wrinkling in graphene sheets and graphene oxide papers. Carbon, 66, 84–92. 224. Uddin, M. N., Huang, Z. D., Mai, Y. W., & Kim, J. K. (2014). Tensile and tearing fracture properties of graphene oxide papers intercalated with carbon nanotubes. Carbon, 77, 481–491. 225. Politano, A., Marino, A. R., Campi, D., Farias, D., Miranda, R., & Chiarello, G. (2012). Elastic properties of a macroscopic graphene sample from phonon dispersion measurements. Carbon, 50, 4903–4910. 226. Liu, F., Ming, P. M., & Li, J. (2007). Ab initio calculation of ideal strength and phonon instability of graphene under tension. Physical Review B, 76, 064120. 227. Bera, S., Arnold, A., Evers, F., Narayanan, R., & Wolfle, P. (2010). Elastic properties of graphene flakes: Boundary effects and lattice vibrations. Physical Review B, 82, 195445. 228. Cadelano, E., Palla, P. L., Giordano, S., & Colombo, L. (2010). Elastic properties of hydro- genated graphene. Physical Review B, 82, 235414. 229. Zheng, Q. B., Geng, Y., Wang, S. J., Li, Z. G., & Kim, J. K. (2010). Effects of functional groups on the mechanical and wrinkling properties of graphene sheets. Carbon, 48, 4315–4322. 230. Kim, J., Cote, L. J., Kim, F., & Huang, J. X. (2010). Visualizing graphene based sheets by fluorescence quenching microscopy. Journal of the American Chemical Society, 132, 260–267. 231. Zakharchenko, K. V., Katsnelson, M. I., & Fasolino, A. (2009). Finite temperature lattice properties of graphene beyond the quasiharmonic approximation. Physical Review Letters, 102, 046808. 232. Cadelano, E., Palla, P. L., Giordano, S., & Colombo, L. (2009). Nonlinear elasticity of monolayer graphene. Physical Review Letters, 102, 235502. 233. Reddy, C. D., Ramasubramaniam, A., Shenoy, V. B., & Zhang, Y. W. (2009). Edge elastic properties of defect-free single-layer graphene sheets. Applied Physics Letters, 94, 101904. 234. Suk, J. W., Piner, R. D., An, J. H., & Ruoff, R. S. (2010). Mechanical properties of mono layer graphene oxide. Acs Nano, 4, 6557–6564. 235. Zheng, Q. B., Li, Z. G., Geng, Y., Wang, S. J., & Kim, J. K. (2010). Molecular dynamics study of the effect of chemical functionalization on the elastic properties of graphene sheets. Journal of Nanoscience and Nanotechnology, 10, 7070–7074. 236. Pei, Q. X., Zhang, Y. W., & Shenoy, V. B. (2010). A molecular dynamics study of the mechanical properties of hydrogen functionalized graphene. Carbon, 48, 898–904. 237. Pei, Q. X., Zhang, Y. W., & Shenoy, V. B. (2010). Mechanical properties of methyl functionalized graphene: A molecular dynamics study. Nanotechnology, 21, 115709. 238. Dikin, D. A., Stankovich, S., Zimney, E. J., Piner, R. D., Dommett, G. H. B., Evmenenko, G., Nguyen, S. T., & Ruoff, R. S. (2007). Preparation and characterization of graphene oxide paper. Nature, 448, 457–460. 239. Park, S., Lee, K. S., Bozoklu, G., Cai, W., Nguyen, S. T., & Ruoff’, R. S. (2008). Graphene oxide papers modified by divalent ions—enhancing mechanical properties via chemical cross-linking. ACS Nano, 2, 572–578. References 93

240. Park, S., Dikin, D. A., Nguyen, S. T., & Ruoff, R. S. (2009). Graphene oxide sheets chemically cross-linked by polyallylamine. Journal of Physical Chemistry C, 113, 15801–15804. 241. Guo, P., Song, H. H., & Chen, X. H. (2009). Electrochemical performance of graphene nanosheets as anode material for lithium-ion batteries. Electrochemistry Communications, 11, 1320–1324. 242. Lin, X. Y., Liu, X., Jia, J. J., Shen, X., & Kim, J. K. (2014). Electrical and mechanical properties of carbon nanofiber/graphene oxide hybrid papers. Composites Science and Technol- ogy, 100, 166–173. 243. Casablanca, L. B., Shaibat, M. A., Cai, W. W. W., Park, S., Piner, R., Ruoff, R. S., & Ishii, Y. (2010). NMR-based structural modeling of graphite oxide using multidimensional C-13 solid-state NMR and ab initio chemical shift calculations. Journal of the American Chemi- cal Society, 132, 5672–5676. 244. Paredes, J. I., Villar-Rodil, S., Solis-Fernandez, P., Martinez-Alonso, A., & Tascon, J. M. D. (2009). Atomic force and scanning tunneling microscopy imaging of graphene nanosheets derived from graphite oxide. Langmuir, 25, 5957–5968. 245. Kim, J., Kim, F., & Huang, J. X. (2010). Seeing graphene-based sheets. Materials Today, 13, 28–38. 246. Zheng, J. A., Di, C. A., Liu, Y. Q., Liu, H. T., Guo, Y. L., Du, C. Y., Wu, T., Yu, G., & Zhu, D. B. (2010). High quality graphene with large flakes exfoliated by oleyl amine. Chemical Communications, 46, 5728–5730. 247. Meyer, J. C., Geim, A. K., Katsnelson, M. I., Novoselov, K. S., Obergfell, D., Roth, S., Girit, C., & Zettl, A. (2007). On the roughness of single- and bi-layer graphene membranes. Solid State Communications, 143, 101–109. 248. Gomez-Navarro, C., Meyer, J. C., Sundaram, R. S., Chuvilin, A., Kurasch, S., Burghard, M., Kern, K., & Kaiser, U. (2010). Atomic structure of reduced graphene oxide. Nano Letters, 10, 1144–1148. 249. Meyer, J. C., Kisielowski, C., Erni, R., Rossell, M. D., Crommie, M. F., & Zettl, A. (2008). Direct imaging of lattice atoms and topological defects in graphene membranes. Nano Letters, 8, 3582–3586. 250. Hansma, P. K., & Tersoff, J. (1987). Scanning tunneling microscopy. Journal of Applied Physics, 61, R1–R23. 251. The Nobel Prize in Physics. (1986). http://www.nobelprize.org/nobel_prizes/physics/ laureates/1986/press.html. Accessed 01 Dec 2014. 252. Hansma, P. K., Elings, V. B., Marti, O., & Bracker, C. E. (1988). Scanning tunneling microscopy and atomic force microscopy—application to biology and technology. Science, 242, 209–216. 253. Tapaszto, L., Dobrik, G., Lambin, P., & Biro, L. P. (2008). Tailoring the atomic structure of graphene nanoribbons by scanning tunnelling microscope lithography. Nature Nanotech- nology, 3, 397–401. 254. Schouteden, K., Volodin, A., & Moorkens, T., Van Haesendonck, C. (2011). Peeling off graphene from Co nanoparticle covered graphite in a scanning tunneling microscope. Carbon, 49, 2258–2263. 255. Harrison, S. E., Capano, M. A., & Reifenberger, R. (2010). Scanning tunneling microscope study of striated carbon ridges in few-layer epitaxial graphene formed on 4 H-silicon carbide (0001). Applied Physics Letters, 96, 081905. 256. Breitwieser, R., Hu, Y. C., Chao, Y. C., Li, R. J., Tzeng, Y. R., Li, L. J., Liou, S. C., Lin, K. C., Chen, C. W., & Pai, W. W. (2014). Flipping nanoscale ripples of free-standing graphene using a scanning tunneling microscope tip. Carbon, 77, 236–243. 257. Ferrari, A. C., Meyer, J. C., Scardaci, V., Casiraghi, C., Lazzeri, M., Mauri, F., Piscanec, S., Jiang, D., Novoselov, K. S., Roth, S., & Geim, A. K. (2006). Raman spectrum of graphene and graphene layers. Physical Review Letters, 97, 187401. 258. Voggu, R., Das, B., Rout, C. S., & Rao, C. N. R. (2008). Effects of charge transfer interaction of graphene with electron donor and acceptor molecules examined using Raman spectroscopy and cognate techniques. Journal of Physics-Condensed Matter, 20, 472204. 94 2 Synthesis, Structure, and Properties of Graphene and Graphene Oxide

259. Ferrari, A. C., & Robertson, J. (2000). Interpretation of Raman spectra of disordered and amorphous carbon. Physical Review B, 61, 14095–14107. 260. Barpanda, P., Fanchini, G., & Amatucci, G. G. (2011). Structure, surface morphology and electrochemical properties of brominated activated carbons. Carbon, 49, 2538–2548. 261. Pimenta, M. A., Dresselhaus, G., Dresselhaus, M. S., Cancado, L. G., Jorio, A., & Saito, R. (2007). Studying disorder in graphite-based systems by Raman spectroscopy. Physical Chemistry Chemical Physics, 9, 1276–1291. Chapter 3 Fabrication of Graphene-Based Transparent Conducting Thin Films

3.1 Introduction

Chemical vapor deposition (CVD)-grown graphene and graphene oxide (GO) have been the main starting materials to produce graphene-based transparent conductors (TCs) [1, 2]. For the CVD-grown graphene, the underlying substrates need to be removed so that the graphene sheets can be transferred onto the device substrates. Several strategies have been developed to transfer graphene sheets and they include the etching and stamping method [3, 4], thermal release method [5], photoresist method [6], roll-to-roll transfer method [7], and general method [8]. Another low- cost route to produce graphene-based TCs on a large scale is to synthesize GO thin films and then reduce them. The ease of solution process of GO sheets due to their high solubility in aqueous solutions has made it a more viable and favorable approach [2]. Once a GO dispersion is produced, GO films can be formed on a substrate using different deposition techniques, including electrophoretic deposition (EPD), spin coating, spray coating, dip coating, transfer printing, Langmuir– Blodgett (L–B) assembly, rod coating, and inkjet coating [1].

3.2 CVD-Grown Graphene-Based TCs

3.2.1 Etching Method

After graphene is grown on a metal substrate as described in Sect. 2.2, graphene needs to be transferred to the device substrate via a post-transfer process, which usually involves two main steps: (i) etching of the metal substrate and (ii) transfer of graphene onto the target substrate. The removal of growth substrate, such as Cu and Ni, is usually performed by immersing the substrate with the graphene film into the etching bath until a free-standing graphene membrane floating on the solution can be readily observed [9–11]. The typical etching recipes include FeCl3 [3], HCl [12], HNO3 [13], Fe(NO3)3 [10], and (NH4)2SO8 [7]. It should be noted that reactions during etching of © Springer Science+Business Media New York 2015 95 Q. Zheng, J.-K. Kim, Graphene for Transparent Conductors, DOI 10.1007/978-1-4939-2769-2_3 96 3 Fabrication of Graphene-Based Transparent Conducting Thin Films

Fig. 3.1 Etching method to transfer CVD-grown graphene. a Flow chart of the synthesis, etching, and transfer processes for the large-scale and patterned graphene films. b A centimeter-scale graphene film grown on a Ni(300 nm)/SiO2(300 nm)/Si substrate. c A floating graphene film after etching the nickel layers in 1 M FeCl3 aqueous solution [3] RT Room temperature, HF Hydroflu- oric acid, BOE Buffered oxide etchant. (Reprinted with permission from [3]. Copyright (2009) by Nature Publishing Group)

Cu with nitric acid lead to the formation of H2 bubbles which cause cracking in the graphene film. It is also found that HCl or HNO3 releases corrosive vapor and the etching rate of copper is very slow. Since FeCl3 is able to etch the copper or nickel without forming gaseous products or precipitates, it has been widely used recently [3].

Figure 3.1a shows the typical etching transfer process by using FeCl3 (or acids). After graphene was grown on nickel substrate, an aqueous FeCl3 solution was used as an oxidizing etchant to remove the nickel layers. It is found that FeCl3 etches the nickel layers effectively within a mild pH range without forming gaseous products or precipitates. The graphene film can be separated from the substrate within a few minutes. 3.2 CVD-Grown Graphene-Based TCs 97

Figure 3.1b and c show the graphene film grown on nickel substrate before and after

FeCl3 etching, respectively. It is seen that the graphene film floating on the surface of the solution is then ready to be transferred to any kind of substrates [3].

3.2.2 Stamping Method

The stamping transfer is typically done by depositing a protective polymeric coating, such as polydimethylsiloxane (PDMS) [3], ploy(methylmethacrylate) (PMMA) [14], or silicone [12], on top of the graphene thin film and removing the underlying substrate. Figure 3.2a shows a method of transferring graphene in wafer scale via

Fig. 3.2 Etching and stamping method to transfer CVD-grown graphene. a Transferring and patterning of graphene films grown on a metal/SiO2/Si wafer [9]. b Graphene transfer by polyethylene terephthalate (PET)/silicone [12]. (Reprinted with permission from [9, 12]. Copyright (2010, 2013) by ACS and Elsevier) 98 3 Fabrication of Graphene-Based Transparent Conducting Thin Films stamping method. The polymer supports such as soft PDMS stamps and thermal-release tapes are first attached to the graphene films grown on metal substrates. After soaking in water with gentle ultrasonication for several minutes, the polymer support/graphene/metal layers are detached from SiO2 by water intervening between metal and SiO2. The metal layers were then removed by soaking in FeCl3 solution. The resulting graphene film on the polymer support is ready to be transferred onto arbitrary substrates, such as poly(ethylene terephthalate) (PET) film or rubber substrate [9]. A two-layer structure consisting of PET and silicone was also developed to transfer graphene grown by chemical vapor deposition onto various rigid and flexible substrates through dispersive adhesion [12]. As shown in Fig. 3.2b, three essential steps, which include adhesion of the PET/silicone to the graphene/Cu, etching of the Cu layer, and transferring of released graphene film, are involved in the PET/silicone transfer process. The advantage of the two-layer structure is that it only takes a few seconds to transfer graphene from PET/silicone to the target substrates [12]. Kang et al. [4] modified the transfer technique by inking the stamp process, enabling the transfer of single-layer graphene onto a Si/SiO2 substrate in a pattern defined by the geometry of the PDMS stamp. As shown in Fig. 3.3a, large-scale graphene was grown on Cu foil which was then laminated onto a microscale-patterned

PDMS stamp. The Cu foil was removed by etching with (NH4)2 S2O8 (ammonium persulfate) solution, and the PDMS stamp was thoroughly rinsed with distilled (DI) water. The raised features in the stamp were “inked” with graphene that has the same pattern as the PDMS stamp, which was then transferred to the target substrate by pressing and gently removing the PDMS mold. Figure 3.3b shows the microscale features of graphene on a SiO2 substrate, presenting graphene patterns over a large area of 20 × 1000 µm, confirming the technique’s capability to transfer large-area patterns. The transfer mechanism is that the adhesion between the graphene and the target substrate is higher than that of the PDMS/graphene interface [4, 15]. Gra- phene films with a variety of shapes were transferred onto different substrates using the same technique, such as poly(4-vinylphenol) (PVP; Fig. 3.3c, d). To realize the transferring of graphene onto arbitrary substrates, a general method was developed to perform reliable transfer [8]. This general method relies on a sacrificial “self-releasing” polymer layer placed between a conventional PDMS elastomer stamp and the graphene film. As shown in Fig. 3.4a, the general method employs a polymer thin film—“self-release layer” (SRL)—which is inserted between the elastomer stamp and the graphene sheet. The work of adhesion between the SRL and the stamp has to be smaller than that between the graphene and the destination substrate, making dry transfer of the release polymer/graphene bilayer thermodynamically favorable. The SRL protects graphene from the contamination by the low-molecular weight siloxane oligomers present in the PDMS stamp. After patterning by oxygen plasma, a 0.2–1.0-mm thick SRL is spun-cast over the graphene and brought into conformal contact with the PDMS stamp, then the excess graphene is removed by oxygen plasma. The Cu substrate is subsequently etched 3.2 CVD-Grown Graphene-Based TCs 99

Fig. 3.3 Inking elastomeric stamp method. a Schematics of the fabrication procedure for transferred micro-patterned graphene electrode-based organic field effect transistor using rubrene single-crystal semi-conductor. b An optical microscope image of patterned graphene electrodes on a SiO2 layer fabricated by stamping method and a magnified view of patterned graphene in inset. c An optical microscope image of graphene patterned electrode fabricated on PET/graphene/PVP layer and the transparent PET/graphene/PVP/patterned graphene structure in inset. d A scanning electron microscope (SEM) image of the hexagonal arrays of graphene electrodes micro-patterned onto a SiO2 substrate. (Reprinted with permission from [4]. Copyright (2011) by Wiley) off to transfer the polymer/graphene bilayer to the stamp. Through the graphene face, the stamp is brought into contact with the destination substrate. After baking for several minutes to enhance the conformal contact and adhesion, the stamp is lifted off, leaving the release polymer/graphene bilayer on the destination substrate. The SRL is then dissolved to obtain the graphene film transferred on the substrate.

Figure 3.4b–e shows the images of transferred graphene films onto SiO2/Si and organic polymer substrates. The graphene films transferred using the SRL is largely intact at the optical length (Fig. 3.4b), while the film transferred without the SRL is highly fragmented (Fig. 3.4c). The atomic force microscopy (AFM) image in Fig. 3.4d shows that the integrity of the graphene monolayer is preserved and no cracks or contaminant residues are found. More importantly, the patterned graphene films can be placed to align to features, allowing the integration of graphene into more complex device structures. For example, millimeter-sized graphene pads were transferred onto a polymer film while aligned with the underlying gold source– drain electrode array (Fig. 3.4e). 100 3 Fabrication of Graphene-Based Transparent Conducting Thin Films

Fig. 3.4 A general stamping method for transferring graphene. a Schematic of the SRL methodology in combination with a pick-and-place elastomer stamp. b, c Optical images of a centimeter-sized graphene film successfully transferred to a 300-nm thick SiO2/Si substrate b and failed transfer when the SRL is not present c. d AFM images of graphene film on SiO2/Si at low and high ( inset) magnifications (inset z-scale is 3 nm). e Digital camera image of pre-patterned graphene films transferred to an organic semiconductor device structure. CVD chemical vapor deposition. (Reprinted with permission from [8]. Copyright (2013) by Nature Publishing Group) 3.2 CVD-Grown Graphene-Based TCs 101

3.2.3 Thermal Release Method

Since there is strong adhesion between gold and carbon materials [16, 17], a thermal release method was developed recently using gold and thermal releasing tapes [5]. Figure 3.5a–c shows the schematic flowchart of the transfer-printed graphene patterns through gold-stamp-assisted exfoliation, including three steps. Firstly, micrometer-scale line features with different sizes were patterned on the surface of a highly ordered pyrolytic graphite (HOPG) disk using a photolithographic technique followed by an oxygen plasma-etching process. Secondly, a gold film was deposited onto the patterned HOPG disk surface, where a thermal releasing tape was used to peel off the gold film together with the graphene patterns. Finally, the gold film with graphene patterns was pressed onto the target substrate. The thermal tape was released and removed from the gold film after heating, and the gold film was etched away using an etchant solution [5]. Figure 3.5d,e shows the atomic force microscopy (AFM) images of graphene patterns on a silicon substrate, displaying different graphene thicknesses over the printed line patterns, ranging from 0.8 to 2.5 nm (Fig. 3.5f). The histogram of the average thickness shown in Fig. 3.5g suggests that most of the graphene patterns had thicknesses about 1–3 nm. Although the thermal release method is complex and costly, its major advantage is that it can create graphene patterns for electronic applications [5].

3.2.4 Photoresist Method

Figure 3.6a; presents the procedure employed to fabricate graphene device arrays based on the photoresist method. Graphene is grown on a Cu- or Ni-coated Si wafer substrate, which is used to fabricate a large-scale device array with a simple photolithography process [6]. The device consists of two large pads connected by a thin strip. The photoresist covering the devices acts as a protective layer and oxygen plasma etch is used to remove unwanted photoresist residue and graphene. After the exposure to a continually refreshed etchant solution to remove Cu/Ni in the unprotected areas and beneath the connecting photoresist/graphene strips, two large pads of graphene/Cu laminate connected by a narrow channel of graphene can be obtained. Figure 3.6b, c shows the optical image of the fabricated device, indicating that there exhibits a clear undercut around the edges of Cu/Ni as well as the intact graphene channel. Figure 3.6d provides the differential interface contrast image of a long graphene channel, confirming that the surface of the device channel is very clean without any visible residue underneath. The inset shows that the graphene channel has a uniform Raman signature over a large area, further confirming the high quality of the device channel. 102 3 Fabrication of Graphene-Based Transparent Conducting Thin Films

Fig. 3.5 Schematic diagram and photographs of transfer- printed graphene patterns by gold-film-assisted exfoliation. The process includes (i) fabricating graphene patterns on highly ordered pyrolytic graphite (HOPG) by photolithography and O2 plasma etching, (ii) depositing a gold film on the patterns, (iii) peeling off the gold film with graphene patterns, (iv) transfer-printing graphene patterns onto a substrate, and (v) etching off the gold film. a, b Images of graphene patterns on HOPG surface and gold film, respectively. Dashed lines and arrows are used to highlight the line patterns. c An SEM image of graphene patterns transfer-printed onto a silicon substrate with 300-nm thick thermally grown SiO2. d–f Low- d and high-magnification e AFM images of transfer-printed graphene with 10-µm-thick line patterns exfoliated and printed on a silicon substrate. The solid line presents a scanning trace across the transfer-printed graphene, which is plotted in f. g Average graphene thickness distribution collected from 200 transfer-printed graphene patterns, showing very thin transferred graphene films, including some monolayers. (Reprinted with permission from [5]. Copyright (2009) by ACS) 3.2 CVD-Grown Graphene-Based TCs 103

Fig. 3.6 Fabrication of graphene device array. a Schematic of device fabrication procedure (see main text for details). b Bright field optical image of a typical sample substrate after fabrication. c Close-up bright field image of the same sample. Graphene connecting the copper pad is just visible ( boxed). Inset: Image of the device channel. d Differential interference contrast image of a longer device. Upper inset: Raman spectra across the length of the graphene strip are highly uniform. Lower inset: Bright field image of the sample. PR Photoresist, RIE Reactive ion etching. (Reprinted with permission from [6]. Copyright (2009) by ACS)

3.2.5 Roll-to-Roll Transfer Method

The flexibility of both the grown graphene and the underlying substrate allows roll- to-roll transfer of larger-scale graphene thin films where thermal release tapes are used as the supporting film [7, 18]. Figure 3.7a presents three essential steps consti- tuting the roll-to-roll transfer process, including the adhesion of polymer supports to the graphene film/Cu substrate laminate, etching of the Cu substrate, and release of the graphene layers [7, 19]. Figure 3.7b presents the corresponding photographs of the process. The graphene film is attached to a thermal release tape by applying a soft pressure between two rollers followed by etching the copper foil with an etchant. After rinsing with DI water to remove residual etchant, the graphene film is ready to be transferred to any kind of flat or curved substrate surface. By inserting the graphene film/thermal release tape laminate between the rollers together with a target substrate, successful transfer from the tape to the target substrate is achieved with exposure to mild heat. Multilayer graphene films can be prepared by repeat- ing these steps on the same substrate to achieve enhanced optoelectrical properties. Figure 3.7c shows the 30-in. multilayer graphene film transferred to a roll of PET substrate. Graphene-based touch screens can be assembled after the assembly with electrodes and spacers (Fig. 3.7d, e). Figure 3.7f shows the optical transmittance of the graphene films with different numbers of stacked layers, implying that the trans- 104 3 Fabrication of Graphene-Based Transparent Conducting Thin Films

Fig. 3.7 Roll-to-roll transfer of graphene films. a Schematic of the roll-based production of graphene films grown on a Cu foil. b Roll-to-roll transfer of graphene films from a thermal release tape to a PET film at 120 °C. c A transparent large-area graphene film on a 35 in. PET sheet. d An assembled graphene/PET touch panel showing outstanding flexibility. e A graphene-based touch screen panel connected to a computer with control software. f UV–vis spectra of graphene films on quartz substrates. The inset shows the UV spectra of graphene films with and without HNO3 doping. The corresponding optical images are shown in the right inset. g Sheet resistances of graphene films transferred to a ploy (methylmethacrylate) (PMMA) substrate [7] 3.3 Fabrication of GO-Based TCs 105 mittance is reduced by ~ 2.2–2.3 % for an additional graphene layer. The electrical conductivity of the graphene films was proportional to the number of graphene layers (Fig. 3.7g). After nitric acid (HNO3) doping, the sheet resistance of graphene film was reduced to ~ 125 Ω/sq with 97.4 % transparency.

3.2.6 Challenges of Transferred CVD-Grown Graphene for TCs

Compared with Ni and Pd, Cu is the most promising CVD catalytic substrate to grow continuous layers of high quality graphene over large areas. This is due to the very low carbon solubility in Cu and its catalytic activity toward hydrocarbon gas- ses. CVD-grown graphene has less wrinkles and grain boundaries than chemically derived graphene and therefore excellent optoelectrical properties can be achieved. In spite of the significant progress made so far, there are still some important challenges. First, it is very important to grow large and controllable grain size graphene, such as single-grain graphene with wafer scale size [20]. This will definitely require a better understanding and optimization of the growth process. Second, CVD-grown graphene with controllable number of layers is critically important for technological implementation, especially for large area electronics [18]. For example, bilayer and trilayer graphene may offer functions and properties different from monolayer graphene. Due to the smaller interlayer distance and possible AB stacking, bilayer and trilayer graphene will likely have superior electrical properties. Third, reducing the manufacturing cost is a very important component for future industrial applications. The cost may be reduced by low-temperature graphene growth or direct growth on arbitrary substrates without any post-transfer process. Last but not the least, an easy, reliable, and scalable transfer method is an urgent challenging issue. The aforementioned transfer methods all seem complex, expensive, and time consuming. CVD- grown graphene then has the potential to replace ITO as the ubiquitous TCs if the above challenges are overcome.

3.3 Fabrication of GO-Based TCs

Depositing multilayer GO sheets on a rigid or flexible substrate in a systematic manner has become a preferred choice to fabricate TCs because of ease of process and low cost compared to transferring CVD-grown graphene. Several techniques, such as EPD, spin and spray coating, dip coating, transfer printing, L–B assembly, rod coating, and inkjet printing, have now become well established to deposit GO films on a substrate. Table 3.1 summarizes the operating principles of these techniques and their relative advantages and disadvantages. It can be seen that controlling the film uniformity, surface morphology, thickness, and surface coverage depends on the deposition methods and parameters used [21]. Upon deposition, the van der Waals forces and strong hydrogen bonds between the GO sheets allow them to strongly adhere to the substrate [22]. 106 3 Fabrication of Graphene-Based Transparent Conducting Thin Films ] ], Becerril 26 24 23 ] ] al. [

al. [

25 al. [

al. [

Pham et An et Kim et et References Partial aggregation of GO sheets during drying by heat Randomly stacked GO sheets, only applicable to conductive substrates Uniformity cannot be guaranteed Disadvantages ]. Copyright (2014) by Elsevier) 1 Suitable for mass production Potentially suitable for mass production Suitable for mass production Advantages Schematic figure Fabrication methods of TCFs from GO. (Reprinted with permission [ Fabrication methods of

Spin coating Method Spray coating Electrophoretic deposition Table 3.1 Table 3.3 Fabrication of GO-Based TCs 107 ], ] 10 31 , 33 ], al. [ ], Zheng

al. [ ], Kim e

32 ], Dong 29 34 27 al. [ al. [

] ],Li et

al. [

al. [ 28 30

al. [ al. [

Yamaguchi et Yamaguchi Lee et et Wang Zheng et et Cote et et al. [ 35 ] References sheets of relatively Transfer thick GO films with low transparency Relatively slow deposition speed Disadvantages Simple and cheap process Randomly stacked GO and High efficiency quality Deposition of GO sheets LbL, accurate control of film thickness Advantages Schematic figure (continued)

L–B assembly Method Transfer printing Transfer Dip coating Table 3.1 Table 108 3 Fabrication of Graphene-Based Transparent Conducting Thin Films ] 37 ] ] al. [

38 36 al. [

al. [

Torrisi et Torrisi Wang et Wang Bonaccorso et References Randomly stacked GO sheets for large Low efficiency scale fabrication Disadvantages Suitable for large scale Suitable for large fabrication High precision Advantages LbL layer-by-layer Schematic figure (continued)

Method Inkjet printing Rod coating GO graphene oxide, L–B Langmuir–Blodgett, Table 3.1 Table 3.3 Fabrication of GO-Based TCs 109

3.3.1 Electrophoretic Deposition

The EPD technique includes a broad range of industrial processes where charged particles in suspension move toward an electrode of opposite charge by an electric field and deposit to form a compact film [39]. EPD is a versatile technique that can be applied to solid powder particles with diameters ranging from micro- to nanome- ters, and requires a suitable medium for the formation of a stable suspension. It has many advantages, including facile process, short processing time, cost effectiveness, and potential scaling up [40]. Due to the relatively high density, good chemical stability, and low conductivity, organic solvents such as alcohols and ketones are commonly used. However, organic solvents are usually toxic and flammable, thus an aqueous medium that is cheaper and environmental friendly is often used for the deposition of sensitive materials [41]. The EDP technique was applied to fabricate a variety of GO-based thin films [23, 42, 43]. When a direct current (DC) voltage is applied, GO sheets dispersed in a suspension migrate toward the positive electrode and form a solid thin film. The deposition rate is influenced by several processing parameters, such as the concentration of GO suspension, the applied DC voltage, and the conductivity of the substrate. By modifying the deposition time, the film thickness can be controlled. As shown in Fig. 3.8a, b, the color of the GO films changed from light to dark brown with increasing thickness, especially after the reduction of GO using KOH

Fig. 3.8 EPD of GO films. a, b Digital photographs of GO a and rGO b films deposited on ITO-coated glass electrodes. c, d SEM images of GO c and rGO d films (the insets show the corresponding optical images). (Reprinted with permission from. Copyright (2009) by ACS) 110 3 Fabrication of Graphene-Based Transparent Conducting Thin Films and hydrazine. It is also found that wrinkling occurred along the boundaries between the individual GO sheets after reduction (Fig. 3.8c, d). The fabrication of TCs using large-area GO sheets via EPD was specifically investigated [23, 44]. Reduced graphene oxide (rGO) was obtained using the intrinsic EPD process without the addition of chemical agents or high-temperature treatments [18]. It is shown that the prepared GO films can be electrochemically reduced by an explicit constant potential reduction step. During the EPD process, the oxygen-containing functional groups of GO were substantially removed. The final films showed improved electrical conductivities and low adhesion to the substrate for easy detachment. The conductivity of the rGO film was 1.43 × 104 S m−1, which is much higher than the GO paper obtained by the filtration method, which was 0.53 × 10−3 S m−1 [45]. Ishikawa et al. [44] deposited GO sheets obtained from exfoliation of graphite oxide on a SiO2/Si substrate by an EPD process. The optimal conditions with an EPD voltage of 10 V for 5 min resulted in a few layers of reduced graphene with no wrinkles and reduction was confirmed by the XPS analysis. Its sheet resistance was 4.59 × 104 Ω/sq at a transmittance of 83.8 %. Although the technique enjoys a number of advantages in the preparation of thin films, such as high deposition rate, good thickness controllability, and simplicity of scaling up, its applicability is limited to only electrically conductive substrates, such as ITO-coated glass, Cu, Ni, Al, stainless steel, and p-type Si. For the same reason, it cannot be applied to flexible insulating plastic substrates. In addition, the stacking quality of GO sheets is rather difficult to control, making this method unable to produce uniform deposition over a large surface area [1].

3.3.2 Spin Coating

One of the most convenient and popular methods developed to deposit uniform thin films on a flat substrate is spin coating. It involves the dropping of an excess amount of solution on a substrate and rotating at a high speed to spread the fluid over the entire substrate surface by a centrifugal force. The desired thickness of the film can be controlled by rotating speed and the concentration of the GO solution. GO films were deposited on quartz substrate and the deposition behaviors were studied after different reduction processes [20]. Due to the fast dynamics of deposition by spin coating and the flexible nature of GO sheets, a significant degree of disorder was noted from the measurement of film thickness. It is found that rGO films reduced by hot hydrazine solution resulted in fragmentation and delamina- tion of the films. Thermal annealing of GO films under nitrogen flow, argon flow, and vacuum at temperatures ranging from 400–1100 °C were also investigated. The results indicate that a thermal graphitization procedure was most effective for reduction, producing films with sheet resistances as low as 100–1000 Ω/sq with 80 % transmittance. Wu et al. [46] produced GO film via spin coating and used them as transparent conductive anodes for an organic photovoltaic (OPV) cell. After reduction by a combination of hydrazine and high temperature annealing (400–1100 °C), 3.3 Fabrication of GO-Based TCs 111

Fig. 3.9 Spin coating of graphene oxide (GO) film. a Schematic of modified spin-coating technique. b AFM height image of a GO film. (Reprinted with permission from [47]. Copyright (2008) by ACS) the optical transmittance of the best films was higher than 80 %, while the sheet resistance varied from 5 kΩ/sq to 1 MΩ/sq. A modified spin coating technique [43] was also devised as shown in Fig. 3.9a. It is found that accelerated solution evaporation by blowing dry nitrogen while spin-casting resulted in continuous films with GO sheets lying fairly flat on the surface (Fig. 3.9b).

3.3.3 Spray Coating

Spray coating is a type of painting technique where a spraying device based on forced air is used to apply the coating material through air onto the substrate surface. The method is fast, scalable, and facile to operate, and ideally suited to producing electrodes for liquid-crystal displays (LCDs) [26, 48], as well as conductive graphene coatings on various substrates [49]. Once the graphene colloids are dried after the deposition, they are not re-dispersible in water, resulting in a water resistant coating. Due to the high aspect ratio of the graphene sheet, spray coating technique can produce a continuous conducting network, which is thin and transparent. An rGO coating was produced on a glass slide, achieving a sheet resistance of 2.0 × 107 Ω/ sq with a transmittance higher than 96 % [45]. GO–hydrazine dispersions were also spray deposited to produce large rGO films [21]. The GO–hydrazine dispersion was prepared by mixing GO dispersion with an excess amount of hydrazine monohy- drate, as shown in Fig. 3.10. Therefore, rGO dispersion was sprayed onto preheated substrates to form rGO films with a sheet resistance of 2.2 × 103 Ω/sq and a transmittance of 84 %. Although the technique holds advantages of simple and fast process and low cost, it is very difficult to avoid partial aggregation and crumpling or wrinkling of GO/rGO sheets generated during the coating process [1]. 112 3 Fabrication of Graphene-Based Transparent Conducting Thin Films

Fig. 3.10 Schematic of rGO film fabrication by spray deposition. (Reprinted with permission from [26]. Copyright (2010) by Elsevier)

3.3.4 Dip Coating

Dip coating involves three steps, that is, immersion of a substrate into a tank containing a coating material, removal of the substrate from the tank, and draining of the coating material. 3.3 Fabrication of GO-Based TCs 113

Fig. 3.11 Schematic illustration of layer-by-layer (LbL) assembly of an rGO film consisting − + of alternate layers of negatively charged rGO–COO and positively charged rGO–NH3 . EDC N-ethyl-N′-(3-dimethyl aminopropyl)carbodiimide methiodide. (Reprinted with permission from [27]. Copyright (2011) by RSC)

The coated substrate is then dried using forced air or by baking. By changing the durations of these three steps, the thickness of the film can be easily controlled. Graphene films of 10 nm thickness were obtained with a high transparency and conductivity after coating with GO, followed by thermal reduction [50]. Pretreated hydrophilic quartz substrates were immersed into a hot, aqueous GO dispersion to coat GO films. Scrolled or folded GO sheets were observed arising from the film fabrication, which is detrimental to transparency. Su et al. [51] reported the fabrication of rGO films by dip-coating the cleaned quartz substrates with large-size GO sheets in a standard Piranha process, which were annealed at an elevated temperature in 20 % H2/Ar. The obtained rGO films showed a sheet resistance of 188 kΩ/sq with 98 % transparency. A novel layer-by-layer (LbL) sequential assembly of positively and negatively charged rGO sheets was developed based on the dip-coating method to avoid folding of graphene sheets during deposition [27]. As illustrated in Fig. 3.11, this approach employed the concept of electrostatic attraction between the two oppositely charged suspensions of rGO sheets, which allowed the preparation of multilayer rGO films with a tunable thickness, resistance, and transmittance by controlling the number of bilayers. 114 3 Fabrication of Graphene-Based Transparent Conducting Thin Films

3.3.5 Transfer Printing of GO Films

A transfer-printing process is aimed to transferring part of or an entire thin film from one substrate to the precise positions on another substrate [52]. Eda et al. [22] first reported the fabrication of large-area ultrathin films of rGO as a TCF material. A GO aqueous dispersion was obtained from exfoliation of graphite after oxidation. The GO dispersion was filtrated under vacuum using a cellulose/ester membrane, where the liquid passed through the pores while the GO sheets became lodged. It is noted that the permeation rate of the solvent was controlled by the accumulation of GO sheets on the pores, thus the filtration process was self-regulating and allowed nanoscale control of the film thickness by simply varying either the concentration of the GO dispersion in the suspension or the filtration volume [22]. The GO films on the filtrated membrane were transferred onto the target substrates with a yield of nearly 100 % by dissolving the membrane with acetone after the film was placed to face the substrate. Reduction by a combination of hydrazine vapor exposure and low-temperature annealing treatment resulted in a sheet resistance of ~ 1 × 105 Ω/sq with a transmittance of ~ 60 % for the rGO films. Figure 3.12a shows the schematic fabrication process of rGO films on a pretreated quartz substrate through vacuum filtration and transfer printing. After dissolving the membrane with acetone, TCFs are obtained by annealing and further graphitization. The transparency was well over 80 % and the electrical conductivity was over 200 S/cm (or Rs 1–2 kΩ/sq) at a typical wavelength of 550 nm depending on the film thickness [49]. Because the van der Waals interactions gave rise to strong cohesive forces between the GO sheets as well as between the GO film and the substrate, the yield of the transfer process was nearly 100 % independent of the substrate material. A soft stamping process was also developed using PDMS stamps (Fig. 3.12b) to transfer large-scale rGO films [52]. Based on the distinct strengths of non-covalent adhesion at the PDMS–graphene and graphene–substrate interfaces, the process involved in coating of an rGO film onto a glass substrate and selective areas of the rGO film were transferred to the PDMS stamp. To fully transfer rGO films to the substrates at room temperature, the rGO-registered stamps were brought into contact with the Si/SiO2 substrates. Although the soft transfer-printing process is known to be less durable and has a lower feature resolution than the hard ones, it possesses the advantages of mechanical flexibility and well-controlled transfer of selective areas [52].

3.3.6 Langmuir–Blodgett Method

The L–B technique is a method to systematically control the interfacial molecular orientation and packing, making it possible to perform controllable fabrication of laterally patterned structures on solid supports. This method is based on LbL deposition of molecular-ordered domains formed at the air–solvent interface onto a solid substrate without intermediate transfer processes. An L–B trough (Fig. 3.13a) is usually used to perform the LbL assembly. The amphiphilic GO sheets floating 3.3 Fabrication of GO-Based TCs 115

Fig. 3.12 Transfer-printing process of reduced graphene oxide (rGO ) films. a Transfer-printing process of rGO and the finished rGO films with different thicknesses [53]. b Schematic diagrams ( left) and the corresponding optical microscope images ( right) depicting the PDMS-based soft transfer-printing process of rGO [52]. (Reprinted with permission from [52]. Copyright (2009) by Wiley) on the surface of a given subphase (usually water) are compressed by moving the two opposing barriers toward each other to increase the surface pressure and thus the density of GO sheets. The surface pressure is a measure of the GO density on the water surface, which in turn controls the quality of L–B assemblies. Prior to deposition, a platinum plate is used to monitor the surface pressure during the compression and isothermal curves are recorded to identify the target pressure (Fig. 3.13b) [54]. The prepared substrate is immersed into the water and pulled out at a desired speed once the desired surface pressure is reached, realizing uniform deposition of a layer of GO sheets. The L–B assembly method offers several unique advantages over the other coating techniques: (i) the layered architecture of the films could be accurately controlled by simply varying the L–B assembly parameters, including surface pressure and pulling speed of the substrate, (ii) the LbL deposition process is amenable to assembling any combination of molecular carbon layers on a substrate, thus capable of tailoring the properties and structures of the films, (iii) the assembly process is operative at room temperature with high throughputs, which favors automation and mass production [1]. 116 3 Fabrication of Graphene-Based Transparent Conducting Thin Films

a Pressure Dipper measure plate b

Barriers

c d e

50 µm 50 µm 50 µm

fgh

Fig. 3.13 L–B assembly of GO films. a L–B trough equipped with two movable Teflon barriers, pressure measurement plate, and dipper for film transfer onto solid substrates [54]. b A typical isothermal curve recorded during compressing of GO sheets prior to film transfer [54]. c–h SEM images of GO sheets deposited on a polyester slide by drop casting c. d, e Spin coating d and L–B assembly e [24]. f Close-packed monolayer GO sheets as the first layer. g, h Double layers with a dilute top layer g, and double layers with a high-density top layer h prepared by L–B assembly [34]. (Reprinted with permission from [1]. Copyright (2014) by Elsevier)

Due to the amphophilic nature of GO sheets, a highly stable monolayer of GO sheets was uniformly spread on water without using any surfactant or stabilizing agent [34]. The overlapping during compression at a low surface pressure was naturally prevented because of the edge-to-edge repulsion between the individual GO layers. Therefore, the densities of GO were tuned from dilute, close-packed to over-packed monolayers, which were then transferred to both a rigid and flexible substrate [51]. Figure 3.13c–e compares the SEM images of GO sheets obtained by different methods, including drop casting, spin coating, and L–B assembly. The drop casting and spin coating make GO sheets heavily wrinkled and folded during the deposition process, whereas the film produced by the L–B assembly process is 3.3 Fabrication of GO-Based TCs 117 relatively free of wrinkles or folds. It is also claimed that the L–B method can produce a large-area flat monolayer GO with much higher surface coverage on the substrate with an accurately controlled thickness [55]. It is also possible to stack double layers or multilayers of GO by sequential, LbL coating. As shown in Fig. 3.13f, the first layer is collected at close-packed density, which was either aged in air overnight or baked in an oven to enhance its adhesion to the substrate. Due to the repulsion from both their neighbors and those in the underlayer, the second layer of GO tends to be wrinkled. Figure 3.13g, h show the double layers of GO with low- and high-packing density for the second layer, respectively. The heavy degree of folding and wrinkling of the second layer with high-packing density (Fig. 3.13h) suggests strong repulsion between the two layers [34].

3.3.7 Rod Coating

The rod coating method is a scalable technique for making solution-processed thin films, and has been widely used in the coating industry to prepare liquid thin films [36]. Coating is done using a Meyer rod which is a metal bar with a wire wrapped around its surface. The wire is used to draw a solution over a substrate surface. A wet film is left when the Meyer rod is rolled over the substrates, which is then dried to obtain a solid film. The thickness of the final dry film can be controlled by the diameter of the wire wrapped around the bar and the concentration of the fluid. The preparation of the coating fluid is critically important because the coating fluid should possess a low surface tension and carry sufficient coating material, that is, GO sheets. Rod coating can be performed in a scalable way for roll-to-roll production in industry [56]. Wang et al. [36] demonstrated to fabricate a large-scale GO film on a PET substrate in a controllable manner using the rod-coating technique (Fig. 3.14a). After room-temperature reduction, highly flexible rGO films were used to assemble 4.5- in. touch screens (Fig. 3.14b). The thickness of GO films was controlled from a single layer to tens of layers by adjusting the concentration of GO solution or the diameter of the rod wire. The film thickness is shown to increase with increasing GO concentration from 0.05 to 1 mg/mL, see Fig. 3.14c–f. The GO films reduced using hydrogen with the assistance of a small amount of a palladium (Pd) catalyst had a sheet resistance ranging 1.68–20.1 kΩ/sq with a transmittance ranging 64.6–92.6 %.

3.3.8 Inkjet Printing

Inkjet printing is a process involving propelling droplets of ink onto papers, plastics, or other substrates, and possesses many advantages, such as direct writing, additive patterning, low cost, and scalability for large area manufacturing [57]. Kong et al. [58] directly printed the rGO electrode onto flexible polymeric materials by inkjet 118 3 Fabrication of Graphene-Based Transparent Conducting Thin Films

Fig. 3.14 Rod coating of rGO films. a Rod-coating setup. b A flexible touch screen fabricated using an rGO/PET film as electrode. c–f AFM images and height profiles of the GO films fabricated by rod coating with 0.2-mm-thick rod wires with different concentrations of GO dispersions, 0.05 mg/mL c, 0.1 mg/mL d, 0.2 mg/mL e, 1 mg/mL f. (Reprinted with permission from [36]. Copyright (2012) by Wiley) printing. GO was reduced using an infrared heat lamp at a temperature of ~ 200 °C for 10 min. The sheet resistance of the printed rGO film was tailored by adjusting the space between the adjacent ink droplets and the number of printing layers. The sheet resistance was relatively high (0.3 MΩ/sq) with an optical transparency as high as 86 %. The rGO electrodes prepared thereby were stable even after mechanical bending, and they exhibited rapid decreases in electrical resistance with temperature increase, suggesting potential application as a writable, mechanically flexible, and transparent temperature sensor. Dua et al. [59] demonstrated the one- step conversion of GO into rGO using aqueous vitamin C, which were then deposited onto a PET substrate by inkjet printing (Fig. 3.15a, b). Figure 3.15c shows the surface morphology of the inkjet-printed rGO film. To fabricate large-area graphene devices, Torrisi et al. [38] prepared an ink by liquid-phase exfoliation of graphite in N-methylpyrrolidone (NMP). The printed thin films showed mobilities up to ~ 95 cm2 V−1 s−1, as well as a sheet resistance of ~ 30 kΩ/sq with an 80 % transmittance. References 119

Fig. 3.15 Inkjet-printed rGO films. a Digital images of the vials containing GO and rGO aqueous dispersions (TX-100 = Triton-X100 surfactant). b Inkjet-printed GO film lifted from the PET surface.c AFM image of rGO film obtained by reduction of the film in b with ascorbic acid. (Reprinted with permission from [59]. Copyright (2010) by Wiley)

References

1. Zheng, Q., Li, Z., Yang, J., & Kim, J.-K. (2014). Graphene oxide based transparent conductive films. Progress in Materials Science, 64, 200–247. 2. Wassei, J. K., & Kaner, R. B. (2010). Graphene, a promising transparent conductor. Materials Today, 13, 52–59. 3. Kim, K. S., Zhao, Y., Jang, H., Lee, S. Y., Kim, J. M., Kim, K. S., Ahn, J. H., Kim, P., Choi, J. Y., & Hong, B. H. (2009). Large-scale pattern growth of graphene films for stretchable transparent electrodes. Nature, 457, 706–710. 4. Kang, S. J., Kim, B., Kim, K. S., Zhao, Y., Chen, Z. Y., Lee, G. H., Hone, J., Kim, P., & Nu- ckolls, C. (2011). Inking elastomeric stamps with micro-patterned, single layer graphene to create high-performance OFETs. Advanced Materials, 23, 3531–3535. 5. Song, L., Ci, L. J., Gao, W., & Ajayan, P. M. (2009). Transfer printing of graphene using gold film. ACS Nano, 3, 1353–1356. 6. Levendorf, M. P., Ruiz-Vargas, C. S., Garg, S., & Park, J. (2009). Transfer-free batch fabrication of single layer graphene transistors. Nano Letters, 9, 4479–4483. 7. Bae, S., Kim, H., Lee, Y., Xu, X. F., Park, J. S., Zheng, Y., Balakrishnan, J., Lei, T., Kim, H. R., Song, Y. I., Kim, Y. J., Kim, K. S., Ozyilmaz, B., Ahn, J. H., Hong, B. H., & Iijima, S. (2010). Roll-to-roll production of 30-inch graphene films for transparent electrodes. Nature Nanotechnology, 5, 574–578. 8. Song, J., Kam, F. Y., Png, R. Q., Seah, W. L., Zhuo, J. M., Lim, G. K., Ho, P. K. H., & Chua, L. L. (2013). A general method for transferring graphene onto soft surfaces. Nature Nano- technology, 8, 356–362. 9. Lee, Y., Bae, S., Jang, H., Jang, S., Zhu, S. E., Sim, S. H., Song, Y. I., Hong, B. H., & Ahn, J. H. (2010). Wafer-scale synthesis and transfer of graphene films. Nano Letters, 10, 490–493. 10. Li, X. S., Zhu, Y. W., Cai, W. W., Borysiak, M., Han, B. Y., Chen, D., Piner, R. D., Colombo, L., & Ruoff, R. S. (2009). Transfer of large-area graphene films for high-performance transparent conductive electrodes. Nano Letters, 9, 4359–4363. 120 3 Fabrication of Graphene-Based Transparent Conducting Thin Films

11. Cai, W. W., Zhu, Y. W., Li, X. S., Piner, R. D., & Ruoff, R. S. (2009). Large area few-layer graphene/graphite films as transparent thin conducting electrodes. Applied Physics Letters, 95, 123115. 12. Chen, X. D., Liu, Z. B., Zheng, C. Y., Xing, F., Yan, X. Q., Chen, Y. S., & Tian, J. G. (2013). High-quality and efficient transfer of large-area graphene films onto different substrates. Carbon, 56, 271–278. 13. Lee, Y. H., & Lee, J. H. (2010). Scalable growth of free-standing graphene wafers with copper (Cu) catalyst on SiO2/Si substrate: Thermal conductivity of the wafers. Applied Physics Letters, 96, 083101. 14. Reina, A., Jia, X. T., Ho, J., Nezich, D., Son, H. B., Bulovic, V., Dresselhaus, M. S., & Kong, J. (2009). Layer area, few-layer graphene films on arbitrary substrates by chemical vapor deposition. Nano Letters, 9, 30–35. 15. Kim, H., Yoon, B., Sung, J., Choi, D. G., & Park, C. (2008). Micropatterning of thin P3HT films via plasma enhanced polymer transfer printing. Journal of Materials Chemistry, 18, 3489–3495. 16. Wang, C., Ryu, K. M., Badmaev, A., Patil, N., Lin, A., Mitra, S., Wong, H. S. P., & Zhou, C. (2008). Device study, chemical doping, and logic circuits based on transferred aligned single- walled carbon nanotubes. Applied Physics Letters, 93, 033101. 17. Ryu, K., Badmaev, A., Wang, C., Lin, A., Patil, N., Gomez, L., Kumar, A., Mitra, S., Wong, H. S. P., & Zhou, C. W. (2009). CMOS-analogous wafer-scale nanotube-on-insulator approach for submicrometer devices and integrated circuits using aligned nanotubes. Nano Let- ters, 9, 189–197. 18. Mattevi, C., Kim, H., & Chhowalla, M. (2011). A review of chemical vapour deposition of graphene on copper. Journal of Materials Chemistry, 21, 3324–3334. 19. Ahn, S. H., & Guo, L. J. (2008). High-speed roll-to-roll nanoimprint lithography on flexible plastic substrates. Advanced Materials, 20, 2044–2049. 20. Zhang, Y., Zhang, L. Y., & Zhou, C. W. (2013). Review of chemical vapor deposition of graphene and related applications. Accounts of Chemical Research, 46, 2329–2339. 21. Eda, G., & Chhowalla, M. (2010). Chemically derived graphene oxide: Towards large-area thin-film electronics and optoelectronics. Advanced Materials, 22, 2392–2415. 22. Eda, G., Fanchini, G., & Chhowalla, M. (2008). Large-area ultrathin films of reduced graphene oxide as a transparent and flexible electronic material. Nature Nanotechnology, 3, 270–274. 23. An, S. J., Zhu, Y. W., Lee, S. H., Stoller, M. D., Emilsson, T., Park, S., Velamakanni, A., An, J. H., & Ruoff, R. S. (2010). Thin film fabrication and simultaneous anodic reduction of deposited graphene oxide platelets by electrophoretic deposition. Journal of Physical Chemistry Letters, 1, 1259–1263. 24. Kim, J., Kim, F., & Huang, J. X. (2010). Seeing graphene-based sheets. Materials Today, 13, 28–38. 25. Becerril, H. A., Mao, J., Liu, Z., Stoltenberg, R. M., Bao, Z., & Chen, Y. (2008). Evaluation of solution-processed reduced graphene oxide films as transparent conductors. Acs Nano, 2, 463–470. 26. Pham, V. H., Cuong, T. V., Hur, S. H., Shin, E. W., Kim, J. S., Chung, J. S., & Kim, E. J. (2010). Fast and simple fabrication of a large transparent chemically-converted graphene film by spray-coating. Carbon, 48, 1945–1951. 27. Lee, D. W., Hong, T. K., Kang, D., Lee, J., Heo, M., Kim, J. Y., Kim, B. S., & Shin, H. S. (2011). Highly controllable transparent and conducting thin films using layer-by-layer assembly of oppositely charged reduced graphene oxides. Journal of Materials Chemistry, 21, 3438–3442. 28. Dong, X. C., Su, C. Y., Zhang, W. J., Zhao, J. W., Ling, Q. D., Huang, W., Chen, P., & Li, L. J. (2010). Ultra-large single-layer graphene obtained from solution chemical reduction and its electrical properties. Physical Chemistry Chemical Physics, 12, 2164–2169. 29. Wang, X., Zhi, L., & Muellen, K. (2008). Transparent, conductive graphene electrodes for dye-sensitized solar cells. Nano Letters, 8, 323–327. References 121

30. Zheng, Q. B., Gudarzi, M. M., Wang, S. J., Geng, Y., Li, Z. G., & Kim, J. K. (2011). Improved electrical and optical characteristics of transparent graphene thin films produced by acid and doping treatments. Carbon, 49, 2905–2916. 31. Yamaguchi, H., Eda, G., Mattevi, C., Kim, H., & Chhowalla, M. (2010). Highly uniform 300 mm wafer-scale deposition of single and multilayered chemically derived graphene thin films. Acs Nano, 4, 524–528. 32. Zheng, Q., Ip, W. H., Lin, X., Yousefi, N., Yeung, K. K., Li, Z., & Kim, J.-K. (2011). Trans- parent conductive films consisting of ultra large graphene sheets produced by Langmuir- Blodgett assembly. Acs Nano, 5, 6039–6051. 33. Zheng, Q., Zhang, B., Lin, X., Shen, X., Yousefi, N., Huang, Z.-D., Li, Z., & Kim, J.-K. (2012). Highly transparent and conducting ultralarge graphene oxide/single-walled carbon nanotube hybrid films produced by Langmuir-Blodgett assembly. Journal of Materials Chemistry, 22, 25072–25082. 34. Cote, L. J., Kim, F., & Huang, J. X. (2009). Langmuir-Blodgett assembly of graphite oxide single layers. Journal of the American Chemical Society, 131, 1043–1049. 35. Kim, F., Cote, L. J., & Huang, J. X. (2010). Graphene oxide: Durface activity and two- dimensional assembly. Advanced Materials, 22, 1954–1958. 36. Wang, J., Liang, M. H., Fang, Y., Qiu, T. F., Zhang, J., & Zhi, L. J. (2012). Rod-coating: Towards large-area fabrication of uniform reduced graphene oxide films for flexible touch screens. Advanced Materials, 24, 2874–2878. 37. Bonaccorso, F., Lombardo, A., Hasan, T., Sun, Z. P., Colombo, L., & Ferrari, A. C. (2012). Production and processing of graphene and 2d crystals. Materials Today, 15, 564–589. 38. Torrisi, F., Hasan, T., Wu, W. P., Sun, Z. P., Lombardo, A., Kulmala, T. S., Hsieh, G. W., Jung, S. J., Bonaccorso, F., Paul, P. J., Chu, D. P., & Ferrari, A. C. (2012). Inkjet-printed graphene electronics. Acs Nano, 6, 2992–3006. 39. Chavez-Valdez, A., Shaffer, M. S. P., & Boccaccini, A. R. (2013). Applications of graphene electrophoretic deposition. A review. Journal of Physical Chemistry B, 117, 1502–1515. 40. Besra, L., & Liu, M. (2007). A review on fundamentals and applications of electrophoretic deposition (EPD). Progress in Materials Science, 52, 1–61. 41. Boccaccini, A. R., Keim, S., Ma, R., Li, Y., & Zhitomirsky, I. (2010). Electrophoretic deposition of biomaterials. Journal of The Royal Society Interface, 7, S581–S613. 42. Lee, V., Whittaker, L., Jaye, C., Baroudi, K. M., Fischer, D. A., & Banerjee, S. (2009). Large- area chemically modified graphene films: electrophoretic deposition and characterization by soft X-ray absorption spectroscopy. Chemistry of Materials, 21, 3905–3916. 43. Chen, Y., Zhang, X., Yu, P., & Ma, Y. W. (2009). Stable dispersions of graphene and highly conducting graphene films: A new approach to creating colloids of graphene monolayers. Chemical Communications, 30, 4527–4529. 44. Ishikawa, R., Ko, P. J., Kurokawa, Y., Konagai, M., & Sandhu, A. (2012). Electrophoretic deposition of high quality transparent conductive graphene films on insulating glass substrates. Asia-Pacific Interdisciplinary Research Conference 2011 (Ap-Irc 2011), 352. 45. Park, S., An, J. H., Piner, R. D., Jung, I., Yang, D. X., Velamakanni, A., Nguyen, S. T., & Ruoff, R. S. (2008). Aqueous suspension and characterization of chemically modified graphene sheets. Chemistry of Materials, 20, 6592–6594. 46. Wu, J. B., Becerril, H. A., Bao, Z. N., Liu, Z. F., Chen, Y. S., & Peumans, P. (2008). Organic solar cells with solution-processed graphene transparent electrodes. Applied Physics Letters, 92, 263302. 47. Robinson, J. T., Zalalutdinov, M., Baldwin, J. W., Snow, E. S., Wei, Z. Q., Sheehan, P., & Houston, B. H. (2008). Wafer-scale reduced graphene oxide films for nanomechanical devices. Nano Letters, 8, 3441–3445. 48. Min, K., Han, T. H., Kim, J., Jung, J., Jung, C., Hong, S. M., & Koo, C. M. (2012). A facile route to fabricate stable reduced graphene oxide dispersions in various media and their transparent conductive thin films. Journal of Colloid and Interface Science, 383, 36–42. 49. Li, D., Muller, M. B., Gilje, S., Kaner, R. B., & Wallace, G. G. (2008). Processable aqueous dispersions of graphene nanosheets. Nature Nanotechnology, 3, 101–105. 122 3 Fabrication of Graphene-Based Transparent Conducting Thin Films

50. Wang, X., Zhi, L. J., & Mullen, K. (2008). Transparent, conductive graphene electrodes for dye-sensitized solar cells. Nano Letters, 8, 323–327. 51. Su, C. Y., Xu, Y. P., Zhang, W. J., Zhao, J. W., Tang, X. H., Tsai, C. H., & Li, L. J. (2009). Electrical and spectroscopic characterizations of ultra-large reduced graphene oxide monolayers. Chemistry of Materials, 21, 5674–5680. 52. Allen, M. J., Tung, V. C., Gomez, L., Xu, Z., Chen, L. M., Nelson, K. S., Zhou, C. W., Kaner, R. B., & Yang, Y. (2009). Soft transfer printing of chemically converted graphene. Advanced Materials, 21, 2098–2102. 53. Wang, S. J., Geng, Y., Zheng, Q. B., & Kim, J. K. (2010). Fabrication of highly conducting and transparent graphene films. Carbon, 48, 1815–1823. 54. Li, X. L., Zhang, G. Y., Bai, X. D., Sun, X. M., Wang, X. R., Wang, E., & Dai, H. J. (2008). Highly conducting graphene sheets and Langmuir-Blodgett films. Nature Nanotechnology, 3, 538–542. 55. Zheng, Q. B., Ip, W. H., Lin, X. Y., Yousefi, N., Yeung, K. K., Li, Z. G., & Kim, J. K. (2011). Transparent conductive films consisting of ultra large graphene sheets produced by Langmuir-Blodgett assembly. Acs Nano, 5, 6039–6051. 56. Hu, L. B., Kim, H. S., Lee, J. Y., Peumans, P., & Cui, Y. (2010). Scalable coating and properties of transparent, flexible, silver nanowire electrodes. Acs Nano, 4, 2955–2963. 57. Li, J. T., Ye, F., Vaziri, S., Muhammed, M., Lemme, M. C., & Ostling, M. (2013). Efficient inkjet printing of graphene. Advanced Materials, 25, 3985–3992. 58. Kong, D., Le, L. T., Li, Y., Zunino, J. L., & Lee, W. (2012). Temperature-dependent electrical properties of graphene inkjet-printed on flexible materials. Langmuir, 28, 13467–13472. 59. Dua, V., Surwade, S. P., Ammu, S., Agnihotra, S. R., Jain, S., Roberts, K. E., Park, S., Ruoff, R. S., & Manohar, S. K. (2010). All-organic vapor sensor using inkjet-printed reduced graphene oxide. Angewandte Chemie-International Edition, 49, 2154–2157. Chapter 4 Improvement of Electrical Conductivity and Transparency

4.1 Introduction

Transparent conductors (TCs) made from as-deposited reduced graphene oxide (rGO) sheets significantly underperform those made from the chemical vapor deposition (CVD)-grown graphene in terms of optoelectrical properties because of (i) the inherently inferior electrical conductivities of rGO, (ii) the remaining oxygenated functional groups, and (iii) the intersheet junctions with high contact resistance between the rGO sheets [1]. To further improve the electrical conductivities of graphene or graphene oxide (GO)-based TCFs, several approaches have been taken, such as chemical doping, hybridization, and using large-size GO sheets. Chemical doping is an effective method to tailor the electrical properties of carbon materials, including graphite, carbon nanotube, and graphene [2]. There are two types of chemical doping, that is, p-type and n-type, depending on the functions of the dopants used, as electron acceptor and donor, respectively. Doping is achieved either through surface transfer doping or substitutional doping. Surface transfer doping is usually performed by adsorption of strong electron-donating or electron-with- drawing chemical species. Substitutional doping is usually performed by replacing the carbon atoms in the honeycomb lattice of graphene by atoms with a different number of valence electrons, which would result in the disruption of the sp2 hybridization of carbon atoms [2]. Due to the reactions taking place in the surrounding molecules, such as oxygen or water, the carbon materials doped by surface transfer would not have long-term stability. Although the stability of chemical doping treatment needs to be further investigated, chemical doping is considerably simple and high throughput. It is expected that doping treatment will be an efficient way to produce graphene-based electronic devices with high performance. The vast majority of GO or rGO sheets in current use for the fabrication of TCs is very small, mostly with an area of hundreds of micrometer [2–6]. The small size of graphene sheets results in high intersheet contact resistance between them due to a large amount of intersheet junctions and the presence of oxygenated functional groups on the edge. It was shown that small GO sheets with longer peripheries contained more carboxyl groups among different oxygenated groups than larger ones

© Springer Science+Business Media New York 2015 123 Q. Zheng, J.-K. Kim, Graphene for Transparent Conductors, DOI 10.1007/978-1-4939-2769-2_4 124 4 Improvement of Electrical Conductivity and Transparency

[7]. One of the most effective approaches to reduce the number of intersheet tunneling barriers is producing large-size graphene sheets. Another successful approach is to bridge these intersheet junctions with 1D highly conducting nanofillers, such as single-walled nanotubes (SWNTs), metal nanowire (NW), and nanogrids that can facilitate the restoration of the inherent electrical conductivities while sacrificing a little of the transparency of graphene. Apart from reducing the intersheet contact resistance, the intercalation of conducting fillers between the GO layers may bring about another important benefit by forming 3D conductive networks through the film thickness [8].

4.2 Chemical Doping

4.2.1 Chemical Doping of Carbon Materials

Doping of Graphite Nanoplatelets

It is demonstrated that the Br2 vapor treatment enhanced the electrical conductivity of graphite nanoplatelets (GNPs) by increasing the ionic component of Br functionalities [9]. The GNPs were prepared by thermal expansion of graphite intercalation compounds (GIC) following by ultrasonication. The thickness and diameter of the produced GNPs were estimated to be 4.5 nm and 46 µm on average, respectively

[9]. GNPs were then exposed to Br2 vapor at room temperature in a closed desic- cator for varying durations. The uptake of Br by GNP was evidenced by a weight increase, through the increases in both intercalation complexes and covalent C–Br groups. Figure 4.1a shows the changes in weight and Br atomic concentration as a function of bromination duration. The weight increased rapidly with bromination duration and then stabilized or decreased slightly after about 10 h, which may be caused by the dynamic process of absorption and desorption of Br atoms into graphite. Br was absorbed rapidly because of the ample availability of doping/reactive sites in the GNPs for treatments below 10 h. Further treatment did not induce additional absorption upon saturation of these sites, which can be attributed to gradual changes in the nature and balance of the C–Br bond types. Another evidence for Br uptake was the increase in Br atomic concentration. An effective way to measure the extent of intercalation is the (002) d-spacing of graphite, which can be performed by X-ray diffraction (XRD) spectra. As shown in Fig. 4.1b, the peak corresponding to the 3 h treatment differs in position and breadth from those corresponding to the other bromination durations. Table 4.1 shows that with a 3 h treatment the d-spacing exhibited a minimum while the width-at-half-maximum (WHM) of the peak showed a maximum. The disorder of graphite crystals during the intercalation process was responsible for the broadened peak and low d- spacing after the 3 h treatment. Two possible types of bonds, that is, charge transfer complexes and covalent bonds, were distinguished by time-of-flight secondary ion mass spectrometry (ToF-SIMS) as shown in Fig. 4.1c. The atomic bromine had two 4.2 Chemical Doping 125

Fig. 4.1 Chemical doping of graphite nanoplatelet (GNP ). a Weight changes and Br atomic concentration determined by XPS as a function of bromination duration. b XRD spectra of brominated graphite with different treatment durations. c ToF-SIMS spectra of brominated GNP. XPS X-ray photoelectron spectroscopy. (Reprinted with permission from [9]. Copyright (2007) by Elsevier)

isotopic peaks at molar masses 79 and 81 with an intensity ratio of 50.5:49.5; Br2 showed a triplet at 158, 160, and 162 with a respective intensity ratio of about 1:2:1;

CBr exhibited a doublet at 91 and 93; and C2Br exhibited a doublet at 103 and 105 [9]. 126 4 Improvement of Electrical Conductivity and Transparency

Table 4.1 XRD results with bromination duration. (Reprinted with permission from [9]. Copyright (2007) by Elsevier) Duration (h) 0 1 3 10 72 168 d-spacing (Å) 3.342 3.355 3.348 3.361 3.361 3.361 WHM (°) 0.345 0.365 0.5 0.335 0.32 0.32 WHM width-at-half-maximum

Doping of Carbon Nanotubes

Considerable progress has been made to enhance the electrical conductivities of thin films made from carbon nanotubes (CNTs) while maintaining their high transmittance through chemical doping treatments before graphene thin films became popular. Various techniques, including simple immersion of CNT films in HNO3 [10], SOCl2 [11], HNO3 followed by subsequent treatments by SOCl2 [12] or SOBr2, [13] have been developed. CNT films were fabricated using CNT dispersion containing sodium dodecyl sulfate (SDS) and washed using HNO3 [10], which showed an excellent conductivity with a negligible change in transmittance in the visible light range. Since HNO3 can efficiently remove the residual SDS among CNTs and also enhance the metallicity of the CNTs, the electrical conductivity of the CNT TCFs is significantly improved [10]. A combined room-temperature treatment with

HNO3 and SOCl2 was also shown to be an effective approach to reduce the sheet resistivity through the formation of acyl chloride functional groups [12]. As shown in

Fig. 4.2, the sheet resistance decreased after the initial exposure to HNO3 for 3 h and further decreased after the treatment with SOCl2. Immersion of the film in SOCl2 and complete drying with gentle nitrogen flow leads to instant improvement in the conductivity while longer immersion times (2 weeks in SOCl2) did not provide any further change in conductivity. An interesting result was reported that the CNT films modified using SOBr2 outperformed the counterparts modified using SOCl2 [13]. The observation was attributed to the formation of new conducting paths be-

Fig. 4.2 Decrease in sheet resistance of SWNT thin hŶƚƌĞĂƚĞĚ films as a function of time showing the effects of HNO3 ϯŚƌƐ͘ƚƌĞĂƚĞĚ and SOCl2 treatments. (Reprinted with permission ŝŶ,EKϯ from [12]. Copyright (2007) by AIP) ŌĞƌϮǁĞĞŬƐ dƌĞĂƚĞĚŝŶ^KůϮ 4.2 Chemical Doping 127

Table 4.2 Sheet resistances of SWNT films with different numbers of layers measured before and after chemical treatments. (Reprinted with permission from [13]. Copyright (2008) by Wiley) −1 −1 −1 Pristine (Ω sq ) SOCl2 (Ω sq ) SOBr2 (Ω sq ) One layer 5.04 × 103 4.23 × 103 4.02 × 103 Two layers 456 322 298 Three layers 303 187 168 Four layers 184 76 56

tween CNTs via S atoms when SOBr2 was used, while SOCl2-induced conductivity was mainly due to the formation of SWNT/SOCl2 charge-transfer complexes. In contrast to SOCl2, SOBr2 is a relatively larger molecule, which may cause larger binding energy between Br and SWNTs than that of Cl and SWNTs. Table 4.2 compares the sheet resistances of the SWNT films before and after different chemical treatments. It is noticed that the sheet resistance gradually decreased with increasing number of layers, consistent with the fact that sheet resistance is determined by film thickness.

4.2.2 Chemical Doping of Graphene

Because graphite, CNTs, and graphene share essentially the same chemical structure, it is not surprising that chemical doping would also be an important strategy for improving the optoelectrical properties of graphene films [14–19]. Several different acids and halogenating agents, like HNO3 [15], SOCl2 [20], SOBr2 [17], or AuCl3 [14, 16, 19] have been successfully employed for treating rGO films. Graphene is classified as a zero-band-gap semiconductor, where the density of states vanishes at the Dirac point [15]. As a result, undoped graphene has a low carrier density and thus a high sheet resistance. Chemical doping is an effective approach to tailor the electrical properties of graphene because it can increase the carrier concentration [2]. The chemical doping of graphene can be divided into two categories including surface transfer doping and substitutional doping, see Sect. 4.1 for further details of these doping methods.

Nitric Acid

Acid treatment, especially using nitric acid, is a simple and effective way to realize the surface transfer doping. HNO3 is known to be a p-type dopant, that is, electron acceptor, for graphitic materials. As shown in Eq. (4.1), an electron is transferred from graphene to HNO3 when a charge-transfer complex is formed [15]:

+− 6HNO3 +→ 25C C25 NO 3 • 4HNO3+ NO 2 + H 2 O (4.1) 128 4 Improvement of Electrical Conductivity and Transparency

Fig. 4.3 Acid treatments to improve the optoelectrical properties of graphene TCFs. a Illustration of the graphene band structure, showing the reduction in Fermi energy level due to p-type doping [15]. b Schematic of two different acid doping methods, that is, interlayer doping and last-layer doping [15]. c UV–vis spectra of graphene films of different thicknesses obtained at different treatment stages. (Thermal treatment at 1100 °C and additional treatments using %HIRUHWKHUPDOWUHDWPHQW $IWHUWKHUPDOWUHDWPHQW HNO and SOBr ) [17]. $IWHUDGGLWLRQDOWUHDWPHQWV 3 2 (Reprinted with permission W QP

from [15, 17]. Copyright

(2010, 2011) by ACS and FH W QP Elsevier) WWDQ

W QP

7UDQVPL

:DYHOHQJWK QP

The charge transfer results in a shift of the Fermi level, leading to an increased carrier concentration and a decreased sheet resistance, see Fig. 4.3a. It is also found that the interlayer doping yielded better optoelectrical properties than the last-layer doping (Fig. 4.3b) due to the more efficient doping effect [15]. Besides the doping effect, nitric acids can also clean the graphene film surface, leading to an improved optical transparency (Fig. 4.3c) [20].

Thionyl Chloride

Apart from nitric acids, halogenating agents have been popularly used to realize charge transfer. For example, it was shown that the exposure to SOCl2 induced functional groups on graphene surface, which acted as electron acceptors thus increasing the hole density in graphene [20]. The sheet resistance of the resulting graphene decreased by 50–80 % depending on the type of graphene [20]. The chlorine decorated along the edges and the basal plane of graphene increased the number of 2 holes in the conjugated sp network. The p-type doping of graphene due to SOCl2 has been further confirmed by electronic structure calculations, which indicated a Fermi level shift into the valence band and the results are shown in Fig. 4.4d–f [11]. 4.2 Chemical Doping 129

Fig. 4.4 SOCl2 doping of graphene. a Equilibrium structure of a SOCl2 molecule adsorbed on a pyrene molecule, which represents a graphene or CNT segment. b Total change density of the system in a plane. c Different charge densities in the same plane, indicating regions of charge depletion and accumulation with respect to a superposition of neutral atoms. (Reprinted with permission from [11]. Copyright (2005) by ACS)

An equilibrium structure of a SOCl2 molecule adsorbed on a pyrene molecule, rep- resenting a graphene or CNT fragment, is shown in Fig. 4.4a. Figure 4.4b shows the corresponding charge distribution in SOCl2 that is adsorbed on pyrene. A net charge transfer of about 0.1 electrons from pyrene to SOCl2 was observed according to the calculation results. Figure 4.4c displays the charge flow in the system, suggesting that the largest charge accumulation occurred on oxygen and charge depletion was uniform across pyrene [11].

To evaluate the doping effect of SOCl2 on thermally reduced GO films, the XPS analysis was performed and the elemental compositions and the functionalities of the GO films were identified at various stages [20]. Table 4.3 presents the corresponding atomic concentrations of the elements at different stages including after chemical oxidation, after thermal treatment, and after SOCl2 doping. After the thermal treatment, the C/O atomic ratio surged from 2.42 to 28.8, as a result of the recovery of the sp2-hybridized benzene rings. The C/O ratio was reduced to 5.0 along with an increase in oxygen content to 15.3 % after SOCl2 doping. The recovered oxygen concentration was a reflection of the newly created species, such as SO2 or SO3, which also gave rise to a relatively high concentration of sulfur at 1.96 %. The 1.38 % nitrogen and 4.04 % chlorine originated from the doping agents of HNO3 and SOCl2, respectively. The convoluted XPS spectra given in Fig. 4.5 provided further evidence of thermal reduction and SOCl2 doping treatment. The C1s signal consisted mainly of five different chemically shifted components including: (i) C = C ( sp2) in aromatic rings at ~ 284.8 eV, (ii) C–C ( sp3) in aromatic rings at ~ 285.6 eV, (iii) C–O (hydroxyl and epoxy) at ~ 286.6 eV, (iv) C = O (carbonyl) at 287.8 eV, and (v) O–C = O (carboxyl) at ~ 290.3 eV [21, 22]. The concentrations of the C–O, C = O and O–C = O groups

Table 4.3 Summary of elemental compositions of GO at different treatment stages. (Reprinted with permission from [20]. Copyright (2011) by ACS) Element (atom %) CONS Cl C/O ratio Ultralarge GO 66.14 27.32 2.69 – – 2.42 (UL-GO) Thermally reduced GO 94.43 3.28 0.75 0.16 – 28.79

SOCl2-doped rGO 76.88 15.37 1.38 1.96 4.04 5.00 UL-GO ultralarge graphene oxide, GO graphene oxide, rGO reduced graphene oxide 130 4 Improvement of Electrical Conductivity and Transparency

Fig. 4.5 Deconvoluted C 1s a spectra of as-prepared GO a, thermally reduced GO b, and SOCl2-doped rGO &G c. (Reprinted with permission &J from [20]. Copyright (2011) &2 by ACS)

& 2 2& 2 ,QWHQVLW\ &RXQWV

%LQGLQJHYHUJ\ H9 b &J

,QWHQVLW\ &RXQWV &2 2& 2 & 2

%LQGLQJHYHUJ\ H9 c

&6 2& 2 &2 & 2 ,QWHQVLW\ &RXQWV

%LQGLQJHYHUJ\ H9 4.2 Chemical Doping 131 were 15, 36, and 8 %, which were drastically reduced to 9, 4, and 3 %, respectively, after the thermal treatment. SOCl2 doping induced a strong peak at 286.1 eV attributed to C–S [3], with about 15 % signal composition. It is also worth noting that the –COOH group increased from 3 to about 10 % after SOCl2 doping. Previous theoretical [23] and experimental [24] studies reveal that proper carboxyl functionalization of CNT was beneficial to improving the electrical conductivity. As such, the increase in –COOH functional group possibly helped stabilize the p-doping owing to the electron-deficient property of doped CNT or graphene. It is suspected that the sheet resistance may start to increase again if the –COOH concentration is too high because of the deterioration of the π–π conjugation structure in graphene.

Thus, a proper control of the SOCl2 doping parameters, such as concentration and immersion time, is of critical importance to achieve the highest possible electrical conductivity.

Figure 4.6 shows the S 2p, N 1s, and Cl 2p spectra of the SOCl2-doped rGO film. In the S 2p spectrum (Fig. 4.6a), the peak at ~ 169.5 eV can be attributed to S with 2− an oxidation state VI (SO4 ) and SO covalent bonds [25]. An additional peak occurred at ~ 163.6 eV, identical to that of an organic C–S bond [26], indicating that

–C–S–C– bonds existed in the SOCl2-doped rGO film. The possibility of covalent cross-linking between the rGO sheets through –C–S–C– bonds may also contribute to the enhanced electrical conductivity. In the N 1 s spectrum (Fig. 4.6b), the components near 395–405 eV are attributed to the NO, CN groups that originated mainly from the previous HNO3 treatment, confirming the attachment of N atoms on graphene surface via covalent bonding. Although the treatment by HNO3 was likely to introduce some defects on rGO, it appeared not to cause detrimental effect on the electrical properties of rGO films. This is because the dangling bonds or defects formed by HNO3 are immediately passivated by –OH or –COOH groups. Once –OH or –COOH groups are in contact with SOCl2, a nucleophilic substitution by chloride takes place. The ionic bonds with –Cl or –SOCl groups can improve the conductivity of the rGO films. The Cl 2p core level spectrum (Fig. 4.6c) revealed two nonequivalent chlorine sites from the 3/2 and 1/2 levels, which are separated by 1.4 eV due to spin–orbit coupling. The components are located at 202.0 and 200.4 eV corresponding to covalent C–Cl bonds in the organic chlorocarbon compound [27]. The p-type doping associated with –Cl led to the formation of charge transfer complexes in the rGO sheets, resulting in a charge redistribution in the system that in turn increases the rGO’s electrical conductivity [9]. In summary, SOCl2 doping introduced –Cl or –SOCl functional groups with a strong electronegativity onto the rGO surface which served as electron acceptors to improve the electrical conductivity. SOBr2 doping also showed desired effects of enhanced electrical conductivity and transparency of graphene films [17]. The mechanisms behind the improvemens in these characteristics were identified by establishing the correlation with the surface chemistry of the thin films. In a typical process, the thermally reduced GO film was subject to two additional processes: namely, (i) dipping in a HNO3 bath (70 %) for 3 h and drying with gentle nitrogen flow; and (ii) dipping in a SOBr2 bath for 24 h and drying with gentle nitrogen flow. The deconvoluted C1s spectra taken before 132 4 Improvement of Electrical Conductivity and Transparency

Fig. 4.6 XPS detail spectra a of the SOCl2-doped thin 62 films: a S 2p, b N 1s, and c Cl 2p. (Reprinted with permission from [20]. Copyright (2011) by ACS) 62

&6 ,QWHQVLW\ &RXQWV

%LQGLQJHYHUJ\ H9 b 12

&1 ,QWHQVLW\ &RXQWV

%LQGLQJHYHUJ\ H9

c ,QWHQVLW\ &RXQWV

%LQGLQJHYHUJ\ H9 4.2 Chemical Doping 133

a b && &%U

& 2 ,RQLF %URPLQH ,QWHQVLW\ DX ,QWHQVLW\ DX &22 &6

%LQGLQJ(QHUJ\ H9 %LQGLQJ(QHUJ\ H9

Fig. 4.7 XPS curve fitting of C1s a and Br 3d b spectra of SOBr2-doped rGO. (Reprinted with permission from [17]. Copyright (2011) by Elsevier) and after thermal reduction were all essentially similar to those shown in Fig. 4.5 because the materials and process employed were similar. Major differences arose when the rGO films were subjected to additional treatments using HNO3 and SOBr2 and the corresponding XPS results are shown in Fig. 4.7a and Table 4.4. The deconvoluted Br 3d spectrum in Fig. 4.7b shows that there were two types of C–Br bonds, namely the charge-transfer complexes and the covalent bonds, similar to the C–Cl bonds in thionyl chloride-doped rGO films. The intense Br 3d component at a core-level binding energy of ~ 70.1 eV is typical of C–Br covalent bonds, whereas the less intense component with a lower binding energy of ~ 69.3 eV is assigned to ionic bromine [13]. These two different types of C–Br bonds were confirmed by the ToF-SIMS analysis. The p-type doping associated with –Br led to the formation of charge transfer complexes in the rGO sheets, giving rise to enhanced electrical conductivities of rGO films [9]. A nucleophilic substitution by bromide takes place once –OH or –COOH groups are in contact with SOBr2. Then, the rGO sheets form ionic bonds with –Br or –SOBr groups, further contributing to the conductivity of graphene films. Figure 4.8 shows the resulting sheet resistance values of the rGO films measured at different stages. The sheet resistance of the rGO films changed from 104–106 to 100–2000 Ω/sq after the thermal treatment, which is about three orders of magnitude different. After the doping treatments, it was further reduced by about 20–50 % to 50–1600 Ω/sq depending on the film thickness. To further explain the enhanced

Table 4.4 Elemental compositions of GO, thermally reduced GO, and SOBr2-doped rGO. (Reprinted with permission from [17]. Copyright (2011) by Elsevier) Element (atom %) C N O S Br GO 71.91 0.49 27.48 0.11 – Thermally reduced GO 80.91 0.52 18.43 – –

SOBr2-doped rGO 71.40 2.93 16.79 4.80 4.08 GO graphene oxide, rGO reduced graphene oxide 134 4 Improvement of Electrical Conductivity and Transparency

Fig. 4.8 Comparison of sheet resistance at different treatment stages. (Reprinted with permission from VT Ω/

[17]. Copyright (2011) by Elsevier)

%HIRUHWKHUPDOWUHDWPHQW $IWHUWKHUPDOWUHDWPHQW $IWHUGRSLQJWUHDWPHQWV

6KHHWUHVLVWDQFH

)LOPWKLFNQHVV QP electrical conductivities, the carrier type and sheet carrier concentrations at different stages were determined by a Hall measurement system using the Van der Pauw method. The results are summarized in Table 4.5, showing that the graphene films remained as a positive carrier, that is, hole carrier, throughout the whole processes. The sheet carrier concentration increased by about three orders of magnitude after thermal treatment due to the removal of the oxygenated functional groups and the graphitization; and it further increased by nearly 200 % due to the chemical doping.

Auric Chloride

AuCl3 is also known to be an effective p-type chemical doping agent to reduce the sheet resistance of graphene TCFs [14, 16]. The Raman spectroscopy of the AuCl3- doped graphene indicates that AuIII was reduced to Au particles by charge transfer from graphene to AuIII [16]. It is proposed that graphene was p-doped through the following reactions when nitromethane is used as the solvent [16]:

+−I − III 2 graphene+→ 2AuCl32 2 graphene++ AuClA()uAuClA4 ()u (4.2)

−−0 − 3 AuCl24↔↓ 2 Au ++ AuCl 2 Cl (4.3)

In the presence of excess AuCl3, −− Cl +→AuCl34 AuCl . (4.4)

Table 4.5 Carrier type and sheet carrier concentration of graphene films at different stages. (Reprinted with permission from [17]. Copyright (2011) by Elsevier)

Sample As prepared rGO film After thermal treatment After SOBr2 treatment (cm−2) (cm−2) (cm−2) 38.7 nm thick + 2.30 × 1011 + 2.66 × 1014 + 5.64 × 1014 78.0 nm thick + 7.11 × 1011 + 3.63 × 1014 + 7.09 × 1014 rGO reduced graphene oxide 4.2 Chemical Doping 135

Figure 4.9a shows a schematic of layer-by-layer (LbL) doping strategy by AuCl3. Single-layer graphene was first grown on a Cu foil by a CVD process, which was then transferred onto a polyethylene terephthalate (PET) substrate. AuCl3 solution was spin-casted on the surface of graphene film followed by transferring another graphene layer directly onto the previous layer. This process was repeated to form up to four layers of graphene. In comparison, a four-layer graphene film was prepared and only the topmost layer was spin-casted with AuCl3. Figure 4.9b summarizes the resulting sheet resistance and transmittance of the doped graphene films. Due to the inadequately defined crystallinity of the film and the finite domain sizes,

Fig. 4.9 AuCl3 doping of graphene films. a The top steps indicate layer-by-layer (LbL) doping and the bottom steps indicate the topmost layer doping. b Sheet resistance and transmittance of various samples as a function of the number of graphene layers. PET polyethylene terephthalate, ITO indium tin oxide. (Reprinted with permission from [14]. Copyright (2010) by ACS) 136 4 Improvement of Electrical Conductivity and Transparency the pristine single graphene sheet showed a large sheet resistance of 725 Ω/square with a remarkable transmittance of 97.6 % at 550 nm. Regardless of the thickness, the sheet resistance was greatly reduced by about 80 % with AuCl3 doping (see filled circle). It is also worth noting that the transmittance was slightly decreased compared with pristine graphene films due to the light scattering from the Au nanoparticles formed during the reduction reaction [14, 28]. The Au-doped graphene films had at least a few folds higher electrical conductivities than those obtained after nitric acid treatment. However, the electrical conductivities of the AuCl3-doped films were essentially similar regardless of whether the individual graphene layers were doped or only the top layer of the multilayer graphene films was doped.

4.2.3 Stability of Doped Graphene Films

The degradation of the electrical conductivity and transmittance enhanced by various chemical processes in service environments is a critical issue for practical applications [15, 17]. To evaluate the stability of SOBr2-doped rGO films, the transmittance and Rs values were measured after exposure to ambient air (Fig. 4.10) [17]. It is revealed that the exposure to air for 3 months caused only marginal deterioration of the overall absorbance with the transmittance sustaining basically unchanged. The Rs value increased by 5–20 % after the initial 1.5 months of exposure, but it remained nearly unaltered for up to 3 month of exposure [17]. The initial degradation of the conductivity was ascribed to the loss of bromide functional groups, which were responsible for enhancing the electrical conductivity. The acyl bromide groups are reactive with water, and thus the doped functional groups were likely to react with the moisture in air during ageing. One likely way to mitigate the decomposition and thus to preserve the enhanced electrical conductivity is to coat a thin protection layer, such as poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT/PSS), on the graphene film [29].

The graphene films doped with AuCl3 in two different methods, including LbL and topmost layer doping, were exposed in air for over a few weeks to confirm the stability of the film [14]. It is revealed that the sheet resistance of the LbL-doped film was more stable than that of topmost layer-doped film, as shown in Fig. 4.10b. The initial sheet resistance values of the films doped by both methods were essentially identical for a given number of graphene layers. The LbL-doped graphene films also showed better stability when subjected to bending than other films without AuCl3 doping. As shown in Fig. 4.10c, the sheet resistance of LbL-doped four- layer graphene film increased by only 16.3 % after 1000 cycles of bending, lower than the 24.2 % increase for the HNO3-doped films, resulting in over 100 Ω/square difference in resistance between the two. 4.2 Chemical Doping 137

Fig. 4.10 Stability of doped a graphene film. a Comparison $IWHUWKHUPDOWUHDWPHQW of sheet resistance (Rs) and $IWHUGRSLQJWUHDWPHQW transmittance (T) of doped $IWHUPRQWKV rGO films at different stages $IWHUPRQWKV of exposure to air [17]. VT b Sheet resistance change as a function of time under vari- Ω ous conditions [14]. V c Bending stability test 5 results for four-layer pristine (top), HNO -doped (middle) 3 and Au-doped (bottom) graphene films [14]. LbL layer-by-layer. (Reprinted with permission from [14]. 7UDQPLWWDQFH Copyright (2010) by ACS) b

c 138 4 Improvement of Electrical Conductivity and Transparency

4.3 Hybridization

4.3.1 Hybridization with CNTs

Another useful approach is to bridge the intersheet junctions between graphene or rGO sheets with 1D highly conducting nanofillers, such as SWNTs, which facili- tates the restoration of graphene’s inherent electrical conductivity while sacrificing a little of the transparency of graphene [30]. Apart from reducing the intersheet contact resistance, the intercalation of CNTs may bring about another synergy of forming 3D conductive networks through the film thickness [8, 31]. As such, the development of graphene or rGO hybrids with CNTs is an attractive option that has been devised for improving the optoelectrical properties of TCFs. These hybrid films have been fabricated using several different techniques, such as the CVD method [32], solution mixing [33, 34], self-assembly at the solvent–water interface [35], LbL electrostatic self-assembly [36–38], the bubble deposition method (BDM) [39], and the Langmuir–Blodgett (L–B) technique [30]. The principles of these techniques are summarized in Table 4.6 along with typical TCFs produced based on the respective techniques. The corresponding optoelectrical performances of these hybrid films are compared in Table 4.7.

CVD Method

Kim et al. [32] proposed a facile methodology to form graphene/SWNT hybrid films for use in high-performance TCs via the CVD growth method. As shown in Table 4.6, SWNTs were first spin coated onto Cu foil, and graphene films were subsequently synthesized on the SWNT/Cu foil using the CVD growth method to decrease the contact resistance between graphene and SWNTs. The density and alignment of precoated SWNTs were controlled by adjusting the spin-coating speed, which is a crucial factor for achieving high-performance TCs. The obtained graphene/SWNT hybrid films showed a notable sheet resistance of 300 Ω/sq with 96.4 % transparency. Freestanding transparent graphene/CNT paper was assembled via a solid-phase layer-stacking approach [41]. This method has an advantage that the structural integrity and continuity of both components can be maintained and their intrinsic electrical and mechanical properties can be preserved. As shown in Fig. 4.11a, large area CNT and graphene films were first grown by the CVD method. The CNT film was then lifted from the substrate using a glass rod and gently placed on the graphene-coated copper foil. To secure a strong adhesion between graphene and CNTs, ethanol was dropped onto the CNT film followed by drying. By etching the copper away with a Fe(NO3)3 aqueous solution, freestanding graphene/CNTs hybrid films were obtained which were rinsed with deionized water (Fig. 4.11b). After drying, the obtained graphene/CNTs composite films were found highly flexible, conductive, and transparent, with a sheet resistance of 735 Ω/sq at 90 % transmittance. 4.3 Hybridization 139 33 ] 32 ] al. [

al. [

References Kim et et Tung 40 ]. Copyright (2014) by Elsevier) Typical images Typical hybrid TCFs. (Reprinted with permission from [ hybrid Schematics of synthesis process Fabrication of graphene/CNT and rGO/CNT Fabrication of graphene/CNT

able 4.6 Method CVD method Solution mixing T 140 4 Improvement of Electrical Conductivity and Transparency 35 ] al. [ 38 ] al. [

References Chen et Hong et Typical images Typical Schematics of synthesis process (continued)

able 4.6 Method Self-assembly at solvent–water interface Electrostatic self-assembly T 4.3 Hybridization 141 39 ] al. [

References Azevedo et Typical images Typical Schematics of synthesis process (continued)

able 4.6 Method Bubble deposition method T 142 4 Improvement of Electrical Conductivity and Transparency 30 ] al. [

References Zheng et Typical images Typical Schematics of synthesis process L–B Langmuir-Blodgett (continued)

chemical vapor deposition, Method L–B assembly Table 4.6 Table CVD 4.3 Hybridization 143 ] ] ] ] ] 38 ] 35 33 42 ] 32 37 al. 41

al. [ al. [ al. [ al. [

al. [ al. [

al. [

34 ] Reference [ / DC OP

σσ 0.21 Hong et 81 T 93 0.034 Kim et 0.3 96.4 34.0 Kim et 8 Rs (kΩ/sq) 0.735 90 4.7 Li et 0.24 86 10.0 et Tung 0.1 80 16.0 King et 0.954 88.6 3.2 Huang et 3.1 73 0.36 Chen et 151 °C °C,

min at

day at 80

°C in an argon °C in an argon

°C for 1

Hydrazine solution, 95 – – – Hydrazine vapor (Aldrich) at 70 – overnight 500 Hydrazine mono- hydrate solution for 1 atmosphere thermally annealed for 60 solution spin 3 Spin coating of SWNT solutionSpin coating of SWNT – coated on piranha solution cleaned substrate Deposition method of CNT layerDeposition method of CNT Reduction method CVD grown CNT film is transferred CVD grown CNT on top of graphene film using a glass rod – – Carboxylic acid functionalized SWNT Carboxylic acid functionalized SWNT film formed at pentane–water interface and transferred to substrate – Electrostatic self-assembly, nega- Electrostatic self-assembly, GO film immersed in tively charge aminated MWNT positively charged solution Electrostatic self-assembly, positively Electrostatic self-assembly, MWNT-NH charged optoelectrical properties of TCFs and hybrid graphene/CNT rGO/CNT Comparison of

CVD method Deposition method of graphene or GO layer CVD method GO powders mixed with oxygen in anhydrous functionalized SWNTs hydrazine followed by spin coating Graphene and CNT mixture in water Graphene and CNT with sodium cholate (NaC) surfactant followed by transfer printing GO film formed at pentane–water interface and transferred to substrate Dry powders of GO and SWCNTs Dry powders of GO and SWCNTs are dispersed directly in anhydrous hydrazine followed by spin coating Electrostatic self-assembly, aminated Electrostatic self-assembly, substrate immersed into negatively GO solution charged Electrostatic self-assembly, nega- Electrostatic self-assembly, rGO (or GO) solution tively charged film spin coated on top of MWNT Table 4.7 Table approaches assembled via different 144 4 Improvement of Electrical Conductivity and Transparency L–B al.

al.

39 ] 30 ] [ Reference Azevedo et [ / DC OP

σσ SWNT single-walled nanotubes, T ––– 0.56 86 4.3 Zheng et Rs (kΩ/sq) CNT carbon nanotubes, – – CVD chemical vapor deposition, Bubble deposition method, adhesion of the film to GO L–B assembly, SWNTs on water sur- SWNTs L–B assembly, face assembled onto GO film Deposition method of CNT layerDeposition method of CNT Reduction method rGO reduced graphene oxide, (continued)

Bubble deposition method, adhesion of the film to substrate L–B assembly, GO on water surface L–B assembly, assembled onto substrate Deposition method of graphene or GO layer GO graphene oxide, Table 4.7 Table Langmuir–Blodgett 4.3 Hybridization 145

a b

ŐƌĂƉŚĞŶĞ

EdĮůŵ

'ƌĂƉŚĞŶĞ

ƚĐŚŝŶŐ

ϭĐŵ

Fig. 4.11 Hybridization of chemical vapor deposition (CVD)-grown graphene and carbon nanotubes (CNT). a Schematic for the assembly of the graphene/CNT composite film. b Graphene/CNT composite film floating on water surface. (Reprinted with permission from [41]. Copyright (2010) by ACS)

Solution Mixing

Solution mixing of GO or rGO with CNTs before codeposition is another simple method for producing rGO/CNT hybrid films [34]. Tung et al. [33] first produced a hybrid layer of CNT and rGO via a solution mixing method, which is facile, inexpensive, scalable, and compatible with flexible substrates. As schematically shown in Table 4.6, SWNTs were refluxed in a mixture of nitric acid and sulfuric acid to activate the surface with oxygen functionalities and most of the SWNTs were terminated with hydroxyl and carboxylic moieties. GO powers were then mixed with functionalized SWNTs and dispersed directly in anhydrous hydrazine followed by stirring for 1 day. A uniform dark-gray suspension was formed upon the reduction of GO and functionalized SWNTs. rGO/SWNT dispersions were deposited onto substrates by spin-coating and subsequently heated to 150 °C to remove excess solvent. The obtained hybrid film presented a sheet resistance of 240 Ω/sq at 86 % transmittance after chemical doping. Using a similar process, Huang et al. [34] directly dispersed dry powders of GO and SWNTs in anhydrous hydrazine to produce hybrid suspensions of rGO and SWNTs. rGO/SWNT films were produced by a spin 146 4 Improvement of Electrical Conductivity and Transparency coating method. For the film formed with one and five layers, the sheet resistance reached as low as 954 and 254 Ω/sq, respectively. However, increasing the number of spin-cast layers meant thicker films with increasingly reduced optical transmittance from 88.8 to 58.7 %.

Self-Assembly at Solvent–Water Interface

Since GO sheets are amphiphilic in nature, they can form thin films at organic solvent–water interfaces based on a self-assembly process [43, 44]. Chen et al. [35] presented an ethanol-assisted self-assembly method for the quick formation of GO/ CNT composite thin films with tunable composition, transmittance, and surface resistivity at pentane–water interface. As shown in Table 4.6, ethanol serves as a nonsolvent when it is injected into the aqueous solution containing GO or carboxylic acid functionalized SWNTs. The injected nonsolvent (ethanol) has a significant influence on the stability of GO in water, providing a driving force to allow GO or SWNTs to aggregate at the water–pentane interface. GO/SWCNT (SWCN Single- walled carbon nanotubes) composite thin films were produced based on the self- assembly process at the water–pentane interface. Reduced GO/SWCNT composite films had a much lower surface resistivity of 3.1 kΩ/sq at 73 % transmittance than the neat rGO thin films, 8.3 kΩ/sq at 72 % transmittance.

Electrostatic Self-Assembly

The LbL electrostatic self-assembly is a versatile fabrication method of hybrid thin films by repeated, sequential immersion of a substrate into aqueous solutions of complementarily functionalized materials, that is, positively or negatively charged GO sheets or CNTs. Hong et al. [38] reported that rGO/CNTs hybrid films were − + prepared by integrating rGO-COO with MWNT-NH3 via the LbL assembly. A significant increase in electrical conductivity was achieved by incorporating CNT layers onto the rGO layers, with a sheet resistance of 8 kΩ/sq at 81 % transparency. In a similar study, Yu et al. [36] reported the preparation of positively charged rGO sheets after hydrazine reduction and using polyethyleneimine (PEI) as the stabilizer (Fig. 4.12a). The PEI-modified rGO sheets were employed for sequential self-assembly along with negatively charged acid-oxidized Multi-walled carbon nanotube (MWNT) to form hybrid carbon films with interconnected carbon structures and well-defined nanoscale pores (Fig. 4.12b–c). The similar strategy was employed to fabricate large, uniform, and durable rGO/MWNT hybrid double layers (Fig. 4.12d), [37] showing that the adsorption of MWNTs onto rGO films considerably decreased the sheet resistance of the films without much compromis- ing the transparency. 4.3 Hybridization 147

Fig. 4.12 Electrostatically self-assembled reduced graphene oxide (rGO)/CNT hybrid films. a Illustration of deposition of positively charged PEI functionalized rGO and negatively charged MWNTs on a silicon wafer or ITO-coated glass substrate. [36] SEM images of b the first rGO layer and c the first bilayer deposited on a silicon substrate [36]. d Fabrication of rGO/MWNT hybrid layer on a substrate [37]. (Reprinted with permission from [36, 37]. Copyright (2010, 2009) by ACS)

The BDM

The BDM is another simple method to precisely order and deposit GO films onto any substrates [39]. The BDM is based on the confinement of nano-objects in the core of water bubbles. The core thickness decreases under drainage and change the interference phenomena and the film color. There are mainly four major steps for the BDM process (Table 4.6): (i) formation of bubbles using GO or GO/CNT hybrid solution in a closed chamber, (ii) start of water drainage, (iii) deposition of films on the substrate, and (iv) fast evaporation of the residual water after the busting of the bubbles. The BDM can produce densely packed monolayer to multilayer GO sheets as well as GO/CNT composite thin films in ambient conditions. However, it should be noted that this method is incompatible with large-scale production and is only suitable for fundamental studies. 148 4 Improvement of Electrical Conductivity and Transparency

L–B Assembly

It is well known that the L–B assembly technique is the most sophisticated and accurately controllable among all available techniques to fabricate GO/CNT hybrid films, where amphiphilic ultralarge graphene oxide (UL-GO) and functionalized SWNTs are consecutively deposited via an LbL manner [30]. The deposition of SWNT layers is a crucial part of the whole process, thus the fabrication of COOH- functionalized SWNTs needs special attention. Due to the strong van der Waals forces between SWNTs, they are highly insoluble and aggregated, making pristine SWNTs unsuitable for the L–B assembly. The conventional solution is to apply non- covalent functionalization, including wrapping SWNTs with a variety of surfactant or polymers, such as poly(m-phenylene vinylene-co-2,5-dioctyloxy-pphenylene vinylene) (PmPV) [45] or poly(2,5-dioctyloxy-1,4-phenylene-alt-2,5-thienylene) (POPT) [46], to avoid the formation of insoluble bundles. However, these additives are insulating and difficult to be completely eliminated after the film formation, which is harmful to electrical conductivity of the films due to the additional contact resistance between them [13]. In the absence of surfactant treatment, SWNTs can also be dispersed using special solvents [47], such as N-methyl-2-pyrrolidone (NMP) [48], N,N-Dimethylform (DMF) [49], o-dichlorobenzene (DCB) [50], and dichloroethane (DCE) [45], but they are unsuitable for the L–B assembly because of their high boiling points [48–50] or toxic nature [45, 50]. Hence, carboxylic moieties were introduced non-uniformly along the SWNTs length, making them amphiphilic and ready for the L–B assembly. The XPS results of the COOH– functionalized SWNTs shown in Fig. 4.13a confirmed COOH– functional groups upon sulfuric acid and nitric acid (typically 3: 1 volume ratio) treatments [30]. Typically, the COOH– functionalized SWNTs were dispersed in a 1:3 water/methanol solution before spreading. The surface pressure– area isotherm curves in Fig. 4.13b shows the changes in slope due to the phase transitions of the SWNT monolayer from gas to condensed liquid and to a solid state at ~ 2 and ~ 10 mN/m, respectively. There were also small shifts of the gas–liquid and liquid–solid phase transition points toward a smaller area with increasing the cycles probably because of the loss of a small amount of material from the monolayer after each cycle [43]. The COOH– functionalization was effective in dispersing the bundles into individual SWNTs (Fig. 4.13c). Alternate monolayers of GO and SWNTs were deposited on the substrate one after another by carefully controlling the surface pressure. The scanning electron microscope (SEM; Fig. 4.14b) images suggest partially wrinkled edges when the GO sheets were squeezed toward each other during compression and pullout. The transmission electron microscope (TEM) image and the corresponding selected area electron diffraction (SAED) pattern (Fig. 4.14c) confirmed the primarily amorphous nature of the GO sheets [17]. Due to the amphiphilic GO and SWNT surfaces, strong bonds were formed though π–π interactions and van der Waals forces when the SWNT layer was deposited on the GO layer [51]. The SWCNTs functioned as conductive bridges connecting the separate GO sheets. 4.3 Hybridization 149

Fig. 4.13 L–B assembly a & of SWNT. a Fabrication of J amphiphilic SWNT-COOH and the corresponding C1s XPS spectrum. b Isotherm plots of three sequential compression/expansion cycles in the L–B assembly process,

,QWHQVLW\ DX & confirming highly reversible &2 G &22 and stable SWNT-COOH & 2 monolayer against compression. c Typical SEM image of monolayer SWNT-COOH collected at a surface pressure of 15 mN/m on top of a %LQGLQJHQHUJ\ H9 close-packed UL-GO layer. b (Reprinted with permission VWF\FOH from [30]. Copyright (2012) QGF\FOH UG by RSC) P F\FOH

P1 UH

UHVVX F S

6XUIDFH

$UHD FP c 150 4 Improvement of Electrical Conductivity and Transparency

Fig. 4.14 Fabrication of GO/SWNT hybrid films via an L–B assembly process: schematic a, SEM b, and TEM c images of a monolayer of GO; schematic d, SEM e, and TEM f images for one bilayer GO/SWNT hybrid film, with SWNTs bridging GO sheets; schematic g, and SEM h, i images of four bilayers GO/SWNT hybrid film. (Reprinted with permission from [30]. Copyright (2012) by RSC)

The typical surface morphologies of the GO/SWNT hybrid thin films with four bilayers are shown in Figs. 4.14h, 4.14i and 4.15a. Figure 4.15b shows the mean surface roughness values of the hybrid films as a function of number of bilayers before and after thermal treatment. As the film thickness increased, the surface roughness presented a generally increasing trend. Because of the deposition of porous SWNT monolayer and flat GO layer, there were sudden surges and drops in roughness. Due to the graphitization effect, the thermal reduction generally decreased the surface roughness with less significant drops and surges. The compaction of the films resulting from the removal of oxygenated functional groups and graphitization during annealing were mainly responsible for the reduction in roughness. After thermal treatment, the 30-nm-thick hybrid films had a surface roughness of ~ 10 nm, which is far lower than those prepared by other coating techniques, typi- 4.3 Hybridization 151

Fig. 4.15 a AFM image of the GO/single-walled nanotube (SWNT) hybrid film with 4.0 bilayers. b Film thickness and surface roughness as a function of the number of bilayers. UL-GO ultralarge graphene oxide. (Reprinted with permission from [30]. Copyright (2012) by RSC)

cally ~ 35 nm for the hybrid films with a similar thickness [34]. The GO/SWNT hybrid films without additional doping treatments delivered excellent sheet resistance ranging 180–560 Ω/sq with an optical transmittance ranging 77–86 % depending on the number of hybrid layers.

4.3.2 Hybridization with Metal Wires

Thanks to the high electrical conductivity and optical properties, metal NW films are considered promising materials for TC applications [52]. The sheet resistances of Au NW [53, 54], Ag NW [55, 56], and Cu NW films [57–59] are comparable to or lower than commonly used indium tin oxide (ITO) films at the same optical transmittance. However, the low oxidation resistance, poor adhesion to the substrate, the existence of porous open spaces, and low stability in harsh environments of metal NW films limit their application as TCs [52]. One possible way to address these problems is the hybridization of metal NWs or nanogrids with rGO. Vertically grown metal NWs can reduce the contact resistance, while metal nanogrids can bridge the rGO sheets, both decreasing the electrical resistance. 152 4 Improvement of Electrical Conductivity and Transparency

ZnO Nanorods/NWs

Choi et al. [60] fabricated a heterogeneous 3D nanostructure consisting of 1D ZnO nanorods grown epitaxially on a 2D graphene electrode with no damage using a low temperature solution growth. As shown in Fig. 4.16, a large-scale graphene film was grown on a Ni-coated SiO2/Si wafer by a CVD method. Then, the transparent graphene film was transferred to a flexible polyethylene naphthalate (PEN) substrate. After ZnO nanorods were grown on the graphene sheet, a rollable graphene-based transparent nanogenerator was integrated. A flexible TCF material composed of vertically aligned ZnO NWs grown on rGO-PDMS substrates was also reported

[61]. The rGO films were obtained by spin coating of GO dispersion on a Si/SiO2 substrate and annealing at a high temperature. After peeling them from the silicon substrate in a hydrogen floride (HF) solution, the rGO films were transferred to PDMS substrates. ZnO NWs were hydrothermally grown on the rGO film with a uniform structure even in highly deformed states, showing a typical metal–semiconductor ohmic contact without a contact barrier.

Silver NWs

Because Ag NW networks had a relatively low sheet resistance lower than 80 Ω/ sq at a transmittance of ~ 90 %, Ag NWs were combined together with graphene to form a high-performance TCF [62]. CVD grown graphene, rGO and GO were employed to improve the optoelectrical properties and stabilities of the hybrid TCFs with varied successes. The optoelectrical properties of graphene/Ag NWs hybrid TCFs assembled with different approaches are compared in Table 4.8. It was found that Ag-NWs could decrease the resistance of CVD grown graphene films [63]. Ag NWs were spin coated on glass and SiO2/Si substrates for integration with graphene, as shown in Fig. 4.17a. The CVD grown graphene was then transferred onto the Ag-NW films using the dry transfer technique. The sheet resistance of the resulting graphene/Ag-NW films was comparable to the intrinsic Rs of

“perfect” graphene, namely ~ 30 Ω/sq for graphene/SiO2 system [63, 64]. In another study, rGO was hybridized with Au nanoparticles (NPs) in the presence of Ag-NWs to improve the conductivity without adversely affecting the transparency [65]. The fabrication process involved spin coating of Ag-NW solution on the substrate and covered by a GO/Au-NPs coating, followed by reduction using a hydrazine vapor to prepare hybrid films (Fig. 4.17b). The resulting rGO/Au-NP films had a sheet resistance of 28.6 kΩ/sq, while the final rGO/Au-NP/Ag-NW hybrid films showed a much lower sheet resistance of 26 Ω/sq at a transparency of 83 %. It is believed that the presence of Ag-NWs had an extra benefit of minimizing the junction resistance between the rGO platelets, even at sub-percolation concentrations, similar to the bridging effect seen in GO/SWNT hybrid TCFs [30]. 4.3 Hybridization 153

Fig. 4.16 Fabrication process of graphene-based fully rollable transparent nanogenerator. (Reprinted with permission from [60]. Copyright (2010) by Wiley)

Another simple approach to hybrid with Ag NWs is grafting Ag NWs onto GO sheets before reduction [66]. As shown in Fig. 4.17c, Ag NWs was functionalized with cysteamine, endowing a number of –NH2 functional groups. Once the functionalized NH2–Ag NWs were mixed with GO, the epoxy functional groups of GO react with NH2–Ag NWs, forming Ag NW/GO hybrids. After a two-step reduction process, the Ag NW/GO was reduced to Ag NW/rGO. The Ag NWs on rGO benefited from the reduced contact resistance between the interlayers, producing hybrid TCFs with low surface resistance and high optical transmittance [66]. Liang et al. found that the one atom thickness, mechanical flexibility, and strong bonding with Ag NWs enabled the soft GO sheets to wrap around and solder the Ag NW junctions, which in turn dramatically reduced the inter-NW contact resistance without heat treatment or high force pressing [67]. As illustrated in Fig. 4.17d, the process to fabricate a AgNW/GO network started with bar-coating a layer of Ag- NWs on a glass substrate. The sheet resistance and transmittance of the Ag NW coating was determined by the coating density of the Ag NW percolation network, which was controlled by the concentration of Ag NW. To solder the Ag NW junctions, the dried Ag NW coating was soaked into a GO aqueous dispersion for several min without any heating or stirring. After rinsing with distilled water and blow drying, the GO-soldered Ag NW network has a sheet resistance of 14 Ω/sq with 88 % transmittance [67]. 154 4 Improvement of Electrical Conductivity and Transparency ] ] ] ] 65 65 65 63 ] ] al. [ al. [ al. [ al. [ ]

] 74 ] ] 73 ] ] ] ] ] ] ] 67 68 66 71 70 72 69 69 69 75 75 62 ] al. [ al. [ al. [

al. [

al. [ al. [ al. [ al. [ al. [ al. [ al. [ al. [

al. [

Reference / DC OP

σσ 129.8 Chen et 204.0 Liang et 88 T (%) T 88 22 86 80 18.6 et Tien 1460 91 4.124.8 92 Kholmanov et 2748 178.5 80 Moon et 91.2 59.1 83.3 Zhang et Shi et 16 91.18626 246.9– 85 Xu et 83 25.9 – 74.3 Kholmanov et 44 Kholmanov et Liu et 33 94 181.8 Lee et Rs (Ω/sq) 22532.530.4 91.3 81.5 18.0 71.2 53.9 33.5 Liu et Liu et Liu et 34.4 92.8 143.9 Lee et 24 91 162.7 Kholmanov et 13 71.9 80.9 Shi et 14 NW nanowire, L–B Langmuir–Blodgett TCFs assembled with different approaches TCFs assembled with different Drop casting – – Spin coating Bar or spray coating Spin coating Drop casting Spin coating Spin coating Spin coating Spin coating Deposition method Ag NWs layer of Spin coating Spin coating Spin coating Bar coating etching and transfer etching and transfer etching and transfer Spin coating Spin coating Spin coating and transfer 3 3 3 3 Dip coating Transfer printing Transfer Spin coating Spray coating L–B assembly Spin coating Spin coating Spin coating Etching and transfer Deposition method of graphene layer and transfer L–B assembly Soaking in GO solution followed by rinse and blow-dry GO graphene oxide, rGO reduced Comparison of optoelectrical properties of Ag NW/graphene hybrid Comparison of optoelectrical properties

CVD grown graphene on copper foil Plasma etching and transfer Ag NW-rGO mixture solution Ag NW-rGO Ag NW-rGO mixture solution Ag NW-rGO Gold-decorated rGO GO GO Gold-decorated rGO Gold-decorated rGO GO CVD grown graphene Graphene layer CVD grown graphene on copper foilCVD grown graphene on copper foil HNO CVD grown graphene on copper foil HNO Ammonium persulfate etching CVD grown graphene on copper foil FeCl CVD grown graphene on copper foil Modified dry transfer CVD grown graphene on copper foil HNO GO GO Table 4.8 Table CVD chemical vapor deposition, 4.3 Hybridization 155

Fig. 4.17 Ag nanowire (NW)/graphene hybrids for TCFs. a Schematic illustration of Ag NW/graphene films fabrication [63]. b Schematic of reduced graphene oxide (rGO)/Au nanoparticle (NP)/ Ag NW hybrid film preparation [65]. c Schematic of the fabrication process of the 2D AgNW/ rGO hybrid nanomaterials prepared through a two-step reduction process by sodium borohydride

(NaBH4) and ethylene glycol [66]. d Schematic illustration of the fabrication of a Ag NW/GO network on a glass substrate at room temperature [67]. (Reprinted with permission from [63, 65–67]. Copyright (2012, 2013, 2014) by ACS and Elsevier) 156 4 Improvement of Electrical Conductivity and Transparency

Cu NW/Cu Mesoscale Wire

Kholmanov et al. [52] found that the rGO film could act as an oxidation resistant layer for Cu NWs. The rGO film acts as a conductive and continuous transparent film that fills in the open spaces between the NWs, and as a cover that protects the NWs from harsh environments. As shown in Fig. 4.18a, the deposition process of rGO films onto the Cu NW films includes spin coating of rGO solution (left side of Fig. 4.18b), spray coating of NW solution (right side of Fig. 4.18b) and dry transfer of the hybrid film. rGO and Cu NWs were produced by spin and spray coating, respectively (Fig. 4.18c). By spin coating of a layer of Poly(methyl methacrylate) (PMMA) on the rGO films (Fig. 4.18d), the delaminated PMMA/rGO films were then transferred on the Cu NW films using a dry transfer method [63, 76]. By removing the PMMA layer with acetone, the final rGO/Cu NW hybrid films were obtained (Fig. 4.18e) which showed much lower sheet resistances than the neat rGO films with the same transmittances after thermal annealing, see Fig. 4.18f. Since the lateral size of rGO platelets was as large as several micrometers, they can

Fig. 4.18 Reduced graphene oxide (rGO)/Cu nanowire (NW) hybrid films for TCs. a Schematic of preparation of rGO/Cu NW hybrid films. b Photographs of GO aqueous dispersion and Cu NWs dispersed in IPA. c rGO (top) and Cu NW (bottom) films on glass substrates. d PMMA/rGO film delaminated from the glass substrate in NaOH aqueous solution. e Photograph of rGO/Cu NW films on a glass substrate after the PMMA layer was removed. f Sheet resistance versus transmittance of the neat Cu NW and rGO/Cu NW hybrid films. g, h SEM images of individual rGO platelets g and rGO/Cu NW film h. (Reprinted with permission from [52]. Copyright (2013) by ACS) 4.3 Hybridization 157 bridge two or more disconnected Cu NWs separated by any distance smaller than the lateral size of the platelet (Fig. 4.18g), leading to higher electrical conductivities of the hybrid films (Fig. 4.18h).

4.3.3 Hybridization with Metal Grids

The sheet resistance of metal nanogrids was also reduced by incorporating graphene to form hybrid TCFs [77]. As shown in Fig. 4.19a, metal grids were formed on a transparent substrate, followed by the transfer of CVD grown graphene on a sacrificial PMMA layer to the top of the grid. Hybrid transparent electrodes were formed once the sacrificial PMMA layer was removed by etching. Although the metal nanogrids themselves are opaque, they can form thin transparent percolation networks, making them conductive and highly transparent (Fig. 4.19b). To evaluate the flexibility of the TCFs, the thin films fabricated on PET substrates were bent multiple times (Fig. 4.19c). The flexibility test results showed that the sheet resis-

Fig. 4.19 a Schematic and b SEM image of graphene/metal nanogrids hybrid TCFs. c Image of a hybrid electrode on the PET substrate bent around a cylinder of d = 1 cm. d Sheet resistance of hybrid electrodes as a function of bending cycle. (Reprinted with permission from [77]. Copyright (2011) by ACS) 158 4 Improvement of Electrical Conductivity and Transparency tances of the hybrid films increased by 20–30 % after the initial 50 bending cycles, originating from the graphene film itself (Fig. 4.19d). It is worth noting that the sheet resistance stabilized even up to 500 bending cycles because the weak areas on the graphene did not survive during the initial bending and changed their forms to stabilize upon further cycles [77].

4.4 Using UL-GO

Although mechanical cleavage of graphite could prepare high-quality graphene with a millimeter size, the yield of this method is extremely low, being unsuitable for mass production [78]. Alternatively, graphitization of Si-terminated SiC (0001) in an argon atmosphere could produce monolayer graphene films with a domain size of several tens of micrometers [79]. However, the graphene obtained thereby was difficult to transfer to other substrates and the yield was very low. The CVD technique has been extensively explored to grow extremely large-area graphene on Ni films or Cu foils [80–82]. However, the CVD method usually requires specific substrate materials, which have to be removed chemically after the growth of graphene. The high cost of single crystal substrates and the ultrahigh vacuum conditions necessary to be maintained for the CVD growth significantly limit the use of the method for large-scale applications [81]. Because GO sheets are hydrophilic and produce stable and homogeneous colloidal suspensions in aqueous and various polar organic solvents, they are easy to be processed to produce TCs on a substrate [83]. However, these GO sheets are usually small, mostly with an area in the order of 100 µm2 [3–5]. Due to a large amount of intersheet junctions, the small-size GO sheets result in high intersheet contact resistance even after reduction [1]. Thus, efficient synthesis and assembly of large- area GO sheets are essential to improving the optoelectrical properties of GO-based TCs. Different deposition techniques have been used to prepare TCs using UL-GO sheets, and a summary is presented along with their optoelectrical properties, as shown in Table 4.9.

4.4.1 Solution Casting of UL-GO

Through a simple solution casting of exfoliated platelets from the edge-selectively functionalized graphite (EFG), Bae et al. [84] fabricated a large area, flexible, conductive, and transparent graphene films. EFG dispersions in dichloromethane with different concentrations were drop coated onto an SiO2 surface and subsequently heat treated at 600 °C in Ar for 3 h (Fig. 20a), obtaining heat-treated EFG (HEFG). PMMA solution in tetrahydrofuran (THF) was spin coated on an HEFG film

(Fig. 20b) to transfer them onto other substrates. After the SiO2 layer was etched by floating on aqueous hydrofluoric acid (HF) solution (Fig. 20c), the HEFG on 4.4 Using UL-GO 159

Table 4.9 Comparison of optoelectrical properties of UL/GO-based TCFs assembled with different approaches Size of Deposition Reduction method Rs (kΩ/ T σσ/ Reference UL-GO method of sq) DC OP (µm) UL-GO ~ 20 Solution Thermal annealing, 0.52 63 1.4 Bae et al. casting 600 °C in argon for [84] 3h ~ 20 Solution Thermal annealing, 3.110 90 1.1 Bae et al. casting 600 °C in argon for [84] 3h Up to ~ 200 Dip coating HI aqueous solution 19.1 79 0.079 Zhao et al. (55 %) at 100 °C for [1] 30 s Up to ~ 1–5 Dip coating Thermal annealing, 1005 98 0.018 Su et al.

20 % H2, 800 °C, 2h [85] Up to Dip coating Thermal annealing, 413 98.1 0.047 Su et al.

~ 20–100 20 %H2, 900 °C, 2h [85] Up to Dip coating Thermal annealing, 188 98 0.099 Su et al.

~ 1000 20 %/H2, 1000 °C, 2h [85] Up to ~ 200 L–B Thermal annealing, 0.5 90 6.97 Zheng assembly 1100 °C in argon, et al. [20] 0.5 h Up to ~ 200 L–B HI solution (57 %), 1.1 91 3.55 Lin et al. assembly 90 °C, 10 min [86] UL-GO ultralarge graphene oxide, L–B Langmuir–Blodgett

PMMA (HEFG/PMMA) film was transferred to a PET film. HEFG on PET (HEFG/ PET; Fig. 20d) was obtained once PMMA was removed in acetone. The optical transmittances of the HEFG/PET TCFs were in the range of 63–90 % with sheet resistances of 0.52–3.11 kΩ/sq. It should be noticed that the edge of the HEFG film was much thicker than the central part due to the nonuniform evaporation during the solution casting process.

4.4.2 Dip Coating of UL-GO

Zhao et al. [1] produced GO-based TCFs by using GO sheets with different areas, finding that the sheet resistance was strongly correlated to the GO area. The GO films were fabricated using a dip-coating method on a liquid/air interface (Fig. 4.21a) and reduced by HI acid (Fig. 4.21b). The sheet resistance of the rGO films decreased with increasing the area of GO sheets at the same transmittance because of the decrease in the number of intersheet tunneling barriers. A reduction in sheet resistance by one order of magnitude was achieved when the area of GO sheets increased from ~ 100 to ~ 7000 µm2 (Fig. 4.21c). For example, at a transmittance of ~ 78 %, 160 4 Improvement of Electrical Conductivity and Transparency

Fig. 4.20 Solution casting of edge-selective functionalization of graphite (EFG) for the production of large-area uniform graphene films. (Reprinted with permission from [84]. Copyright (2011) by ACS) the TCF made from sample I (~ 7000 µm2 in area) had a sheet resistance of ~ 840 Ω/ sq while the TCF made from sample II (~ 1000–3000 µm2 in area) and sample III ( ~ 100–300 µm2 in area) had sheet resistance of ~ 5.6 and ~ 19.1 Ω/sq, respectively. Because three types of TCFs were reduced under the same condition, it is believed that the lower sheet resistance of sample I was mainly due to the increase in average sheet area and consequent decrease in the number of intersheet tunneling barriers in a continuous rGO film [1]. A similar trend was also found by Su et al. [85]. As shown in Table 4.9, the sheet resistance of the TCF prepared from the UL-GO sheets with typical sizes approximately a millimeter after 1 h sonication was significantly lower than those prepared from the sample ll GO (S-GO) sheets with typical lateral sizes ~ 1–5 μm after 6 h sonication and those from the L-GO with typical sizes 20–100 μm after 2 h sonication. 4.4 Using UL-GO 161

Fig. 4.21 TCFs produced from UL-GO sheets. a, b Optical images of GO films produced by dip coating a before and b after HI acid reduction. c Sheet resistance and transmittance at 550 nm of TCFs assembled with GO sheets with different sizes. (Reprinted with permission from [1]. Copy- right (2010) by ACS)

4.4.3 L–B Assembly of UL-GO

S-GO and UL-GO dispersions were obtained after three consecutive runs of centrifugation for the L–B assembly. The sizes of S-GO and UL-GO sheets were around 3 and 50 μm, respectively. The typical surface pressure–area isotherm monitored using a tensiometer during the L–B assembly is shown in Fig. 4.22. The changes in the slope corresponded to the phase transition of GO sheets from gas to condensed liquid and to solid state (Fig. 4.22a). An initial gas phase existed where the surface pressure remained basically constant (stage a). The pressure began to increase as the compression continued (stage b). The GO sheets were about to touch one another, tiling over the entire surface. The surface pressure increased because of the electrostatic repulsion between the GO sheets, leading to slight darkening of the monolayer color due to the increase in material density on the water surface. When the monolayer was compressed beyond the close-packed stage, a further increase in surface pressure ensued (stage c) because the GO sheets began to fold along their edges instead of overlapping on top of another. Partial overlapping and wrinkling happened at a higher pressure (stage d). 162 4 Improvement of Electrical Conductivity and Transparency

a D E F G

UIDFHSUHVVXUH P1P

$UHD FP

b VWFRPSUHVVLRQ VW H[SDQVLRQ VWFRPSUHVVLRQ VW H[SDQVLRQ

UIDFHSUHVVXUH P1P 6X

$UHD FP

Fig. 4.22 Isothermal curves of the UL-GO. a Surface pressure vs. area plot showing the corresponding four stages (a–d) of the formation of GO monolayers in L–B assembly. b Isotherm plots of two sequential compression/expansion cycles, showing essentially overlapped two curves over the whole area, except at the early stage of compression. (Reprinted with permission from [20]. Copyright (2011) by ACS)

It was shown that the compression/expansion behavior of the GO sheets was fully reversible even after many cycles (Fig. 4.22b), thus the 2D GO monolayers thereby produced were expected to be stable with the same quality. The representative surface pressure curves for two cycles had nearly the same shape and final pressure. A small shift of the gas–liquid phase transition point toward a smaller area as the cycles continued were likely caused by the loss of a small amount of material from the monolayer. 4.4 Using UL-GO 163

Control of GO Structure

It was found that the GO structure in the TC films can be controlled by varying the L–B assembly parameters such as surface pressure and pulling speed [20, 87]. Sev- eral unique microscopic morphologies, such as wrinkles, folds and overlaps, were observed as a result of the interactions between the neighboring GO sheets when they were brought together side by side during the L–B assembly. The wrinkles, folds, and overlaps are undesirable as they reduce the optical transparency of the GO films [88]. For S-GO, the rigid S-GO sheets collected from the L–B assembly were free of wrinkles, while other deposition methods, such as drop-casting, spin- coating, and spraying, usually produced wrinkled sheets [89]. The packing density increased continuously as the surface compression increased from (a) isolated S-GO sheets, to (b) close-packed S-GO sheets, (c) over-packed GO sheets with folded edges, and (d) over-packed S-GO sheets with folded edges and overlapping on top of another (Fig. 4.23) [44]. UL-GO sheets tend to be softer and more flexible than S-GO sheets due to the large sizes ranging from a few tens to ~ 200 μm, presenting microscopic morphologies distinct from those observed in S-GO sheets depending on the pressure applied (Fig. 4.24). With increasing the surface pressure or the packing density, the typical features varied from: (a) isolated UL-GO sheets, (b) close-packed UL-GO sheets, (c) overlapped UL-GO sheets with some wrinkles, to (d) overlapped UL- GO sheets with extensive wrinkles. It is worth noting that at a high surface pressure the UL-GO sheets tended to wrinkle while the S-GO sheets were more susceptible to overlapping. For example, the UL-GO films collected at a low surface pressure (0 to ~ 10 mN/m) consisted of dilute, well isolated, flat individual UL-GO sheets (Fig. 4.24a, b). The UL-GO sheets were forced to squeeze each other with increasing the surface compression beyond the close-packed region, leading to overlapping and buckling. Figure 4.24c, d shows typical graphene oxide wrinkles (GOWs) with different degrees of wrinkling. The hydrogen bonds present between the carboxylic acid edge groups encouraged the interactions between UL-GO edges, preventing them from sliding and producing wrinkled UL-GO. Due to the irregularly shaped polyhedrons with polydisperse sizes of UL-GO sheets, they would squeeze each other from random directions, resulting in random wrinkling orientations. In view of the fact that these wrinkles tended to be aligned along the contact lines of neighboring sheets, the source of wrinkling was confirmed to be caused by buckling developed under in-plane compression. Different pulling speeds were found an important parameter in controlling the degree of wrinkling. A much higher degree of wrinkling was observed at a high pulling speed of 1.0 mm/min for UL-GO films, designating it “concentrated GOWs” (CGOWs), see Fig. 4.24e, f. The L–B transfer of flat GO sheets is a self-assembly process whose quality depends on the evaporation of water molecules trapped between the UL-GO sheets and substrate. Wrinkle-free S-GO films could be produced (Fig. 4.23) because the S-GO allowed water to evaporate easily. However, the relatively larger size of UL-GO often entrapped the water droplets between the UL-GO sheets and the capillary force induced by water evaporation caused wrinkling of the 164 4 Improvement of Electrical Conductivity and Transparency

Fig. 4.23 SEM images of S-GO sheets collected on a Si substrate at different stages of surface pressure. The packaging density increases from a isolated S-GO sheets, to b close-packed S-GO sheets, c over-packed S-GO sheets with folded edges, and d over-packed S-GO sheets with overlapping. e and f Over-packed S-GO sheets at a high magnification. (Reprinted with permission from [20]. Copyright (2011) by ACS) 4.4 Using UL-GO 165

Fig. 4.24 a–d SEM images of UL-GO sheets collected on a Si substrate at different stages of surface pressure at a pulling speed of 0.1 mm/min. The packaging density increases from a isolated UL-GO sheets, to b close-packed UL-GO sheets, c overlapped UL-GO sheets with some wrinkles, and d overlapped UL-GO sheets with extensive wrinkles. e, f SEM images of UL-GO sheets at stage d of the surface pressure curve at a pulling speed of 1.0 mm/min: concentrated GO wrinkles (CGOWs): taken at e low and f high magnifications. (Reprinted with permission from [20]. Copy- right (2011) by ACS) 166 4 Improvement of Electrical Conductivity and Transparency

Fig. 4.25 Schematics for the generation of concentrated graphene oxide wrinkles (CGOWs) due to capillary forces. UL-GO ultralarge graphene oxide. (Reprinted with permission from [20]. Copyright (2011) by ACS) sheets [90, 91]. In particular, if a high pulling speed were to be applied, there would be too short a time for the UL-GO sheets to relax into a flat state and transfer onto the substrate. Due to the capillary force and gravity (Fig. 4.25), the trapped water caused wrinkling during the transfer process. This explains why the UL-GO sheets assembled at a high pulling speed and a high surface pressure became CGOWs after transfer. It is reported that GO nanoribbons can be produced by plasma etching of GOWs [92]. By accurate control of the plasma etching conditions, single- or double-layer GO nanoribbons were obtained so that the top layers of GOWs acted as the sacrificial layers. The method of producing GOWs by L–B assembly offers a facile and environment-friendly approach to prepare large-size GOs with controllable amounts of wrinkles on their surface. Other than using a fast pulling speed, the compression of the films can also cause wrinkling even before deposition. Another way of controlling the GO structure in 4.4 Using UL-GO 167

Fig. 4.26 a Schematic of the formation of carbon nanoscroll (CNS)-like structures and the loose– dense pattern during the LB process. b–d TEM b, optical microscope image c, and AFM image d of CNS-like structures transferred onto a glass substrate. (Reprinted with permission from [87]. Copyright (2010) by Elsevier) the L–B assembly was proposed by Gao et al. [87]. It is found that the L–B method is a highly efficient fabrication approach for producing carbon nanoscrolls (CNSs) using functionalized GO sheets (Fig. 4.26a). The CNSs had a tubular structure without caps at its ends (Fig. 4.26b) and aligned parallel to the moving barriers of the LB equipment, exhibiting a loose-dense pattern during the LB compression process (Fig. 4.26c–d). A possible formation mechanism of the CNS-like structure is that the scrolling of a GO sheet is determined by the competition between the free energy and the elastic energy [93]. The overlapping of GO sheets decreases the free energy while the bending increases the elastic energy. It follows then that stable CNSs can be spontaneously formed when the van der Waals energy gain overweighs the bending energy. When moving the barriers, the functionalized GO sheets are bent due to the compression force. Once a close-packed monolayer is formed, the edge of GO sheets starts to scroll upon further compression. Thus, the CNS-like structure is formed near the barriers, which expands into the center of the trough. The CNS-like structure and bundles pack closely and align parallel to the moving barriers with the progress of the assembly. The simultaneous formation of the CNS-like structure and the collapse of CNS monolayer result in the loose–dense pattern. Because CNS, GOWs, and CGOWs have no caps at their ends, it may be easier to encapsulate functional molecules or nanomaterials in the internal cavities of GOs than in CNTs [94, 95]. In addition, the interlayer distance of the wrinkles can also be tailored to better accommodate the intercalants of various sizes [87]. The UL-GO sheets with high-density wrinkles, such as CNS, GOWs, and CGOWs, are considered promising candidates for many applications, including hydrogen storage, supercapacitors, biosensors, and nanomechanical devices. Table 4.10 summarizes 168 4 Improvement of Electrical Conductivity and Transparency

Table 4.10 Various GO structures obtained at different pulling speeds and surface pressures [20, 87] Type of GO Pulling speed Surface pressure Structure Potential applications (mm/min) (mN/m) S-GO 0.1–1.0 Any Flat GO Nanoelectronic devices UL-GO 0.1 0–15 Flat GO Micro- and Nanoelec- tronic devices UL-GO 0.1 > 20 GOWs Fabrication of GO nanoribbons UL-GO 1.0 > 20 CGOWs Hydrogen storage, microcircuit interconnects Functionalized 0.1–1.0 ~ 50 CNSs Field effect transistors, GO biosensors, and nanomechanical devices GO graphene oxide, UL-GO ultralarge graphene oxide, GOWs graphene oxide wrinkles, CGOWs concentrated graphene oxide wrinkles, CNSs carbon nanoscrolls

various GO structures that can be obtained by varying pulling speeds and surface pressures, along with their potential applications.

Surface Morphology

The electrostatic repulsion between the ionized carboxylic and phenol hydroxyl groups facilitate the formation of GO colloidal solution in water [96]. When the second GO layer is deposited on top of the first layer, these two layers are likely to experience both electrostatic repulsion and van der Waals attraction. It should be noted that the scaling law of van der Waals potential versus separation depends on the geometry of the interaction bodies [43]. When the GO sheets are brought together in a face-to-face manner, they can be treated as two parallel plane and their van der Waals potential then scales with (1/d2), where d is the distance between the two GO sheets. Besides van der Waals force, the residual π-conjugated domains can also contribute to the attraction between the GO sheets. These attractive forces dominate and lead to successful LbL deposition of GO sheets. However, the electrostatic repulsion that GO sheets experience from both their neighbors and those already deposited can cause wrinkling. Since the substrate is no longer flat due to the presence of GO sheets deposited previously, wrinkling becomes serious when depositing a large number of layers [43]. Figure 4.27 shows the typical surface morphologies of as prepared UL-GO and rUL-GO films after thermal treatment on a quartz substrate. The corresponding arithmetical mean roughness, root mean square roughness, and peak-to-peak roughness values are summarized in Fig. 4.28a–c, while the thicknesses of the deposited UL-GO films measured using atomic force microscopy (AFM) are plotted in Fig. 4.28d. It is noted that the surface roughness of both films increased consistently 4.4 Using UL-GO 169

Fig. 4.27 AFM images of UL-GO films consisting of two layers a, b and eight layers c, d of monolayer GO sheets taken before a, c and after thermal treatment b, d. (Reprinted with permission from [20]. Copyright (2011) by ACS) with increasing the number of GO layers. Instead of a linear increase, the parabolic increase may indicate that the wrinkles have been accumulated after the deposition of each GO layer deteriorating the flatness of the films. The surface roughness was consistently reduced after the thermal treatment which removed oxygenated functional groups and graphitization of the films [17, 97]. Although the wrinkles and defects cannot be completely removed after thermal treatment, the surface roughness of the films produced by L–B assembly was much lower than the films produced by other techniques. It is interesting to note that the average thickness of the films made from one layer of UL-GO was ~ 1.9 nm, which is about 50 % larger than the literature value [98, 99]. This observation is not surprising because the corresponding roughness of the film was about 1 nm due to wrinkling of UL-GO sheets [20]. The effects of substrate material on UL-GO film morphology and surface roughness were also investigated [86]. To study the morphology of UL-GO on a soft substrate, such as PET, the substrate surface needs to be made hydrophilic for proper wetting to take place by water and efficient deposition of UL-GO. One effective way 170 4 Improvement of Electrical Conductivity and Transparency

Fig. 4.28 Surface roughness of GO films in terms a 8/*2

QP U8/*2 of a arithmetical mean, Ra;

b root mean square, R ; c 5D q peak-to-peak roughness, Rpp; and d film thickness as a function of number of layers. b QP

P c

5SS

P d

FNQHVV Q

7KL 1XPEHURIOD\HUV to modify the PET surface is using oxygen plasma, for enhanced hydrophilicity of PET. Figure 4.29a shows the contact angles and the AFM images of the PET substrate surfaces obtained at different stages. It is found that the oxygen plasma treatment enhanced the hydrophilicity of the PET substrate, reducing the contact angle from 69 to 22°. The oxidation effect during the oxygen plasma treatment process endowed the PET surface with moieties of polar groups, which in turn reduced the C/O ratio while increasing the surface energy. Once a UL-GO layer was deposited on the PET substrate, the contact angle increased notably from 22° to 74.5°, due mainly to the amphiphilic GO sheets that had both hydrophilic groups (–COOH, –OH, and C = O) and hydrophobic (C–C, C–H) groups. The contact angle increased to 103.2° after HI reduction due to the removal of the oxygenated groups, and dropped back to 83.5° after the chemical doping with HNO3 and SOCl2 introducing of new functionalities. Figure 4.29b summarizes the corresponding thickness and mean roughness values of the UL-GO films on the PET substrate. Similar to UL-GO films on the quartz substrate, the average thickness of the film increased with increasing the number of GO layers. It is worth noting that the initially very high roughness of the film rapidly decreased with the increasing number of layers due possibly to the flattened PET substrate surface on the microscopic scale. From the AFM images shown in Fig. 4.29a, it is seen that the sharp peaks were removed by the deformation caused by the attractive forces between the hydrophilic PET surface and the amphiphilic UL-GO sheets. The larger the number of UL-GO layers, the larger the deformation and the more flattening of the sharp peaks, which helped mitigate the overall surface roughness [86]. 4.4 Using UL-GO 171

Fig. 4.29 a Contact angles and the corresponding AFM images of graphene oxide (GO) films on a polyethylene terephthalate (PET) substrate measured at different stages. b Film thickness and surface roughness as a function of number of GO layers. rGO reduced graphene oxide. (Reprinted with permission from [86]. Copyright (2013) by RSC)

Optical Transmittance and Electrical Conductivity

Figure 4.30 shows the comparison of optical and electrical properties of UL-GO films with different numbers of layer obtained at different stages of treatment. As expected, a thicker film resulted in a higher degree of absorption of light and thus a lower transparency at all treatment stages (Fig. 4.30a). The transparency was sig- 172 4 Improvement of Electrical Conductivity and Transparency

Fig. 4.30 Optoelectrical properties of L–B assembled ultralarge graphene oxide (UL-GO) films. a Comparison of optical and electrical properties between UL-GO films of different numbers of layers taken at different stages of treatment (transmittance measured at 550 nm wavelength). b Sheet resistance at different stages. c Sheet resistance and transmittance measured at 550 nm for transparent conductors consisting of rS-GO, rUL-GO, and C-rUL-GO. (Reprinted with permission from [20]. Copyright (2011) by ACS) nificantly deteriorated after the thermal treatment due to the reduction of GO and the adsorption of impurity particles on the other side of quartz substrates, while part of the lost transparency was restored after the chemical treatments [17]. However, the removal of these impurities after acid treatment contributed to the improvement of transparency. There appeared to be a strong interaction between the rUL-GO and quartz substrate after thermal treatment arising from the graphitization effect of the high-temperature annealing. After the thermal treatment, the sheet resistance of the rUL-GO films was in the range of 277–605 Ω/sq for film thickness 3.7–18.5 nm (Fig. 4.30b). The sheet resistance was further reduced by 30–50 % to 197–459 Ω/sq after the chemical treatments. To evaluate whether these properties remained stable, which is critical for practical applications [15], the sheet resistance of the chemically doped, reduced ultralarge graphene oxide (C-rUL-GO) film was measured after 4 months of exposure to ambient air (Fig. 4.30b). It was found that the sheet resistance increased by about 10–30 % after exposure depending on the film thickness. The loss of ameliorating chloride functional groups I was mainly responsible for the degradation of the elec- References 173 trical conductivity. Because the acyl chloride groups were reactive with water, it was likely that the functional groups doped on the graphene film may have reacted with moisture present in air during ageing. Employing a protective coating could reduce the possibility of decomposition and thus retain the improved electrical conductivity [29]. The sheet resistance values are compared between the films made from GO sheets of two different sizes using the same processing conditions, as shown in Fig. 4.30c. The rUL-GO sheets showed a much lower sheet resistance than the rS- GO sheets, by a remarkable one order of magnitude, for a given transmittance of the films. The reduced number of intersheet tunneling barriers in a continuous rUL-GO film because of the large area of GO sheets was responsible for this observation [1].

References

1. Zhao, J. P., Pei, S. F., Ren, W. C., Gao, L. B., & Cheng, H. M. (2010). Efficient preparation of large-area graphene oxide sheets for transparent conductive films. ACS Nano, 4, 5245–5252. 2. Liu, H. T., Liu, Y. Q., & Zhu, D. B. (2011). Chemical doping of graphene. Journal of Materi- als Chemistry, 21, 3335–3345. 3. Becerril, H. A., Mao, J., Liu, Z., Stoltenberg, R. M., Bao, Z., & Chen, Y. (2008). Evaluation of solution-processed reduced graphene oxide films as transparent conductors. ACS Nano, 2, 463–470. 4. Eda, G., Fanchini, G., & Chhowalla, M. (2008). Large-area ultrathin films of reduced graphene oxide as a transparent and flexible electronic material. Nature Nanotechnology, 3, 270–274. 5. Wang, X., Zhi, L. J., & Mullen, K. (2008). Transparent, conductive graphene electrodes for dye-sensitized solar cells. Nano Letters, 8, 323–327. 6. Zhu, Y. W., Cai, W. W., Piner, R. D., Velamakanni, A., & Ruoff, R. S. (2009). Transpar- ent self-assembled films of reduced graphene oxide platelets. Applied Physics Letters, 95, 103104. 7. Lin, X. Y., Shen, X., Zheng, Q. B., Yousefi, N., Ye, L., Mai, Y. W., & Kim, J. K. (2012). Fab- rication of highly-aligned, conductive, and strong graphene papers using ultra large graphene oxide sheets. ACS Nano, 6, 10708–10719. 8. Huang, Z. D., Zhang, B. A., Oh, S. W., Zheng, Q. B., Lin, X. Y., Yousefi, N., & Kim, J. K. (2012). Self-assembled reduced graphene oxide/carbon nanotube thin films as electrodes for supercapacitors. Journal of Materials Chemistry, 22, 3591–3599. 9. Li, J., Vaisman, L., Marom, G., & Kim, J. K. (2007). Br treated graphite nanoplatelets for improved electrical conductivity of polymer composites. Carbon, 45, 744–750. 10. Geng, H. Z., Kim, K. K., So, K. P., Lee, Y. S., Chang, Y., & Lee, Y. H. (2007). Effect of acid treatment on carbon nanotube-based flexible transparent conducting films. Journal of the American Chemical Society, 129, 7758–7759. 11. Dettlaff-Weglikowska, U., Skakalova, V., Graupner, R., Jhang, S. H., Kim, B. H., Lee, H. J., Ley, L., Park, Y. W., Berber, S., Tomanek, D., & Roth, S. (2005). Effect of SOCl2 treatment on electrical and mechanical properties of single-wall carbon nanotube networks. Journal of the American Chemical Society, 127, 5125–5131. 12. Parekh, B. B., Fanchini, G., Eda, G., & Chhowalla, M. (2007). Improved conductivity of transparent single-wall carbon nanotube thin films via stable postdeposition functionalization. Applied Physics Letters, 90, 121913. 13. Wang, Y., Di, C. A., Liu, Y. Q., Kajiura, H., Ye, S. H., Cao, L. C., Wei, D. C., Zhang, H. L., Li, Y. M., & Noda, K. (2008). Optimizing single-walled carbon nanotube films for applications in electroluminescent devices. Advanced Materials, 20, 4442–4449. 174 4 Improvement of Electrical Conductivity and Transparency

14. Gunes, F., Shin, H. J., Biswas, C., Han, G. H., Kim, E. S., Chae, S. J., Choi, J. Y., & Lee, Y. H. (2010). Layer-by-layer doping of few-layer graphene film. ACS Nano, 4, 4595–4600. 15. Kasry, A., Kuroda, M. A., Martyna, G. J., Tulevski, G. S., & Bol, A. A. (2010). Chemical doping of large-area stacked graphene films for use as transparent, conducting electrodes. ACS Nano, 4, 3839–3844. 16. Kim, K. K., Reina, A., Shi, Y. M., Park, H., Li, L. J., Lee, Y. H., & Kong, J., (2010). Enhanc- ing the conductivity of transparent graphene films via doping. Nanotechnology, 21, 285205. 17. Zheng, Q. B., Gudarzi, M. M., Wang, S. J., Geng, Y., Li, Z. G., & Kim, J. K. (2011). Improved electrical and optical characteristics of transparent graphene thin films produced by acid and doping treatments. Carbon, 49, 2905–2916. 18. Eda, G., Lin, Y. Y., Miller, S., Chen, C. W., & Su, W. F., & Chhowalla, M. (2008). Transpar- ent and conducting electrodes for organic electronics from reduced graphene oxide. Applied Physics Letters, 92, 233305. 19. Shin, H. J., Kim, K. K., Benayad, A., Yoon, S. M., Park, H. K., Jung, I. S., Jin, M. H., Jeong, H. K., Kim, J. M., Choi, J. Y., & Lee, Y. H. (2009). Efficient reduction of graphite oxide by sodium borohydrilde and its effect on electrical conductance. Advanced Functional Materi- als, 19, 1987–1992. 20. Zheng, Q., Ip, W. H., Lin, X., Yousefi, N., Yeung, K. K., Li, Z., & Kim, J.-K. (2011). Trans- parent conductive films consisting of ultra large graphene sheets produced by Langmuir- Blodgett assembly. ACS Nano, 5, 6039–6051. 21. Wang, X., Zhi, L. J., Tsao, N., Tomovic, Z., Li, J. L., & Mullen, K. (2008). Transparent carbon films as electrodes in organic solar cells. Angewandte Chemie-International Edition, 47, 2990–2992. 22. Geim, A. K., & Novoselov, K. S. (2007). The rise of graphene. Nature Materials, 6, 183–191. 23. Wang, C. C., Zhou, G., Wu, J., Gu, B. L., & Duan, W. H. (2006). Effects of vacancy-carboxyl pair functionalization on electronic properties of carbon nanotubes. Applied Physics Letters, 89, 173130. 24. Tantang, H., Ong, J. Y., Loh, C. L., Dong, X. C., Chen, P., Chen, Y., Hu, X., Tan, L. P., & Li, L. J. (2009). Using oxidation to increase the electrical conductivity of carbon nanotube electrodes. Carbon, 47, 1867–1870. 25. Adams, L., Oki, A., Grady, T., McWhinney, H., & Luo, Z. P. (2009). Preparation and characterization of sulfonic acid-functionalized single-walled carbon nanotubes. Physica E-Low- Dimensional Systems & Nanostructures, 41, 723–728. 26. Cavalleri, O., Gonella, G., Terreni, S., Vignolo, M., Pelori, P., Floreano, L., Morgante, A., Canepa, M., & Rolandi, R. (2004). High resolution XPS of the S 2p core level region of the L-cysteine/gold interface. Journal of Physics-Condensed Matter, 16, S2477–S2482. 27. Fedoseeva, Y. V., Bulusheva, L. G., Okotrub, A. V., Asanov, I. P., Troyanov, S. I., & Vyalikh, D. V. (2011). Electronic structure of the chlorinated fullerene C60Cl30 studied by quantum chemical modeling of X-Ray absorption spectra. International Journal of Quantum Chemis- try, 111, 2688–2695. 28. Kim, K. K., Bae, J. J., Park, H. K., Kim, S. M., Geng, H. Z., Park, K. A., Shin, H. J., Yoon, S. M., Benayad, A., Choi, J. Y., & Lee, Y. H. (2008). Fermi level engineering of single- walled carbon nanotubes by AuCl3 doping. Journal of the American Chemical Society, 130, 12757–12761. 29. Jackson, R., Domercq, B., Jain, R., Kippelen, B., & Graham, S. (2008). Stability of doped transparent carbon nanotube electrodes. Advanced Functional Materials, 18, 2548–2554. 30. Zheng, Q., Zhang, B., Lin, X., Shen, X., Yousefi, N., Huang, Z.-D., Li, Z., & Kim, J.-K. (2012). Highly transparent and conducting ultralarge graphene oxide/single-walled carbon nanotube hybrid films produced by Langmuir-Blodgett assembly. Journal of Materials Chemistry, 22, 25072–25082. 31. Zhang, B., Zheng, Q. B., Huang, Z. D., Oh, S. W., & Kim, J. K. (2011). SnO(2)-graphene- carbon nanotube mixture for anode material with improved rate capacities. Carbon, 49, 4524–4534. References 175

32. Kim, S. H., Song, W., Jung, M. W., Kang, M. A., Kim, K., Chang, S. J., Lee, S. S., Lim, J., Hwang, J., Myung, S., & An, K. S. (2014). Carbon nanotube and graphene hybrid thin film for transparent electrodes and field effect transistors. Advanced Materials, 26, 4247–4252. 33. Tung, V. C., Chen, L. M., Allen, M. J., Wassei, J. K., Nelson, K., Kaner, R. B., & Yang, Y. (2009). Low-temperature solution processing of graphene-carbon nanotube hybrid materials for high-performance transparent conductors. Nano Letters, 9, 1949–1955. 34. Huang, J. H., Fang, J. H., Liu, C. C., & Chu, C. W. (2011). Effective work function modula- tion of graphene/carbon nanotube composite films as transparent cathodes for organic optoelectronics. ACS Nano, 5, 6262–6271. 35. Chen, F. M., Liu, S. B., Shen, J. M., Wei, L., Liu, A. D., Chan-Park, M. B., & Chen, Y. (2011). Ethanol-assisted graphene oxide-based thin film formation at pentane-water interface. Lang- muir, 27, 9174–9181. 36. Yu, D. S., & Dai, L. M. (2010). Self-assembled graphene/carbon nanotube hybrid films for supercapacitors. Journal of Physical Chemistry Letters, 1, 467–470. 37. Kim, Y. K., & Min, D. H. (2009). Durable large-area thin films of graphene/carbon nanotube double layers as a transparent electrode. Langmuir, 25, 11302–11306. 38. Hong, T. K., Lee, D. W., Choi, H. J., Shin, H. S., & Kim, B. S. (2010). Transparent, flexible conducting hybrid multi layer thin films of multiwalled carbon nanotubes with graphene nanosheets. ACS Nano, 4, 3861–3868. 39. Azevedo, J., Costa-Coquelard, C., Jegou, P., Yu, T., & Benattar, J. J. (2011). Highly ordered monolayer, multilayer, and hybrid films of graphene oxide obtained by the bubble deposition method. Journal of Physical Chemistry C, 115, 14678–14681. 40. Zheng, Q., Li, Z., Yang, J., & Kim, J.-K. (2014). Graphene oxide based transparent conductive films. Progress in Materials Science, 64, 200–247. 41. Li, C. Y., Li, Z., Zhu, H. W., Wang, K. L., Wei, J. Q., Li, X. A., Sun, P. Z., Zhang, H., & Wu, D. H. (2010). Graphene Nano-“patches” on a carbon nanotube network for highly transparent/conductive thin film applications. Journal of Physical Chemistry C, 114, 14008–14012. 42. King, P. J., Khan, U., Lotya, M., De, S., & Coleman, J. N. (2010). Improvement of transparent conducting nanotube films by addition of small quantities of graphene. ACS Nano, 4, 4238–4246. 43. Cote, L. J., Kim, F., & Huang, J. X. (2009). Langmuir-Blodgett assembly of graphite oxide single layers. Journal of the American Chemical Society, 131, 1043–1049. 44. Kim, F., Cote, L. J., & Huang, J. X. (2010). Graphene oxide: Durface activity and two- dimensional assembly. Advanced Materials, 22, 1954–1958. 45. Li, X. L., Zhang, L., Wang, X. R., Shimoyama, I., Sun, X. M., Seo, W. S., & Dai, H. J. (2007). Langmuir-Blodgett assembly of densely aligned single-walled carbon nanotubes from bulk materials. Journal of the American Chemical Society, 129, 4890–4891. 46. Giancane, G., Ruland, A., Sgobba, V., Manno, D., Serra, A., Farinola, G. M., Omar, O. H., Guldi, D. M., & Valli, L. (2010). Aligning single-walled carbon nanotubes by means of Lang- muir-Blodgett film deposition: optical, morphological, and photo-electrochemical studies. Advanced Functional Materials, 20, 2481–2488. 47. Coleman, J. N. (2009). Liquid-phase exfoliation of nanotubes and graphene. Advanced Func- tional Materials, 19, 3680–3695. 48. Furtado, C. A., Kim, U. J., Gutierrez, H. R., Pan, L., Dickey, E. C., & Eklund, P. C. (2004). Debundling and dissolution of single-walled carbon nanotubes in amide solvents. Journal of the American Chemical Society, 126, 6095–6105. 49. Giordani, S., Bergin, S. D., Nicolosi, V., Lebedkin, S., Kappes, M. M., Blau, W. J., & Cole- man, J. N. (2006). Debundling of single-walled nanotubes by dilution: Observation of large populations of individual nanotubes in amide solvent dispersions. Journal of Physical Chem- istry B, 110, 15708–15718. 50. Coe-Sullivan, S., Steckel, J. S., Woo, W. K., Bawendi, M. G., & Bulovic, V. (2005). Large- area ordered quantum-dot monolayers via phase separation during spin-casting. Advanced Functional Materials, 15, 1117–1124. 176 4 Improvement of Electrical Conductivity and Transparency

51. Cote, L. J., Kim, J., Tung, V. C., Luo, J. Y., Kim, F., & Huang, J. X. (2011). Graphene oxide as surfactant sheets. Pure and Applied Chemistry, 83, 95–110. 52. Kholmanov, I. N., Domingues, S. H., Chou, H., Wang, X. H., Tan, C., Kim, J. Y., Li, H. F., Piner, R., Zarbin, A. J. G., & Ruoff, R. S. (2013). Reduced graphene oxide/copper nanowire hybrid films as high-performance transparent electrodes. ACS Nano, 7, 1811–1816. 53. Lyons, P. E., De, S., Elias, J., Schamel, M., Philippe, L., Bellew, A. T., Boand, J. J., & Cole- man, J. N. (2011). High-performance transparent conductors from networks of gold nanowires. Journal of Physical Chemistry Letters, 2, 3058–3062. 54. Azulai, D., Belenkova, T., Gilon, H., Barkay, Z., & Markovich, G. (2009). Transparent metal nanowire thin films prepared in mesostructured templates. Nano Letters, 9, 4246–4249. 55. Leem, D. S., Edwards, A., Faist, M., Nelson, J., Bradley, D. D. C., & de Mello, J. C. (2011). Efficient organic solar cells with solution-processed silver nanowire electrodes. Advanced Materials, 23, 4371–4375. 56. Lee, J. Y., Connor, S. T., Cui, Y., & Peumans, P. (2008). Solution-processed metal nanowire mesh transparent electrodes. Nano Letters, 8, 689–692. 57. Wu, H., Hu, L. B., Rowell, M. W., Kong, D. S., Cha, J. J., McDonough, J. R., Zhu, J., Yang, Y. A., McGehee, M. D., & Cui, Y. (2010). Electrospun metal nanofiber Webs as high-performance transparent electrode. Nano Letters, 10, 4242–4248. 58. Rathmell, A. R., & Wiley, B. J. (2011). The synthesis and coating of long, thin copper nanowires to make flexible, transparent conducting films on plastic substrates. Advanced Materials, 23, 4798–4803. 59. Zhang, D. Q., Wang, R. R., Wen, M. C., Weng, D., Cui, X., Sun, J., Li, H. X., & Lu, Y. F. (2012). Synthesis of ultralong copper nanowires for high-performance transparent electrodes. Journal of the American Chemical Society, 134, 14283–14286. 60. Choi, D., Choi, M. Y., Choi, W. M., Shin, H. J., Park, H. K., Seo, J. S., Park, J., Yoon, S. M., Chae, S. J., Lee, Y. H., Kim, S. W., Choi, J. Y., Lee, S. Y., & Kim, J. M. (2010). Fully rollable transparent nanogenerators based on graphene electrodes. Advanced Materials, 22, 2187–2192. 61. Hwang, J. O., Lee, D. H., Kim, J. Y., Han, T. H., Kim, B. H., Park, M., No, K., & Kim, S. O. (2011). Vertical ZnO nanowires/graphene hybrids for transparent and flexible field emission. Journal of Materials Chemistry, 21, 3432–3437. 62. Xu, S. C., Man, B. Y., Jiang, S. Z., Liu, M., Yang, C., Chen, C. S., & Zhang, C. (2014). Graphene-silver nanowire hybrid films as electrodes for transparent and flexible loudspeakers. Crystengcomm, 16, 3532–3539. 63. Kholmanov, I. N., Magnuson, C. W., Aliev, A. E., Li, H., Zhang, B., Suk, J. W., Zhang, L. L., Peng, E., Mousavi, S. H., Khanikaev, A. B., Piner, R., Shvets, G., & Ruoff, R. S. (2012). Improved electrical conductivity of graphene films integrated with metal nanowires. Nano Letters, 12, 5679–5683. 64. Chen, J. H., Jang, C., Xiao, S. D., Ishigami, M., & Fuhrer, M. S. (2008). Intrinsic and extrin- sic performance limits of graphene devices on SiO2. Nature Nanotechnology, 3, 206–209. 65. Kholmanov, I. N., Stoller, M. D., Edgeworth, J., Lee, W. H., Li, H. F., Lee, J. H., Barnhart, C., Potts, J. R., Piner, R., Akinwande, D., Barrick, J. E., & Ruoff, R. S. (2012). Nanostructured hybrid transparent conductive films with antibacterial properties. ACS Nano, 6, 5157–5163. 66. Tien, H. W., Hsiao, S. T., Liao, W. H., Yu, Y. H., Lin, F. C., Wang, Y. S., Li, S. M., & Ma, C. C. M. (2013). Using self-assembly to prepare a graphene-silver nanowire hybrid film that is transparent and electrically conductive. Carbon, 58, 198–207. 67. Liang, J. J., Li, L., Tong, K., Ren, Z., Hu, W., Niu, X. F., Chen, Y. S., & Pei, Q. B. (2014). Silver nanowire percolation network soldered with graphene oxide at room temperature and its application for fully stretchable polymer light-emitting diodes. ACS Nano, 8, 1590–1600. 68. Chen, R. Y., Das, S. R., Jeong, C., Khan, M. R., Janes, D. B., & Alam, M. A. (2013). Co- percolating graphene-wrapped silver nanowire network for high performance, highly stable, transparent conducting electrodes. Advanced Functional Materials, 23, 5150–5158. 69. Liu, Y., Chang, Q. H., & Huang, L. (2013). Transparent flexible conducting graphene hybrid films with a subpercolating network of silver nanowires. Journal of Materials Chemistry C, 1, 2970–2974. References 177

70. Lee, D., Lee, H., Ahn, Y., Jeong, Y., Lee, D. Y., & Lee, Y. (2013). Highly stable and flexible silver nanowire-graphene hybrid transparent conducting electrodes for emerging optoelectronic devices. Nanoscale, 5, 7750–7755. 71. Lee, M. S., Lee, K., Kim, S. Y., Lee, H., Park, J., Choi, K. H., Kim, H. K., Kim, D. G., Lee, D. Y., Nam, S., & Park, J. U. (2013). High-performance, transparent, and stretchable electrodes using graphene-metal nanowire hybrid structures. Nano Letters, 13, 2814–2821. 72. Liu, B. T., & Kuo, H. L. (2013). Graphene/silver nanowire sandwich structures for transparent conductive films. Carbon, 63, 390–396. 73. Moon, I. K., Kim, J. I., Lee, H., Hur, K., Kim, W. C., & Lee, H. (2013). 2D graphene oxide nanosheets as an adhesive over-coating layer for flexible transparent conductive electrodes. Scientific Reports, 3, 1112. 74. Zhang, X., Yan, X. B., Chen, J. T., & Zhao, J. P. (2014). Large-size graphene microsheets as a protective layer for transparent conductive silver nanowire film heaters. Carbon, 69, 437–443. 75. Shi, L., Yang, J., Yang, T., Qiu, H., Li, J., & Zheng, Q. (2014). Molecular level controlled fabrication of highly transparent conductive reduced graphene oxide/silver nanowire hybrid films. RSC Advances, 4, 43270–43277. 76. Suk, J. W., Kitt, A., Magnuson, C. W., Hao, Y. F., Ahmed, S., An, J. H., Swan, A. K., Gold- berg, B. B., & Ruoff, R. S. (2011). Transfer of CVD-grown monolayer graphene onto arbitrary substrates. ACS Nano, 5, 6916–6924. 77. Zhu, Y., Sun, Z. Z., Yan, Z., Jin, Z., & Tour, J. M. (2011). Rational design of hybrid graphene films for high-performance transparent electrodes. ACS Nano, 5, 6472–6479. 78. Meyer, J. C., Geim, A. K., Katsnelson, M. I., Novoselov, K. S., Booth, T. J., & Roth, S. (2007). The structure of suspended graphene sheets. Nature, 446, 60–63. 79. Chen, S. S., Wu, Q. Z., Mishra, C., Kang, J. Y., Zhang, H. J., Cho, K. J., Cai, W. W., Balandin, A. A., & Ruoff, R. S. (2012). Thermal conductivity of isotopically modified graphene. Nature Materials, 11, 203–207. 80. Kim, K. S., Zhao, Y., Jang, H., Lee, S. Y., Kim, J. M., Kim, K. S., Ahn, J. H., Kim, P., Choi, J. Y., & Hong, B. H. (2009). Large-scale pattern growth of graphene films for stretchable transparent electrodes. Nature, 457, 706–710. 81. Reina, A., Jia, X. T., Ho, J., Nezich, D., Son, H. B., Bulovic, V., Dresselhaus, M. S., & Kong, J. (2009). Layer area, few-layer graphene films on arbitrary substrates by chemical vapor deposition. Nano Letters, 9, 30–35. 82. Bae, S., Kim, H., Lee, Y., Xu, X. F., Park, J. S., Zheng, Y., Balakrishnan, J., Lei, T., Kim, H. R., Song, Y. I., Kim, Y. J., Kim, K. S., Ozyilmaz, B., Ahn, J. H., Hong, B. H., & Iijima, S. (2010). Roll-to-roll production of 30-inch graphene films for transparent electrodes. Nature Nanotechnology, 5, 574–578. 83. Dreyer, D. R., Park, S., Bielawski, C. W., & Ruoff, R. S. (2010). The chemistry of graphene oxide. Chemical Society Reviews, 39, 228–240. 84. Bae, S. Y., Jeon, I. Y., Yang, J., Park, N., Shin, H. S., Park, S., Ruoff, R. S., Dai, L. M., & Baek, J. B. (2011). Large-area graphene films by simple solution casting of edge-selectively functionalized graphite. ACS Nano, 5, 4974–4980. 85. Su, C. Y., Xu, Y. P., Zhang, W. J., Zhao, J. W., Tang, X. H., Tsai, C. H., & Li, L. J. (2009). Electrical and spectroscopic characterizations of ultra-large reduced graphene oxide monolayers. Chemistry of Materials, 21, 5674–5680. 86. Lin, X. Y., Jia, J. J., Yousefi, N., Shen, X., & Kim, J. K. (2013). Excellent optoelectrical properties of graphene oxide thin films deposited on a flexible substrate by Langmuir-Blodgett assembly. Journal of Materials Chemistry C, 1, 6869–6877. 87. Gao, Y., Chen, X. Q., Xu, H., Zou, Y. L., Gu, R. P., Xu, M. S., Jen, A. K. Y., & Chen, H. Z. (2010). Highly-efficient fabrication of nanoscrolls from functionalized graphene oxide by Langmuir-Blodgett method. Carbon, 48, 4475–4482. 88. Cote, L. J., Kim, J., Zhang, Z., Sun, C., & Huang, J. X. (2010). Tunable assembly of graphene oxide surfactant sheets: wrinkles, overlaps and impacts on thin film properties. Soft Matter, 6, 6096–6101. 178 4 Improvement of Electrical Conductivity and Transparency

89. Gilje, S., Han, S., Wang, M., Wang, K. L., & Kaner, R. B. (2007). A chemical route to graphene for device applications. Nano Letters, 7, 3394–3398. 90. Kim, J., Kim, F., & Huang, J. X. (2010). Seeing graphene-based sheets. Materials Today, 13, 28–38. 91. Kim, J., Cote, L. J., Kim, F., & Huang, J. X. (2010). Visualizing graphene based sheets by fluorescence quenching microscopy. Journal of the American Chemical Society, 132, 260–267. 92. Zhou, X. Z., Lu, G., Qi, X. Y., Wu, S. X., Li, H., Boey, F., & Zhang, H. (2009). A method for fabrication of graphene oxide nanoribbons from graphene oxide wrinkles. Journal of Physi- cal Chemistry C, 113, 19119–19122. 93. Braga, S. F., Coluci, V. R., Legoas, S. B., Giro, R., Galvao, D. S., & Baughman, R. H. (2004). Structure and dynamics of carbon nanoscrolls. Nano Letters, 4, 881–884. 94. Cao, L., Chen, H. Z., Li, H. Y., Zhou, H. B., Sun, J. Z., Zhang, X. B., & Wang, M. (2003). Fabrication of rare-earth biphthalocyanine encapsulated by carbon nanotubes using a capillary filling method. Chemistry of Materials, 15, 3247–3249. 95. Gao, X. P., Zhang, Y., Chen, X., Pan, G. L., Yan, J., Wu, F., Yuan, H. T., & Song, D. Y. (2004). Carbon nanotubes filled with metallic nanowires. Carbon, 42, 47–52. 96. Aboutalebi, S. H., Gudarzi, M. M., Zheng, Q. B., & Kim, J.-K. (2011). Spontaneous formation of liquid crystals in ultralarge graphene oxide dispersions. Advanced Functional Materi- als, 21, 2978–2988. 97. Wang, S. J., Geng, Y., Zheng, Q., & Kim, J.-K. (2010). Fabrication of highly conducting and transparent graphene films. Carbon, 48, 1815–1823. 98. Jung, I., Vaupel, M., Pelton, M., Piner, R., Dikin, D. A., Stankovich, S., An, J., & Ruoff, R. S. (2008). Characterization of thermally reduced graphene oxide by imaging ellipsometry. Journal of Physical Chemistry C, 112, 8499–8506. 99. Li, X. L., Zhang, G. Y., Bai, X. D., Sun, X. M., Wang, X. R., Wang, E., & Dai, H. J. (2008). Highly conducting graphene sheets and Langmuir-Blodgett films. Nature Nanotechnology, 3, 538–542. Chapter 5 Application of Graphene-Based Transparent Conductors (TCs)

5.1 Introduction

As graphene has several potential advantages over indium tin oxide (ITO) including weight, robustness , flexibility, chemical stability, and cost, many applications, such as touch panels, displays, solar cells, organic light-emitting diode, transistors and other new areas, have been demonstrated [1]. Although the application of graphene for transparent conductors (TCs) is still in its early stage and the performances of some devices presented in this book are in a preoptimized state, the unique functional characteristics can make graphene a strong candidate to replace the currently commercially dominant TC materials [2]. These devices, with their functional, structural, and mechanical requirements, where graphene has been considered to apply are discussed in this chapter.

5.2 Touch Screen

A touch screen is an electronic visual display that detects the presence and location of a touch [2]. A variety of touch-screen technologies, such as resistive, surface acoustic wave, capacitive, surface capacitance, projected capacitance, has been developed [3]. The most commonly used touch screens are the resistive and capacitive types, which require a sheet resistance of ~ 300–1500 Ω/sq at a transparency of ~ 86–90 % [4]. Graphene has several advantages including flexibility, wear resistance, chemical durability, and low toxicity (Fig. 5.1a–b) compared to the traditional ITO. Based on the successful fabrication of graphene films, with outstanding sheet resistance and transparency, and a large size of tens of centimeters, Bae et al. [5] incorporated them into touch-screen panel devices (Fig. 5.1b). It is revealed that the touch-screen display made from graphene outperformed that of ITO in terms of the applied strain. The former touch screen could handle twice as much strain as conventional ITO-based devices (Fig. 5.1c) [5]. The graphene-based panel resisted up to 6 % strain, which is limited mainly by the silver electrode and not by graphene itself, while the ITO-based touch panel easily broke at just 2–3 % strain. © Springer Science+Business Media New York 2015 179 Q. Zheng, J.-K. Kim, Graphene for Transparent Conductors, DOI 10.1007/978-1-4939-2769-2_5 180 5 Application of Graphene-Based Transparent Conductors (TCs)

Fig. 5.1 a Structure of graphene-based resistive-type touch screen [6]. b Image of flexible touch screen based on graphene films. c Electromechanical properties of graphene-based touch-screen devices compared with ITO/PET electrodes in tension. The inset shows the resistance change with compressive and tensile strains applied to the upper and lower graphene/PET panels, respectively [5] 5.3 Displays 181

5.3 Displays

Nearly 1.7 billion display device panels are produced annually [4] as major components for mobile phones, televisions, laptops, desktops, machine interfaces, monitors, etc. Transparent electrodes have critical functions in display devices, such as liquid crystal displays (LCD) and light-emitting diode (LED) [4] and the functions in these displace devices are discussed as follows.

5.3.1 Liquid Crystal Displays

An LCD is a thin, flat electronic visual display, which consists of thin films of optically transparent polymers with micrometer-sized liquid crystal (LC) droplets placed within the pores of the polymer [7]. Due to its ability to switch from translucence to opacity, the LCD has become an attractive material for display applications. Although ITO was the conventional transparent conductive film (TCF) material to apply the electric field across the LCD, the development was hindered by its instability and poor flexibility. Blake et al. [8] demonstrated LCDs with electrodes made of graphene that showed excellent performance with a high contrast ratio. Figure 5.2a shows the schematic diagram of LCDs fabricated with mechanically exfoliated monolayer graphene. Planar-aligned LCDs were fabricated using graphene-on-glass films, which were first located using an optical microscope (Fig. 5.2f, g) and then were further identified as monolayer graphene using Raman microscopy. The LC director was re- oriented by applying an AC (square-wave) voltage across the cells. By placing an optical microscope between the crossed polarizers while keeping the rubbing direction oriented 45° with respect to the polarizers, the electro-optic properties were observed. As shown in Fig. 5.2b–e, i, a strong change in the transmission was observed both in white and monochromatic lights above the expected threshold voltage of around

0.9 Vrms. Figure 5.2h shows a control sample with an opening in the metallization (Cr/ Au) not covered by graphene. It is suggested that the graphene had no negative effect on LC alignment because the whole graphene electrode area changed uniformly. The contrast ratio was better than 100 under white light, showing that graphene could indeed function as a transparent electrode for LCDs [8]. In addition, graphene showed additional advantages of chemical stability and mechanical flexibility [7].

5.3.2 Light-Emitting Diodes

LEDs have received great interests as they can be used in many areas such as flash- lights, traffic signals, and text and video displays [7]. It is expected that both the LED TV and LED backlights market will grow steadily in the future. ITO has been the dominant material for LED applications. However, the price fluctuations and its ceramic nature have greatly limited the future development. Novel TCF material with outstanding optoelectrical and mechanical properties is urgently needed 182 5 Application of Graphene-Based Transparent Conductors (TCs)

Fig. 5.2 a Schematic diagram of the structure of an LCD with typical layer thicknesses in brackets: 1 glass (1 mm), 2 graphene, 3 Cr/Au contact surrounding graphene flake (5 nm Cr + 50 nm Au), 4 alignment layer (PVA; 40 nm), 5 liquid crystal (20 μm), 6 alignment layer (40 nm), 7 ITO (150 nm), 8 glass (1 mm). The graphene flake is surrounded by a nontransparent Cr/Au contact. Optical micrographs of an LCD using green light (505 nm, fwhm 23 nm) with different voltages applied across the cell: b V = 8 Vrms, c V = 13 Vrms, d V = 22 Vrms, and e V = 100 Vrms. Overall image width is 30 μm. The central hexagonal window is covered by graphene, surrounded by the opaque Cr/Au electrode. f An optical micrograph (in reflection, using white light) of a graphene flake on the surface of a 1-mm-thick glass slide. The contrast is in the order of 6 %. Overall image width is 10 μm. g The same image but in a transmission mode. The flake is practically invisible. h Control device with no graphene in the opening of the Cr/Au contacts with V = 100 Vrms applied across the cell. i Light transmission through the LCD as a function of voltage applied across the cell, normalized to the maximum transmission. (Reprinted with permission from [8]. Copyright (2008) by ACS) 5.3 Displays 183

Fig. 5.3 a Processes of batch fabrication of GaN-based LEDs with patterned MLG electrodes; b schematic cross-sectional view of GaN-based LED structure with a transparent MLG electrode; and c–f optical micrographs of GaN LEDs with transparent MLG electrodes. c Large-area patterned multiple LED devices and d an individual LED. e LED with tip probes attached before applying the input current and f after applying an input current of 100 μA [9] to replace ITO in LED devices. It was recently demonstrated that patterned multilayer graphene (MLG) could be used for a large-scale fabrication of GaN LEDs [9]. Figure 5.3a shows the fabrication process of the LED devices with MLG electrodes where the chemical vapor deposition (CVD)-synthesized graphene film was used as a top anode for GaN LEDs. Figure 5.3b shows the schematic cross-sectional view of the LED, which consists of an MLG anode, an active luminescent layer (p- GaN/multiple quantum wells (MQW)/n-GaN), a Cr/Au cathode, undoped GaN, and the sapphire substrate. Figure 5.3c–d presents large-scale, batch-processed multiple devices with patterned MLG electrodes. The optical micrographs of an MLG electrode LED before (Fig. 5.3e) and after (Fig. 5.3f) applying an input current show that blue light emission is clearly visible even at a low input current of 100 μA [9]. Graphene thin films were used as transparent electrodes for organic light-emitting diodes (OLEDs) [10]. It is shown that the electrical and optical performance of a small molecule OLED on graphene was similar to that of control devices on an ITO, although there were marked differences in total thickness of the optical stack. It is believed that graphene is a viable alternative to ITO [10, 11], while further 184 5 Application of Graphene-Based Transparent Conductors (TCs) investigations, such as developing methods to deposit high-quality, thin layers of graphene on low-cost plastic substrates, are needed.

5.4 Solar Cells

Many kinds of graphene-based solar cells, including dye-sensitized solar cells [12], organic bulk-heterojunction (BHJ) photovoltaic cells [13], hybrid ZnO/poly(3-hexyl- thiophene) (P3HT) solar cells [14], Si Schottky junction solar cells [15], and InGaN p–i–n solar cells [16], have been developed recently [7]. One important advantage of graphene-based solar cell is that graphene can be used for flexible photovoltaic device applications. Arco et al. [17] demonstrated a feasible, scalable, and an effective method to employ CVD-grown graphene as a highly transparent, continuous, and flexible electrode for organic photovoltaic (OPV) cells. As shown in Fig. 5.4a, graphene films were synthesized by CVD and transferred to transparent substrates. The TCFs were then evaluated in organic solar cell heterojunctions (TCE/poly-3,4- ethylenedioxythiophene:poly styrenesulfonate (PEDOT:PSS)/copper phthalocyanine/fullerene/bathocuproine/aluminum). The key to success was the continuous nature of the CVD graphene films. The comparison study shows that graphene offers comparable performance with ITO. Graphene solar cells demonstrated an outstanding capability to operate under bending conditions up to 138°, whereas the ITO- based devices displayed cracks and irreversible failure with 60° bending [17]. A major challenge to successful implementation in these applications is the difficulty to effectively separate photogenerated electron–hole pairs and transfer the separated charge carriers to the electrodes [7]. Graphene-based materials have shown controllable surface and interfacial properties as well as tailored work functions via functionalization during synthesis and/or posttreatment [7, 18, 19]. Be- cause the potential created by the different work functions helps to separate the exciton pairs, it is important to facilitate better transport of charge carriers to each electrode, which can be achieved by an additional functional layer [19]. Choe et al. [13] reported the application of MLG films grown by the CVD method to OVP cells. Figure 5.4b shows the device structure of a photovoltaic cell with graphene as a transparent and conducting electrode. The cross-sectional transverse electromagnetic mode (TEM) image of the photovoltaic cell (Fig. 5.4c) presents distinctive interfaces formed between the layers of the individual components. It is shown that the optimized cell structure with an inserted TiOX layer enhanced the power conversion efficiency up to 2.58 ± 0.45 % (Fig. 5.4d–e).

5.5 Transistors

Since graphene has rapidly evolved from the exclusive domain of condensed-matter physicists to explore by pioneers in a variety of scientific and engineering communities, graphene-based transistors have attracted much interest [20]. It is well known that graphene is now considered as an option for post-silicon electronics. Due to 5.5 Transistors 185

Fig. 5.4 a Implementation of continuous, highly flexible, and transparent graphene films obtained by CVD as transparent conductive electrodes (TCE) in OVP cells [17]. b Schematic diagram of photovoltaic device structure with MLG electrodes and a hole-blocking TiOX layer. c TEM cross- sectional image of a photovoltaic device. The insets show HRTEM images near the TiOX layer ( top) and near the MLG films ( bottom). d J–V curves of photovoltaic devices with 1000 °C-grown

MLG electrodes ( circles) and with ITO electrodes ( diamonds). The curves without TiOX layer ( filled symbols) are compared to the ones with TiOX layer ( open symbols). e PCEs for graphene- electrode photovoltaic devices in comparison with those for the ITO-electrode photovoltaic devices with and without TiOX layers (pristine) [13] PCE Power conversion efficiency. (Reprinted with permission from [13, 17]. Copyright (2010) by Elsevier and ACS) the potential applications in large-area, flexible, and low-cost electronics, organic field-effect transistors (OFETs) have been rapidly developed in the past decades [21]. Due to the favorable work function, gold is normally used as the source and drain (S/D) electrodes for p-type organic semiconductors [2]. It is recently reported that the work functions of Cu and Ag electrodes could be tuned by depositing thin graphene films on their surfaces [21]. Graphene film can grow on the Cu and Ag electrodes that are patterned on a highly n-doped silicon wafer with a thermally oxidized SiO2 dielectric layer, as shown in Fig. 5.5a. By heating the patterned 186 5 Application of Graphene-Based Transparent Conductors (TCs)

Fig. 5.5 a Schematic illustration of the fabrication of OFETs with patterned graphene electrodes. b Transfer characteristics of pentacene-based OFETs with graphene and Ag electrodes. c Transfer characteristics of pentacene-based OFETs with graphene and Cu electrodes [21]. d Schematic illustration of the approach to fabricate patterned graphene electrodes [25]. e Structure of the CuPc monolayer transistor device with metal electrodes protected by a 50 nm layer of silicon dioxide [23]. f OM and 5.6 Other Applications 187

Cu and Ag electrodes in an ethanol/H2/Ar gas to 700–800 °C, the electrodes were easily modified. The device performance and contact resistance were dependent mainly on graphene electrodes. The devices with the heated pure Cu or Ag electrodes exhibited lower field-effects and higher contact resistance than those made with graphene electrodes, see Fig. 5.5b–c. This result is attributed mainly to the decreased work function of the S/D electrodes and the reduced contact resistance between the electrodes and the organic semiconductors after the deposition of graphene layer [21]. Full graphene S/D electrodes were also fabricated to prove the benefits of the graphene electrodes over normal gold contacts. Well-defined patterned graphene S/D electrodes were fabricated using the solution-processed GO films by means of a novel oxygen-plasma etching approach, which is an efficient way of patterning graphene on a large scale, as schematically shown in Fig. 5.5d. In particular, monolayer graphene can also act as the electrode in OFETs and pho- todetectors [8, 22–24]. Cao et al. [23] fabricated a new class of high-performance photoresponsive molecular FETs using the Langmuir–Blodgett (L–B) monolayers of copper phthalocyanine (CuPc) and 2D ballistically conductive single-layer graphene as planar contacts (Fig. 5.5e). The L–B techniques offered a promising and reliable method to prepare large-area, ordered ultrathin films with well-defined architectures. Thus, the unique feature of the FETs was the integration of L–B techniques with the fabrication of nanogap electrodes to build functional molecular electronic devices [23]. Based on the monolayer graphene contacts, a straight- forward methodology was developed to fabricate high-performance photosensitive nanoscale transistors [24]. The contacts were directly prepared by oxidative cutting of the individual 2D planar graphene (PG) sheets using the electron beam lithography and oxygen-plasma etching. Efficient transistors, on the nanometer scale, were readily formed (Fig. 5.5f).

5.6 Other Applications

5.6.1 Electromagnetic Interference (EMI) Shielding

Electromagnetic interference (EMI) or radio–frequency interference (RFI) refers to the disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source, like radio and TV stations, cell phones, or electric power transmission lines. Shielding can be achieved by reflection and absorption of electromagnetic radiation to shield against the penetration of the radiation energy [26]. Due to the high demand for sophisticated electronic devices and rapid growth of radio frequency (RF) radiation sources, EMI shielding in electronic devices has become a serious concern in modern society [26, 27]. In addition, EMI shielding is essential to protect the environment

AFM images of a representative device. The average thickness of a monolayer graphene is ~ 0.8 nm, and the gap size between the graphene ends is ~ 100 nm. Inset is the height profile across the nanogap [24]. (Reprinted with permission from [21, 23, 24, 25]. Copyright (2008, 2010, 2009) by Wiley) 188 5 Application of Graphene-Based Transparent Conductors (TCs) and human body. Due to the light weight, resistance to corrosion, flexibility, and processing advantages, electrically conducting composites have become popular to replace conventional metal-based EMI shielding materials [28, 29]. Graphene is an excellent choice for high-performance EMI shielding in the form of either sheets [26], papers [30–32], polymer-based composites, or coatings [33–36] because of its high conductivity, saturation velocity, flexibility, and mechanical strength [37]. Monolayer graphene prepared by the CVD method was found able to serve as an ultrathin, transparent, weightless, and flexible EMI shield [26]. The measurement setup for the EMI shielding effectiveness (SE) of monolayer graphene is shown in Fig. 5.6a–b. The results show that the monolayer CVD graphene has an average SE value of 2.27 dB, corresponding to ~ 40 % shielding of incident waves (Fig. 5.6c), while the defective graphene provided almost no shielding effect (Fig. 5.6d). It is suggested that manufacturing an ultrathin, transparent, weightless, and flexible EMI shield by a single or a few atomic layers of graphene would be tremendously important for portable electronic devices, transparent electronics and displays, automobiles, and EM field isolation in 3D ICs [26]. It was also demonstrated that patterned graphene/insulator stacks could be used as tunable far-infrared notch filters (Fig. 5.6e–f), which could lead to the development of transparent mid- and far-infrared photonic devices such as detectors, modulators, and three-dimensional metal material systems [37]. Freestanding graphene papers also show excellent and specific EMI SE [30–32].

Gupta et al. [31] demonstrated that MnO2 decorated graphene nanoribbons (GNRs) in paper form possessed outstanding microwave shielding properties. MnO2 in GNRs effectively enhanced the electronic polarization, interfacial polarization, and anisotropy energy in the presence of microwaves. A maximum SE of − 57 dB in the Ku band, that is, 12–18 GHz, was achieved for a 3.0-mm-thick sample. By using nickel pellet as a catalyst template during the CVD synthesis, Zhang et al. [32] developed a polymer-free process for synthesis of three-dimensional graphene structures and graphene papers. The obtained graphene papers with thickness below 100 µm already showed excellent EMI SE. For example, the graphene paper with thickness of 50 µm showed 60 dB EMI SE. Recently, efforts have also been made for the development of high-performance EMI shielding graphene/polymer nanocomposite materials [33–36]. For example, Chen et al. [36] developed a graphene/poly(dimethyl siloxane) (PDMS) foam composite. As shown in Fig. 5.7a–b, the fabricated graphene/PDMS foam composite is lightweight, flexible, and highly porous. The results show that its SE is as high as 30 dB in the 30 MHz–1.5 GHz frequency range and 20 dB in the X-band frequency range. Kim’s group [35] in HKUST further found that self-aligned rGO/ epoxy nanocomposites (Fig. 5.7c) with highly anisotropic mechanical and electrical properties present high-performance EMI shielding with a remarkable shielding efficiency of 38 dB. It is proposed that the relatively high shielding efficiency is associated with two unique features of the composites: namely, (i) the aligned rGO sheets contributed positively to shield the electromagnetic waves that emanate through the thickness direction (Fig. 5.7d), and (ii) the capability of absorbing the 5.6 Other Applications 189

Fig. 5.6 Schematic drawings of the measurement setup for the electromagnetic interference shielding effectiveness of graphene. a Waveguide measurement system (frequency range: 2.2–7 GHz) with two waveguide-to-coaxial adapters and a vector network analyzer. b Measurement setup using a horn antenna, a TEM cell, and a vector network analyzer. The SE, absorbance loss (AL), and reflectance loss (RL) of c a monolayer graphene, and d defective graphene [26]. e Extinc- tion in transmission, 1-T/T0, using a single layer of unpatterned graphene in the far-infrared and terahertz wavelength range for undoped graphene on quartz without ( gray squares) and with ( red squares) the polymer buffer layer underneath, and for doped graphene on quartz with the polymer buffer layer ( green squares). Solid lines are corresponding fitted curves. Inset: schematic of the measurement. f Fitted Drude weight and scattering width as a function of graphene layer number in the stacked devices [37] 190 5 Application of Graphene-Based Transparent Conductors (TCs)

Fig. 5.7 Graphene/polymer nanocomposites with high-performance EMI: a and b photograph and SEM image of the graphene/PDMS foam composites [36], c SEM and TEM (inset) image of rGO/epoxy nanocomposites (2.0 wt%) showing aligned nanostructures [35], and d schematic of electromagnetic wave shielding through the thickness of rGO/epoxy nanocomposites with aligned rGO sheets [35]. (Reprinted with permission from [35, 36]. Copyright (2013, 2014) by Wiley) incidental electromagnetic waves by polarization in the electric field due to the high charge storage capacities of the rGO/epoxy composites [35].

5.6.2 Functional Glasses

Most functional glass products would not have the desired functional properties without proper coatings. The unique properties of graphene offer the opportunities for improving functionalities of glass. Nowadays, graphene/glass defoggers (Fig. 5.8a) are fabricated, revealing that the graphene-based heating system has a better heating efficiency in term of temperature vs. power density than any other existing heating systems [38]. Figure 5.8b shows the assembly process of a graphene/glass defogger. The results show that the graphene/glass defoggers had

(with different power densities). d Saturated temperature vs. electrical power density of graphene and Cr thin-film defoggers. e Schematics of direct scattering, trapping–desorption process of ambient gases and thermal energy transfer between molecules and electrode surface for graphene and Cr thin-film defoggers [38]. f Schematic diagram of extremely rapid growth process on glass substrates at a low temperature without using metal catalyst. g Optical images of graphene-like carbon based films with different thicknesses obtained for different growing periods [40] 5.6 Other Applications 191

Fig. 5.8 a Schematic of heat transfer in a graphene defogger, showing substrates and expected temperature profile over the cross-section. b Layer-by-layer (LbL) transfer process of graphene on to glass substrate. c Temperature profile of two defoggers reaching the similar saturated temperature 192 5 Application of Graphene-Based Transparent Conductors (TCs) shorter response times and higher saturated temperatures than Cr/glass defoggers (Fig. 5.8c–d), which can be explained by the competition between direct scattering and trapping–desorption of ambient gases (Fig. 5.8e) [39]. Because the adsorption energies of ambient gases like O2, N2, and H2O in graphene are relatively low, the thermal accommodation coefficient is also low. The novel interfacial property opens a new possibility for a variety of flexible and transparent heating systems, such as outdoor displays and vehicle front-window defrosters and defoggers [38]. It is also reported that transparent and conductive graphene-like carbon films were deposited on a glass substrate at a low temperature by a fast and noncatalytic growth method, see Fig. 5.8f, g [40]. The fabrication process is extremely rapid and performed on a 2 in. wide scale dielectric substrate at a relatively low temperature (< 550 °C) without using a metal catalyst, so that the damaging and expensive transfer processes of graphene-based films could be avoided, making it compatible with current fabrication technologies [40].

5.6.3 Transparent Loudspeakers

Since the invention of loudspeaker more than a century ago, many kinds of loudspeakers have been proposed and commercialized [41]. The next generation display devices require transparent and flexible loudspeakers for new audio environments. Because graphene is electrically conducting and has a low mass density, it is an ideal building material for small, efficient, high-quality broad-band audio speakers [42, 43]. Xu et al. fabricated graphene-based loudspeakers by transferring the graphene film on the PVDF piezoelectric film [44], demonstrating that the graphene- based loudspeaker could be a practical magnet-free loudspeaker by simply applying an audio frequency field through it. Compared with the commercial thin-film speakers, graphene-based film loudspeakers can generate sound with a wide frequency range, a high sound pressure level (SPL) and low total harmonic distortion (THD), and has much less power consumption [44]. Especially, it is transparent and superior to those made from commercial films such as aluminum-based films. Figure 5.9a shows the structure and the sound generation mechanism of a graphene- based PVDF thin-film speaker. Once a time variable voltage is applied to the graphene electrodes, an audio frequency vibration on the PVDF piezoelectric film is produced by the attractive and repulsive forces between the internal charges as the external field varies. A photograph of the graphene-film loudspeaker is shown in Fig. 5.9b. This kind of transparent and flexible film loudspeakers has wide potential applications, especially in transparent and flexible devices [44].

5.6.4 Transparent Heaters

Due to a wide range of applications including outdoor displays, vehicle window defrosters, heating retaining windows, and other heating systems, transparent and 5.6 Other Applications 193

Fig. 5.9 a Schematic illustration of the structure and function. b Photograph of a graphene-based thin-film loudspeaker connected to the sound source and amplifier. (Reprinted with permission from [44]. Copyright (2013) by AIP) flexible film heaters have attracted growing interest [45, 46]. Although ITO has been widely used to prepare transparent heating films, the drawbacks of ITO, including limited availability of indium, intolerance to acid or base and fragil- ity under mechanical bending, limit its future applications [12]. The exceptional optoelectrical and thermal properties of graphene offer many advantages for producing transparent heaters [47]. It is suggested that the CVD-grown large-scale graphene films could be used for the fabrication of transparent heaters [45, 47]. As illustrated in Fig. 5.10a, graphene films were placed on a polyethylene terephthalate (PET) substrate and the copper layer was used to enhance the contact with graphene at the edges. Figure 5.10b–c shows the fabricated graphene-based heater, indicating the highly transparent and flexible nature. The time-dependent temperature results 194 5 Application of Graphene-Based Transparent Conductors (TCs)

Fig. 5.10 a A schematic structure of a transparent, flexible graphene heater combined with a plastic substrate and Cu electrodes. b An optical image of the assembled graphene-based heater showing its outstanding flexibility. c An infrared picture of the assembled graphene-based heater while applying an input voltage under bending conditions. d Temperature profiles of graphene-based heaters with two different doping agents and an ITO-based heater measured using an infrared scanner. e Mechanical stability test results of the graphene-based heater. (Reprinted with permission from [47]. Copyright (2011) by ACS)

(Fig. 5.10d) show that the graphene-based heater had a faster heating rate and a more homogeneous temperature distribution than the ITO-based heater. Due to the flexibility of graphene-based heater (Fig. 5.10e), it can be used on a curved window surface or a rollable screen [47].

5.6.5 Transparent Actuators

Transparent flexible electronics, especially those having excellent mechanical ac- tuating capabilities have received much interest [10, 48]. Transparent soft actuators can also satisfy our daily lives with artificial intelligence devices [48]. Although much progress has been made in developing soft actuators, the realization of high transparency in soft actuators is hampered by the actuator component materials [49]. Kim et al. [50] proposed a transparent, stretchable graphene-based actuator consisting of a dielectric elastomer substrate and multiple stacked graphene electrodes. Figure 5.11a depicts the working principle of the proposed actuator. The designed

FLG-driven actuator captured by the digital single lens reflex (DSLR) camera in the states of voltage e “OFF” and f “ON”. The manuscript images through the FLG-driven actuator is compared in g and h, each corresponding to the voltage “OFF” and “ON,” respectively [51] 5.6 Other Applications 195

Fig. 5.11 a Operating principle of graphene-based actuator. b Displacement of the actuator based on three-layer graphene electrodes. The actuator is maintained for 10 s per single voltage level from 0.3 to 3.0 kV as a sine wave of frequency 0.5 Hz. c Performance of the actuator when the input voltage is applied from 0 to 3 kV [50]. d Preparation of FLG-driven actuator. e–h Photographs of 196 5 Application of Graphene-Based Transparent Conductors (TCs) graphene-based actuator was able to generate the motion along the thickness direction while it has outstanding durability and robustness. At a frequency of 0.5 Hz in a button-like motion, the high displacement of 1050 µm was achieved for a 100-µm-thick graphen-based actuator (Fig. 5.11b–c). In addition, it is shown that the graphene-based actuator could still work well even under 25 % stretching while preserving its electrical and mechanical properties. It is suggested that the graphene-based actuator could be used for functional touch-screen panel devices [50]. Hwang et al. [51] also fabricated a transparent dielectric elastomer actuator driven by few-layer-graphene (FLG) electrode. The silicone elastomer substrate was sand- wiched between the two layers of FLG electrodes to form the transparent actuator, see Fig. 5.11d. The FLG-driven actuator had an optical transparency of over 57 % at a wavenumber of 600 nm, and produced bending displacement performance ranging from 29 to 946 µm as functions of frequency and voltage, demonstrating its application feasibility in variable focus lens and opto-electro-mechanical devices, see Fig. 5.11e–h [51].

5.6.6 Transparent Sensors

Sensors are essential components of many electronic and optoelectronic devices, and if these sensors are made transparent, they could lead to the development of skin- like multifunctional sensors [52]. Due to the excellent stretchability and transparency, graphene is an excellent candidate to realize a new class of human-interface devices [53, 54]. Bae et al. [54] demonstrated a graphene-based strain sensor that was capable of monitoring the motion of body parts. As shown in Fig. 5.12a, graphene- based transparent strain sensors were fabricated on a flexible plastic or stretchable rubber substrate using reactive ion etching and stamping techniques. Figure 5.12b shows a rosette gauge of graphene fabricated on a thinner PDMS substrate. The graphene strain sensor had high transparencies of 75–80 % over wavelengths ranging 400–700 nm, see Fig. 5.12c. A motorized tensile machine (Fig. 5.12d) was used to apply an uniaxial tensile strain to a thin-film material so that the piezoresistive properties of the graphene strain sensors were investigated in tension. The presence of a strong van der Waals force between graphene and the PDMS substrate allowed the same strain level in graphene as in the PDMS film. The measurement results (Fig. 5.12e) indicate a nonmonotonic resistance change against tensile strain up to 7.1 %, attributed to the presence of defects, disorders, and micro-cracks in graphene [54]. It is also demonstrated that the transparent rosette gauge on a stretchable and wearable hand glove was able to detect the bending motion of a finger. Graphene woven fabrics (GWFs) were utilized to design highly sensitive strain sensors for human motion monitoring [52]. GWFs were obtained by atmospheric- pressure CVD growth on crisscross copper meshes, as shown in Fig. 5.13a. After the copper meshes were etched away in FeCl3/HCl solution, GWFs were transferred to a composite film consisting of a medical tape and PDMS. The GWF/PDMS/tape was dried and connected to silver wires with silver paste on both ends to obtain 5.6 Other Applications 197

Fig. 5.12 a Schematic representation of various steps in fabrication process; b transparent graphene strain sensor; c transmittance spectrum for wavelength ranging 400–700 nm and Raman spectrum of graphene film (inset); d graphene strain sensor fixed in motion controller under stretching test, and the initial distance (~ 0 %) and final distance (~ 7.1 %) between the two fixed points (inset), and e variation of resistance with respect to stretching up to ~ 7.1 % of graphene strain sensor. (Reprinted with permission from [54]. Copyright (2013) by Elsevier) the final GWF strain sensor. The assembled transparent sensor exhibited relatively good sensitivity to follow human skin deformation (Fig. 5.13b). Its relative resistance change could be 10 times at 2 % strain or 104 times at 8 % strain, and also could be 0.07 time at 2 % strain depending on deformation strain which was formed by the motions, and these values are large enough for ordinary instruments to detect the motion signals (Fig. 5.13c). Weak motions were chosen to test the resistance change, including hand clenching, phonation, expression change, blink, breath, and 198 5 Application of Graphene-Based Transparent Conductors (TCs)

Fig. 5.13 Transparent graphene strain sensors for human motion monitoring: a schematic illustrations of fabrication procedure of a human motion sensor based on GWF/PDMS/medical tape film; b images of GWF/PDMS/tape at various positions; and c relative change of resistance between 0 and 2 % strain. (Reprinted with permission from [55]. Copyright (2014) by Wiley) pulse. Due to the distinctive features of high sensitivity and reversible extensibility, the transparent GWF-based piezoresistive sensors can find wide potential applications, such as robotics, fatigue detection, and body monitoring [55].

5.6.7 Transparent Supercapacitors

The high specific surface area, excellent chemical stability, and outstanding electrical conductivities make graphene an ideal material for the next generation energy storage devices [56]. Stretchable and transparent supercapacitors with highly stable performance can open up new possibilities for multifunctional applications of electronics in various energy, biomedical, and wearable optoelectronic systems [57]. However, most of the existing electrodes are either brittle (e.g., ITO), or with poor transparency (e.g., conducting polymers) [57]. Graphene films that are optically 5.6 Other Applications 199

Fig. 5.14 a Photographs of transparent thin films of varying thicknesses on glass slides. b TEM image of graphene collected from dispersion before filtration. c SEM image of 100 nm graphene film on a glass slide [58]. d–k Highly stretchable and flexible transparent supercapacitors made from WE sheets: d and e photographs of the supercapacitors d before and e after stretching up to 40 % strain. f and g Photographs of the supercapacitors f before and g after bending. h CV curves of the supercapacitor measured at different tensile strains at a scan rate of 0.1 V s−1. i Nor- malized specific capacitances of the supercapacitors made from either the PG or WE sheets as a function of tensile strain. j CV curves of the supercapacitor with different stretching cycles at a scan rate of 0.1 V s−1. k Normal- ized specific capacitances of supercapacitors made from PG or WG sheets as a function of stretching cycle [57]. (Reprinted with permission from [57, 58]. Copyright (2014,2010) by ACS and AIP)

transparent and mechanically flexible have demonstrated excellent capacitive behaviors [57, 58]. Yu et al. [58] presented a simple method for the preparation of ultrathin, transparent graphene films for use in supercapacitors. The ultrathin 200 5 Application of Graphene-Based Transparent Conductors (TCs) graphene films produced by vacuum filtration had high optical transparencies and homogeneous morphology, see Figs. 5.14a–c. For the film of ~ 25 nm in thickness, a specific capacitance of 135 F/g was obtained at a transmittance of ~ 70 %. Trans- parent and stretchable graphene-based supercapacitors were also developed by Chen et al. [57] where wrinkled graphene (WE) films were stretched to avoid the reduction in electrical conductivity. As shown in Fig. 5.14d–g, highly transparent (up to 60 % at 550 nm) and stretchable multilayer WE sheets were transferred onto a polydimethylsiloxane (PDMS) substrate. The WE films were used as both the current collector and active electrodes, which showed high transparency of 57 % and flexibility of up to 40 % strain. Even after the films were stretched to 40 % strain, their capacitance–voltage (CV) profiles and charge/discharge performance (Fig. 5.14h) as well as the specific capacitance (Fig. 5.14i) were almost unchanged. The mechanical cycleability was also measured by stretch/unloading tests of 40 % strain for over 100 cycles (Fig. 5.14j–k). Both the CV curves and charge/discharge characteristics remained almost unchanged, confirming highly stretchable and mechanically durable graphene-based transparent supercapacitors [57].

References

1. Wassei, J. K., & Kaner, R. B. (2010). Graphene, a promising transparent conductor. Materials Today, 13, 52–59. 2. Pang, S. P., Hernandez, Y., Feng, X. L., & Mullen, K. (2011). Graphene as transparent electrode material for organic electronics. Advanced Materials, 23, 2779–2795. 3. Bonaccorso, F., Sun, Z., Hasan, T., & Ferrari, A. C. (2010). Graphene photonics and optoelectronics. Nature Photonics, 4, 611–622. 4. Hecht, D. S., Hu, L. B., & Irvin, G. (2011). Emerging transparent electrodes based on thin films of carbon nanotubes, graphene, and metallic nanostructures. Advanced Materials, 23, 1482–1513. 5. Bae, S., Kim, H., Lee, Y., Xu, X. F., Park, J. S., Zheng, Y., Balakrishnan, J., Lei, T., Kim, H. R., Song, Y. I., Kim, Y. J., Kim, K. S., Ozyilmaz, B., Ahn, J. H., Hong, B. H., & Iijima, S. (2010). Roll-to-roll production of 30-inch graphene films for transparent electrodes. Nature Nanotechnology, 5, 574–578. 6. Lee, Y., & Ahn, J. H. (2013). Graphene-based transparent conductive films. Nano, 8, 1330001. 7. Jo, G., Choe, M., Lee, S., Park, W., Kahng, Y. H., & Lee, T. (2012). The application of graphene as electrodes in electrical and optical devices. Nanotechnology, 23, 112001. 8. Blake, P., Brimicombe, P. D., Nair, R. R., Booth, T. J., Jiang, D., Schedin, F., Ponomarenko, L. A., Morozov, S. V., Gleeson, H. F., Hill, E. W., Geim, A. K., & Novoselov, K. S. (2008). Graphene-based liquid crystal device. Nano Letters, 8, 1704–1708. 9. Jo, G., Choe, M., Cho, C. Y., Kim, J. H., Park, W., Lee, S., Hong, W. K., Kim, T. W., Park, S. J., Hong, B. H., Kahng, Y. H., & Lee, T. (2010). Large-scale patterned multi-layer graphene films as transparent conducting electrodes for GaN light-emitting diodes. Nanotechnology, 21, 175201. 10. Kim, K. S., Zhao, Y., Jang, H., Lee, S. Y., Kim, J. M., Kim, K. S., Ahn, J. H., Kim, P., Choi, J. Y., & Hong, B. H. (2009). Large-scale pattern growth of graphene films for stretchable transparent electrodes. Nature, 457, 706–710. 11. Reina, A., Jia, X. T., Ho, J., Nezich, D., Son, H. B., Bulovic, V., Dresselhaus, M. S., & Kong, J. (2009). Layer area, few-layer graphene films on arbitrary substrates by chemical vapor deposition. Nano Letters, 9, 30–35. References 201

12. Wang, X., Zhi, L. J., & Mullen, K. (2008). Transparent, conductive graphene electrodes for dye-sensitized solar cells. Nano Letters, 8, 323–327. 13. Choe, M., Lee, B. H., Jo, G., Park, J., Park, W., Lee, S., Hong, W. K., Seong, M. J., Kahng, Y. H., Lee, K., & Lee, T. (2010). Efficient bulk-heterojunction photovoltaic cells with transparent multi-layer graphene electrodes. Organic Electronics, 11, 1864–1869. 14. Yin, Z. Y., Wu, S. X., Zhou, X. Z., Huang, X., Zhang, Q. C., Boey, F., & Zhang, H. (2010). Electrochemical deposition of ZnO nanorods on transparent reduced graphene oxide electrodes for hybrid solar cells. Small, 6, 307–312. 15. Li, X. M., Zhu, H. W., Wang, K. L., Cao, A. Y., Wei, J. Q., Li, C. Y., Jia, Y., Li, Z., Li, X., & Wu, D. H. (2010). Graphene-on-silicon Schottky junction solar cells. Advanced Materials, 22, 2743–2748. 16. Shim, J. P., Choe, M., Jeon, S. R., Seo, D., Lee, T., & Lee, D. S. (2011). InGaN-based p-i-n solar cells with graphene electrodes. Applied Physics Express, 4, 052302. 17. De Arco, L. G., Zhang, Y., Schlenker, C. W., Ryu, K., Thompson, M. E., & Zhou, C. W. (2010). Continuous, highly flexible, and transparent graphene films by chemical vapor deposition for organic photovoltaics. Acs Nano, 4, 2865–2873. 18. Huang, J. H., Fang, J. H., Liu, C. C., & Chu, C. W. (2011). Effective work function modu- lation of graphene/carbon nanotube composite films as transparent cathodes for organic optoelectronics. Acs Nano, 5, 6262–6271. 19. Jo, G., Na, S. I., Oh, S. H., Lee, S., Kim, T. S., Wang, G., Choe, M., Park, W., Yoon, J., Kim, D. Y., Kahng, Y. H., & Lee, T. (2010). Tuning of a graphene-electrode work function to enhance the efficiency of organic bulk heterojunction photovoltaic cells with an inverted structure. Applied Physics Letters, 97, 213301. 20. Schwierz, F. (2010). Graphene transistors. Nature Nanotechnology, 5, 487–496. 21. Di, C. A., Wei, D. C., Yu, G., Liu, Y. Q., Guo, Y. L., & Zhu, D. B. (2008). Patterned graphene as source/drain electrodes for bottom-contact organic field-effect transistors. Advanced Mate- rials, 20, 3289–3293. 22. Liu, W., Jackson, B. L., Zhu, J., Miao, C. Q., Chung, C. H., Park, Y. J., Sun, K., Woo, J., & Xie, Y. H. (2010). Large scale pattern graphene electrode for high performance in transparent organic single crystal field-effect transistors. Acs Nano, 4, 3927–3932. 23. Cao, Y., Wei, Z. M., Liu, S., Gan, L., Guo, X. F., Xu, W., Steigerwald, M. L., Liu, Z. F., & Zhu, D. B. (2010). High-performance Langmuir–Blodgett monolayer transistors with high responsivity. Angewandte Chemie-International Edition, 49, 6319–6323. 24. Cao, Y., Liu, S., Shen, Q., Yan, K., Li, P. J., Xu, J., Yu, D. P., Steigerwald, M. L., Nuckolls, C., Liu, Z. F., & Guo, X. F. (2009). High-performance photoresponsive organic nanotransistors with single-layer graphenes as two-dimensional electrodes. Advanced Functional Materials, 19, 2743–2748. 25. Pang, S. P., Tsao, H. N., Feng, X. L., & Mullen, K. (2009). Patterned graphene electrodes from solution-processed graphite oxide films for organic field-effect transistors. Advanced Materi- als, 21, 3488–3491. 26. Hong, S. K., Kim, K. Y., Kim, T. Y., Kim, J. H., Park, S. W., Kim, J. H., & Cho, B. J. (2012). Electromagnetic interference shielding effectiveness of monolayer graphene. Nanotechnol- ogy, 23, 455704. 27. Li, N., Huang, Y., Du, F., He, X. B., Lin, X., Gao, H. J., Ma, Y. F., Li, F. F., Chen, Y. S., & Eklund, P. C. (2006). Electromagnetic interference (EMI) shielding of single-walled carbon nanotube epoxy composites. Nano Letters, 6, 1141–1145. 28. Chu, C. W., Ouyang, J., Tseng, H. H., & Yang, Y. (2005). Organic donor-acceptor system exhibiting electrical bistability for use in memory devices. Advanced Materials, 17, 1440–1443. 29. Joo, J., & Lee, C. Y. (2000). High frequency electromagnetic interference shielding response of mixtures and multilayer films based on conducting polymers. Journal of Applied Physics, 88, 513–518. 30. Sun, D. P., Zou, Q., Qian, G. Q., Sun, C., Jiang, W., & Li, F. S. (2013). Controlled synthesis of porous Fe3O4-decorated graphene with extraordinary electromagnetic wave absorption properties. Acta Mater, 61, 5829–5834. 202 5 Application of Graphene-Based Transparent Conductors (TCs)

31. Gupta, T. K., Singh, B. P., Singh, V. N., Teotia, S., Singh, A. P., Elizabeth, I., Dhakate, S. R., Dhawanc, S. K., & Mathur, R. B. (2014). MnO2 decorated graphene nanoribbons with superi- or permittivity and excellent microwave shielding properties. Journal of Materials Chemistry A, 2, 4256–4263. 32. Zhang, L., Alvarez, N. T., Zhang, M., Haase, M., Malik, R., Mast, D., & Shanov, V. (2015). Preparation and characterization of graphene paper for electromagnetic interference shielding. Carbon, 82, 353–359. 33. Zhang, H. B., Yan, Q., Zheng, W. G., He, Z. X., & Yu, Z. Z. (2011). Tough graphene-polymer microcellular foams for electromagnetic interference shielding. Acs Applied Materials & Interfaces, 3, 918–924. 34. Chen, Y., Wang, Y., Zhang, H.-B., Li, X., Gui, C.-X., & Yu, Z.-Z. (2015). Enhanced electromagnetic interference shielding efficiency of polystyrene/graphene composites with magnetic Fe3O4 nanoparticles. Carbon, 82, 67–76. 35. Yousefi, N., Sun, X. Y., Lin, X. Y., Shen, X., Jia, J. J., Zhang, B., Tang, B. Z., Chan, M. S., & Kim, J. K. (2014). Highly aligned graphene/polymer nanocomposites with excellent dielectric properties for high-performance electromagnetic interference shielding. Advanced Materials, 26, 5480–5487. 36. Chen, Z. P., Xu, C., Ma, C. Q., Ren, W. C., Cheng, H. M. (2013). Lightweight and flexible graphene foam composites for high-performance electromagnetic interference shielding. Advanced Materials, 25, 1296–1300. 37. Yan, H. G., Li, X. S., Chandra, B., Tulevski, G., Wu, Y. Q., Freitag, M., Zhu, W. J., Avouris, P., & Xia, F. N. (2012). Tunable infrared plasmonic devices using graphene/insulator stacks. Nature Nanotechnology, 7, 330–334. 38. Bae, J. J., Lim, S. C., Han, G. H., Jo, Y. W., Doung, D. L., Kim, E. S., Chae, S. J., Huy, T. Q., Luan, N. V., & Lee, Y. H. (2012). Heat dissipation of transparent graphene defoggers. Advanced Functional Materials, 22, 4819–4826. 39. Fan, G. Q., & Manson, J. R. (2009). Theory of direct scattering, trapping, and desorption in atom-surface collisions. Physical Review B, 79, 045424. 40. Munoz, R., & Gomez-Aleixandre, C. (2014). Fast and non-catalytic growth of transparent and conductive graphene-like carbon films on glass at low temperature. J Phys D Appl Phys, 47, 045305. 41. Sugimoto, T., Ono, K., Ando, A., Kurozumi, K., Hara, A., Morita, Y., & Miura, A. (2009). PVDF-driven flexible and transparent loudspeaker. Appl Acoust, 70, 1021–1028. 42. Bunch, J. S., van der Zande, A. M., Verbridge, S. S., Frank, I. W., Tanenbaum, D. M., Par- pia, J. M., Craighead, H. G., & McEuen, P. L. (2007). Electromechanical resonators from graphene sheets. Science, 315, 490–493. 43. Zhou, Q., & Zettl, A. (2013). Electrostatic graphene loudspeaker. Applied Physics Letters, 102, 223109. 44. Xu, S. C., Man, B. Y., Jiang, S. Z., Chen, C. S., Yang, C., Liu, M., Gao, X. G., Sun, Z. C., & Zhang, C. (2013). Flexible and transparent graphene-based loudspeakers. Applied Physics Letters, 102, 151902. 45. Sui, D., Huang, Y., Huang, L., Liang, J. J., Ma, Y. F., & Chen, Y. S. (2011). Flexible and transparent electrothermal film heaters based on graphene materials. Small, 7, 3186–3192. 46. Kim, J. H., Du Ahn, B., Kim, C. H., Jeon, K. A., Kang, H. S., & Lee, S. Y. (2008). Heat generation properties of Ga doped ZnO thin films prepared by rf-magnetron sputtering for transparent heaters. Thin Solid Films, 516, 1330–1333. 47. Kang, J., Kim, H., Kim, K. S., Lee, S. K., Bae, S., Ahn, J. H., Kim, Y. J., Choi, J. B., & Hong, B. H. (2011). High-performance graphene-based transparent flexible heaters. Nano Letters, 11, 5154–5158. 48. Wu, C. Z., Feng, J., Peng, L. L., Ni, Y., Liang, H. Y., He, L. H., & Xie, Y. (2011). Large- area graphene realizing ultrasensitive photothermal actuator with high transparency: new prototype robotic motions under infrared-light stimuli. Journal of Materials Chemistry, 21, 18584–18591. References 203

49. Jiang, H. Y., Kelch, S., & Lendlein, A. (2006). Polymers move in response to light. Advanced Materials, 18, 1471–1475. 50. Kim, U., Kang, J., Lee, C., Kwon, H. Y., Hwang, S., Moon, H., Koo, J. C., Nam, J. D., Hong, B. H., Choi, J. B., & Choi, H. R. (2013). A transparent and stretchable graphene-based actuator for tactile display. Nanotechnology, 24, 145501. 51. Hwang, T., Kwon, H. Y., Oh, J. S., Hong, J. P., Hong, S. C., Lee, Y., Choi, H. R., Kim, K. J., Bhuiya, M. H., & Nam, J. D. (2013). Transparent actuator made with few layer graphene electrode and dielectric elastomer, for variable focus lens. Applied Physics Letters, 103, 023106. 52. Lipomi, D. J., Vosgueritchian, M., Tee, B. C. K., Hellstrom, S. L., Lee, J. A., Fox, C. H., & Bao, Z. N. (2011). Skin-like pressure and strain sensors based on transparent elastic films of carbon nanotubes. Nature Nanotechnology, 6, 788–792. 53. Hwang, S. H., Ahn, H. J., Yoon, J. C., Jang, J. H., & Park, Y. B. (2013). Transparent graphene films with a tunable piezoresistive response. Journal of Materials Chemistry C, 1, 7208–7214. 54. Bae, S. H., Lee, Y., Sharma, B. K., Lee, H. J., Kim, J. H., & Ahn, J. H. (2013). Graphene-based transparent strain sensor. Carbon, 51, 236–242. 55. Wang, Y., Wang, L., Yang, T. T., Li, X., Zang, X. B., Zhu, M., Wang, K. L., Wu, D. H., & Zhu, H. W. (2014). Wearable and highly sensitive graphene strain sensors for human motion monitoring. Advanced Functional Materials, 24, 4666–4670. 56. Choi, H. J., Jung, S. M., Seo, J. M., Chang, D. W., Dai, L. M., & Baek, J. B. (2012). Graphene for energy conversion and storage in fuel cells and supercapacitors. Nano Energy, 1, 534–551. 57. Chen, T., Xue, Y. H., Roy, A. K., & Dai, L. M. (2014). Transparent and stretchable high-performance supercapacitors based on wrinkled graphene electrodes. Acs Nano, 8, 1039–1046. 58. Yu, A. P., Roes, I., Davies, A., Chen, Z. W. (2010). Ultrathin, transparent, and flexible graphene films for supercapacitor application. Applied Physics Letters, 96, 253105. Chapter 6 Conclusions and Perspectives

The exceptional transport properties combined with excellent optical transparency make graphene thin films an excellent alternative to indium tin oxide (ITO) films as transparent conductors (TCs). As almost all modern portable and household electronics are driven by optoelectronics, the use of graphene to produce TCs has tre- mendous implications. As extensively discussed in the preceding chapters, a myriad of studies have been made toward developing highly conductive, transparent, and durable thin films based on graphene or reduced graphene oxide (rGO), as well as their derivatives. To characterize the relative performance of transparency and sheet conductivity between transparent conductive films (TCFs) of different thicknesses and those prepared using different synthesis routes and materials, the term “direct current (DC) to optical conductivity ratio,” σσDC/ Op, [1] was used to compare their optoelectrical properties. A high σσDC/ Op ratio represents a high transmittance and a low sheet resistance [2]. It is generally agreed that the materials to potentially replace ITO should possess at least a sheet resistance lower than 100 Ω/sq with a transmittance higher than 90 % [1]. This means that the conductivity ratio should be higher than 35. Table 6.1 summarizes the optoelectrical performances taken from the representative literature for TCFs made by different techniques and using different graphene- based materials. The techniques used include chemical vapor deposition (CVD), electrophoretic deposition, Langmuir–Blodgett (L–B) deposition, transfer printing, spin coating, spray coating, and dip coating. The employed materials include CVD grown graphene and rGO sheets with different sizes, different surface functionalities, and hybridizations with other materials. The comparison provides an overview of the current, state of the art graphene-based TCFs. It is worth noting that the TCFs produced using CVD-grown graphene sheets in general have the highest conductivity ratios, that is, in the range of 3.5–11.1, among those prepared by different techniques. Although the TCFs synthesized using CVD-grown graphene could outperform those based on the traditional ITO films when combined with the doping treatment

Table 6.1 Comparison of optoelectrical properties of graphene- and graphene oxide (GO)-based TCFs collected from representative references. (Reprinted with permission from [2]. Copyright (2014) by Elsevier)

Fabrication Comments Rs (Ω/sq) T (%) σσDC/ OP References method/film type CVD Ni substrate 1000 90 3.48 Reina et al. [4] Ni substrate 280 80 5.70 Kim et al. [3] Ni substrate 1350 91 2.89 Wang et al. [7] Cu substrate 350 90 9.96 Li et al. [8] Cu substrate 200 85 11.1 Cai et al. [9] L–B deposition Expandable graphite, 1.5 × 105 92 0.03 Li et al. [10] exfoliated with DMF High temperature 4.0 × 106 95 0.0018 Kim et al. [11] annealing Electrophoretic GO films are electro- 4.59 × 104 83.8 0.044 Ishikawa et al. [12] depostion chemically reduced during EPD Transfer High temperature 6848 82 0.27 Wang et al. [13] printing annealing Chemical reduction 3.0 × 104 80 0.05 Liu et al. [14] Chemical reduction 3500 85 0.64 Mattevi et al. [15] + high temperature annealing Chemical reduction 1.0 × 105 65 0.008 Eda et al. [16] + high temperature annealing Chemical reduction 7.0 × 104 65 0.011 Eda et al. [17] + high temperature annealing Graphite, exfoliated in 2000 75 0.61 Green et al. [18] sodium cholate solution Spin coating High temperature 1000 80 1.60 Becerril et al. [19] annealing High temperature 800 82 2.26 Wu et al. [20] annealing High temperature 5000 80 0.32 Wu et al. [21] annealing High temperature 1750 70 0.55 Liang et al. [22] annealing Spray coating Graphite exfoliated with 5000 90 0.697 Blake et al. [23] DMF Dip coating Chemical reduction 1000 85 2.23 Zhao et al. [24] Chemical reduction 1.1 × 104 87 0.23 Zhu et al. [25] High temperature 1800 70 0.54 Wang et al. [26] annealing High temperature 8000 70 0.12 Zhao et al. [27] annealing Chemical reduction 11 × 106 95 0.0007 Kim et al. [28] + high temperature annealing Conclusions and Perspectives 207

Table 6.1 (continued)

Fabrication Comments Rs (Ω/sq) T (%) σσDC/ OP References method/film type

Doped rGO Reduced by NaBH4 and 9900 93.7 0.58 Shin et al. [29] doped by AuCl3 Reduced by hydrazine 2 × 104 65 0.04 Eda et al. [17] vapor and doped by

SOCl2 Thermal reduction and 1600 82 1.13 Zheng et al. [30]

SOBr2 doping Large size rGO Changing oxidation con- 840 78 1.70 Zhao et al. [24] ditions and HI reduction Edge-selective function- 3110 90 1.12 Bae et al. [31] alization of graphite and thermal reduction intercalation, thermal 500 90 6.97 Zheng et al. [32] shock exfoliation, chemical oxidation and thermal reduction Hybridization Spin coating of SWNT 300 96.4 34.0 Kim et al.[33] with CNTs dispersion followed by CVD grown of graphene Electrostatic 2.8 × 105 90 0.01 Kim et al. [28] self-assembly 8000 81 0.21 Hong et al. [34] Solution mixing 954 87 3.17 Huang et al. [35] 100 80 15.97 King et al. [36] 240 86 10.03 Tung et al. [37] Self-assembly at water 8300 72 0.13 Chen et al. [38] interface L–B technique 560 86 4.30 Zheng et al.[39] Hybridization Gold-decorated rGO and 26 83 74.25 Kholmanov et al. with NWs or Ag NW [40] nanogrids CVD grown graphene 16 91.1 246.9 Xu et al.[41] and Ag NW Metallic nanogrid and 20 90 174.24 Zhu et al. [42] graphene CNT carbon nanotube, CVD chemical vapor deposition, DMF N, N-Dimethylform, EPD electrophoretic deposition, GO graphene oxide, HI hydrogen iodine, L–B Langmuir–Blodgett, NWs nanowires, rGO reduced graphene oxide, SWNT single-walled carbon nanotubes with halogen elements [3–6], the complex and expensive transfer process of graphene limits their large scale applications. For example, the CVD method usually requires specific substrate materials, such as copper and nickel, which have to be etched away after the graphene growth. The high cost of the metal substrates, the ultrahigh vacuum conditions necessary for the CVD growth, and the extra steps 208 6 Conclusions and Perspectives required for graphene transfer to the target substrates significantly restrict the wide application of the CVD method [4]. In particular for the next generation flexible electrodes and circuits, the CVD-grown graphene needs roll-to-roll transfer equipment [5], making it highly dependent on initial infrastructural investments. In addition, several other challenges must be overcome before the industrial application of CVD grown graphene is realized. These challenges include (i) growth of graphene with large and controllable grain sizes, such as single-grain graphene with wafer scale size; (ii) CVD growth of graphene with controllable number of layers; (iii) reduction of the synthesis cost by low temperature growth or direct growth on any metal substrates without posttransfer processes; and (iv) developing easy, reliable, and scalable transfer processes. Facile and inexpensive methods for fabricating graphene-based TCFs are highly desirable to achieve the standard quality to replace ITO. Since graphene oxide (GO) dispersions are easy to be obtained and deposited on a substrate in a controllable manner, GO can provide a practical route toward the low-cost and scalable production of TCFs with tunable optoelectrical properties. GO-based TCFs are known to be cheaper and easier to scale up than the CVD-grown graphene, and are thus ap- pealing for applications where cost reduction is essential. Although many studies have been dedicated to the development of GO-based TCFs, their applications in real products are still in their infant stage. This is because the assessment of competition on the materials/manufacturing costs and relative functional performance between the TCFs produced using different materials and techniques are ongoing. There is still large room for improvements in the two major functional criteria. This book offers an overview of the research and development expended thus far on the synthesis and property measurements of GO-based TCFs with particular emphasis on the principles of different techniques devised for GO synthesis and deposition, as well as their influences on the corresponding optoelectrical properties of TCFs. To produce GO-based TCFs, GO or rGO sheets have been deposited via several well-established techniques, including spin or spray coating, transfer printing, dip coating, electrophoretic deposition, and L–B assembly, followed by further chemical reduction and/or thermal annealing. When comparing these approaches, the L–B assembly is found to be the only technique that can realize layer-by-layer (LbL) deposition of GO sheets, ensuring accurate control of the film thickness upon repeated depositions. The optoelectrical properties of the final products can be optimized by varying the deposition parameters, such as surface pressure and pulling speed. A few strategies have been successfully adopted as the postdeposition treatments to improve the optoelectrical properties of graphene or rGO films, and thus to fully exploit their fascinating properties. These strategies include: (i) doping with acids, especially diluted nitric acid; (ii) doping with nitrogen or halogenating agents, such as SOBr2, SOCl2 and AuCl3 solutions; (iii) use of large size GO sheets; (iv) hybridization with carbon nanotube (CNTs), metal nanowires (NWs), or nanogrids. Nota- ble examples with high conductivity ratios based on the techniques include the use of large-size GO sheets with a conductivity ratio of 7.0 [32], GO/CNT hybrid TCFs prepared by solution mixing with a conductivity ratio of 16.0 [36], CVD grown Conclusions and Perspectives 209 graphene/CNT hybrid film with a conductivity ratio of 34.0, rGO or graphene hybridized with Ag NWs with a conductivity ratio of 246.9 [41], and graphene hybridized with nanogrids with a conductivity ratio of 174.2 [42]. It should also be noted that apart from using large-size GOs and hybridizing with CNTs, metal NWs like Ag NWs or Cu NWs, or nanogrids that function as bridges between the isolated GO sheets, the conductivity of graphene-based TCFs can be further improved though simple doping treatments by immersing the TCFs into halogenating agents, like

AuCl3, SOCl2, or SOBr2. After the reduction of GO films, additional treatments have been applied sepa- rately or in a series of two or more processes. For example, it is found that the acid treatment helped remove the impurities present on graphene films as well as substrate surfaces, while the dopant functional groups arising from the halogenating agents and nitrogen increased the carrier densities in rGO films. It is also found that the TCFs made from ultralarge graphene oxide (UL-GO) sheets showed a much lower sheet resistance than that of small size GO sheets, by one to two orders of magnitude for a given transmittance of the film. Due to the increase in sheet area of GO from several to thousands of μm2, the reduction in the number of inter-sheet tunneling barriers in a continuous rGO film was responsible for this observation. Besides employing large size GO sheets, producing pure rGO sheets with a minimal number of defects is another efficient approach for improving the optoelectrical properties of TCFs. The purpose of avoiding defects is to eliminate any noncarbon atoms that may disrupt the perfect hexagonal pattern of graphene, which is essential for establishing solid conducting networks in rGO. Hybridizing rGO sheets with highly conducting 1D materials, such as CNTs, metal nanowires, or nanogrids is shown to be another particularly attractive option to improve the optoelectrical properties. The conducting 1D nanofillers or nanogrids can bridge isolated rGO sheets, where the extended conjugated network structure serve as fast electronic conducting channels. However, there are still some challenges in the process and use of these hybrid thin films. For example, weak bonds are formed between GO and CNTs because CNTs are usually physisorbed onto the hydrophilic surface of GO sheets, resulting in high contact resistance at the junctions. After high temperature reduction, the bonds between GO and CNTs can be even more deteriorated, necessitating the functionalization of these nanocarbons to allow covalent bonds to form between them, as proposed by molecular dynamics (MD) simulations [43, 44]. The rGO hybrids with metal nanowires and nanogrids also have potential issues that need to be addressed, such as high contact resistance, poor environmental stability, lack of scalable fabrication and uniformity, and electrical shorts due to the large aspect ratios of these hybrid fillers. In summary, doped metal oxides, such as ITO and fluorine tin oxide (FTO) , have well served for over 60 years for TCF applications. However, these materials suffer from several drawbacks including brittleness, lack of chemical stability, and high manufacturing cost. Emerged over the last 20 years, conducting polymers have potential to solve the issues inherent to doped metal oxides. Unfortunately, however, conducting polymers suffer from poor environmental stability and no- ticeable color. CNTs have shown excellent potential for commercial use in TCF 210 6 Conclusions and Perspectives applications, but the similar challenges, including the unacceptably large surface roughness and high cost, still exist. Metallic NWs and nanogrids also possess high electrical conductivities, however, the large film apertures require the combination with another more continuous, transparent electrode material. With about 10 years of short history of the discovery of graphene, extensive and in-depth research efforts have been directed toward understanding the characteristics and properties of graphene and its derivatives. Thanks to these strenuous efforts, graphene is now finding a number of useful applications as sensors and actuators, additives and free-standing electrode materials for energy conversion and storage devices, conducting and strong nanofillers to reinforce composites, and especially TCFs for various optoelectronic devices, like touch screens, liquid crystal displays (LCDs), organic photovoltaics (OPV) cells, and organic light emitting diodes (OLEDs). It is proven that graphene is an attractive alternative to currently dominant ITO for TCF applications. CVD grown graphene films show excellent electrical conductivity, but they cannot be produced in sufficient quantities. GO based TCFs have more defects and possess lower electrical conductivity, but they are capable of mass production with low cost. Nevertheless, many challenges are still ahead, such as the relatively low conductivity, before full-scale industrial applications of these materials are realized. If these desirable attributes are not to be compromised with others weaknesses, producing large-size GO sheets from natural graphene flakes and reduction to defect-free or near defect-free rGO sheets to achieve recovery of their inherent electrical conductivities are among the ongoing issues that need to be addressed. The relatively little understanding of their long-term functional durability to sustain the desired optoelectrical properties and structural stability under different service environments is another challenging issue that has seldom received attention. As the existing synthesis methods are becoming matured, future research efforts should also focus on developing novel processing technologies that were previously inconceivable and exploiting exciting properties of graphene-based TCFs in new applications, especially for a revolutionary new breed of transparent and flexible electronics [2].

References

1. De, S., & Coleman, J. N. (2010). Are there fundamental limitations on the sheet resistance and transmittance of thin graphene films? Acs Nano, 4(5), 2713–2720. 2. Zheng, Q., Li, Z., Yang, J., & Kim, J.-K. (2014). Graphene oxide based transparent conductive films. Progress in Materials Science, 64, 200–247. 3. Kim, K. S., Zhao, Y., Jang, H., Lee, S. Y., Kim, J. M., Kim, K. S., Ahn, J. H., Kim, P., Choi, J. Y., & Hong, B. H. (2009). Large-scale pattern growth of graphene films for stretchable transparent electrodes. Nature, 457(7230), 706–710. 4. Reina, A., Jia, X. T., Ho, J., Nezich, D., Son, H. B., Bulovic, V., Dresselhaus, M. S., & Kong, J. (2009). Layer area, few-layer graphene films on arbitrary substrates by chemical vapor deposition. Nano Letters, 9(1), 30–35. References 211

5. Bae, S., Kim, H., Lee, Y., Xu, X. F., Park, J. S., Zheng, Y., Balakrishnan, J., Lei, T., Kim, H. R., Song, Y. I., Kim, Y. J., Kim, K. S., Ozyilmaz, B., Ahn, J. H., Hong, B. H., & Iijima, S. (2010). Roll-to-roll production of 30-inch graphene films for transparent electrodes. Nature Nanotechnology, 5(8), 574–578. 6. De Arco, L. G., Zhang, Y., Schlenker, C. W., Ryu, K., Thompson, M. E., Zhou, C. W. (2010). Continuous, highly flexible, and transparent graphene films by chemical vapor deposition for organic photovoltaics. Acs Nano, 4(5), 2865–2873. 7. Wang, Y., Chen, X. H., Zhong, Y. L., Zhu, F. R., Loh, K. P. (2009). Large area, continuous, few-layered graphene as anodes in organic photovoltaic devices. Applied Physics Letters, 95(6), 063302. 8. Li, X. S., Zhu, Y. W., Cai, W. W., Borysiak, M., Han, B. Y., Chen, D., Piner, R. D., Colombo, L., & Ruoff, R. S. (2009). Transfer of large-area graphene films for high-performance transparent conductive electrodes. Nano Letters, 9(12), 4359–4363. 9. Cai, W. W., Zhu, Y. W., Li, X. S., Piner, R. D., & Ruoff, R. S. (2009). Large area few-layer graphene/graphite films as transparent thin conducting electrodes. Applied Physics Letters, 95(12), 123115. 10. Li, X. L., Zhang, G. Y., Bai, X. D., Sun, X. M., Wang, X. R., Wang, E., & Dai, H. J. (2008). Highly conducting graphene sheets and Langmuir-Blodgett films. Nature Nanotechnology, 3(9), 538–542. 11. Kim, F., Cote, L. J., Huang, J. X. (2010). Graphene oxide: durface activity and two-dimensional assembly. Advanced Materials, 22(17), 1954–1958. 12. Ishikawa, R., Ko, P. J., Kurokawa, Y., Konagai, M., Sandhu, A. (2012). Electrophoretic deposition of high quality transparent conductive graphene films on insulating glass substrates. Asia-Pacific Interdisciplinary Research Conference 2011 (Ap-Irc 2011), 352. 13. Wang, S. J., Geng, Y., Zheng, Q. B., & Kim, J. K. (2010). Fabrication of highly conducting and transparent graphene films. Carbon, 48(6), 1815–1823. 14. Liu, Y. Q., Gao, L., Sun, J., Wang, Y., & Zhang, J. (2009). Stable nafion-functionalized graphene dispersions for transparent conducting films. Nanotechnology, 20(46), 465605. 15. Mattevi, C., Eda, G., Agnoli, S., Miller, S., Mkhoyan, K. A., Celik, O., Mostrogiovanni, D., Granozzi, G., Garfunkel, E., & Chhowalla, M. (2009). Evolution of electrical, chemical, and structural properties of transparent and conducting chemically derived graphene thin films. Advanced Functional Materials, 19(16), 2577–2583. 16. Eda, G., Fanchini, G., & Chhowalla, M. (2008). Large-area ultrathin films of reduced graphene oxide as a transparent and flexible electronic material. Nature Nanotechnology, 3(5), 270–274. 17. Eda, G., Lin, Y. Y., Miller, S., Chen, C. W., Su, W. F., & Chhowalla, M. (2008). Transpar- ent and conducting electrodes for organic electronics from reduced graphene oxide. Applied Physics Letters, 92(23), 233305. 18. Green, A. A., & Hersam, M. C. (2009). Solution phase production of graphene with controlled thickness via density differentiation. Nano Letters, 9(12), 4031–4036. 19. Becerril, H. A., Mao, J., Liu, Z., Stoltenberg, R. M., Bao, Z., & Chen, Y. (2008). Evaluation of solution-processed reduced graphene oxide films as transparent conductors. Acs Nano, 2(3), 463–470. 20. Wu, J. B., Agrawal, M., Becerril, H. A., Bao, Z. N., Liu, Z. F., Chen, Y. S. & Peumans, P. (2010). Organic light-emitting diodes on solution-processed graphene transparent electrodes. Acs Nano, 4(1), 43–48. 21. Wu, J. B., Becerril, H. A., Bao, Z. N., Liu, Z. F., Chen, Y. S., & Peumans, P. (2008). Organic solar cells with solution-processed graphene transparent electrodes. Applied Physics Letters, 92(26), 263302. 22. Liang, Y. Y., Frisch, J., Zhi, L. J., Norouzi-Arasi, H., Feng, X. L., Rabe, J. P., Koch, N., & Mullen, K. (2009). Transparent, highly conductive graphene electrodes from acetylene- assisted thermolysis of graphite oxide sheets and nanographene molecules. Nanotechnology, 20(43), 434007. 212 6 Conclusions and Perspectives

23. Blake, P., Brimicombe, P. D., Nair, R. R., Booth, T. J., Jiang, D., Schedin, F., Ponomarenko, L. A., Morozov, S. V., Gleeson, H. F., Hill, E. W., Geim, A. K., & Novoselov, K. S. (2008). Graphene-based liquid crystal device. Nano Letters, 8(6), 1704–1708. 24. Zhao, J. P., Pei, S. F., Ren, W. C., Gao, L. B., & Cheng, H. M. (2010). Efficient preparation of large-area graphene oxide sheets for transparent conductive films. Acs Nano, 4(9), 5245–5252. 25. Zhu, Y. W., Cai, W. W., Piner, R. D., Velamakanni, A., & Ruoff, R. S. (2009). Transparent self-assembled films of reduced graphene oxide platelets. Applied Physics Letters, 95(10), 103104. 26. Wang, X., Zhi, L. J., & Mullen, K. (2008). Transparent, conductive graphene electrodes for dye-sensitized solar cells. Nano Letters, 8(1), 323–327. 27. Zhao, L., Zhao, L., Xu, Y. X., Qiu, T. F., Zhi, L. J., & Shi, G. Q. (2009). Polyaniline electrochromic devices with transparent graphene electrodes. Electrochimica Acta, 55(2), 491–497. 28. Kim, Y. K., & Min, D. H. (2009). Durable large-area thin films of graphene/carbon nanotube double layers as a transparent electrode. Langmuir, 25(19), 11302–11306. 29. Shin, H. J., Kim, K. K., Benayad, A., Yoon, S. M., Park, H. K., Jung, I. S., Jin, M. H., Jeong, H. K., Kim, J. M., Choi, J. Y., & Lee, Y. H. (2009). Efficient reduction of graphite oxide by sodium borohydrilde and its effect on electrical conductance. Advanced Functional Materi- als, 19(12), 1987–1992. 30. Zheng, Q. B., Gudarzi, M. M., Wang, S. J., Geng, Y., Li, Z. G., & Kim, J. K. (2011). Improved electrical and optical characteristics of transparent graphene thin films produced by acid and doping treatments. Carbon, 49(9), 2905–2916. 31. Bae, S. Y., Jeon, I. Y., Yang, J., Park, N., Shin, H. S., Park, S., Ruoff, R. S., Dai, L. M., Baek, J. B. (2011). Large-area graphene films by simple solution casting of edge-selectively functionalized graphite. Acs Nano, 5(6), 4974–4980. 32. Zheng, Q. B., Ip, W. H., Lin, X. Y., Yousefi, N., Yeung, K. K., Li, Z. G., Kim, J. K. (2011). Transparent conductive films consisting of ultra large graphene sheets produced by Lang- muir-Blodgett assembly. Acs Nano, 5(7), 6039–6051. 33. Kim, S. H., Song, W., Jung, M. W., Kang, M. A., Kim, K., Chang, S. J., Lee, S. S., Lim, J., Hwang, J., Myung, S., & An, K. S. (2014). Carbon nanotube and graphene hybrid thin film for transparent electrodes and field effect transistors. Advanced Materials, 26(25), 4247– 4252. 34. Hong, T. K., Lee, D. W., Choi, H. J., Shin, H. S., & Kim, B. S. (2010). Transparent, flexible conducting hybrid multi layer thin films of multiwalled carbon nanotubes with graphene nanosheets. Acs Nano, 4(7), 3861–3868. 35 Huang, J. H., Fang, J. H., Liu, C. C., & Chu, C. W. (2011). Effective work function modula- tion of graphene/carbon nanotube composite films as transparent cathodes for organic optoelectronics. Acs Nano, 5(8), 6262–6271. 36. King, P. J., Khan, U., Lotya, M., De, S., & Coleman, J. N. (2010). Improvement of transparent conducting nanotube films by addition of small quantities of graphene. Acs Nano, 4(7), 4238–4246. 37. Tung, V. C., Chen, L. M., Allen, M. J., Wassei, J. K., Nelson, K., Kaner, R. B., & Yang, Y. (2099). Low-temperature solution processing of graphene-carbon nanotube hybrid materials for high-performance transparent conductors. Nano Letters, 9(5), 1949–1955. 38. Chen, F. M., Liu, S. B., Shen, J. M., Wei, L., Liu, A. D., Chan-Park, M. B., & Chen, Y. (2011). Ethanol-assisted graphene oxide-based thin film formation at pentane–water interface. Lang- muir, 27(15), 9174–9181. 39. Zheng, Q., Zhang, B., Lin, X., Shen, X., Yousefi, N., Huang, Z.-D., Li, Z., & Kim, J.-K. (2012). Highly transparent and conducting ultralarge graphene oxide/single-walled carbon nanotube hybrid films produced by Langmuir–Blodgett assembly. Journal of Materials Chemistry, 22(48), 25072–25082. References 213

40. Kholmanov, I. N., Stoller, M. D., Edgeworth, J., Lee, W. H., Li, H. F., Lee, J. H., Barnhart, C., Potts, J. R., Piner, R., Akinwande, D., Barrick, J. E., & Ruoff, R. S. (2012). Nanostructured hybrid transparent conductive films with antibacterial properties. Acs Nano, 6(6), 5157–5163. 41. Xu, S. C., Man, B. Y., Jiang, S. Z., Liu, M., Yang, C., Chen, C. S., & Zhang, C. (2014). Graphene-silver nanowire hybrid films as electrodes for transparent and flexible loudspeakers. Crystengcomm, 16(17), 3532–3539. 42. Zhu, Y., Sun, Z. Z., Yan, Z., Jin, Z., & Tour, J. M. (2011). Rational design of hybrid graphene films for high-performance transparent electrodes. Acs Nano, 5(8), 6472–6479. 43. Dimitrakakis, G. K., Tylianakis, E., & Froudakis, G. E. (2008). Pillared graphene: a new 3-D network nanostructure for enhanced hydrogen storage. Nano Letters, 8(10), 3166–3170. 44. Varshney, V., Patnaik, S. S., Roy, A. K., Froudakis, G., & Farmer, B. L. (2010). Modeling of ehermal transport in pillared-graphene architectures. Acs Nano, 4(2), 1153–1161. Index

Symbols B π-conjugated domains, 168 Bending energy, 167 Bending motion, 196 A Binary compounds, 7 Abrasion resistance, 4 Binding energy, 127, 133 Absorbance loss (AL), 189 Biosensors, 167, 168 Absorption Body monitoring, 197 and desorption, 124 Boiling point, 148 of electromagnetic radiation, 187 Bromination, 124, 125 optical Bromination duration, 124, 125 of polymers, 9 Bubble deposition method (BDM), 138, 147 AB stacking, 105 Buckling, 163 Accumulation, 114, 129 Bulk-heterojunction (BHJ), 184 charge, 129 Active electrodes, 200 C Acyl chloride, 126, 172 Capacitance-voltage (CV), 200 Adsorption energy, 190 Capacitive type, 179 Aluminum-doped zinc oxide (AZO), 7, 8 Capillary force, 163, 166 Ammonium persulfate, 98, 153 Carbonaceous byproducts (CB), 19 Amorphous substrates, 7 Carbon nanoscrolls (CNSs), 167 Amphiphilic Carbon nanotubes (CNTs), 4, 18, 19 GO sheets, 115, 146, 170 chemical structure of, 127 UL-GO, 148 doping of, 126 Anhydrous hydrazine, 145 hybridization with, 138, 146, 208, 209 Anisotropy energy, 188 Carboxyl groups, 123 Aqueous solution, 95, 96, 138, 146, 156 Carrier concentration, 127, 128, 134 Arithmetical mean roughness, 168 Carrier type, 134 Aromatic ring, 14, 129 Catalyst template, 188 Ascorbic acid, 119 Catalytic activity, 105 Aspect ratio, 17, 18, 111, 209 Cathode ray tubes (CRT), 4 Atomic concentration, 124, 129 Cellulose/ester membrane, 114 Atomic force microscopy (AFM), 101 Centrifugal force, 110 Audio frequency field, 192 Charge accumulation, 129

Charge distribution, 129 Degree of absorption, 171 Charged particles, 109 Degree of polymerization, 14 Charge flow, 129 Degree of wrinkling, 163 Charge redistribution, 131 Deionized (DI), 138 Charge-transfer complex, 127, 133 Density, 161 Chemical doping, 123, 145, 170 and alignment, 138 n-type, 123 of GO sheets, 115 of carbon materials, 124 Deposition rate, 109, 110 of graphene, 127 Deposition time, 109 p-type, 123 Desorption treatment, 20, 126 of Br atoms into graphite, 124 Chemical durability, 4, 179 Detector, 188 Chemically doped, reduced ultra-large Dichloroethane (DCE), 19, 148 graphene oxide (C-rUL-GO), 172 Diethylene glycol (DEG), 15 Chemical reduction, 208 Digital single lens reflex (DSLR), 196 Chemical vapor deposition (CVD), 19, 158 Dimethyl sufoxide (DMSO), 15 Chlorocarbon compound, 131 Dip coating, 95, 105, 112, 113, 156, 158, 205, Chlorosulfonic acid (CSA), 19 208 Close-packed, 116, 117, 149, 161, 163, 165, of UL-GO, 159, 161 167 Direct current (DC), 109 Coating density, 153 Disorder, 110, 196 Coating fluid, 117 Dispersive adhesion, 98 Cold cathode fluorescent lamp (CCFL), 2 Doping effect, 128 Compression force, 167 of SOCl2, 129 Computer monitors, 4 Driving force, 146 Concentrated graphene oxide wrinkles Drop casting, 116 (CGOW), 164–167 and spin coating, 117 Conducting nanofillers, 124, 138 Dry transfer, 98, 152, 156 Conducting paths, 126 Durability, 1, 4 Conductive networks, 124, 138 long-term functional, 210 Contact angle, 170 Contact barrier, 152 E Contaminant residues, 99 Edge-selective functionalization, 208 Contrast ratio, 181 Elastic energy, 167 Copper nanowire (Cu NW), 17, 18, 151, 156, Electrical conductivity, 5, 9, 14, 19, 131, 136, 157, 209 138, 148, 151, 173, 210 Copper phthalocyanine (CuPc), 187 and optical transmittance, 171 Core level spectrum, 131 and optical transparency, 6 Corrosion devices, 15 and transmittance, 136 Corrosive vapor, 96 and transparency of graphene films, 131 Covalent bonding, 131 of graphite nanoplatelets, 124 Covalent functionalization, 19 of PPy thin films, 14 Cracking, 96 of the CNT TCFs, 126 Cu foils, 158 Electric field, 109, 181, 190 Current collector, 200 Electrochemical properties, 18 Current leakage, 9 Electrochromic mirrors, 2, 4 Curved window surface, 194 Electromagnetic interference shielding, 189 Electron D acceptor, 123, 128, 131 DC to optical conductivity ratio, 19, 205 beam evaporation, 6 Decomposition, 136, 173 donor, 123 Defect, 209 scattering, 17 structural, 9 Electronegativity, 131 Defogger, 190, 192 Electronic polarization, 188 Cr/glass, 190 Electronic structure, 128 Index 217

Electrophoretic deposition, 95, 105, 109, 110, Graphite nanoplatelet (GNP), 124 205, 208 Graphitic materials, 127 Electrostatic repulsion, 161, 168 Graphitization, 110, 134, 150, 169, 172 Electrostatic self-assembly, 138, 146 Gravity, 166 Energy efficiency, 1 Energy storage, 198 H Epitaxial growth, 19 Halogenating agents, 127, 128, 208, 209 Equilibrium structure, 129 Harmonic distortion, 192 Etchant solution, 101 Heating rate, 193 Etching bath, 95 Height profile, 118, 187 Etching method, 95, 97 Highly ordered pyrolytic graphite (HOPG), Etching rate, 96 101 High temperature annealing, 6, 110, 172 F Human interface devices, 196 Face-to-face manner, 168 Human motion monitoring, 196, 197 Fatigue detection, 197 Human skin deformation, 196 Feature resolution, 114 Hybridization, 205, 208 Fermi level, 128 Hydrazine, 110 Few-layer-graphene (FLG), 196 Hydrocarbon gas, 105 Field effect transistors, 168 Hydrogen bonds, 105, 163 Flat panel display (FDP), 2 Hydrogen floride (HF), 152 Flexibility, 9, 103, 114, 153, 157, 181, 188, Hydrogen storage, 167, 168 194, 200 Hydrophilic, 158, 170, 209 Flexible Hydrophobic, 14, 170 EMI, 188 Hydroxyl groups, 168 film heaters, 193 GO sheets, 110 I polyethylene naphthalate (PEN), 152 Impedance of free space, 20 polymeric materials, 118 Indium-doped cadmium-oxide (ICO), 7 steel or plastic substrates, 1 Indium-free oxides, 8 substrates, 145 Indium tin oxide (ITO), 4, 5 TCFs, 4 Industry applications, 19 touch screen-based graphene films, 180 Infrared heat lamp, 118 Fluorine tin oxide (FTO), 209 Infrared scanner, 194 Fluoroalkyl trichlorosilane (FTS), 9, 13 Ink droplets, 118 Free energy, 167 Inkjet printing, 105, 117 Front electrodes, 1 In-plane compression, 163 Fullerene, 19, 184 Input voltage, 194, 195 Functional glasses, 190 Insulator Functional groups, 10, 123, 134, 148, 169, stacks, 188 173, 209 Intelligence devices, 194 Functionalization, 209 Intensity ratio, 125 Interfacial polarization, 188 G Interlayer distance, 105, 167 Gaseous product, 96 Intermediate transfer processes, 115 Glass slide, 111, 182, 200 Intersheet junctions, 123, 124, 158 Grain boundaries, 105 Invisible security circuits, 4 Grain size, 105, 208 Ion diffusion, 9 Graphene, 4, 19, 20, 95 Ionic bond, 131, 133 Graphene nanoribbons (GNR), 188 Ionic bromine, 133 Graphene oxide wrinkles (GOW), 163, 166 Ionic liquid, 15 Graphene patterns, 98, 100–102 Isopropanol (IPA), 15 Graphene woven fabrics (GWFs), 196 Isothermal curve, 115, 116, 162 Graphite intercalation compounds (GIC), 124 Isotopic peaks, 125 218 Index

L Neutral atoms, 129 Laboratory ovens, 4 Nitric acid, 96, 127, 128, 136, 208 Langmuir-Blodgett (L-B), 114 Nitrogen flow, 110, 126, 131 Large-scale manufacturing, 9 N-Methyl-2-pyrrolidone (NMP), 148 Lateral size, 160 Non-covalent functionalization, 18, 19, 148 Layer-by-layer (LbL), 113, 114 Nonpolar environments, 9 Layered architecture, 115 Nonsolvent, 146 L-B trough, 115, 116 Nucleophilic substitution, 131, 133 Light emitting diode (LED), 181 Light scattering, 136 O Liquid/air interface, 159 O-dichlorobenzene (DCB), 148 Liquid crystal display (LCD), 1, 181 Opacity f, 181 Liquid phase exfoliation, 118 Optical microscope, 99, 181 Low-emissivity glass windows, 1 Optical reflectance, 6 Optical transmittance, 6, 19, 111, 146, 151, M 153, 159 Machine interface, 181 and electrical conductivity, 171 Magnetron sputtering (MS), 8 Optoelectronic devices, 1, 4, 6, 9, 210 Mass density, 192 Organic light emitting diodes (OLEDs), 1, 4, Mass production, 116, 158, 210 15, 183 Mechanical bending, 118, 193 Organic photovoltaics (OPVs), 4 Mechanical cleavage, 158 Organic solvents, 109, 158 Mechanical properties, 9, 183, 194 Outdoor displays, 192 Medical tape, 196, 198 Overlapped, 162, 163, 165 Memory devices, 15 Oxidizing agent, 15 Metal nanogrids, 9, 17, 157 Oxidizing etchant, 96 Metal nanowires, 17, 20, 209 Oxygenated groups, 123, 170 Meyer rod, 117 Oxygen-containing functional groups, 110 Micro-cracks, 196 Mild heat, 103 P Modulator, 188 Packing density, 117, 163 Molar mass, 125 Patterned structures, 114 Molecular beam epitaxy, 6 Peak to peak roughness, 168, 170 Molecular orientation, 114 Percolation, 153 Molecular structure, 9 Permeation rate, 114 Motion signals, 197 Phase transition, 148, 161 Multi-component oxides, 7, 8 Photolithography, 100, 102 Multifunctional sensors, 196 Photons, 17 Multi-layer graphene (MLG), 183 Photoresist method, 95, 101 Multiple quantum wells (MQW), 183 Photovoltaic cell, 184, 185 Multi-walled carbon nanotube (MWCNT), 9 Piezoelectric film, 192 N Piezoresistive sensors, 197 Nanocomposites, 188, 190 Planar graphene, 187, 199 Nanogenerator, 152 Plasma display panel (PDP), 2 Nanomechanical devices, 167 Plasma etching, 102, 166, 187 Nanoparticles (NPs), 6, 136 Plasma wavelength, 1, 2 Nanorods, 152 Plastic substrates, 1, 9, 110 Nanowires (NWs), 9 Platinum plate, 115 Near-infrared region, 9 Polar groups, 170 Negatively charged Polar molecules, 9 MWNTs, 147 Poly(2,5-dioctyloxy-1,4-phenylene-alt-2,5- rGO sheets, 113, 146 thienylene) (POPT), 148 Index 219

Poly(3,4-ethylenedioxythiophene) (PEDOT), Screen printing, 6 9, 12, 15 Secondary doping, 15 Poly(4-vinylphenol) (PVP), 98 Self-assembly, 9 Polyaniline (PANI), 9, 12, 14 Self-polymerization, 9 Polydimethylsiloxane (PDMS), 97, 198, 200 Self-release layer (SRL), 100 Polyethyleneimine (PEI), 146 Semiconductor, 185 Polyethylene terephthalate (PET), 98 Sensitivity, 196 Poly(methyl methacrylate) (PMMA), 97 Sensors, 1, 196, 210 Poly(m-phenylene vinylene-co-2,5- Sheet resistance, 2, 4, 6, 17, 19, 20, 110, 113, dioctyloxy-pphenylene vinylene) 118, 128, 134, 136, 146 (PmPV), 148 Shielding effectiveness (SE), 189 Poly(para-phenylene vinylene) (PPV), 9, 10, Silicon dioxide, 187 12 Silicone, 97, 98, 196 Polypyrrole (PPy), 9, 12, 14 Silver nanowire (Ag NW), 208, 209 Polystyrene sulfonate) (PSS), 15, 184 Single crystal, 158 Polythiophene (PT), 9, 12 Single-walled carbon nanotube (SWCNT), 18 Positively charged, 113, 146, 147 Small graphene oxide (S-GO), 160, 161, 163, Post-transfer process, 95 164 Precipitate, 96 Smart windows, 2, 9 Printable Sodium cholate (SC), 19 electronics, 4 Sodium dodecyl benzene sulfonate (SDBS), Processability, 10 19 Processing condition, 11, 173 Sodium dodecyl sulfate (SDS), 19, 126 Projected capacitance, 179 Soft pressure, 103 Pulling speed, 115, 163–166, 168, 208 Soft transfer printing, 114, 115 Pulsed laser deposition (PLD), 6, 8 Solar cell, 1, 4, 9, 15, 184 Pyrene, 129 Solar spectrum, 1 Sol-gel techniques, 6 Q Solid solution, 5 Quartz substrates, 104, 113 Solid supports, 114 Solubility, 15, 105 R Solvent-water interface, 138, 146 Radiation energy, 187 Sound pressure level (SPL), 192 Radio frequency interference (RFI), 6 Source-drain electrode, 99 sp2 hybridization, 123 Radio frequency (RF), 6 3 Raman spectroscopy, 134 sp hybridization, 129 Reactive ion etching, 196 Specific capacitance, 199, 200 Rear-projection (RP), 2 Spin coating, 205, 208 Rechargeable batteries, 15 Spin–orbit coupling, 131 Reflectance loss (RL), 189 Spray coating, 19, 95, 105, 111, 205, 208 Resistive-type touch screen, 179 Stability, 8, 10, 123, 136, 137, 151 Robotics, 197 Stamping method, 95, 98, 99 Robustness, 179, 194 Strain boundaries, 17 Rod coating, 95, 105, 117 Strain level, 196 Rollable screen, 194 Strain sensor, 196, 197 Roll-to-roll transfer, 95, 103, 105, 208 Substitutional doping, 123, 127 Roll-up displays, 4 Sulfuric acid, 15, 18, 145, 148 Root mean square roughness, 168 Supercapacitors, 167, 198, 199, 200 Rotating speed, 110 Surface acoustic wave, 179 Surface capacitance, 179 Surface coverage, 105, 117 S Surface morphology, 105, 118, 168 Scaling law, 168 Surface pressure, 115, 148, 149, 161, 163, Schottky junction, 184 165, 208 220 Index

Surface pressure-area isotherm, 148, 161 Transparent sensors, 196 Surface roughness, 6, 150, 151, 168, 169, 210 Transparent supercapacitors, 198–200 Surface tension, 117 Triton X-100, 19 Surface transfer doping, 123, 127 Surfactants, 18, 19 U Ultralarge graphene oxide (UL-GO), 159, 163, T 172 Teflon barriers, 116 Ultrasonication, 124 Television tubes, 4 Uniaxial tensile strain, 196 Temperature distribution, 193 Tensile strain, 181, 196, 199 V Thermal reduction, 150 Vacuum arc plasma deposition (VAPE), 8 Thermal release method, 95, 101 Vacuum filtration, 114, 198 Thermal releasing tapes, 101 Valence band, 128 Thermal stability, 1, 2 Van der Pauw method, 134 Thionyl bromide (SOBr2), 19, 126, 127, 131, van der Waals force, 105, 148, 168, 196 133, 208 Visible range, 6, 9 Thionyl chloride (SOCl2), 19 Vitamin C, 118 Time-of-flight secondary ion mass spectrometry (ToF-SIMS), 124 W Titanium nitride, 1 Water-pentane interface, 146 Total harmonic distortion (THD), 192 Water resistant coating, 111 Touch screen, 1, 179, 210 Wearable optoelectronic systems, 198 Toxicity, 7, 9, 179 Wear resistance, 179 Transfer printing, 19, 105, 114, 205, 208 Width-at-half-maximum (WHM), 124 of GO films, 114 Work function, 184, 185 Transistor, 179, 184, 187 Work of adhesion, 98 Transparent actuators, 194 Wrinkled graphene, 199, 200 Transparent conducting oxides (TCOs), 5, 7, 9 Wrinkles, 105, 117 Transparent conducting polymers, 9 Transparent conductive films (TCFs), 1, 4, 20, 138, 153 X Transparent conductors (TCs), 2 Xerographic copiers, 4 Transparent heaters, 193 X-ray diffraction (XRD), 124 Transparent loudspeakers, 192