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The Pennsylvania State University

The Graduate School

College of Earth and Mineral Sciences

SCALABLE SYNTHESIS OF GRAPHENE BASED

HETEROSTRUCTURES AND THEIR USE IN ENERGY

SENSING, CONVERSION AND STORAGE

A Dissertation in

Material Science and Engineering

Ganesh Rahul Bhimanapati

 2017 Ganesh Rahul Bhimanapati

Submitted in Partial Fulfillment of the Requirements for the Degree of

Doctor of Philosophy

May 2017

The dissertation of Ganesh Rahul Bhimanapati was reviewed and approved* by the following:

Joshua A Robinson Associate Professor in Material Science and Engineering Dissertation Advisor Chair of Committee

James Adair Professor in Material Science and Engineering, , Biomedical Engineering and Pharmacology

Tom Mallouk Professor in Chemistry, Biochemistry and molecular biology, Physics and Engineering science and Mechanics.

Mauricio Terrones Professor in Physics, Chemistry and Material Science & Engineering

Susan Mohney Graduate Program Head, Professor in Material Science & Engineering

*Signatures are on file in the Graduate School

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ABSTRACT

2D materials are a unique class of materials system which has spread across the entire spectrum of materials including semi-metallic graphene to insulating boron nitride. Since graphene there has been many other 2D material systems (such as boron nitride (hBN), transition metal dichalcogenides (TMDs)) that provide a wider array of unique chemistries and properties to explore for applications specifically in optoelectronics, mechanical and energy applications.

Specifically tailored heterostructures can be made which can retain the character of single-atom thick sheets while having an entirely different optical and mechanical properties compared to the parent materials. In the current work, heterostructures based on graphene, hBN and TMDs have been made, which were used to study the fundamental process-property relations and their use in energy conversion and storage have been studied.

The first part of this dissertation focusses on scalable approach for liquid phase exfoliation of graphene oxide (GO) and hBN (Chapter 2). The current work successfully shows an exfoliation efficiency of ~25% monolayer material for hBN, which was not previously achieved. These exfoliated materials were further mixed in the liquid environment to form a new heterostructure BCON (Chapter 3). This newly formed heterostructure was studied in detail for its process-property relations. At pH 4-8, BCON was highly stable and can be dried to form paper or ribbon like material. New bonds were observed in BCON which could be linked to the GO linkage at the nitrogen sites of the hBN. This free standing BCON was tested under various radiation sources like x-rays, alpha, beta, gamma sources and heavy ion like Ar particles and was found that it is very robust to radiation (Chapter 5). By understanding the chemistry, stability and properties of these materials, this could lay a foundation in using these materials for integration in iv conductive and insulating ink development, polymer composite development to improve the thermal and mechanical properties.

Another major focus of this dissertation work is combining TMDs and graphene for energy applications specifically hydrogen evolution reactions (HERs) and Lithium ion batteries

(LiBs). TMD’s specifically MoS2 and WSe2 were grown on graphite paper using powder vaporization and metal organic chemical vapor deposition (MOCVD) (Chapter 4). Control over the architecture of the MoS2 and WSe2 was achieved by varying the precursor concentration and pressure, which was observed by using scanning electron microscopy. These samples were further characterized using cross-sectional transmission electron microscopy, x-ray photoelectron spectroscopy and Raman microscopy confirming the high quality of the material that was grown.

The MoS2/graphite flowers were tested for hydrogen evolution reactions and were found that they are highly active for catalysis and by modifying the surface using simple UV-Ozone treatments, this activity can be increased by 4x (reducing the Tafel slope from 185 to 54 mV/Dec). Similar performance was observed for WSe2/Graphite heterostructure where the tiny 100 nm vertical flakes on graphite paper showed one of the lowest reported Tafel slope of 64 mV/Dec (Chapter

6). MoS2/Graphite was further tested for lithium ion batteries and was found that it had a higher cyclic capacity of 750 mV/Dec. This enhanced stability and performance for energy applications was achieved because of the direct growth technique on graphite. Hence this technique could be used as a scalable alternative to make anodes for lithium ion batteries.

TABLE OF CONTENTS

List of Figures ...... ix List of Tables ...... xx List of Symbols/acronyms ...... xxi Acknowledgements ...... xxvii

Chapter 1 INTRODUCTION/ LITERATURE REVIEW ...... 1 What is a 2D Material? ...... 1 Graphene? ...... 1 History and Structure evolution ...... 1 Beyond Graphene Materials: Introduction ...... 4 Inorganic 2D Materials ...... 7 Introduction To Boron Nitride ...... 7 Structure And Properties Of 2D Boron Nitride ...... 8 Synthesis Of BNNS ...... 13 Mechanical Exfoliation ...... 13 Solvent assisted ultra-sonication ...... 17 Acid Exfoliation ...... 20 Chemical Functionalization of h-BN ...... 20 Single And Few-Layer h-BN Via Chemical Vapor Deposition ...... 25 Transition Metal Dichalcogenides ...... 33 Large-Area, Morphology-Controlled Synthesis of TMDs ...... 35 Synthesis of 2D-TMD Heterostructures ...... 37 TMD’s and Graphene based materials for Hydrogen evolution reactions and Lithium ion batteries ...... 39 Current Research Focus ...... 42 vi

Chapter 2 Exfoliation and Functionalization of Graphene oxide and Hexagonal Boron Nitride ...... 45 Introduction ...... 45 Exfoliation of Graphene Oxide (GO) ...... 46 Synthesis Procedure ...... 46 Structural and Chemical characterization of GO ...... 48 Exfoliation of Hexagonal Boron Nitride (hBN) ...... 49 Synthesis Procedure ...... 49 Structural Characterization of hBN ...... 50 XRD analysis ...... 55 Thickness Characterization of hBN ...... 56 Chemical Functionalization Study of exfoliated hBN ...... 58 X-ray Photoelectron Spectroscopic analysis ...... 58 Fourier transform Infra-red spectroscopy analysis ...... 62

Chapter 3 Liquid Phase synthesis of Graphene heterostructures ...... 64 Introduction ...... 64 Synthesis of BCON: Structure-property relations ...... 65 pH dependence with time ...... 69 Structural Characterization Of BCON ...... 71 Auger electron spectroscopic analysis of BCON ...... 73 Chemical analysis of BCON ...... 75 X-Ray photoelectron spectroscopy studies ...... 75 Raman Study on BCON ...... 77 FTIR Study of varying composition of GO:hBN in BCON ...... 78

Synthesis of Reduced Graphene oxide/ MoS2 composites ...... 79

Chapter 4 Hybrid synthesis of 2D heterostructures ...... 82 Introduction ...... 82 vii

Integration of graphene and graphene oxide as substrates ...... 83 Graphite paper characterization ...... 84

Growth of MoS2 via powder vaporization ...... 86 Precursor Control ...... 86 Effect of Pressure ...... 93 Growth on Various substrates ...... 93

Characterization of MoS2 flowers on graphite paper ...... 95 Raman Study of MoS2 flowers/ graphite paper ...... 95 AES and XPS analysis of MoS2 flowers/graphite paper ...... 97 Cross-Sectional TEM analysis of MoS2 flowers/Graphite paper ...... 99

Growth of WSe2/ Graphite based substrates via MOCVD ...... 101 Effect of Pressure on the growth ...... 102 Effect of Precursor residence time ...... 103 Effect of Se:W ratio on the growth ...... 105 Effect of substrate defects on growth ...... 106

Improved Stability of WSe2 on GP ...... 108

Chapter 5 Radiation interaction of graphene heterostructures ...... 110 Introduction ...... 110 In-Situ XPS for Understanding the radiation interaction ...... 111 Interaction mechanism simulation using Stopping Range in Matter (SRIM) ...... 113 Al k-α X-Ray Interaction with BCON...... 115 Effect of The Gamma Particles On BCON ...... 121 Alpha Irradiation of BCON (Boro-carbon-Oxy-nitride) ...... 124 In-Situ Measurements With Beta Source ...... 128 Neutron Interaction with BCON ...... 129 Heavy Ion (Ar ion) interaction with BCON ...... 132 Environmental Impacts on X-ray interaction with GO ...... 134 viii

Chapter 6 Applications of 2D-TMD heterostructures ...... 137

MoS2/Graphite as Super hydrophobic surface ...... 137 Surface Wettability Study ...... 137 Electrochemical impedance spectroscopy (EIS) ...... 141

Hydrogen Evolution Reaction (HERs) for MoS2/Graphite heterostructure ...... 143 HER Setup ...... 143

HER Performance for UV-Ozone treated MoS2 samples ...... 145

Hydrogen Evolution Reactions (HERs) for WSe2 based materials ...... 149

UV-Ozone Treatment on WSe2/Graphite samples ...... 151 Alloying effect on HERs ...... 153

Chapter 7 Summary and Future Work ...... 155 Summary...... 155 Future work ...... 160 Radiation study of 2D materials ...... 160 Alloying Effects of TMDs on HERs ...... 162

Lithium ion battery performance of Vertical MoS2/Graphite ...... 163

Bibliography ...... 168

LIST OF FIGURES

Figure 1-1. Image showing the history of graphene evolution over the years...... 3

Figure 1-2. Publication trends in 2D materials beyond graphene ...... 6

Figure 1-3. Boron nitride allotropes (a) few layer hBN, (b) mono layer h-BN nanosheet or BNNS, (c) Boron nitride nano ribbons (BNNR) with different edge termination leading to Zigzag B-edge and armchair edge structures, (d) boron nitride nano tubes (BNNT) and (e) BN fullerenes...... 8

Figure 1-4. (a) Low magnification TEM image of an exfoliated BNNS, (b) HR-TEM image showing the number of layers at the folded edges, (c) (d) AFM height map of CVD grown BNNS showing the monolayer thickness of 0.4 nm, (e) Chemically exfoliated BNNS showing a height of 1 nm, (f) White light microscopy image of exfoliated h-BN flake showing the optical contrast on 90 nm SiO2/Si, (g) Improved optical image of h-BN flake using a 590 nm light source showing the different contrast between layers, (h) Raman spectra for BNNS to few layer h-BN comparing the difference in the peak width vs number of layers and (I) Peak position variation as observed for monolayer to bulk BNNS ...... 11

Figure 1-5. (a) Phase diagram of h-BN showing the stability of h-BN vs cBN at various temperatures and pressures., (b) Close up image of the phase transition and calculations from solozenko(dotted line)...... 12

Figure 1-6. (a) (i), (iii) Wet ball mill process used for the mechanical exfoliation process1, (ii), (iv) SEM images showing the exfoliation process mechanism, (b) (i) Mechanism of vortex fluidic process2 which was operated at 8000 rpm. This process works on centrifugal force for exfoliation, which can be observed in the Figure and (ii) Image showing the uniform dispersion before and after the process...... 15

Figure 1-7. (a) Flow diagram showing the micro fluidization process used for exfoliation, (b) TEM image showing the presence of ultrathin BNNS achieved from this process, (c) Elemental mapping and lattice spacing obtained in high resolution TEM confirming the presence of h-BN. (d) Mechanism of dry exfoliation process using molten hydroxides (NaOH, KOH), (e) and (f) SEM images showing the curling of h-BN sheets and formation of nano scrolls of h-BN...... 16

Figure 1-8. Proposed mechanism for liquid phase exfoliation by matching the solvent surface tension with the 2D material3. (b) Image showing the dispersion of various solvents, (c) and (d) Dispersion of h-BN as a function of total surface tension and polar dispersive ratio (Figure d is fitted with a Lorentzian peak function)...... 18

Figure 1-9. (a) Image showing the sonicated assisted hydrolysis of h-BN to form BNNS4. Using a high polar solvent, exfoliation and cutting of the nanosheets are possible, (b) Image showing the BNNS flakes obtained after the exfoliation process with a thickness x of 1nm, (c) Optical absorption of BNNS suspended in solvents, (d), (e) TEM images showing the BNNS flakes obtained and by looking at the edges, different thickness BNNS are obtained, (f) and (g) HRTEM image showing the monolayer BNNS structure and the defects in the material...... 19

Figure 1-10. (a) Noncovalent functionalization of BNNS using polyvinyl alcohol (PVA) which improves the dispersion of the BNNS in solvents, used in thermal and mechanical composites5, (b) Ab-initio calculations for optimized structure configurations for different nucleobases adsorbed on BNNS for bio-applications[75], (c) Amine functional group addition on the BNNS and NMR spectra confirming the presence of ODA-BN functionalization8...... 23

Figure 1-11. Covalent functionalization of h-BN nanosheets using different functional groups. (a) hydroxyl functionalization of the BNNS sheets and later decorating it with silver nanoparticles, (b) XPS spectra confirming the presence of the functionalization and the decoration of Ag on BNNS, (c) Nitrene addition on BNNS, covalently bonded on to the phase and edge sites (Inset image showing the dispersion and absorption change in the BNNS before and after functionalization), (d) Dibromocarbene (DBC) addition on BNNS via aqueous heating in sodium hydroxide...... 25

Figure 1-12. a) an optical top-down view of the as-deposited h-BN on Silicon, b) a close- up TEM micrograph of the hexagonal patterning in h-BN, and c) a cross-sectional view of few-layer h-BN growth9. Similar images from Shi, et al.,10 in d) e) and f). The last column in Figure 1 is from Kim, et al.,11 with g) an SEM image of monolayer triangle formation of h-BN, and h) optical image of h-BN growth on unpolished and i) highly polished copper foil...... 27

Figure 1-13. a) Representative schematic of the triangle formation of h-BN, where the Nitrogen termination is the more energetically favorable termination from Kim et al.,11, and b) the change in h-BN initial grain formation shape as a function of nitrogen and boron chemical concentrations in the CVD furnace from Stehle et al.,12...... 29

Figure 1-14. a) Setup of reactive magnetron sputtering for h-BN films, b) cross-sectional TEM image of few-layer growth on Ru (0001), c) XPS analysis of thickness for the different growth conditions, and d) outlining the three-step process for arbitrarily thick h- BN coatings, all from Sutter, et al.,13 The next two images portray h-BN growth with PLD processing of e) the B:N ratio measured by XPS as a function of background gas nitrogen pressure and working distance in the chamber, and f) a cross-sectional image of few-layer h-BN on HOPG. The two final images are of MBE of h-BN with g) initial h- BN grain formation and h) a close-up of a similar image, from Nakhaie, et al.,14 window sizes of 30 µm and 9 µm, respectively...... 32

Figure 1-15. Structure of TMDs (a) About 40 different layered TMD compounds exist by using the transition metals (Group IV – X) and the three chalcogen elements (S, Se and Te). The materials that predominantly crystallize in these layered arrangements are xi highlighted in the periodic table. Partial highlights in the periodic table for Co, Rh, Ir and Ni indicates that only some of the dichalcogenides form layered structures. Image showing a single-layer TMD with (b) trigonal prismatic and (c) octahedral coordination. (The purple atom in the image corresponds to metal and the yellow corresponds to chalcogen atom)15...... 34

Figure 1-16. Synthesis of 2D TMD heterostructures, (a) Cross-sectional HRTEM of MoS2/Quasi-free-standing epitaxial graphene (QFEG) showing the nucleation and subsequent lateral growth of MoS2 on a SiC step edge covered with the top graphene layer is flat, (b), (c) Scanning TEM images of stacked MoS2-WSe2-EG vertical stacked heterostructures showing pristine interfaces with no intermixing of Mo-W or S-Se after synthesis. (d) 2D monolayer MoS2/WS2 heterostructures with lateral hetero interfaces. Synthesis schematic and chemical composition analysis.79 (e) HRTEM image showing 81 the hetero interface of 2D monolayer MoS2/WS2. (Adapted with permission from Ref ). ... 39

Figure 2-1. Image showing the 5 stage synthesis process for graphene oxide paper or thin film. Stage 1 involves the mixing of 99.9% pure graphite with potassium permanganate (KMnO4) and acid mixture of sulfuric acid and phosphoric acid (H2SO4+H3PO4), Stage 2 involves the heating this mixture to 50oC while stirring it constantly at 200 rpm for 18-24 hours, Stage 3 and Stage 4 involves the centrifugation and sonication of the acid mixture to remove the excess ions of permanganates, sulfates and phosphates in the solution. This is achieved by careful monitoring of the pH, which should reach a value of 4...... 47

Figure 2-2. (a) SEM image of GO sheet, (b) AFM map of GO dried on Si substrate, (c) XPS spectra of GO showing the effective removal of sulfur and other impurities, (d) High resolution C 1s spectra showing the various functional groups present after exfoliation process and (e) Raman spectra showing the presence of D at ~1350 cm-1 and G at 1580 cm-1...... 49

Figure 2-3. (a) SEM image showing the initial hBN powder before exfoliation; (b), (c) Exfoliated hBN with a large flake size (6 µm); (d) Stability of the suspension in DI Water; (e) Stability of hBN in iso propyl alcohol, DI water, acetone and ethanol., and (f) UV-Vis spectra showing an optically active defect at 320 nm (~3.9 eV) in hBN due to the addition of sulfur functionalization...... 52

Figure 2-4. (a), (c) TEM images showing mono layer and few layer hBN, (b), HR-TEM images showing the atomic structure of the hBN, (d) XRD spectra of hBN on Si indicating high crystallinity after exfoliation and (e) EELS spectra showing the presence of sulfur on the surface of hBN at spot 2...... 54

Figure 2-5. X-ray diffraction pattern for (a) hBN before and after exfoliation, (b) hBN and bare Si substrate...... 55

Figure 2-6. AFM image of exfoliated hBN and its corresponding height measurements at different spots. As it can be observed in Figure (a), spot 2 has a monolayer whereas the spot 1 is a bilayer. In order to obtain the AFM scans, samples were dispersed in ethanol xii and drop casted on the Silicon dioxide substrate, which made the exfoliated flakes stack on top of each other...... 56

Figure 2-7. Raman spectra of bulk hBN Vs exfoliated hBN, showing a shift of 3.3 cm-1 indicating a significant reduction of sheet thickness...... 58

Figure 2-8. Plot showing the survey spectrum for hBN (a) before exfoliation and (b) after exfoliation. After exfoliation we can see that there is an enhancement in the O 1s and addition of sulfur and phosphorous peaks along with the standard boron and nitrogen...... 59

Figure 2-9. High resolution XPS spectra of (a) B 1s, (b) O 1s, (c) N 1s before exfoliation showing no additional bonding in the individual species, (d) B 1s spectrum showing the addition of sulfur and oxygen bonding of Boron, (e) O 1s shows an increase in oxidation due to B O bonding and (f) Unchanged N 1s peak structure suggests the functionality is 2 3 added to the boron sites...... 61

Figure 2-10. High resolution spectra of B 1s and O 1s showing the addition of the oxygen and sulfur functionalization in B1s on a different spot. This shows that the presence of the oxygen bonding is slightly different at different spots...... 62

Figure 2-11. (a) FTIR spectra of hBN before and after exfoliation. (b) Picture showing the presence of a new peak at 1054 cm-1 which corresponds to B-O bonding...... 63

Figure 3-1. SEM image showing the cross section view of (a) free standing ultra-thin BCON (~33 nm) and (b) Stability of BCON at pH ranging from 1-13, Image showing the BCON solution with (c) 9:1 GO: hBN (d) 1:1 GO:hBN, (e) 1:9 GO:hBN, (f), (g) SEM image showing the dried BCON solution corresponding to (c) and (d), and (h) SEM image corresponding to (e), showing the formation of ribbon like structures...... 67

Figure 3-2. Image showing the comparison of different pH solutions of BCON. pH corresponding SEM images showing the structure vs pH of the dried BCON. 1 ml with a concentration of 0.5-1 mg/ml of the material was dried on Silicon substrate in all cases...... 68

Figure 3-3. Image showing the pH dependence with time. It can be clearly observed that with the increasing time, higher and lower pH materials tend to settle faster than the solutions at pH 4-8. (All the samples were taken from the same batch with a concentration of 1 mg/ml)...... 70

Figure 3-4. (a), (b) Low magnification and high magnification TEM Images showing the distribution of hBN in GO, (c) XRD spectrum confirming the bulk-crystallinity of BCON as most of the flakes are (002) oriented and (d) Auger electron spectroscopy (AES) images showing the nanoscale integration of hBN with GO...... 72 xiii

Figure 3-5. TEM images showing the presence of hBN and GO in a single layer, (b) EELS spectra confirming the presence of parent hBN and GO, (c)&(d) Diffraction image of BCON showing the crystalline nature of BCON after the exfoliation process...... 73

Figure 3-6. (a) & (b) AES topography of BCON at sample 1 and sample 2 showing the presence of boron, nitrogen, carbon and oxygen at varying concentrations...... 74

Figure 3-7. High resolution XPS scan of (a) C 1s showing the presence of O=C-NH bond ,(b) O 1s confirming the presence of O=C-NH bond, (c) B 1s showing the addition of B- O and (d) N 1s showing the presence of additional peak...... 76

Figure 3-8. Raman spectra showing the BCON spectra where the D band of GO overlap with hBN at 1371 cm-1 and the G band at 1600 cm-1. The peak broadening in the D band overlap shows the presence of GO and hBN...... 77

Figure 3-9. FTIR spectra of various compositions of GO: hBN showing the difference in the peak intensity. The B-N-B (~800cm-1) and B-N (1370 cm-1) vibration corresponds to the hBN where as the C-O, O-H, C=O correspond to the GO...... 78

Figure 3-10. (a) Image showing the setup used for the exfoliation process, (b) Vacuum filtration setup used to remove the ethylene glycol and clean the material with solvents like ethanol, DI water, (c) final ethylene glycol free composite on Teflon lined filter and (d) Dried composite on Teflon filter which forms like a paper...... 79

Figure 3-11. (a), (b) SEM images showing the dispersion of MoS2 flakes in rGO matrix. As it can be observed that some of the flakes are electron transparent, showing the evidence of few layer flakes after the exfoliation, (c) Optical image showing the 4 layer flake as observed on a 270 nm SiO2 substrate and (d) Raman spectra corresponding to the -1 MoS2/rGO composite. The peaks of MoS2 were observed at 387 and 408 cm and for rGO at 1350 and 1600 cm-1. (e) High resolution XPS spectra for C 1s showing the reduction of the graphene oxide evident from reduced C=O and C-O peaks and Mo 3d, S 2p peaks remain unchanged and there is no oxidation of Mo or S...... 81

Figure 4-1. (a) Image showing the flexibility of graphite paper used as the substrate, (b) SEM image showing the surface morphology of the graphite paper, (c) Raman map showing the G/D ratio of the graphite paper, (d) XPS spectra showing the composition of Graphite paper and inset picture showing the high resolution C1s spectra showing the bonding of graphite paper and (e) Raman spectra corresponding to (c) showing the high quality of the graphite paper...... 85

Figure 4-2. Schematic of the MoS2 obtained using blender inc., (b) Image showing before and after growth of MoS2 on silicon substrates on the 3D printed crucible...... 87 xiv

Figure 4-3. (a) 5 nm Mo film deposited on graphite paper, sulfurized Mo film on graphite and its corresponding SEM images, which is similar to the observations from literature, (b) Powder vaporization of MoO3 and S on graphite and Silicon substrates forming MoS2 across the edges and center using a special 3D printed crucible and SEM image corresponding to this growth process...... 88

Figure 4-4. (a) SEM image showing the growth of low density vertical flakes at the center of the graphite substrate and (b) SEM images showing the growth of high density flakes towards the edge of the graphite substrate...... 89

Figure 4-5. (a) Raman spectra of the graphene paper treated with different oxidation times using UV-Ozone. As we can see that at 20 minutes, we can see the formation of D- band in graphene, (b), (d) SEM images showing the growth on the 20 minute UV-Ozone treated sample, (c) and (e) SEM images showing the growth on 30 minute treated UV- Ozone sample...... 91

Figure 4-6. Growth of MoS2 at various times the MoO3 was introduced in the system. The effect of precursor times can be observed for 5 minutes, 10 minutes and 15 minutes.

...... 92

Figure 4-7. Effect of pressure on the growth of MoS2 flowers on graphite paper. (a) SEM image showing the representative growth at <250 torr, (b) Image showing the growth at a pressure 400-500 torr and (c) Growth of MoS2 film like material at higher pressures (>500 torr)...... 93

Figure 4-8. Growth of MoS2 was nearly uniform on all the different substrates that were used. In the images above, growth on (a) Silicon and (b) graphite, (c) Silicon dioxide 270 nm/Silicon (SiO2/Si) and (d) sapphire substrates was shown for comparison...... 94

Figure 4-9. Raman spectra showing the evolution of MoS2 flowers from MoO2 particles on graphite paper. (a) Raman spectra showing the presence of large flakes of MoO2 as observed in section, (b) Reduced intensity of MoO2 spectra indicates the increased conversion to MoS2 and (c) After 30 minute growth, majority of the flakes are converted to MoS2...... 96

Figure 4-10. (a),(b) Mo and S elemental maps as collected using Auger electron spectroscopy, (c), (d) High resolution Mo spectra as collected in XPS showing the Mo 3d5/2 an Mo 3d3/2 with a peak-peak separation of 3.135, an optimal separation for MoS2, (d) high resolution S spectra showing the S 2p3/2 and S 2p ½ with a peak to peak separation of 1.2 eV and (e) Survey spectra as collected in XPS showing the presence of Mo and S only and no presence of impurities...... 98

Figure 4-11. (a), (b), (c) FIB images showing the sample locations where the sections were made, (d), (e) and (f) Sample sections corresponding to (a), (b) and (c) which is used for TEM imaging...... 99 xv

Figure 4-12. High angle annular dark field image as collected in TEM on the cross section sample and its corresponding elemental maps showing the presence of carbon, molybdenum and sulfur and small amounts of oxygen everywhere. The substrate graphite can be clearly seen and MoS2 that was grown on top which made a clean interface indicating there was no mixing of MoS2 into the graphite paper...... 100

Figure 4-13. MOCVD growth of tungsten diselenide (WSe2) in an RF induced heating system...... 102

Figure 4-14. SEM images showing the effect of pressure on the WSe2 growth. Representative images for the growth were taken for the different pressure ranges at (a) P<100 torr, (b) 100

Figure 4-15. Effect of time on the WSe2 growth can be seen in the above image with varying time of the precursors in the system for (a) 2 minutes, (b) 5 minutes, (c) 15 minutes and (d) 30 minutes. The growth conditions for the time study were studied at 700 torr, Se:W at 10,800, 800oC...... 104

Figure 4-16. SEM images showing the surface morphology of the growth with various ratios of Se: W for (a) 21,670, (b) 10,800 and (c) 500. It is clearly observed that at higher ratios of Se:W, there is no formation of WSe2 on graphite, when the ratio was lowered, the growth is much prominent, (d) Raman intensity map showing the presence of bilayer WSe2 growth on Graphite paper and (e) PL intensity peak at the WSe2 regions in figure 2(d)...... 106

Figure 4-17. (a) SEM image of Graphene Oxide (GO) which was used as initial substrate, (b) High resolution XPS spectra showing the reduction of GO into RGO in the reactor before the growth of WSe2, (c) image showing the impact of the substrate defects on the growth, (d) GP before the growth, (e) High resolution XPS showing that no change occurred in the GP before the growth and (f) SEM image showing the growth of WSe2 on the GP...... 107

Figure 4-18. High Resolution XPS spectra for (a) Tungsten (W 4f) showing the WSe2 bonding and the stability of WSe2 on Graphite (b) Selenium 3d corresponding to the WSe2, (c) Oxygen (O 1s) is nearly same in all the sample spots collected over a period of time...... 109

Figure 5-1. Image showing the set of radioisotopes obtained from spectrum techniques which included Cs-137, Co-60, Sr-90, Tl-204 and Po-210. Each disc is 1” in diameter and all radioisotopes are completely sealed in the epoxy except for the Po-210 source, table showing the activity, half-life and energies of the different radiation sources used in the current work...... 111

Figure 5-2. Image showing the setup for the radioisotope in-situ XPS measurements. (a) The XPS load lock chamber was used as an isolated vacuum chamber in between XPS measurements and (b) Image showing the radioisotope laminated disc source and the xvi

BCON paper which were wrapped in aluminum foil and mounted onto the XPS sample mount bar...... 112

Figure 5-3. (a),(b) Travel depth for Alpha particles in BCON for model 1 (uniform BCON sheet) and model 2 (heterogeneous distribution of hBN and GO). Helium ions were used to simulate alpha particles as they resemble very closely with the energy of the alpha particles with an energy of 5304 keV, (b),(c) Energy loss as simulated in SRIM for the two models showing the energy deposition in BCON structure at various thicknesses and (e),(f) Simulated image showing the complete stopping range for the alpha particles in BCON at 27.5 μm for model 1 and 25.5 μm for model 2...... 114

Figure 5-4. XPS survey spectra showing the composition of BCON with a very little amount of sulfur contamination...... 117

Figure 5-5. The degradation rate of (a) GO and (b) BCON as a function of absorbed x-ray irradiation. The value of X=O/C ratio, where Xo is the initial O/C and Xa is the O/C ratio after “a” amount of radiation. (c) and (d) High resolution scans from XPS showing the reduction of the phenolic, carbonyl species without a change in the O=C-NH species...... 118

Figure 5-6. High resolution Spectra for (a) C 1s and (b) O 1s with charge shift for BCON before and after irradiation of X-Rays. (c) & (d) showing the high resolution spectra of B1s and N 1s with the charge shift...... 120

Figure 5-7. Plot showing the effect of Gamma radiation on the BCON and GO. (a) Plot showing the change in the O/C ratio with radiation dosage absorbed and (b) High resolution C 1s scan showing the reduction in the R2 and R1 components in BCON. It can also be observed that through the interaction, the R3 component remained unchanged...... 122

Figure 5-8. Image showing the XPS plot of BCON, (b) Image showing the Oxygen 1s spectra...... 123

Figure 5-9. In-situ XPS measurements of alpha irradiated BCON sample, comparing the change in (a) O/C and (b) the changes in B/N as a function of alpha irradiation time...... 125

Figure 5-10. (a) Energy loss as a function of attenuation distance for a Po-210 alpha particle in BCON. The alpha particle is completely absorbed after traveling through 26 μm of BCON. As the particles travels through the material, the maximum energy deposition occurred at 25-26 um. (b) Energy loss of alphas in hBN. The total energy is deposited at 24.2 um into the material. This information was acquired from running simulation using SRIM (stopping range for ions in matter). As there are two different material systems in BCON, the model that was used for this study was a mixture of GO& hBN in the same layer...... 127

Figure 5-11. Plot showing the changes in the C/O ratio with the total energy absorbed from beta radiation...... 128 xvii

Figure 5-12. XPS measurement of the O/C ratio for GO after thermal neutron irradiation and a control sample. Because measurements could not be performed in-situ using the XPS, the material had to be transported to the Breazeale reactor. The two samples (GO16 neutron and GO16 control) were transported in vacuum sealed bags and UV impenetrable carrying case. Because the neutron beam exposure could not be performed under high vacuum, the control sample was removed from the sealed container for the same duration as the irradiated GO sample to determine how GO chemistry was altered by the environmental conditions at the reactor...... 130

Figure 5-13. Changes in the high resolution spectra for (a) O1s & (b) C1s showing a minimum change in the peak structure indicating that there was no change before and after the neutron exposure, (c) A slight change in B1s indicating that the concentration of boron atoms should be higher inorder to see any statistically significant change...... 131

Figure 5-14. High resolution spectra of (a) C1s & (b) B1s showing the changes in the peak structure over Ar ion time...... 133

Figure 5-15. Survey spectrum comparing the before and after Ar ion interaction with BCON. As we can see that there is an additional peak of Ar 2p present in the material. The spectra were collected after 1 hour of Ar ion irradiation...... 134

Figure 5-16. Before and after comparison of x-ray exposure in ambient atmosphere showing that there is no significant change in the sample after 16 hours of exposure...... 136

Figure 6-1. Contact angle measurement for (a) Bare graphite paper, (b) As grown MoS2 flowers on GP, (c) As grown MoS2 flowers after 2 minutes of gentle UV-Ozone treatment, (d) 10 minute UV-Ozone treatment showing a significant reduction in contact angle showing an increase in the surface free energy, (e) As grown partially covered MoS2 flowers have a contact angle less than the UV-Ozone treated one and (f) complete wettability using a lower surface tension liquid (ethanol)...... 138

Figure 6-2. Wetting transition from (a) Cassie-Baxter state to (b) Wensel state for water contacting on MoS2 flowers and (c) image showing the reduction of contact angle with the addition of the S-O functional groups on the surface...... 139

Figure 6-3. High resolution XPS spectra for (a) Mo 3d showing the growth of Mo-O bonding when we treat the sample with UV-Ozone and (b) S 2p spectra showing the presence of S-O bonding when treated with UV-Ozone...... 140

Figure 6-4. Impedance measurements of as synthesized MoS2 vs UV-Treated MoS2 showing the reduction of ~ 4 x in the resistance of the MoS2 heterostructure after the 10 minute treatment...... 141

Figure 6-5. Contact angle measurement for (a) Cleaned SiO2, (b) As grown MoS2 flowers on SiO2, (c) As grown MoS2 flowers after 2 minutes of gentle UV-Ozone treatment, (d) 10 minute UV-Ozone treatment of MoS2 flowers on SiO2...... 143 xviii

Figure 6-6. Plot showing the HER performance for as grown 2H-MoS2/graphite vs 1T- MoS2/Graphite. As we can clearly notice from the figure, the onset potential reduced by 150 mV and Tafel slope reduced by 78 mV/Dec...... 145

Figure 6-7. (a) Current density vs voltage plot for as grown MoS2 flowers on graphite and 1,5,10 minute UV-Ozone treated samples and (b) Tafel slope corresponding to the HER plots. As it can be observed that the Tafel slope was reduced by 3.5x from pure MoS2/graphite to 10 minute UV-Ozone treated sample...... 146

Figure 6-8. Cycling performance for 10 minute UV-Ozone treated sample. As it can be observed that the sample is very stable even after 24 hours in solution for over 1000 cycles...... 147

Figure 6-9. (a) Current density vs voltage plot showing the effect of UV-Ozone treatment beyond 10 minutes. As it can be observed that 15, 20 and 25 minute samples had increased onset potential and Tafel slope has been increased from 54 mV/Dec to 209 mV/Dec. Inset image showing the complete wetting of water beyond 10 minute treatment which is evident from the contact angle being 0...... 148

Figure 6-10. (a) Tafel slope for different sample structures of MoS2/graphite and (b) Potential vs current density plot corresponding to the Tafel plots. As it can be observed from the plots, Increasing the sulfur content to increase the density of MoS2 flowers/graphite did not yield a lot of improvement in the HERs reactions...... 149

Figure 6-11. (a) Current vs potential plot of HERs for different WSe2/graphite samples and (b) Tafel slopes corresponding to (a) samples. As it can be noticed that vertical flakes would definitely decrease the Tafel slope...... 150

Figure 6-12. HER plot of various levels of UV-Ozone treated WSe2/graphite sample showing degradation in performance as the onset potential increases and Tafel slope also increases with the increase in UV-Ozone time...... 152

Figure 6-13. High resolution spectra for (a) W 4f showing the formation of W+ 6 states +4 and W corresponding to WSe2 and (b) Se 3d shows the formation of Se-O with increasing UV-Ozone times...... 153

Figure 6-14. (a), (b) Voltage Vs NHE plot and Tafel slope plot showing the enhancement in HERs with transition metal substitution and (c) XPS of Mo 3d showing the peak position shift for Mo 3d by 0.5 eV because of the chalcogen substitution...... 154

Figure 7-1. Shifting in XPS spectra near the valence band maximum (VBM) and for the major tungsten (W 4f) and selenium (Se 3d) peaks, for WSe2 on: a) graphite paper, b) sapphire and c) silicon carbide. The valence band maximum was determined for each sample by applying a linear fit to both the valence band edge and the background noise inside the valence band. The after spectrum shown here corresponds to the maximum X- ray dose of ≈5.3 MGy...... 162 xix

Figure 7-2. Image showing the theoretical capacity and potential for current materials used in lithium ion batteries (LiBs). As it can be observed that S has a very wide range in theoretical capacity, which is because of the presence of various metal atoms that it bonds to inorder to stabilize its structure...... 164

Figure 7-3. Step by Step procedure for lithium ion battery assembly for liquid phase synthesis materials...... 166

Figure 7-4. Cyclic performance for MoS2/rGO showing a uniform charge/discharge performance, (b) Image showing the voltage vs specific capacity for the first two cycles. .... 167

Figure 7-5. Plot showing the higher cyclic performance for CVD MoS2/graphite heterostructure with a cyclic capacity of 750 mAh/g...... 168

LIST OF TABLES

Table 1-1. Table showing the different TMD’s grown till date using various techniques ...... 37

Table 1-2. Table summarizing the use of MoS2/graphene heterostructures for various energy applications...... 41

Table 2-1. Elemental composition of hBN as calculated using CASA XPS...... 60

xxi

LIST OF SYMBOLS/ACRONYMS

% - percentage

1D- one dimensional

2D – two dimensional

3D- three dimensional

ADF- annular dark field

AES- Auger Electron Spectroscopy

AFM – atomic force microscopy

Ag- silver

Al2O3- alumina

ALD- Atomic layer deposition

ARPES- Angle resolved photoemission spectroscopy

Au- gold

BCON- boro carbon oxy nitride

BNNRs- boron nitride nanoribbons

BNNT- boron nitride nanotubes

BP- Black phosphorous

C- Carbon

C2H2- ethylene

C2H4- methane

C60- bucky ball c-BN – cubic boron nitride

CH3- methyl radical

Cl- chlorine atom xxii

Cl2- chlorine cm- centimeter

CNTs- carbon nanotubes

CO- carbon monoxide

Co- Cobalt

CS- cross section

CTE- coefficient of thermal expansion

Cu- copper

CVD- chemical vapor deposition

DBC- dibromocarbene

DI- deionized

DNA- deoxy ribonucleic acid

EELS- Electron energy loss spectroscopy

EG- Epitaxial graphene eV- electron volt

FET- Field effect transistor

FIB- focus ion beam

FWHM- full width at half maximum g- gram

GaSe – Gallium Selenide

GO- Graphene Oxide

H2O- water

H2O2- hydrogen peroxide

H2S- Hydrogen sulfide hBN- Hexagonal Boron Nitride xxiii

HCl- hydrogen chloride

HF- hydrogen fluoride

HOPG- Highly oriented pyrolytic graphite

HRTEM- high resolution transmission electron microscopy

HRTEM- High resolution Transmission electron microscopy

IPA- isopropyl alcohol

J- joules

K- potassium

KMNO4 – potassium permanganate

KOH- potassium hydroxide

LEED- low energy electron diffraction

Li- lithium

Li3BN2- lithium diborate

LiOH – lithium hydroxide

LPCVD- low pressure chemical vapor deposition m- meter

MBE- molecular beam epitaxy

MBE- Molecular beam epitaxy ml- milli-liter

Mn- manganese

Mo- Molybdenum

MoCl5- Molybdenum chloride

MOCVD- Metal organic chemical vapor deposition

MoO3- Molybdenum (VI) oxide

MoS2- Molybdenum disulfide xxiv

MoSe2- Molybdenum diselenide

MPC- methoxylphenyl carbomate

MSA- Methyl sulfonic acid

Na- sodium

NbS2 – niobium disulfide

NH2- ammonia radical nm- nanometer

NMP – N-methyl-2-pyrrolidone

NO2- Nitrogen dioxide

O2- Oxygen oC- centigrade

ODA- octadecylamine

OH- hydroxyl radicals

OLED- organic light emitting diode

Pa- Pascal

Pd- palladium

PEG- polyethylene glycol

PL- Photoluminescence

PLD- pulse laser deposition

PMMA- poly methyl methacrylate

Pt – Platinum

PTFE- polytetrafluroethylene

PVA- polyvinyl alcohol rGO- reduced graphene oxide

RNA- ribonucleic acid xxv rpm- rotations per minute s- second

S- Sulfur

Se- Selenium

SEM- Scanning electron microscopy

Si- Silicon

SiC- Silicon carbide

SiO2- Silicon dioxide

STEM- scanning transmission electron microscopy

STM- Scanning tunneling microscopy

STMDs- semiconducting transition metal dichalcogenides

STS- Scanning tunneling spectroscopy

TaS2- Tantalum disulfide

TaSe2- Tantalum diselenide

TEM- Transmission electron microscopy

TGA- thermogravimetric analysis

TMDs – Transition metal dichalcogenides

UHV- ultra high vacuum

UV- ultra violet

V- volt vdW – van der Waals

W- tungsten

WO3 – Tungsten (VI) oxide

WS2- tungsten disulfide

WSe2- Tungsten diselenide xxvi

XPS- X-ray photoelectron spectroscopy

XRD- X-ray diffraction

μm- micro meter

xxvii

ACKNOWLEDGEMENTS

No words can express how much I would like to thank all the people who have helped me along the way in this wonderful graduate school journey. All the interactions I had, all the valuable lessons learnt in life, which I will remember throughout my life time and they have made me the person I am today. This is dedicated to all the people who have been a part of this wonderful journey.

Firstly, I would like to thank my dad (Mallikarjuna Reddy), mom (Lakshmi) and my sister (Rashmi) without their support I wouldn’t become what I am today. I know it is a challenge in not being with you physically to help and support you but your open ended belief in me has allowed me to travel this far during these years and all of which have been wonderful experiences.

I would like extend my humble thanks to my advisor Josh Robinson. He is an extraordinary individual, who is always open to give advices and is always available. He is probably the best boss anyone can expect as a grad student. His friendly, energetic, always positive attitude kept things on the right track and his motivation to promote positive work culture helped me a lot. I have enjoyed all our meetings, interactions and the best part was the group gatherings. I can never forget the time I had working with you and I will definitely miss working with you.

I cannot thank enough to the Robinson group who helped me a lot and I have learnt a lot with all the interactions that I had with you guys. I had the opportunity to mentor a few and it was always a pleasurable experiences working with and along you guys. Without your help, I wouldn’t have achieved this milestone. Cheers to all those fun group meetings, fun hangouts outside of work.

Life as a grad student cannot be summarized without friends. I had the opportunity to interact and be friends with some of the very best people. I was blessed with awesome roommates

Chethan, Abhinav, Radha, Harsha and had some of the best times with them. I can never forget all those wonderful times I had and you are the best roommates that I can ever ask. I would also like to thank my friends Amrita, Birgitt, Yu-chuan, Amit, Vivek, Koke, Charu, Ganesh, Alp, Ece, James, Chia-hui, Fred, xxviii

Iso, Natalie, Yash, Shruti, Tanu, Nama, Simham, Ryan, Jared and many more with whom I had some great times along the way. All those fun hangouts with you guys will always be remembered through my life. These were some of the best times I had, and some of the best memories in my life.

Life in grad school is challenging without having hobbies. Cricket and photography were such hobbies which helped me focus on my work while still staying active and enthusiastic. Things that I learned outside of work definitely showed a different dimension where I could apply myself without being too distrustful. I would like to extend my thanks to all the people with whom I have played cricket.

The interactions with all the different people made me a better person by looking at things from different perspective. I would specially like to thank my teammates Akhil, Balaji, Sridhar, Abhinav, Hari, Harry,

Phani, Venkat, Subash, Shyam, Arjun, Radha, Siddarth, with whom I had the pleasure of playing the penn state tournaments for 5 years. Cricket at Penn State brings back the good old memories where I was on the winning side and the losing side. Being the captain for the team for over 3 years showed me how to manage people and most importantly having fun even if it meant we lost a few times.

I would also like to thank the MRI staff specifically Vince Bojan, Colette, Josh Stapleton,

Trevor and Maria. Vince, you are an amazing person, patiently answered all of my questions. Never said no to my “quick questions”. The interactions that I had with you made me a better scientist and changed the approach at some of the things I look at both in research and life.

Of course none of this work could have been completed without funding support from

DTRA and ATOMIC. I would also like to thank Angstron materials for providing the high quality graphite which reduced my effort in making graphene and help me focus on the actual science.

xxix

Dedicated to Mallikarjuna Reddy, Lakshmi and Rashmi.

Chapter 1

INTRODUCTION/LITERATURE REVIEW

What is a 2D Material?

Each layered material, when thinned to its physical limits, exhibits novel properties different from its bulk counterpart. Therefore, at the physical limit these materials are referred to as two-dimensional (2D) materials. The most highly studied 2D material is graphene because of its exceptional electronic, optoelectronic, electrochemical and biomedical applications.

Graphene

The term graphene has been derived from the Greek word “Graphein”16,17,18,19. This material existed for over 100 years and still new material properties are being discovered, even today. The first extraordinary properties of this material has been proposed back in the 1940s theoretically which suggested that the electronic characteristics within the plane will have extraordinary20. 60 years later, these predictions came to life after the first isolated layers from graphite by mechanical exfoliation by Geim and Novosolov, where they found the carrier mobilities >200,000 cm2/V.S at an electron density of 2 x 1011 cm-2, exceptional Young modulus of >1 TPa and spring constants of 15 N/m.21–27 This wonder material has an estimated theoretical surface area of >2500 m2/g28 and experimentally measured was ~400-700 m2/g,29–31 which attracted many commercial applications including gas, energy storage and conversion32–

38,biological applications39–41 micro and macro optoelectronics24,42–47.

History and Structure Evolution

A complete yearly timeline for the history and evolution of graphene has been shown in Figure 1-1.

Graphene and graphene like structures were first synthesized back in 1840’s by a German scientist

Schafhaeutl48,49, where he reported an intercalation process to exfoliate graphite by using sulfuric and nitric acid. After this attempt, a wide range of exfoliants and intercalants have been used including alkali 2

metals, fluoride salts, transition metals and organic species. After the first attempt, in 1859 a British chemist Brodie modified the process by adding an oxidant in the process developed by Schafhaeutl50,51.

The use of this technique not only intercalated graphite but also exfoliate and functionalize graphite, forming graphene oxide (GO). These intercalation and oxidation experiments were the first examples of exfoliation of graphite. Nearly a century after the study by Brodie, Boehm etal.52 reduced GO by treating it in a dilute alkaline media like hydrazine, hydrogen sulfide or iron (II) salts. The number of layers present in the structure was determined by using densitometry against a standard set of films with known thickness using transmission electron microscopy (TEM). From his studies, he was able to observe that the thickness of the material has increased to 0.46 nm instead of 0.4 nm, which was assumed to be experimental error53–55. From an IUPAC definition, Boehm et al., isolated reduced GO rather than graphene56. In 1968 Morgan and Somorjai used low-energy electron diffraction (LEED) to observe the adsorption of various organic gas molecules like carbon monoxide(CO), methane (C2H4), ethylene

(C2H2)on Platinum(100) surface at high temperature, and they observed diffraction patterns of graphitic structures on the Pt surface57. After analyzing the data, he deduced that “the first monolayer of graphite minimizes the energy of placement on each of the studied phases of platinum”, which was the first ever observation of graphene under a TEM58. Later, Blakely et al., studied extensively the segregation of mono and few layers of carbon on various metal surfaces59–64. Similar phenomenon was utilized by Von

Bommel in 1975 where they sublimed silicon atoms from silicon carbide and formed monolayer carbon at high temperatures and high pressures65. After confirming the observations using x-ray diffraction, it was concluded that two-three layers of the carbon atoms merged to form monolayer graphite, which was consistent with the theoretical predictions66.

Figure 1-1. Image showing the history of graphene evolution over the years48,49,51,52,54–72 4

Besides the epitaxial growth and chemical/thermal treatment of GO, a new process was developed to isolate graphene, described as micromechanical exfoliation69. This process involves the use of different graphite sources and delaminate the carbon layers by shear force as these materials are bound by van der

Waals forces. In 1999, this approach was used by Ruoff and coworkers to obtain thin lamellae of multiple layers of graphene, although they were not fully exfoliated to monolayer.69 Later in 2004, Geim,

Novosolov and coworkers were successful in obtaining monolayer graphene from highly oriented pyrolytic graphite (HOPG) and were able to transfer it onto silicon dioxide70. The graphene was then characterized using SEM, AFM and studied for its electrical field effect. These samples were free from functional groups which were confirmed by XPS. This laid the foundation for future exfoliation of various other two-dimensional materials. As this approach was very easy and highly successful, Geim and

Novosolov received Nobel Prize in 2009 for their achievement. After 100 years, James Tour and coworkers developed a modified Hummers process which was less toxic than the original process which involved the use of KMnO4 and H2O2 and provided a higher oxidation ratio along with exfoliation of the carbon sheets71. This interest in GO was revived after 2004’s successful exfoliation of graphene and the understanding of their properties. Several different strategies have been used in order to reduce the GO to form rGO, which is an isostructural to graphene. Most recently, Tom Mallouk and coworkers showed a non-oxidative approach to intercalate and exfoliate graphite, to form individual graphene sheets using

Bronstead acids72. This approach is quite significant to form graphene sheets as there is no oxidation process involved and this produced monolayers of graphene sheets directly, which eliminated the need to reduce the GO.

Beyond Graphene Materials: Introduction

Beyond graphene, there is a very wide spectrum of 2D electronic materials that range from insulators to semiconductors to metals and even to superconductors. 2D materials research initially focused on graphene following the seminal paper by Novoselov and Geim,70 but it was also demonstrated very early on that other 2D materials also possess exciting properties.73 Following graphene, 2D hexagonal boron nitride (hBN) was theoretically predicted to induce a bandgap in graphene when graphene was deposited 5

onto it.74 This led to a significant increase in hBN experimental research, and ultimately to an understanding that hBN may be an ideal substrate for graphene electronics.47 Rapidly following graphene and hBN, research on semiconducting 2D layers provided evidence that the band structure of a subset of the 2D materials family changes drastically as they are thinned to monolayer thickness.75 Device fabrication further ignited the interest of many in the electronics community.76 These novel semiconducting 2D materials, known as transition-metal dichalcogenides (TMDs), are now a primary focus of many researchers, as clearly evidenced by the publication record devoted to such materials

(Figure 1-2). There are many layered materials that go beyond TMDs, including monochalcogenides

(GaSe, etc.), monoelemental 2D semiconductors (silicene, phosphorene, germanene), and MXenes

(Figure 1-2). Finally, the true potential of these layered materials may emerge from the ability to stack them, layer-by-layer in any desired sequence, to create novel 3-dimensional (3D) architectures with entirely new functions.

Figure 1-2. Publication trends in 2D materials beyond graphene. Source: Web of Science. Search index:

[Title search (x) or title (y) and topic (z), where x = graphene or boron nitride or transition metal chalcogenides or molybdenum disulfide or monoelements (silicene, germanene, phosphorene, stanene or borophene), y = graphene or BN or TMDs or MoS2) and z = graphene or boron nitride or transition metal chalcogenides (TMDs: MoS2, WS2, MoSe2, WSe2, TaS2, TaSe2, NbS2) or MoS2 or monoelement 2D: silicene, germanene, stanene, borophene, phosphorene.

As from Figure 1-2, we can see that there are several other materials beyond graphene which received significant interest. Boron nitride, a material with similar structure to that of graphene was one of the most attracted materials. Because of its wide bandgap, it was proposed to use as a dielectric layer for graphene devices. Because of this, several strategies were explored to achieve monolayer boron nitride. 7

INORGANIC 2D MATERIALS

(Adapted from 2D Boron nitride: Synthesis and Applications; Ganesh R Bhimanapati et al.,.,

Semiconductors and Semimetals 95, 101-147)

INTRODUCTION TO BORON NITRIDE:

Boron nitride (BN) is a chemical compound that is isoelectronic and isostructural to carbon with equal composition of boron and nitrogen atoms. The first synthesis of boron nitride was in 1842 by Balmain77 using molten boric acid and potassium cyanide; however, stabilizing the material to form powders was a challenge until recently78. Similar to carbon, boron nitride is produced in amorphous (a-BN) and crystalline forms. In its crystalline form, boron nitride exists in three major forms: hexagonal boron nitride (h-BN) resembling graphite (Figure 1-3(a)), sphalerite boron nitride (β-BN) resembling cubic diamond, and wurtzite boron nitride (Ɣ-BN) resembling hexagonal diamond form79. Unlike the carbon fullerenes (C60 bucky balls), BN fullerenes have mostly squares or octagons instead of pentagons to avoid the thermodynamically unfavorable B-B and N-N bonding (Figure 1-3(d)). Similar to 1D carbon nanotubes, BN nanotubes (BNNT) (Figure 1-3(c)) also exist which are isoelectric to carbon nanotubes

(CNTs) in terms of chirality, tube diameters and number of walls. Out of these different phases, h-BN is the most common stable form of BN and most of the interest started after the isolation of graphene sheets in 2004 80. h-BN is also a layered structure, and within each layer the boron and nitrogen atoms are bound by strong covalent bonds in plane and each layer is held together by van der Waals forces. A single layer of h-BN is typically referred as a BN nanosheet or BNNS. This nomenclature is only true for h-BN sheets whose aspect ratio is small. For higher aspect ratio materials where the typical widths will be <50 nm, they are referred as BN nanoribbons or BNNR’s.

Figure 1-3. Boron nitride allotropes (a) few layer hBN, (b) mono layer h-BN nanosheet or BNNS, (c)

Boron nitride nano ribbons (BNNR) with different edge termination leading to Zigzag B-edge and armchair edge structures, (d) boron nitride nano tubes (BNNT) and (e) BN fullerenes79,81.

Although h-BN has a similar structure to graphene, it is a wide bandgap material with an intrinsic band gap (Eg) of 5.9 eV when compared to the highly conductive graphene. It is also thermally conductive, which is attractive for many electronic applications since it can be used as electrically insulating filler material for polymer or ceramic composites82,83, thermal radiators84,85, field emitters86 and UV emitters87,88. In addition to its insulating properties, h-BN is chemically inert in a wide variety of acids, solvents, and oxidizers. h-BN is insoluble in the usual acids but is soluble in alkaline molten salts and nitrides such as LiOH, KOH, Li3BN2. Owing to its high chemical resistance and thermal stability, h-BN is an attractive material for use as a chemically inert coating in hazardous environments84,89,90.

STRUCTURE AND PROPERTIES OF 2D BORON NITRIDE

STRUCTUAL PROPERTIES

The physical form of h-BN is a white slippery powder, analogous to graphite. Commercial availability of h-BN flake size range from few hundreds of nanometers to few tens of microns (information obtained 9

from Sigma Aldrich, Alfa Aesar). Hence, the BNNS sheets obtained by exfoliation of these crystals are often limited to the maximum lateral sizes (few tens of microns) of the initial starting material91.

Individual h-BN monolayers or BNNSs are a honeycomb structure with alternating boron and nitrogen atoms. The bond type for B-N is covalent and bond length is 1.45 Å. The distance between the centers of neighboring borazine rings is 2.5 Å. The edge structure for the BNNS can be armchair or zigzag, similar to graphene. The former is a B or N-edged structure whereas the latter is a BN pair edged structure. The crystal structure for h-BN is hexagonal with P63/mmc space group (Figure 3(a)), lattice constants a = b =

0.2504 nm, c = 0.6661 nm, bond angles alpha = beta = 90o, gamma = 120o .The partially ionic structure of

BN in h-BN reduces covalency and electrical conductivity in these layers unlike graphite, favoring the

AA` stacking92. Typically, this is the most energetically favorable stacking observed in BN, where the electron-deficient B atoms are directly above or below the electron-rich N atoms in the adjacent layer93,94.

The most common way to determine the number of layers in h-BN is by looking at the transmission electron microscopy (TEM) images of the folded edges8–11,84,95–97. Figure 1-4(a) and 1-4(b) shows the high-resolution TEM images, which typically gives the number of layers along with more atomic lattice information. Another simple technique to determine the thickness of h-BN is by using atomic force microscopy (AFM)8–10,91,95,98. The number of layers can be calculated by looking at the step height typically with respect to the surface (Figure 4(d) and 4(e)). Most monolayer h-BN sheets from mechanical exfoliation and chemical vapor deposition have a height of ~ 0.4 nm99. For the chemical exfoliation sheets, the height obtained can vary as much as 1 nm because of the solvent that can be trapped between the h-BN flakes and the substrate4. Hence, HRTEM is usually used to report the number of layers and when reporting the height through AFM, more characterization techniques should be used to confirm the findings.

Another simple and convenient technique using an optical microscope was developed to observe the number of layers of graphene sheets can be employed for h-BN as well100. Usually, a standard silicon wafer was coated with ~300 nm of SiO2 layer (Figure 1-4(c)), is used as a reference substrate and based 10

on the optical contrast that was observed using the optical microscope; the numbers of layers were predicted. In the case of h-BN, as it does not absorb in visible region, the substrate oxide layer had to be changed to 80 nm in order to optically identify the mono to few layered materials. As shown in Figure 1-

4(f), the white light contrast reaches a low of 2.5% for monolayer h-BN, which is 4 times less than that of graphene and the contrast increases with the increase in the number of layers101. Another way to identify the number of layers is by using a different wavelength light, which can be observed in Figure 1-4(c). By using a light as shown in Figure 1-4(g) with ~590 nm wavelength, the contrast between the number of layers was clearly evident.

Figure 1-4. (a) Low magnification TEM image of an exfoliated BNNS, (b) HR-TEM image showing the number of layers at the folded edges8, (c) (d) AFM height map of CVD grown BNNS showing the monolayer thickness of 0.4 nm, (e) Chemically exfoliated BNNS showing a height of 1 nm, (f) White light microscopy image of exfoliated h-BN flake showing the optical contrast on 90 nm SiO2/Si, (g)

Improved optical image of h-BN flake using a 590 nm light source showing the different contrast between layers98, (h) Raman spectra for BNNS to few layer h-BN comparing the difference in the peak width vs number of layers and (I) Peak position variation as observed for monolayer to bulk BNNS98.

This layer identification study can further be extended to Raman microscopy98. Boron nitride has a vibrational mode at 1364-1371 cm-1, which is dependent on the number of layers. Figure 1-4(h) and 1-4(i) shows the comparative study made by Gorbachev et al., for different layers of BNNS. It can be observed in Figure 1-4(h) that for monolayer, the Raman peak broadens and shifts to a higher wavenumber of 1370 cm-1 when compared to the bulk, which is at 1366 eV. This study clearly shows the red shift for BNNS formation, which could later be used as a standard to confirm the presence of BNNS.

THERMAL STABILITY

BN allotropes, especially h-BN and c-BN, exhibit high thermal and chemical stability. h-BN is stable without decomposing at temperatures of over 1000oC in air and 1400oC in vacuum and up to 2850oC in an inert atmosphere102,103. The theoretical thermal conductivity values for h-BN can be close to graphene

(~1700-2000 w/m.k)104,105, one of the best thermally conductive material available to date106. Depending on the type of BNNR structure, the zigzag edged BNNR are 20% larger than the arm chair-edged nanoribbons at room temperature107. In-plane thermal conductivity has been reported as high as 390 W/m-

K at room temperature, 280 times higher than SiO2, making h-BN an attractive dielectric material for heat generating electronic devices107. Similar to graphite, h-BN has a strongly anisotropic coefficient of thermal expansion (CTE) due to its anisotropic bond strength. The CTE in the a-direction (in-plane) is - 12

2.90x10-6 K-1 at room temperature, while the CTE in the c-direction is over ten times larger and 4.05x10-5

K-1 at room temperature108. The large positive thermal expansion in the c-direction is due to weak van der

Waals bonding between planes.

The most commonly accepted phase diagram of boron nitride was calculated from thermodynamic properties of boron nitride phases by Solozhenko et al., in 1999 after refining their original work from

1988109. It was found that the cubic phase of boron nitride (c-BN), rather than h-BN, is thermodynamically stable at ambient conditions.109 This is in contrast to the carbon phase diagram, where the hexagonal phase (graphite) is the stable phase at ambient conditions110. The phase diagram of boron nitride is shown in Figure 1-5(a) and 1-5(b), where the dashed lines indicate the original calculations by

Solozhenko et al., in 1988 and the solid lines indicate the refined diagram. The h-BN/c-BN/liquid triple point occurs at 3480±10K at 5.9±0.1GPa while the h-BN/liquid/vapor triple point occurs at 3400±20K and 400±20Pa109.

From the above phase diagram, it is evident that c-BN is more favorable than the h-BN at temperatures below 1600 K. Although this is true, the CVD growth of h-BN, which will be discussed in section 3(VI), was reported on transition metals at much lower temperatures. The typical growth 13

temperatures for CVD are around 700-1100oC, which are at least 300oC cooler than that observed in the general phase diagram. This unusual behavior can be because of the Gibbs free energy of the system. The transition temperature between h-BN to cBN shifts as a function of Gibbs free energy (Shift ±10 meV/atom). This change in the free energy can vary the transition temperature between 1200- 1800 K, where the lowest value is typically used for the growth of h-BN. These small variations are generally caused by grain size, defects, contaminants or sometimes interactions with the substrate transition metal itself.

SYNTHESIS OF BNNS

BNNS can be obtained via top-down (typical exfoliation type approaches) or bottom-up approaches

(usually CVD or other deposition techniques). Most common methods that have been used are mechanical exfoliation2,83,111–113, chemical exfoliation8,91,94,111,114–121, chemical vapor deposition9–11,97,122–125 and pulsed laser deposition126. Most of these methods have their own advantages and disadvantages as they are used for specific targeted applications. BNNS from exfoliation techniques are usually crystalline although obtaining a high percentage of exfoliated sheets is very difficult. Using these exfoliation techniques, obtaining a higher density of monolayer flakes is really difficult and time consuming. Also, because of the strong lip-lip interactions between the layers, there is high probability of layer stacking on top of each other. In contrast, CVD techniques uses the breakdown of precursors and hence it is highly controllable process of making BNNS. Typically, the crystallinity obtained via CVD is less than that when compared with exfoliation techniques.

MECHANICAL EXFOLIATION

Mechanical exfoliation, also known as the “Scotch tape method”, or micromechanical cleavage method uses the simplest tools for exfoliation i.e., scotch tape70. Using this technique, isolation of layers can be controlled down to monolayer and the flake size depends on the biggest flake available for exfoliation.

This process utilizes the adhesive films to peel off the layers and then pressed onto a targeted substrate. 14

The final BNNS obtained via this technique retains the original lateral size and low defect density as the parent h-BN crystal used. As a variety of nano-sheets can be easily obtained with this technique, fundamental studies in physics and electronics have been done mostly using the materials obtained via this technique73,95,98,127–129. Compared to other exfoliation methods, the mechanical exfoliation method is the least scalable technique especially for large-scale applications. Other reports using mechanical cleavage methods beyond simple mechanical exfoliation have been explored to produce large quantities of BNNS. As these materials are bonded by weak van der Waals forces, techniques with shear forces like a ball mill technique were used112,113. Traditional ball mill techniques are very aggressive, non-selective and yielded significant number of defects in the exfoliated material1,83,111,130. Although this technique yields a better output than mechanical exfoliation, the defect density in the final exfoliated films is higher.

Further improvements to the ball milling technique were also used by using a solvent along with the ball milling process (Figure 1-6(a)). Li et al., reported that using a mild wet ball milling process1, few layered crystalline BNNS with only a slight lateral size reduction was obtained. Various solvents have been tested in wet- ball milling. Benzyl benzoate was an effective lubricant because of its weight, which provided better exfoliation results than water, ethanol or dodecane131. As selection of the solvent is important for exfoliation, which can be obtained by looking at the weight of the solvent and the surface tension properties of the solvent and h-BN. Further, use of planetary mill instead of the more aggressive high- energy mills provide uniform rolling motions of the balls applying predominantly shear force to the h-BN flakes113,132. Further, use of smaller diameter balls instead of the heavier balls added additional lubrication for the interaction, which reduced the loss of the BNNS sheet size.

Another setup that was used for this type of exfoliation is the vortex fluidic device2. As it can be observed in Figure 1-6(b), a small amount of h-BN suspension in N-Methyl-2-pyrrolidone (NMP) was rotated at

8000 rpm in a glass tube with a fixed angle of the fork which resulted in significant exfoliation of the h-

BN crystals. The presence of NMP formed a thin layer on the inner walls of the centrifuge tube and the rotating shaft, which was at an angle, provided enough shear force to partially exfoliate the layers in the liquid mixture. Further, a more scalable approach was proposed following was proposed based on this technique (Figure 1-7(a)). Here, a high pressure micro-fluidization process82 was used in a continuous flow setup by using h-BN flakes and a polar organic solvent which was pumped through a microfluidic processor at a high pressure of 207 MPa, circulated continuously for multiple times. At these high pressures, the liquid generates a large shear force which results in exfoliation of h-BN particles with a reported yield efficiency of 45%. High resolution TEM mapping clearly showed that the final exfoliated material was h-BN without any functional groups and defects.

Dry mechanical process can also be used for the exfoliation process115 which is achieved by using molten hydroxides. Most common ones used in this case are sodium hydroxide (NaOH) and potassium hydroxide 16

(KOH). This process involves that the hydroxide and the h-BN powder was ground together then transferred into a polytetrafluoroethylene (PTFE) lined stainless steel autoclave where the reaction takes place for 180oC for 2 hours. This process is used for making nano scrolls of BNNS. The exfoliation process follows a sequence of steps that are shown in Figure 1-7(d). The first step involves the self- curling of the h-BN sheets at the edges after the molten hydroxide is attached on the surface layer.

Subsequently, the anions and the cations enter the interlayer space and the adsorbed anion (OH-) radicals start curling process in the adsorbed layer and finally the peeling away of the parent material is observed as a result of this curling process. As this process is easy and involves low-cost method, it can be scalable.

The final product obtained can be transferred to any solvents or to any substrates.

Figure 1-7. (a) Flow diagram showing the micro fluidization process82 used for exfoliation, (b) TEM image showing the presence of ultrathin BNNS achieved from this process, (c) Elemental mapping and lattice spacing obtained in high resolution TEM confirming the presence of h-BN. (d) Mechanism of dry exfoliation process using molten hydroxides115 (NaOH, KOH), (e) and (f) SEM images showing the curling of h-BN sheets and formation of nano scrolls of h-BN.

SOLVENT ASSISTED ULTRA-SONICATION

Sonication assisted exfoliation has been a common approach and has been used widely across various 2D materials. General idea in this process is to disperse the boron nitride in a solvent and with the aid of sonication power, the sample tends to exfoliate because of the energy that is generated by the sonication process. Sometimes, surfactants were also added to improve the dispersion of the nano particles.

In order to understand the dispersion ability for 2D materials, Coleman et al.,133 and Shen et al.,3 attempted to understand the direct solvent dispersion of 2D materials by using Hansen solubility parameter theory. They postulated that in order to select a good solvent or a mixture of solvents for direct dispersion should minimize the enthalpy of mixing and hence the energy of the exfoliation is minimized.

This means that the solvent should possess a similar surface energy as that of the nanosheets (Figure 1-

8(a)). In the case of boron nitride, a surface energy of around 65 mJ/m2 was proposed to be ideal and hence BNNS was usually formed in solvents such as isopropylalcohol (IPA) or NMP. This theory is true for most of the 2D materials (Figure 1-8(b)) where the surface energy ~70 mJ/m2 or a surface tension of

~40 mJ/m2 would provide the best possible results133. The dispersion ratio for h-BN with varying concentrations is listed in Figure 1-8(c) and 1-8(d). Certain mixture of solvents based on ethanol and water were also used which showed improved exfoliation and dispersion than when used individually. 18

BN as a function of total surface tension and polar dispersive ratio (Figure d is fitted with a Lorentzian peak function)

Although this model holds for most solvents, certain solvents tend to deviate from this behavior because they tend to interact with the nanomaterials much more strongly and sometimes chemically react with the 19

material. One good example is water. Although water does not wet h-BN at room temperature and was proposed to be hydrophobic, it was found to be counter intuitive when reports claimed to show high dispersion of BNNS in water4,115. Also, it was shown that BNNS dispersion contains a very high fraction of exfoliated sheets (~20%) than that of the other solvents. This unusual behavior can be accounted to the reactivity of h-BN, which could yield boron oxide and ammonia when it was treated in hydrolysis conditions. The mechanism of exfoliation can be observed in Figure 1-9(a). Hence, after the exfoliation process, a small amount of boron oxide and dissolved ammonia was found in the solvent and OH functionality was present on the surface of the h-BN. This presence of OH functionality on the surface further yielded the additional stability and dispersibility in water (Figure 1-9(b)-(g)).

in solvents, (d), (e) TEM images showing the BNNS flakes obtained and by looking at the edges, different thickness BNNS are obtained, (f) and (g) HRTEM image showing the monolayer BNNS structure and the defects in the material.

ACID EXFOLIATION

Boron nitride is typically neutral when reacting to acids. General acids did not show a high degree of exfoliation although using a high protic acid134, methyl sulfonic acid (MSA) was proved to be effective for the exfoliation. Concentrations of up to 0.3 mg/ml were obtained when bulk BN was dispersed and sonicated in MSA, which was comparable to water or other solvents. Similar to graphite exfoliation by hummer’s process, BNNS were also protonated at the edges and surface during this process which lead to have some repulsive forces between the layers resulting in exfoliation of bulk BN. This was further supported by looking at the color change that occurred after the reaction. The BNNS dispersed in MSA turned orange which could be because of the charge transfer between acid molecules and nanosheets. This color was later changed to milky white when redispersed back into other organic solvents. Further characterization using FTIR and XPS showed that there was no presence of additional bonding which indicated that the BNNS were not functionalized134.

CHEMICAL FUNCTIONALIZATION OF h-BN

Utilizing chemical functionalization for exfoliation has proved effective for the case of graphite allotropes, especially CNTs135,136. Similar strategies have been used in the case of BN to achieve exfoliation via functionalization of the surface. Typically, BN is resistant to wet and dry oxidation, although 2D and 1D BN are ionically active to functionalization due to the local polarization of B-N bonding. A simple way to functionalize BNNS includes heating h-BN in air to a high temperature

(~800oC) 116. Heating h-BN in air leads to the incorporation of small amounts of oxygen into the lattice of the h-BN,116 which is followed by stirring in DI water to incorporate a B-O functionality. The stirring mechanism leads to the formation of the functional groups that results in separation of the h-BN layers 21

into BNNSs. As this process involves heating and dispersion in water, it could be termed as a similar approach to sonication in DI water process. The yield obtained through this process is small (~2-4%). In order to completely functionalize the BNNS, the functionalization approaches can be divided into three major categories: noncovalent type functionalization, ionic functionalization or Lewis acid-base functionalization, and covalent functionalization.

Noncovalent Functionalization:

This type of functionalization is typically obtained by using a polymer or a co-polymer. These polymers typically tend to functionalize via the π-π interactions between the polymer and the h-BN sheets. The polarity of the functional molecule plays a major role in its optimal orientation on the h-BN sheets137–139.

The lowest energy molecular interactions will be governed by the electrostatic interactions between the functional polymer and the boron nitride sheets. Several polymers such as poly (m-phenylenevinlene),

2,5-dictoxy-p-phenylenevinylene, tetracyanoquinodimethane, tetrathiafulvalene were used to study the functionalization mechanism. Recently, BNNS were non-covalently functionalized using polyvinyl alcohol (PVA) and later attached to a paper with natural nacre5 (Figure 1-10(a)). This noncovalent functionalized approach drastically improved the mechanical and thermal performance of the natural nacre, which were also proposed to be used as substrates for electronics. Also, several transition-metal- arene complexes were also predicted to non-covalently functionalize h-BN. It was predicted from theory that the stable configuration for the transition metal arenes to functionalize is by having a metal atom at the center of the borazine rings. Most of the time after functionalization, band gap reduction and spin polarization is observed in h-BN, indicating the success of functionalization. Further Ab initio studies showed that h-BN can also be attached with DNA and RNA molecules6,7 (Figure 1-10(b)), where the N and O atoms of the nucleobases are located above the B atoms. Further, the nucleobases prefer to attach facially rather than perpendicularly because the noncovalent interaction is higher than the ππ interactions.

This could lead to a better bio-medical applications using h-BN.

Ionic Functionalization (or Lewis acid-base interactions):

The boron atom in the h-BN is electron deficient and hence it possesses Lewis acid characteristics and is susceptible to attack by a Lewis base molecule such as amines or phosphines to form stable Lewis acid- base complexes. Although there are reports on the boron atom being ionically functionalized8,140,141, there was no experimental reports on nitrogen atom although it is electrophilic in nature. Because of the B-N bonding being locally polarized, in theory it should be a convenient target for both the atoms to get ionically functionalized.

Several attempts were made to demonstrate the Lewis acid-base interactions on h-BN using amino groups of octadecylamine (ODA), amine terminated oligomeric polyethylene glycol (PEG), trioctylamine and trioctylphosphine 8,130. The lipophilic and hydrophilic chains of these molecules intercalated into the h-BN layers and functionalized the sheets which was later extracted by dispersing in water or organic solvents.

The final dispersed material was still transparent for lower concentrations and milky white for higher concentrations. Lateral sizes of ~1 µm and thickness of about 1-7 nm were obtained via this process.

Trace amounts of monolayer materials were also obtained but the lateral sizes were <100nm. To further improve the efficiency of functionalization, defects were created on h-BN sheets by ball milling the h-BN powder130. These defects were confirmed by x-ray diffraction (XRD) and Raman as the (002) peak and the inplane mode (E2g peak) broaden when defects are introduced. After defects were introduced, there was better solubility after the functionalization. This was further confirmed using NMR spectroscopy which is observed in Figure 1-10 (c) and 1-10(d)8,141. The alkyl radicals on the amino-terminated region was observed for its C-H signals which was reduced from the parent material, indicating that this end was attached to the h-BN. Further studies on the defect h-BN starting material showed a shifting position of the same C-H signals of the ODA functional group. This indicated that the amino end of the ODA molecule that was attached to the B atoms and the boron sites contained the defects. 23

Figure 1-10. (a) Noncovalent functionalization of BNNS using polyvinyl alcohol (PVA) which improves the dispersion of the BNNS in solvents, used in thermal and mechanical composites5, (b) Ab-initio calculations for optimized structure configurations for different nucleobases adsorbed on BNNS for bio- applications[75], (c) Amine functional group addition on the BNNS and NMR spectra confirming the presence of ODA-BN functionalization8.

Covalent functionalization:

Direct covalent functionalization is not possible, as BN is very stable material. Although, several functional groups have been added covalently on h-BN sheets typically at its edges and the basal planes.

In order to attach the functionalization and exfoliation via covalent functionalization, several treatments have to be made. Recent reports showed that Hummer’s process with modifications in the temperature worked effectively for creating oxygen bonding on the boron sites. The confirmation of the bonding was observed through XPS and FTIR where B-O bond was present. Other approaches were also made by 24

attaching hydroxyl groups, a nitrene (methylphenylcarbonate) via a thermal oxidation and treatment with strong oxidants. These were mostly used in the applications for polymer composites which improved the performance of the composite. For specific targeted applications, silver nanoparticles were further decorated after hydroxyl functionalization on the BN sheets (Figure 1- 11(a) and 11(b))121. Coleman et al., also functionalized BNNS using dibromocarbene (DBC). In their work, they used BNNS as a two- dimensional phase transfer catalyst for the carbine migration across the organic-aqueous phase boundary.

The BNNS formed B-CBr2 bonding in order to stabilize the carbenes which also act as the reactive substrate for the functionalization. The functionalization reaction is shown in Figure 1- 11 (d). This functionalization resulted in the formation of B-C and B-N bonds on the BNNS lattice by forming dibromo-bridged bicycle BCN linkage. This covalent chemistry was confirmed using multiple different characterization techniques. This CBr2 linkage on the BNNS was later used to interface with solvents, which can later be integrated in polymer composites easily119. Further, similar work was extended to functionalize the BNNS by nitrene addition.120 4-methoxybenzyloxycarbonylazide was used as the source to functionalize it. This type of functionalization mostly attacks the B atoms in the h-BN lattice, resulting in breaking of B-N bonds in the structure. This reaction changes the milky-white color of BNNS to dark brown generating N2 bubbles and creating reactive nitrene radicals. This later yields the formation of methoxylphenyl carbomate (MPC) bound to BNNS via the B-N bond as shown in Figure 1- 11 (c). Most of the functionalization approaches results in highly dispersed BNNS in solvents which can later be integrated into many different applications. Typically, most of these processes were used to make polymer composites to improve thermal and mechanical strength of the polymers. 25

XPS spectra confirming the presence of the functionalization and the decoration of Ag on BNNS, (c)

Nitrene addition on BNNS, covalently bonded on to the phase and edge sites120 (Inset image showing the dispersion and absorption change in the BNNS before and after functionalization), (d) Dibromocarbene

(DBC) addition on BNNS via aqueous heating in sodium hydroxide119.

SINGLE AND FEW-LAYER h-BN via CHEMICAL VAPOR DEPOSITION

Nucleation and growth methodologies for high quality mono- and few-layer h-BN films, as well as many other 2D material counterparts, have been of significant interest within the past decade. The necessity for direct growth, in contrast to the previously discussed exfoliation techniques, is driven by the desire for large area film coverage with strict control of layered thickness as well as eliminating the difficulties 26

associated with the lift-off and transfer processes seen in many exfoliation variations142,143. To date, the progress in growth of 2D insulators including hexagonal-boron nitride has significantly lagged behind comparable conductors and semiconductors (e.g. graphene, transition metal dichalcogenides, and other

2D material systems), as the difficulty in materials processing presents many challenges for device usage144. These difficulties include lack of large-area growth required for electronic devices, compatibilities with substrate materials, and required high deposition temperatures (typically >1000°C), each that will be discussed in following sections.

Even as the overall interest in 2D materials has increased substantially since the isolation and initial studies at the University of Manchester70, monolayer and few-layer h-BN has been directly grown in small domains since the mid 1990’s. One of these first isolations of monolayer h-BN was made possible

145,146 by A. Nagashima and was performed by thermal decomposition of borazene (B3N3H6) at 700-800°C on metallic substrates including nickel (Ni (111)),palladium (Pd (111)), and platinum (Pt(111)), which continue to be common substrates for h-BN growth. Initial studies using EELS and X-ray photoelectron spectroscopy (XPS) led to key observations including the realization of a very low interaction energy between h-BN and the substrate, as well as the extremely slow growth rate for the second layer of h-BN from a lack of reactivity on the surface of the monolayer islands. These initial studies were performed with limited interest at the time, but would later influence the 2D materials community substantially nearly a decade later.

Once the exciting discovery of graphene ignited the research in the topic of 2D materials in the mid

2000’s, chemical vapor deposition (CVD) become the most prominent growth technique used for single and few-layer 2D materials, including h-BN. Demonstration of large area growth of few-layer h-BN by atmospheric pressure chemical vapor deposition (APCVD) in early 2010 yielded highly uniform films on copper foil with thicknesses of a few atomic layers9,10. The precursor reactants in these initial studies included ammonia borane (NH3-BH3), a solid powder requiring sublimation at temperatures of 120-

130°C, as well as a gaseous precursor borazene, also used in initial studies by Nagashima145. Both studies were performed at substrate temperatures of 600-700°C for 20-30 minutes of growth in an Ar/H2 gas 27

mixture, followed by a post-annealing step at 1000°C to further crystallize the multi-layer film. The similarities in film growth and structure for initial efforts in CVD h-BN coatings is represented in Figure

1- 12, where Figure 1- 12 (a), (b), (c) show an optical image of the few-layer h-BN film, high resolution

“top-down” TEM view of the atomic structure, and cross-sectional TEM image of the few-layer h-BN film, respectively, from Song et al.,9 Figure 1- 12 (d), (e), and f display complementary images using the same techniques from Shi et al.,10 First growth procedures for the ultra-thin h-BN coatings identified many critical issues, common in current growth methods as well, including the necessity to reduce wrinkles, cranks, and pinholes, as the need for uniform, large area coverage is required for many applications.

Figure 1-12. a) an optical top-down view of the as-deposited h-BN on Silicon, b) a close-up TEM micrograph of the hexagonal patterning in h-BN, and c) a cross-sectional view of few-layer h-BN growth9. Similar images from Shi, et al.,10 in d) e) and f). The last column in Figure 1- 12 is from Kim, et al.,11 with g) an SEM image of monolayer triangle formation of h-BN, and h) optical image of h-BN growth on unpolished and i) highly polished copper foil.

It wasn’t until 2012, when Kim et al.,11 were able to isolate repeatable, triangular domains of single layer h-BN by performing the growth at lower pressures (technique known as LPCVD) using borazene, a gaseous species slightly more environmentally stable than borazene when decomposed. Advantages of the LPCVD synthesis include increased cleanliness in a vacuum environment, reduced activation energy for h-BN formation at lower pressures147, as well as the reduced effect of kinetics and dynamics of the inlet gas flow for growth148. At a single layer, the reactivity of the h-BN is very low, and can lead to incomplete film coverage in this instance of monolayer triangles. Additionally, the surface conditions of the substrate play a critical role in the nucleation density as well as the growth dynamics125, evidenced by the optical images of the distribution of domains of h-BN seen in Figure 12(h) and 12(i) of the same growth conditions on unpolished and polished copper foil, respectively11.

At the time, it was a surprise that the first domains of h-BN were triangular, in contrast to the traditional hexagonal pattern seen in graphene synthesis on copper foil (Cu)149. Origins of these triangle domains are attributed to low-reactivity of the nitrogen terminated edges (Figure 1- 13(a)) 150 that restrict the lateral growth of the triangular domains. As graphene is a monatomic structure, the lack of contrasting edge site nucleation forms a hexagonal pattern similar to the internal structure of the graphene rings. It was observed by Stehle et al.,12 that by altering the location of the substrate in the tube (i.e. further or closer to reactant source), and thus altering the nitrogen-to-boron active species concentrations in the gas phase at the substrate location, the shape of these few micron-sized domains can be altered from triangle to semi- hexagons, as seen in Figure 1- 13(b). Further manipulation of gas flow dynamics in the CVD tube (more 29

substantial impact at atmospheric pressure growth) has also proven to control the nucleation shape of the h-BN materials from controllable triangles to more advanced structures151.

Beyond h-BN growth on copper foils, 2D h-BN nucleation and growth has continued to expand to other metals including various crystal orientations of Ni152–155, Pt124, Pd156 and alloys of these materials157.

Nickel, in particular the (111) crystal orientation, has shown to facilitate high quality h-BN growth due to the small lattice mismatch (0.4%). This strain induced by the difference in lattice mismatch is compensated by a slight corrugation of the ad layer, and the periodicity of this depends on the degree of lattice mismatch between the substrate156. Grain size of the 2D h-BN can be directly influenced by the surface structure, as exemplified by record high triangular grain size of 7500 µm2 on Cu/Al alloys at growth temperatures of 1050°C157. The authors attribute the large grain size to the small addition of Al in the alloy, where the nucleation density can be decreased to 60 sites per mm2, leading to unimpeded, large crystal growth on the surface.

Other interesting 2D structures can be formed by epitaxial lattice interactions between the substrate and h-

BN. h-BN nano mesh, a porous material consisting of 12 substrate unit cells with aperture structures 2 30

nm in width, can be formed on ruthenium (Ru (0001)) and rhodium (Rh (111)) by LPCVD techniques used for monolayer growth on other foils158,159. These structures form upon the decomposition of borazine vapor at temperatures at 1100°C, and can have a particular benefit for use in patterning and assembly of nanoparticles such as gold nanoclusters. The change in nanostructure simply by altering the surface lattice structure and texturing conditions highlights the fact that h-BN in both the monolayer and few-layer form is extremely surface sensitive during the nucleation and growth process.

The majority of CVD synthesis routes have been focused on metallic substrates, as the foils can withstand high temperature synthesis (800-1000°C) and can have epitaxial-like growth effects. Ideally, the 2D material can be grown directly on substrates of greater interest for electronics and other applications. If this would be possible, the requirement for the time and personnel-intensive lift-off and transfer steps would be eliminated, streamlining the device construction process. Prospects for direct growth of h-BN on alumina (Al2O3) and Silica (SiO2), both common substrates for nano electronic applications, are discussed by Bresnehan et al.,97,142 Key aspects of the h-BN growth such as stoichiometry as a function of growth temperature, crystal size, and thermal transport properties were evaluated to elucidate how the thin h-BN films would behave in a graphene device construct. In addition, the enhancement of graphene hole concentration and mobility was evaluated with respect the quality of the as-deposited films on Al2O3 and SiO2. Direct growth of the turbo-static, 2D h-BN films processed at 400°C was shown to possess a higher boron content, however, show the largest increase in mobility of the graphene layered on top of the film, with a 1.5x and 2.5x increase compared to Al2O3 and SiO2, respectively.

In parallel to the initial studies involving CVD nucleation and growth, physical vapor deposition techniques have also become of great interest for growth of few-layer h-BN. Physical vapor deposition techniques including pulsed laser deposition (PLD), magnetron sputtering, and molecular beam epitaxy

(MBE) have all proven to facilitate h-BN growth at various conditions. In these cases, the use of a high energy plasma (in the case of PLD and sputtering) or molecular beam is initiated in an ultra-high vacuum chamber (typically residual base pressures of 10-7-10-8 Pascals. The plasma or molecular beam then comes in contact with the substrate and condenses to form thin films of h-BN. 31

With the use of high energy plasmas, several benefits over conventional CVD approaches including reduced required temperature for h-BN formation, large area coverage and versatility in device fabrication. The plasma-based techniques including magnetron sputtering as well as PLD have been used to coat very large wafers with uniform, cohesive thin films on the order of 4” in diameter and larger. Due to the high energy plasma species, however, special considerations need to be addressed as to how to control the kinetically driven processes to resist damage to the substrate and reduce the nucleation site density to allow for large area, unimpeded grain formation. Sutter et al.,13 using reactive magnetron sputtering from a boron source in an Ar/N2 gas mixture (see Figure 1- 14(a)), was able to generate a very slow flux of energetic ions and neutral species to the surface to condense and form uniform, highly crystalline multi-layer h-BN on a Ru substrate, seen in Figure 1- 14(b). With a three-step process combination of initial CVD to seed the growth to less than a monolayer, following by sputtering at high and low temperatures for subsequent multi-layer growth, multiple layers can be formed with a varying degree of thicknesses and stoichiometry, as measured by XPS in Figure 1- 14(c) and (d). Glavin et al.,126 describe that in PLD from an amorphous boron nitride target, a highly stoichiometric B-N plasma can be formed, and a recipe can be constructed where the stoichiometry of the as-deposited BN film can be uniform regardless of substrate. By appropriately choosing the working distance from target to substrate and adjusting the background pressure to facilitate shock-wave like plasma dynamics, the appropriate precursors for stoichiometric h-BN formation can be realized, regardless of surface conditions. The structure, however, is very dependent upon the surface, as a highly dense amorphous boron nitride film is formed on most substrates, and a nanocrystalline h-BN is growth on nearly lattice-matched substrates including HOPG at temperatures of 700°C. The growth procedure is fairly quick, at ~0.4 nm/laser pulse, where the laser pulses are at 1 Hz repetition rate. Because of the high deposition rate, the nucleation density is also very high, resulting in grain size formation of 3 nm. The film, however, is cohesive and uniform over the substrates of interest, yielded similar electronic tunneling properties at all measured locations on the substrate. Other PLD studies using very high-energy CO2 lasers have shown to produce 32

high quality h-BN nanosheets that overlap and form on the substrate surface at an even lower temperature of 400°C160.

Studies of molecular beam epitaxy of h-BN growth on nickel foils by Nakhaie, et al.,14 were able to use energy from the molecular ion beam, namely an elemental B and N source, to form large area, cohesive thin films of h-BN. The mixture of the high energy molecular flux, coupled with the high surface temperatures, can lead to interesting meta-stable structures that aren’t necessary bound to triangular film formation seen in CVD. Figure 1- 14(a) and a close-up in Figure 1- 14(b) provides images of MBE growth of h-BN that can result in flower-like configurations, which eventually nucleate and grow in ways that aren’t traditional in CVD reactor chambers. These can be attributed to the additional kinetic energy161 supplied by the incoming beam, where the addition of this energy can lead to a reduction in required growth temperature, evidence by growth of these initial flower-like growths at 730°C.

outlining the three-step process for arbitrarily thick h-BN coatings, all from Sutter, et al.,13 The next two images portray h-BN growth with PLD processing of e) the B:N ratio measured by XPS as a function of background gas nitrogen pressure and working distance in the chamber, and f) a cross-sectional image of few-layer h-BN on HOPG. The two final images are of MBE of h-BN with g) initial h-BN grain formation and h) a close-up of a similar image, from Nakhaie, et al.,14 window sizes of 30 µm and 9 µm, respectively.

2D h-BN is one of the more interest 2D materials to date, and direct growth methods to understand how to enhance and control the film formation are critical to the future of nano-electronic devices, anti- corrosive coatings, and other uses. Advances in CVD processing have resulted in extremely large grain size, stoichiometric films, and alternative processes including PLD and plasma-based processing techniques could allow for realization of direct-growth device constructs.

Transition Metal Dichalcogenides

Another class of the 2D layered materials are the transition metal dichalcogenides or TMDs. As it can seen from Figure 1- 15 (a), they can take the form MX2 where M is the transition metal from group IV, V or VI and X is the chalcogen atom (S,Se or Te). These materials in its monolayer form are very interesting for the scientific community because of their direct band-gap and non-centro symmetric lattice structure as observed in Figure 1- 15(b) and (c). The bulk band gap for this class of materials differ with their monolayer counterparts, for example MoS2 has a layer dependent tunable band gap ranging from 1.2 eV for the bulk to 1.8 eV for monolayer. Also, they undergo an indirect bandgap transition in the bulk to a more direct band gap for the monolayer sheets.

Recent studies on these 2D-TMDs showed that these materials can be potentially used as atomically thin transistors, vertical tunneling transistors, optoelectronic devices like light emitting diodes, catalytic applications like hydrogen evolution reactions and energy storage applications like lithium ion batteries, sodium ion batteries etc., Along with understanding these properties, efforts have been made to grow these materials on a large scale uniformly and have control over the chemical composition, physical 34

dimension (i.e., horizontal flakes vs vertical flakes) and growing heterostructures with other 2D materials or other 2D-TMDs.

Figure 1-15. Structure of TMDs (a) About 40 different layered TMD compounds exist by using the transition metals (Group IV – X) and the three chalcogen elements (S, Se and Te). The materials that predominantly crystallize in these layered arrangements are highlighted in the periodic table. Partial highlights in the periodic table for Co, Rh, Ir and Ni indicates that only some of the dichalcogenides form layered structures. Image showing a single-layer TMD with (b) trigonal prismatic and (c) octahedral coordination. (The purple atom in the image corresponds to metal and the yellow corresponds to chalcogen atom)15.

Large-Area, Morphology-Controlled Synthesis of TMDs

Two-dimensional TMDs have been realized by various synthetic methods, including vapor-phase chemical reactions, wet-chemical synthesis, and liquid exfoliations.15,162,163 Vapor deposition techniques have been most extensively explored due to their potential for high scalability and morphological control.

163 In this process, metal containing precursors [e.g., MoO3, WO3, or (NH4)2MoS4] are vaporized and reacted with chalcogen elements through vapor-solid reactions, leading to the growth of 2D TMDs on a substrate downstream. Most common procedures rely on the co-evaporation of metal and chalcogen precursors both initially in solid phases. These techniques generally yield 2D TMDs with limited spatial homogeneity and uncontrolled morphologies due to the difficulty with controlling growth variables for vaporized reactants. Opposite to the commonly used solid-phase chalcogen elements, gas-phase chalcogen reactants such as hydrogen sulfide (H2S) have been used to improve the scalability and

19 uniformity of 2D MoS2 Also, the synthesis of nano-crystalline 2D MoS2 with controlled layer numbers has been demonstrated on the centimeter scale via the sulfurization of molybdenum chloride (MoCl5) precursors164 The success for the improved scalability is attributed to the balance control between the

18 partial pressure of vaporized MoS2 and its equilibrium pressure . Moreover, pre-deposition of elemental metals on growth substrates and their subsequent reactions with chalcogen vapors have been demonstrated to yield large-area 2D TMDs.165–172 The major advantage of this approach is the morphology and the coverage control of 2D TMDs enabled by metal deposition methods. The goal of uniformity was further extended via seed-promoting molecules such as perylene-3,4,9,10-tetracarboxylic acid tetra-potassium salt (PTAS).173,174 The high solubility of PTAS in water enhances the seed solution to be uniformly distributed on hydrophilic substrate surfaces on a large scale. Another such technique to grow high quality, precisely doped electronic grade 2D materials with abrupt interfaces is through molecular beam epitaxy (MBE).175–180 With the availability of in situ characterization during the growth,

MBE particularly has an advantage of having precise thickness control and the ability to grow on 2D or

3D substrates. Finally, very recently, metal-organic CVD (MOCVD) based on gas-phase precursors181–183 has proven that large-scale growth of 2D TMDs is readily achievable, with the most impressive 36

182 demonstration being a 100-mm wafer of monolayer MoS2 with uniform electrical properties. In addition to scalable growth, the synthesis of 2D TMDs with controlled orientation, layer number, chemical composition, and placement have been extensively pursued. CVD based on the thermal evaporation of elemental metal seed layers has demonstrated the growth of 2D TMDs in two distinct 2D layer orientations of vertical and horizontal.167,184,185 The thickness of the metal seed layers was identified to be the critical parameter to dictate the 2D layer growth orientation.167 The chemical composition of 2D

TMDs can also be controlled by simultaneously reacting multiple metal precursors with multiple chalcogen elements in vapor phases. For example, alloyed 2D monolayer of various compositions such

186,187 188–190 as MoxW1-xS2 and MoSxSe2-x have been synthesized, which present tunable band gap energies with varying ratios of elements. The control of the stoichiometric variations in MoS2 have been demonstrated to provide tunable electrical and optical properties.191–193 In addition to the chemical compositions control during growths, post-growth processes have also been developed to convert the chemical composition of 2D TMDs after their growths. For example, ion-exchange reactions have been

194,195 applied to convert 2D MoS2 into 2D MoSe2, consequently tuning their opto-electrical properties.

Selective area growth of 2D materials has also recently been realized, in part, using a spin-coated ammonium heptamolybdate [AHM, (NH4)6Mo7O24·4H2O] and pre-growth lithographic techniques to

196 achieve site-specific, patterned growth of 2D MoS2. Finally, 2D MoS2 with site-specifically controlled layer numbers has been demonstrated on a single SiO2 growth substrate by modifying the surface

197 properties of the SiO2 via plasma etch treatments. Despite the modest success in improving scalability and uniformity, the chemically-synthesized large-area 2D TMDs at present still exhibit electrical properties far from what are demanded for realistic applications. For example, the room temperature field-effect-transistor (FET) mobility of state-of-the art 2D MoS2 grown on a four-inch wafer with uniform layer numbers is ~30 cm2V-1s-1182, an order-of-magnitude smaller than those of mechanically- exfoliated materials. Such limitation is attributed to the defective nature of chemically synthesized materials: intrinsic defects (e.g., vacancies) in individual 2D grains as well as interfacial defects (e.g., grain boundaries) across individual 2D grains. Further information on the growth of different TMD’s 37

using different techniques is summarized in Table 1. Here, mechanical exfoliation includes scotch tape exfoliation and chemical vapor transport. Powder vaporization covers the general CVD techniques that are based on powder transport to grow 2D materials on various substrates.

Table 1-1. Table showing the different TMD’s grown till date using various techniques

75,133,152,164,167,173,174,181,182,187,189,194,198–262245,263–266

Synthesis of 2D-TMD Heterostructures

Developing new hybrid materials by combining 2D layered materials in vertical stacks offers an enormous amount of possibilities to broaden the versatility of 2D materials, allowing for achieving superior and unusual material properties that cannot be obtained otherwise.143 Thus, securing robust methods that enable controlled stacking of a variety of 2D materials with any desired combination and thicknesses will play a key role for 2D materials to be widely used, in particular, in transparent and flexible electronic and optoelectronic device applications. Many of these structures are summarized in

208,210 Table 1, where a commonly used scheme to fabricate 2D heterostructures such as MoS2/WSe2,

267 268 269 MoS2/WS2, MoS2/MoSe2, and MoSe2/WSe2 is the mechanical transfer of separately exfoliated 2D crystals.211,215,217,270 However, despite the relatively high quality crystals, the dry transfer method may not 38

be suitable when sophisticated control of lattice orientation is required and can introduce various adsorbates—such as hydrocarbons—trapped between layers during the process that may deteriorate the interface quality between the layers.271 As a result, it is very advantageous to develop techniques capable of directly synthesizing these heterostructures. Vertically stacked 2D TMD/graphene247,248,254,272–274 heterostructures (Figure 1- 16(a)-(c)) have been synthesized by the sequential growth of 2D TMDs on top of pre-existing graphene (or hBN275). Additionally, a variety of 2D TMD-only heterostructures have also been demonstrated, including heterostructures with vertically-standing 2D layers such as MoS2/WS2 (or

185,276 MoSe2/WSe2) synthesized by the one-step sulfurization (or selenization) of elemental metals. In- plane 2D heterostructures have been more challenging to synthesize as the formation of alloyed structures is thermodynamically favored. Recently, in-plane heterostructures in lateral and vertical orientations have been demonstrated by the sequential in-situ vapor-solid reactions,256,257 or by the co-reaction of Mo and

W-containing precursors with chalcogens (Figure 1- 16(d),(e)).240,242,257,277,278 Post-growth processes based on ion-exchange reactions have also been employed to develop 2D heterostructures. For example, an exchange of Se ions with S ions on pre-existing MoSe2 has been demonstrated to site-specifically convert

195 MoSe2 into MoS2 realizing in-plane 2D MoSe2/MoS2 heterostructures . Finally, the epitaxy of MoS2,

WS2, and WSe2 on SnS2 microplates was also demonstrated under mild experimental conditions (450 °C

255 for 2-6 min), using MoCl5/S, WCl6/S and Se as precursors, respectively. Despite significant lattice mismatch at the interface with SnS2—15%, 16%, and 11% for MoS2, WS2, and WSe2—the heterostructure can successfully form without a significant strain due to relatively weak vdW forces at the interface. At present, the synthesis of all the above mentioned 2D heterostructures has been limited to very small areas. More effort should be focused in the direction to produce scaled-up 2D heterostructures while maintaining their atomically seamless hetero-interfaces. 39

Figure 1-16. Synthesis of 2D TMD heterostructures, (a) Cross-sectional HRTEM of MoS2/Quasi-freestanding epitaxial graphene (QFEG) showing the nucleation and subsequent lateral growth of MoS2 on a

SiC step edge covered with the top graphene layer is flat, (b), (c) Scanning TEM images of stacked MoS2-

WSe2-EG vertical stacked heterostructures showing pristine interfaces with no intermixing of Mo-W or S-

Se after synthesis. (d) 2D monolayer MoS2/WS2 heterostructures with lateral hetero interfaces. Synthesis schematic and chemical composition analysis.79 (e) HRTEM image showing the hetero interface of 2D

81 monolayer MoS2/WS2. (Adapted with permission from Ref )

TMD’s and Graphene based materials for Hydrogen evolution reactions and Lithium ion batteries

TMD’s have become particularly interesting because of their optical162,257 and electrical

163,247,279 performance and their interest in various fields is expanding. TMD’s specifically MoS2 has gained special interest and many applications have been looked into for this unique material ranging from transistor applications, photodetectors, solar cells, energy applications like hydrogen evolution reactions, lithium ion batteries etc., Because of the limitations in the intrinsic structures available, one simple 40

structure of MoS2 is difficult to satisfy all properties and functional performance in practical applications.

Fabrication of hybrid structures based on 2D materials will take advantage of the individual components, which ultimately will be superior to the actual 2D material itself. For example, graphene has an outstanding electrical performance, but fails in switch control due to gapless band structure, whereas

MoS2 can be used for bandgap engineering depending on the number of layers but its electron mobility is not comparable to that of graphene. Hence, making a heterostructure with graphene/MoS2 will help solve some of the conduction issues, while keeping the properties of MoS2. Specifically in applications such as hydrogen evolution reactions (HERs) and lithium ion batteries where conduction plays a crucial role, these properties can be utilized. Although, straight 2D layers will not provide enough material for commercial use catalysts for HERs or LiB’s, hence 3D structures based on 2D materials need to be developed in order to utilize the potential of these materials. If a 3D structure based on these 2D materials can be constructed, it will provide us with the advantage of larger surface area, larger amount of active catalyst per unit area, higher conduction which ultimately yields higher efficiencies.

Table 2 summarizes the typical synthesis method used to develop MoS2 on graphene based catalysts for their use in energy applications specifically HERs and LiBs. As it can be seen that most of the catalysts that have been successfully developed were using solvo-thermal and solution method as these are viable approaches for large scale manufacturing. CVD based techniques have been looked as an alternative although previous reports suggests that it requires harsh chemicals such as molybdenum chloride as a precursor for the synthesis. Although this would provide a 3D growth of 2D materials, this is not viable as it is not environment friendly. Hence, better approaches need to be developed in order to use these 3D catalysts for commercial applications.

Table 1-2. Table summarizing the use of MoS2/graphene heterostructures for various energy applications

Synthesis Applications Type of Structure Performance/Outcome method Template- Vertical MoS2/carbon Overpotential ~0.12 V, Tafel slope 45 assisted CVD fiber280 mV/Dec (MoCl5, S) Overpotential ~0.15V, cathode current 281 solvothermal 2 MoS2/carbon cloth density- 86 mA/cm , Tafel slope~50 synthesis mV/Dec

281 solvothermal Overpotential~0.19V, Tafel slope~ 95 MoS2/rGO synthesis mV/Dec

MoS2/rGO Solvothermal Tafel slope~150 mV/Dec HERs

Vertical MoS2/carbon CVD 282 Tafel slope 43-47 mV/Dec fiber paper (MoO3, S)

2 MoS2 nanoparticles/ CVD cathode current density- 200 mA/cm , 283 carbon fiber (MoO3, S) Tafel slope~62 mV/Dec

Overpotential ~0.3V, cathode current Templated vertical CVD 2 284 density- 100 mA/cm , Tafel slope~83 MoS2/carbon fiber (5 nm Mo, S) mV/Dec

285 Solution 100 mA/g current, capacity 1100 MoS2/Graphene deposition mAh/g, capacity~100 cycles

Honeycomb Solvothermal 200 mA/g current density, discharge 286 MoS2/graphene foam synthesis capacity 1235 mAh/g, 85.8% efficiency

LiB’s 287 Solvothermal 100 mA/g current, capacity 672 MoS2 microspheres synthesis mAh/g,

288 Hydrothermal 100 mA/g current, capacity 698 MoS2/CNT synthesis mAh/g,

289 Solution phase 1000 mA/g current, capacity 1040 MoS2/Graphene method mAh/g for 50 cycles.

*rGO- reduced graphene oxide, MoS2-Molybdenum disulfide

Current Research Focus

Current work mainly focuses in answering the fundamental properties of graphene based heterostructures.

Large scale exfoliation of boron nitride was achieved with an exfoliation efficiency of ~25%, which helped in making a stable composite with graphene oxide (GO) called BCON. Stability studies have been performed for this heterostructure and were used as a radiation sensing material. Later, chemical vapor deposition (CVD) was used to grow transition metal dichalcogenides (TMDs) especially molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) on graphite, which were used to achieve a high performance catalyst for hydrogen evolution reactions.

Outcomes from the current work

1. Large scale exfoliation of hexagonal boron nitride

Hexagonal boron nitride is a very stable compound and is very inert to acids. Typical exfoliation techniques utilized long times inorder to achieve mg’s of stable exfoliated material in suspensions. In the current work, modified hummer’s process was used and conditions were changed to exfoliate and functionalize hBN, which improved the stability of hBN in various solvents like acetone, IPA, DI water.

Further, oxygen and sulfur functional groups were attached onto hBN which opened an optical bandgap at

3.5 eV. This functional groups aided in the stabilization in solvents for over 24 hours, much higher than the already reported processes. This process yielded ~20-25% exfoliated flakes and the flake size was dependent on the starting hBN flake size.

2. Process-Property Relations of liquid phase heterostructure

Liquid exfoliation was used to obtain graphene oxide and hexagonal boron nitride separately and mixed later to form a heterostructure of hBN and GO, termed as BCON or Borocarbonoxynitride. This new heterostructure that was formed was tested at various pH to understand its stability. Inorder to understand the structural and topological information, SEM, TEM and XRD were used, which confirmed that the 43

BCON that was synthesized still retained its parent properties and was still crystalline. TEM revealed that the distribution of hBN in BCON was heterogeneous. Further XPS showed us that new bonds were formed as a result of this BCON formation. This new linkage aided the stability of this material at pH ranging from 4-8.

3. Radiation interaction with heterostructures

The new heterostructure, BCON that was synthesized was tested for various radiation sources like x-rays, alpha, beta, gamma rays and heavy ion particle interactions. Similar to graphene oxide (GO), BCON had a higher response for alpha and gamma particles when compared with x-rays. As x-rays had much more energy, the surface functionalization removal could be monitored with time. This understanding of radiation effect of BCON will lay a foundation for understanding radiation interaction with this new class of materials and will provide a way to understand radiation impact for future materials.

4. Controlled Large area growth of TMD’s on Graphite

Transition metal dichalcogenides (TMD’s) especially molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) were synthesized using powder vapor deposition and metal organic chemical vapor deposition (MOCVD). Oxide and metal organic precursors were used in this case. As CVD is a much more controllable process, this current work on graphite will provide a direct grown material for hydrogen evolution and lithium ion battery applications. In the case of MoS2, vertical flower like structures were synthesized and the process was controlled to obtain these flowers over large area (1 cm x 1 cm). This was further characterized using techniques like SEM, TEM, AES and was found that the material had

100% conversion of MoO3 to MoS2, and XPS revealed that Mo:S was ~2. FIB and CS-TEM reveled that these flower like structures that were grown were over 1 μm thick, with an individual flake size of ~5-10 nm. In the case of WSe2, MOCVD was used to controllably grow this material on graphite paper. Further experiments were performed to controllably grow vertical vs horizontal flakes of this material. As

MOCVD utilizes metal organics, conversion of the MoS2 to MoSe2 was also performed easily. This 44

controllable conversion of MoS2 to MoSe2 shows the extent of control that we have over the growth process. These synthesized materials were further tested hydrogen evolution reactions and lithium ion batteries.

5. Improvement in hydrogen evolution

As synthesized materials were tested for hydrogen evolution as MoS2 was catalytically active material for hydrogen production. These testing on MoS2 revealed that the as synthesized vertical flowers on graphite have a Tafel slope of 185 mV/Dec, which was comparable to the previously reported values for vertical flakes of MoS2, although synthesized via hydrothermal reactions. This Tafel slope was further reduced to

99 mV/Dec by converting the semiconducting 2H-MoS2 to 1T-MoS2 using lithium intercalation. Further improvement of the Tafel slope was seen when this material was treated with UV-Ozone for ~10 minutes.

The Tafel slope for the 10 minute treated sample was ~54 mV/Dec, which was one of the lowest values reported for this structure. By treating with the UV-Ozone, MoS2 surface had the S-O and Mo-O bonding, which decreased the contact angle of the sample, thereby increasing the surface energy and conduction.

This study was further extended to study the HERs for WSe2. The as synthesized flakes for

WSe2/Graphite showed a Tafel slope of 64 mV/Dec, which was the lowest, reported for this material system. Further, the as converted materials were also tested for HERs and it was shown that for Se based compounds, having S defects on the structure helped reduce the Tafel slope and vice versa was not true.

6. Binder-free anode for Lithium ion batteries

Finally, the as synthesized MoS2 flowers were tested as binder-free anode material for lithium ion batteries. The performance was compared with the liquid phase synthesized material and it was shown that the CVD material outperformed the exfoliated MoS2, and showed a cyclic capacity of ~760 mAh/g.

As there is no further involved with the CVD material like the exfoliated material, it can be used directly as a binder free anode material.

Chapter 2

EXFOLIATION AND FUNCTIONALIZATION OF GRAPHENE OXIDE AND HEXAGONAL BORON NITRIDE

Introduction:

Hexagonal boron nitride (hBN) has been of interest because of its unique properties. It has a band gap of

5.9 eV, with a layered structure similar to that graphene. hBN has the same layer thickness as that of graphene, although insulating unlike graphene. Because of this property, hBN has been of interest to use as a dielectric material. Typical synthesis processes to make hBN involve chemical exfoliation using mechanical exfoliation, exfoliation in various solvents, chemical vapor deposition and pulsed laser deposition. In order to synthesize large quantities of hBN, chemical exfoliation has been used historically.

Similar to graphene exfoliation to form GO, hBN was exfoliated using solvents like ethanol, DI water, acetone etc., Although, the exfoliation efficiencies that were obtained using this process was very low.

Hence a large scale exfoliation process needs to be developed in order to use it to make various composites. This chapter summarizes thesis research focused on chemical exfoliation and functionalization of graphene oxide (GO) and hexagonal boron nitride (hBN). The modified Hummer’s process was explored as a primary means to achieve exfoliation, with specific chemistries and processing conditions being tuned to achieve maximum exfoliation efficiency. The effect of the exfoliation process was characterized by using scanning electron microscopy (SEM), transmission electron microscopy

(TEM), X-Ray diffraction (XRD) and Fourier transform infra-red spectroscopy (FTIR). The chemical information was obtained by using XPS and TEM analysis which showed that there are oxygen functional groups on the surface of hBN which opened up new optically active defects, which was further confirmed by using UV-Vis spectroscopy.

EXFOLIATION OF GRAPHENE OXIDE (GO)

SYNTHESIS PROCEDURE

Graphene oxide (GO) was exfoliated by a process similar to the approach taken by Marcano et.al71Figure

2- 1 summarizes the process, which is described here. First, 3 grams of graphite power was mixed with 18 grams of potassium permanganate. As permanganate is the exfoliating agent, it was added in a ratio of 1:6 to achieve consistent performance. It is also noted that higher permanganate ratio beyond 1:6 did not show too much difference in the exfoliation efficiency of the final exfoliated GO. To this, a mixture of sulfuric (360 ml) and phosphoric acid (40 ml) in the ratio of 9:1 was added slowly. The temperature of the solution was monitored as it raised to 35-40oC after mixing. This was then heated to 50oC for 12-16 hours under a constant mixing using a teflon-coated stir bar. The solution was then removed from the hot plate and cooled to room temperature. This was then added to a mixture of 400 ml frozen DI water and 3 ml of 30% hydrogen peroxide. After the addition, the solution were seperated into various centrifuge tubes. This solution was then washed simultaneously with hydrochloric acid, ethanol (or isopropyl alcohol) and DI water. After every wash, the material was centrifuged at 4000 rpm for 45 minutes to remove the supernatant. The bottom precipitate typically contains heavier ions and flakes with larger thicknesses and the supernatant usually contains mono-few layer flakes dispersed in the solvent. Careful seperation must be done inorder to ensure the complete seperation of the supernatant from the precipitate inorder to have maximum efficiency for the exfoliation. These washings were continued until the solution

- pH reached ~4. This cleaning process ensures the complete removal of the permanganate (KMnO4 ),

- - sulfates (SO4 ) and phosphate (PO4 ) ions, which could act as impurities and readsorb on the surface of

GO. Finally, after the centrifugation and rinsing cycles, the GO is dispersed in water, where it is stable for several months because of the various functional groups that were created during the process of exfoliation.

Free standing GO “paper” can also be obtained by either drop casting or spin coating on hydrophobic surfaces. The concentration of the GO can be varied to tune the thickness of the paper, from 0.01 mg/ml for ultra thin films (~500 nm) to 10 mg/ml for thick films (~10-25 µm). Spin coating or spray casting can 47

be used to coat the substrates with mono-few layer flakes on the surface. Inorder to ensure full coating, multiple iterations needs to be made. Further, the thick paper like material can be ground using a mortal and pistle and can be made into powder form, which could be used to make composites.

o sulfuric acid and phosphoric acid (H2SO4+H3PO4), Stage 2 involves the heating this mixture to 50 C while stirring it constantly at 200 rpm for 18-24 hours, Stage 3 and Stage 4 involves the centrifugation and sonication of the acid mixture to remove the excess ions of permanganates, sulfates and phosphates in the solution. This is achieved by careful monitoring of the pH, which should reach a value of 4.

Structural and Chemical Characterization of GO

Following the exfoliation process, scanning electron microscopy (SEM) was used to identify the bulk thickness and sheet assembly when drying the GO into a paper. As it can be observed in Figure 2- 2 (a), a free standing thin sheet of approximately 25 nm was formed by spin coating the sample onto a silicon substrate, and later peeling it off. Further thickness profile of the exfoliated sheets were characterized using atomic force microscopy (AFM). As it can be observed from Figure 2- 2(b), most of the flakes are monolayer and the bilayer flakes overlap each other. It can also be observed that the flake size for the exfoliated flakes did not change a lot from the bulk graphite that was used. Most of the exfoliated flakes ranged from 10 µm – 50 µm in diameter, which matches the domain size of the initial natural graphite powder. The thickness of the monolayer graphene oxide flakes was ~0.4 nm, where as bilayers have a thickness of 0.74 nm. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy was used to understand the effect of oxidation on the graphene oxide sheets. As it can be observed from Figure 2- 2

(c), very little contamination (<2%) from the ions is left after the cleaning process. The oxygen to carbon ratio is ~1.8, which is a result of the oxidation process. Further understanding of the functional groups were made by looking at the high resolution C 1s spectra (Figure 2- 2(d)). In order to identify the various functional groups such as the C-O, C=O, O=C-OH along with the sp2 and sp3 carbon that were formed as a part of the exfoliation process. Raman analysis (Figure 2- 4e) of the material indicated the two characteristic peaks of GO i.e., D peak at ~1350 cm-1 and G peak at 1570 cm-1. By looking at the ratio of

D/G (~0.8) indicates the amount of disorderness in the structure. This exfoliation process was very effective in functionalizing the graphene oxide as the O/C ratio as measured by XPS is 1.8.

Figure 2-2. (a) SEM image of GO sheet, (b) AFM map of GO dried on Si substrate, (c) XPS spectra of

GO showing the effective removal of sulfur and other impurities, (d) High resolution C 1s spectra showing the various functional groups present after exfoliation process and (e) Raman spectra showing the presence of D at ~1350 cm-1 and G at 1580 cm-1.

EXFOLIATION OF HEXAGONAL BORON NITRIDE:

SYNTHESIS PROCEDURE

To synthesize mono- and few-layer hBN, a process similar to GO exfoliation was used. In this process, 1g of hBN powder (99.5%, particle size 1-5 µm, Alpha Aesar) was mixed with 6g of potassium permanganate (KMnO4, A.C.S Reagent, J.T. Baker) in a 1L glass beaker. An acid mixture of 135 ml was prepared separately by mixing phosphoric acid (H3PO4, 85% w/w, EMD chemicals) and sulphuric acid

(H2SO4, 95%, EMD chemicals) in the ratio 1:8 and was added to the hBN dry mixture. The resultant reaction is slightly exothermic, leading to a temperature rise in the solution to 40oC. The solution was then heated on a hot plate to 75oC under constant mixing for 12 hours. Subsequently, 6 ml of hydrogen 50

peroxide (H2O2, 30% w/w, A.C.S Reagent, J.T. Baker) and 120 ml deionized water (DI water) was added to this mixture to halt the oxidation. This approach was similar to the synthesis of graphene oxide by

Marcono et.al.,71 with conditions tuned for the hBN material system. The resultant suspension was cooled to room temperature and centrifuged at 6000 rpm for 30 minutes, and the supernatant was pipetted out from the solution. Subsequently, 45 ml DI water was added to this solution and centrifuged again at

6000 rpm for 30 minutes. Non-exfoliated material was removed during the centrifugation and the supernatant solution was then subjected to a series of washing and centrifugation stages with DI water, ethanol and HCl. These washings were performed until a pH >3 was reached, which ensures the complete removal of the metal ions. The supernatant solution is then drop-casted or spin coated on Si wafers and dried to obtain the exfoliated and functionalized hBN sheets.

STRUCTURAL CHARACTERIZATION OF EXFOLIATED HBN

The exfoliation process results in mono- and few-layer hBN sheets that are distributed uniformly throughout the suspension. Initial hBN powder (before exfoliation) exhibits a variety of particle clusters, with varying thickness of hBN platelets (Figure 2- 3 (a)). The thickness of the hBN platelets can be controlled by carefully centrifuging the hBN in solvent. Various centrifugation speeds were used to obtain hBN platelets of different thicknesses. Lower centrifugation speeds (~1500-2000 rpm) results in thicker platelets (100-200 nm) while higher centrifugation speeds yields thinner flakes. In order to obtain the thinnest flakes, the solution was centrifuged at 6000 rpm for 30 minutes. The resultant supernatant was pipetted out and drop-casted on to a substrate for further analysis. After separating out the exfoliated flakes by centrifugation, a highly concentrated solution of exfoliated flakes was obtained. From the weight balance, the suspension typically consisted of about 12-25% of the starting hBN weight, providing a significantly higher yield than previously reported.8 The weight balance of hBN samples was made using a micro balance in a TGA. The sample weight after exfoliation was obtained by dispersing the exfoliated hBN in IPA or ethanol and drying it on aluminum boat. Typically, for about 1 gram of starting material, about 230-250 mg of exfoliated hBN is produced. If the temperature is not controlled, the yield 51

can go as low as 12% as the reaction mixture doesn’t have enough reaction time, and the evaporation of the solvent mixture was observed. The optimal temperature 75oC, which pertains to the reaction temperature of the mixture.

Following the exfoliation process, most of the sheets are transparent to the electron beam at 2 kV clearly indicating a near atomic thickness of the hBN sheets (Figure 2- 3 (b),(c)). Importantly, while the sheet thickness is reduced to <5 nm as confirmed by atomic force microscopy (AFM), majority of the flakes were over one micron in size. Ultimately, the lateral hBN flake size is dependent on the bulk hBN powder starting material, where the maximum sheet size observed was always close to that quoted for bulk hBN

(particle size 1-5 µm, Alpha Aesar). This lateral dimensions of the exfoliated sheets were similar to that of graphene oxide obtained by the similar process71. Bending of hBN sheets is also observed (Figure 2-

3(c)), indicating that these sheets are “peeled” from the bulk hBN material during the exfoliation process.

This type of morphology indicates that few layers hBN is highly flexible. Finally, the dispersion stability of the exfoliated hBN (Figure 2- 3(d)) through the current process is significantly more stable and transparent in deionized water compared to previous reports8. As observed in Figure 2- 3(d), the hBN dispersed in DI water was much more stable at a relatively higher concentration (0.2 mg/ml) than the previously reported.8,96,290 Additionally, the suspension stability of the exfoliated hBN was studied in acetone, isopropyl alcohol (IPA), ethanol and DI water. As we can see from the Figure 2- 1e, the dispersion stability of hBN was uniform throughout the solvent and the hBN remained suspended for over

24 hours. This stability is the result of hBN functionalization during the exfoliation process and this property could be used to make composite materials such as polymer composites etc., where stability plays a major role in obtaining uniform material. Further investigation of the optical properties (Figure 2-

3(f)) was accomplished by UV-visible absorption spectroscopy. The hBN before exfoliation exhibits one absorption peak at ~ 210 nm, corresponding to a band gap of 5.8 ± 0.1 eV, similar to previous reports.291

Exfoliated hBN exhibits two absorption peaks: ~210 nm (5.7± 0.2 eV) and 330 nm (3.9 ± 0.1 eV). The first absorption peak at 210 nm can be correlated to the stable ring structure of BN in the basal plane. The 52

exfoliated hBN exhibits another absorption peak at 3.9 ± 0.1 eV which could be correlated to the opening of a mid-gap state in hBN because of the addition of sulfur and oxygen functionalization.

While SEM provides evidence of exfoliation, high resolution transmission electron microscopy

(HRTEM) provides a more comprehensive view of the flake nanostructure. Evident from Figure 2- 4(a) and 4(c), there can be a random aggregation of the sheets, with no registration between layers. Similar aggregation of layers was observed in AFM while determining the thickness of the exfoliated hBN. The atomic structure of hBN in Figure 2- 4(a), at spot 1, is presented in Figure 2- 4(c), and clearly delineates the honeycomb structure of the hBN. Several dark regions in Figure 2- 4(c) indicate the possible location of sulfur functionalization, which is confirmed by electron energy loss spectroscopy (EELS) (Figure 2- 53

4(e)). Further bonding information was observed through x-ray photoelectron spectroscopy, and confirms the presence of sulfur. All observed hBN sheets are highly crystalline, irrespective of the thickness and size indicating that functionalization of hBN may primarily occur at flake edges where the hBN is most reactive. X-ray diffraction (XRD) of hBN drop casted on a Si substrate was also used to verify the crystallinity of the exfoliated hBN. 54

XRD DATA ANALYSIS:

The data obtained through XRD (Figure 2- 5(a)) clearly indicates that the majority of the crystals are

o oriented in the (002) direction which is the characteristic peak observed at 26.7 . Further, from the Joint

Committee on Powder Diffraction Standards (card number 34-0421) for hBN powders, the diffraction peaks can be indexed to the hBN crystals (lattice constants a=b= 2.504 and c=6.656 A0). The observed peaks in the hBN (Figure 2- 5(b)) spectra are at 2θ = 26.7640, 41.5970, 55.1640 with a d-spacing of 3.328

A0, 2.169 A0and 1.663 A0 which can be correlated to the (002), (100) and (004) planes of hBN where the

(004) is parallel to the (002) plane. The intensity of the (004) plane is higher when we compare the ratio of intensities of (004) and (100) planes in bulk hBN and exfoliated hBN. I004/I100 is 2.5 for bulk hBN and

I004/I100 is 6 for the exfoliated hBN indicating the effectiveness of the exfoliation process. As the hBN sheets tend to lie on their widest facets when they are dispersed randomly on a substrate, we found that

(002) planes are the widest according to the measurement, concluding that the exfoliation occurred at the

(002) plane.

( (

a) b)

Figure 2-5. X-ray diffraction pattern for (a) hBN before and after exfoliation, (b) hBN and bare Si substrate.

THICKNESS CHARACTERIZATION OF hBN:

As hBN has AA stacking, it is very difficult to identify the layer thickness just from TEM alone. In order to confirm the thickness of the exfoliated sheets, AFM was used. The samples for AFM were prepared by drop casting the exfoliated hBN on Silicon wafer. This drop casting procedure induced the flakes to stack on top of each other as it can be seen in Figure 2- 6. The two different spots that were analyzed shows that the flakes that are stacked are mono layer and bi layer materials. The thickness for these flakes were 0.413 nm and 0.837 nm, which are consistent with previously reported AFM thickness (0.4-0.5 nm for monolayer, 0.8-0.9 nm for bi layer and 1.2-1.3 nm for three layer system) of few layer hBN.98 The deviation from the standard layer thickness of 0.33 nm could be because of addition of functional groups on the surface of each sheet.

Figure 2-6. AFM image of exfoliated hBN and its corresponding height measurements at different spots.

As it can be observed in Figure (a), spot 2 has a monolayer whereas the spot 1 is a bilayer. In order to 57

obtain the AFM scans, samples were dispersed in ethanol and drop casted on the Silicon dioxide substrate, which made the exfoliated flakes stack on top of each other.

Another correlation for the thickness of the hBN can be made by using Raman microscopy. hBN has a

Raman signature peak at ~1360-1375 cm-1 depending on the layer thickness. The Raman spectra for the bulk vs exfoliated hBN is shown in Figure 2- 7. We can see that the peak broadens when compared with that of the bulk hBN and the intensity of the peak is reduced by 85% when compared with the bulk. This reduction in intensity is observed when the layer thickness is reduced. This shift in the peak position can be linked to the strain which is caused due to the stretching of the sheets and hence a red shift in the peak is observed in the Raman spectra. This observed stain-induced shift is similar to that of graphene, where

-1 98 the G peak is red-shifted as much as 20 cm . As pointed out by Watanabe et. Al., the strain induced shifts in pristine graphene can be covered by doping effects292 which move the G peak in graphene by ~10 cm-1. As these doping effects absent in hBN, we don’t see a lot of shift (~3 cm-1) in the peak position.

Also, FWHM for exfoliated hBN (22 cm-1) does not change significantly when compared with the bulk hBN (20 cm-1) which signifies that there is significantly less strain induced in the material during the exfoliation process.

Figure 2-7. Raman spectra of bulk hBN Vs exfoliated hBN, showing a shift of 3.3 cm-1 indicating a significant reduction of sheet thickness.

CHEMICAL FUNCTIONALIZATION STUDY OF hBN

X-ray Photoelectron Spectroscopic analysis

Chemical functionalization information for the hBN was obtained by using x-ray photoelectron spectroscopy (XPS). Fresh samples were taken for this study as XPS is a surface technique and care was taken to reduce the contamination. Typically, the samples for XPS were deposited on a Si substrate by using drop casting, where hBN were already dispersed in IPA. A higher vapor pressure solvent was used in this case and IPA did not leave a lot of residue after the drying process. As XPS probes the top 10 nm of the sample surface, majority of the functional groups that were observed can be related to the surface of hBN. Figure 2- 8 shows the comparison between the bulk hBN and the exfoliated hBN. As it can be observed from the survey spectrum, hBN sample before exfoliation has its standard boron and nitrogen peaks at 191 eV and 398.5 eV, with a tiny bit of contamination from carbon and oxygen. This presence of carbon can be because of the drying process as hBN was dissolved in IPA. After exfoliation, the contamination from carbon significantly decreases and the presence of oxygen content increases. Along 59

with oxygen, other species like sulfur (S) and phosphorous (P) can be observed in the survey spectrum.

This addition of S and P comes from the acid exfoliation process we use H2SO4 and H3PO4. Most of the

Manganese and sulfates ions are removed which is evident from the absence of these elements in the survey spectrum.

Figure 2-8. Plot showing the survey spectrum for hBN (a) before exfoliation and (b) after exfoliation.

After exfoliation we can see that there is an enhancement in the O 1s and addition of sulfur and phosphorous peaks along with the standard boron and nitrogen.

The individual elemental composition as obtained from the CASA, which is listed in table 2.1, corresponding to Figure 2- 8. The elemental composition was calculated by analyzing the area under the curve for the high resolution peaks. A linear background fit was performed on all the regions. It can be seen that the initial starting hBN was not 100% stoichiometric on the surface, which drastically reduces after the exfoliation. We can clearly observe that there is a significant increase in the oxidation and addition of S and P in the exfoliated hBN. This indicates that the addition of the components have occurred during the exfoliation process.

Table 2.1. Elemental composition of hBN as calculated using CASA XPS.

Species hBN Before exfoliation (%) hBN After exfoliation (%)

B 1s 34 ±3 31±3 N 1s 30±3 20±3 O 1s 4±2 29±3 C 1s 32±3 14±3 S 2p 0 2 P 2p 0 4

High resolution peaks of B 1s, N1s and O 1s are analyzed to understand the bonding characteristics of the samples. The stoichiometry of boron to nitrogen was obtained by taking the ratio under the curves for boron and nitrogen. The background was fitted with a linear fit and a Gaussian-laurentian fit was used to fit the peaks. By looking at the area under the curve, the ratio for B/N increased from 1.1 to 1.63, indicating a change in the nitrogen percentage after the exfoliation process. This decrease in the nitrogen could be supported by accommodating the additional oxygen and sulfur functional groups in the exfoliated hBN. Further analysis was made on the high resolution spectra of individual elements of boron, nitrogen and oxygen. B 1s has a peak corresponding to hBN at ~190 eV. The deconvolution of the B 1s peak clearly indicates a change in the peak structure (Figure 2- 9 (a) & (d)). The additional boron- oxygen293 bonding was observed at 192.5 eV and the boron-sulphur294 bonding at 191.3 eV.

Deconvolution of the O 1s peak (Figure 2- 9(b) and 9(e)) shows an increase in the 532.5 eV peak, corresponding to enhanced oxygen-boron bonding. Spectroscopic evidence of the boron-oxygen bonding was also confirmed by Fourier transform infrared spectroscopy (FTIR). As no literature exists for boron- phosphorous bonding, and hence it cannot be shown accurately. These changes in the high resolution peak structure clearly indicate the addition of functional groups to the boron sites in the exfoliated hBN. 61

Figure 2-9. High resolution XPS spectra of (a) B 1s, (b) O 1s, (c) N 1s before exfoliation showing no additional bonding in the individual species, (d) B 1s spectrum showing the addition of sulfur and oxygen bonding of Boron, (e) O 1s shows an increase in oxidation due to B O bonding and (f) Unchanged N 1s 2 3 peak structure suggests the functionality is added to the boron sites.

As we can see from Figure 2- 10, there is a slight non-uniformity with boron-oxygen bonding that was collected on a different sample region. This could be because of the handling of the sample after the exfoliation process i.e., sampling drying environment and transfer of the sample after drying. These conditions might impact the functionalization of the hBN as it can be noticed from the XPS measurements. 62

FOURIER TRANSFORM INFRA-RED SPECTROSCOPY (FTIR)

FTIR was used to identify the functionality in the exfoliated hBN. As hBN is highly absorbing material in the IR region, the sample had to be diluted using KBr powder (0.1 mg of hBN in 500 mg of KBr) in order to obtain an accurate spectrum. The characteristic B-N stretching mode is typically observed at 1366 cm-1 for the exfoliated sample along with the out of plane bending mode at 814 cm-1 respectively. From Figure

2- 11, we can also see that there is an addition of new peak at 1154 cm-1 and a significant increase in the peak at 3450 cm-1 which could correspond to the addition of oxygen in the sample and these peaks can be indexed to B-O vibrational mode and –OH absorption in the sample295. Also, we can see that the full width half maximum for the exfoliated peak has increased by 15 cm-1 which could be related to the addition of the functional groups to the structure. 63

Figure 2-11. (a) FTIR spectra of hBN before and after exfoliation. (b) Picture showing the presence of a new peak at 1054 cm-1 which corresponds to B-O bonding.

Chapter 3

LIQUID PHASE SYNTHESIS OF GRAPHENE HETEROSTRUCTURES

Introduction

Graphene is preferred choice for nanocomposites because of their synergetic effects which allow the atomic scale control of the functional groups, which is one of the important properties when forming composites. Typically, graphene has been used as a filler material to improve the performance of polymers to improve their thermal and mechanical properties296–298. As graphene is very low density material, forming a composite will aid in improving the properties without adding significant weight to the formed composite material. Formation of composite material via graphene oxide (GO) route is easier because of its dispersion capabilities in multiple solvents and its stability in the solutions. Another layered material of interest for composites is hexagonal boron nitride, which has a relatively high thermal conductivity along with being completely insulating (band gap ~5.6 eV)84,85,87,141. As hBN and graphene have relatively small lattice mismatch of 1.7%, their integration allows in tuning the bandgap which could be different from the graphene and hBN band gap299, hence utilizing in several electronic and optoelectronic applications. Having a mixture of graphene and boron nitride will allow us a way to tune the optical properties by tuning the bandgap based on the composition and atomic configurations in the material300. Formation of these BCN composite structures301–303 showed a strong impact in applications such as hydrogen storage303, electronic devices, field-emission, super capacitors etc. Recently, it has also been shown that forming composites with graphene oxide and hexagonal boron nitride has improved the structural strength of graphene foam alone where the boron nitride acted as an anchor sites to stabilize the 65

graphene structures. It was also shown that adding a mixture of graphene and hBN has improved the thermal conductivity of the PTFE polymer dramatically than adding them individually.

Recently, reduced graphene oxide and hBN were used to form sponges which showed an improvement in the overall mechanical properties of the material system304. It was also shown that mixing of hBN in rGO improved the performance and cyclic efficiency in lithium ion batteries305 and it provided a way to prepare binder free anode materials for lithium ion batteries. Also, recent reports indicated that addition of hBN and GO simultaneously improved the thermal conductivity and dimensional stability in the flexible polyimide films306. Further, BCNO structures have been used as light emitting diodes although synthesized differently307–310. Hence, in the current work, an alternate approach is chosen to synthesize the composites based on graphene via liquid exfoliation using common solvents and relatively faster.

Synthesis of BCON: Structure - Property Relationship

Graphene Oxide (GO) and hexagonal boron nitride (hBN)91 were exfoliated seperately and mixed in a sonicator for 2 hours at 65oC. After this simple mixing, the material was dried on teflon sheets to form film like materials. Depending on the concentration of GO and hBN initially used, different types of materials were obtained. The interaction of GO and hBN in BCON varied with the pH of the starting GO and hBN. The pH of the solution was adjusted with the help of 0.1 M HCl and 0.1 M NaOH. Very high or very low pH did not favor the formation of this BCON structure because of agglomeration in solutions

(Figure 3- 1 b). This pH dependent behavior is observed because of the COO- functional sites that are present in the edge sites of GO. These carboxyl groups are protonated at low pH, hence GO becomes less hydrophilic and it forms suspended GO aggregates. These aggregates then form large sandwich like structures which are stable in the solution and hence we don’t find any agglomeration. At pH>8, these carboxylic groups are deprotonated. The presence of hydroxyl basal plane groups further causes a change in hydrophilicity of GO which causes it to agglomerate and form thick sheets at high pH and hence we can observe this color change in the suspensions as we go higher in pH311–314. At pH 4-8, this effect is 66

minimized and hence the bonding between GO and hBN was observed. Over time, the sheets tend to agglomerate and only stable suspensions were formed at a pH 4-8, which can be observed in Figure 3- 2.

The structural properties of the BCON was analyzed by varying the amount of GO and hBN that are mixed while maintaining a constant pH (~ 4). By varying the percentage of hBN in GO solution, variety of structures was obtained. The GO rich suspensions tend to dry out as a paper like materials whereas the hBN rich suspensions tend to dry out as ribbon like structures. This structural variance can be observed in

Figure 3- 1 (f) – 1 (h). The 50:50 mix of GO and hBN dries out as film with hBN particles embedded in the GO sheets as observed in Figure 3- 1(g).

corresponding to (c) and (d), and (h) SEM image corresponding to (e), showing the formation of ribbon like structures.

pH dependence with time

Further, pH dependence of the BCON suspensions were studied over time to see the stability of the material. It is clearly observed that very low or very high pH suspensions tend to settle down immediately. At a pH of 4-8, the solution is more stable for over 6 hours, which is observed in Figure 3-

3. At lower or higher pH, the aggregation of sheets is evident with time as the COO- functional groups in

GO tend to bond with the next neighbouring sheets forming bulk structures of GO, which tend to change the absorption, hence settling down in the solution. Similar behaviour can be used to explain the higher pH.

Structural Characterization Of BCON

Nanoscale distribution and crystallinity was observed using Transmission Electron Microscopy (TEM).

Inorder to observe the nanoscale distribution, a dilute solution was made which was then drop casted on a

Lacey Cu TEM grid, and dried overnight. The distribution of hBN in GO can be observed in Figure 3- 4a and Figure 3- 4b. Although the distribution of hBN sheets in GO was not uniform, there is large mix of the stacking structure of hBN on GO which can be seen in Figure 3- 4b. The bulk crystallinity was again confirmed by XRD measurements. Free standing films were used for this measurement and we can see from Figure 3- 4c, most of the flakes were oriented in (002) direction indicating the high uniformity after the exfoliation process. The peak at 10o corresponds to GO (002) plane, indicating that the graphene oxide was not reduced after the processing. The peaks for rGO and graphite occurs at 2θ = 25o and 27.5o, showing the efficient exfoliation of graphene oxide. hBN typically has peaks 27.5 and 55 corresponding to the (002) and (004) peaks. As it can be observed that all other peaks corresponding to other planes cannot be observed unlike the exfoliation of hBN. This indicates that most of the flakes tend to orient in

(002) plane after the synthesis of BCON.

Further TEM analysis was conducted on the sample and EELS spectra was studied at multiple regions as observed in Figure 3- 5 (a). The EELS spectra at location A and B show the visual presence of boron and nitrogen at A and carbon and oxygen at B, indicating that they are planar arranged. Further diffraction patterns were obtained at these locations showing the crystallinity of this material. Although accurate d- spacing cannot be calculated as graphene and hBN have similar lattice parameters.

Auger Electron Spectroscopic Analysis (AES) of BCON

Further confirmation of the stacking structure was obtained using Auger electron spectroscopy. As dried films of BCON were used for the study of the surface distribution of hBN in GO. It can be observed from

Figure 3- 6, AES was performed on multiple spots on the sample and presence of the hBN in GO on the surface could be observed. As auger electrons typically appear from the top 5 nm of the surface, it could be confirmed that the dispersion of hBN was throughout the top layers of the BCON. Although, looking at multiple spots, it was confirmed that the distribution was non-uniform as the total signal that was 74

obtained for Boron and nitrogen when compared to carbon and oxygen is not the same at every spot. This distribution was similar to the observations made in TEM.

Figure 3-6. (a) & (b) AES topography of BCON at sample 1 and sample 2 showing the presence of boron, nitrogen, carbon and oxygen at varying concentrations.

Chemical analysis of BCON

X-Ray Photoelectron Spectroscopy studies

Chemical bonding information of BCON was obtained using X-ray photoelectron spectroscopy (XPS). As it can be observed in Figure 3- 7, the high resolution peaks of carbon, oxygen, boron and nitrogen were analyzed to observe the presence of any new bonding. The deconvolution of the individual peaks revealed the presence of new bonds in the material. By analyzing the high resolution B 1s peak at 190 eV, it is clearly evident that the boron-oxygen bond (192 eV) was not completely lost during this process. This peak broadened slightly when compared to the original peak of B1s in hBN. Although, new bonds were observed in the nitrogen (N 1s) peak which is evident by looking at the peak at 402.6 eV, which was assigned to O=C-NH, along with the N 1s peak corresponding to hBN at 398.5 eV. This peak was also evident after deconvolution of the carbon 1s at 289.8 eV corresponding to nitrogen bonded to carbon

(O=C-NH) along with the other peaks which correspond to GO (C-O, C=O, C-C SP2, C-C SP3). These peaks can be further confirmed by looking at the oxygen 1s spectra which shows the presence of this additional peak for O=C-NH bonding at 530.8 eV. All the peaks were fitted with a Shirley background and a Gaussian-laurentian line shape was used for all the peaks.

Figure 3-7. High resolution XPS scan of (a) C 1s showing the presence of O=C-NH bond ,(b) O 1s confirming the presence of O=C-NH bond, (c) B 1s showing the addition of B-O and (d) N 1s showing the presence of additional peak.

Raman Study on BCON

Further confirmation of the bonding information was obtained by looking at the Raman specta for the

BCON. As observed in Figure 3- 8, we can see that the D peak of Graphene oxide and the hBN peak overlaps which are closely located around 1370cm-1. The deconvolution of hBN vibration with D peak of

GO is difficult as the D peak of GO is very sample dependent. Hence, this technique could not be used quantitatively to determine the presence of hBN and GO individually.

Figure 3-8. Raman spectra showing the BCON spectra where the D band of GO overlap with hBN at

1371 cm-1 and the G band at 1600 cm-1. The peak broadening in the D band overlap shows the presence of

GO and hBN.

FTIR Study of varying composition of GO:hBN in BCON

Further characterization to confirm the BCON bonding via spectroscopy was performed using fourier transform infra-red spectroscopy (FTIR). Multiple solutions were dried to form BCON of varying composition with GO: hBN at a pH 4. From Figure 3- 9, it is evident that with increasing hBN in GO, the signal strength rises for B-N vibration mode and B-N-B vibration mode at 1370 and 800 cm-1. This B-N stretch at 1370 cm-1 also covers the C-O vibration mode in GO. As we can see that the peak at 1080 cm-1 also increases, which corresponds to the B-O peak as observed in exfoliated boron nitride. Although, the bonding peaks which are observed in BCON via XPS couldn’t be confirmed as they are masked in the

OH stretch.

Figure 3-9. FTIR spectra of various compositions of GO: hBN showing the difference in the peak intensity. The B-N-B (~800cm-1) and B-N (1370 cm-1) vibration corresponds to the hBN where as the C-

O, O-H, C=O correspond to the GO. 79

Synthesis of Reduced Graphene Oxide/ MoS2 Composite

Another composite structure based on graphene oxide was made by using liquid exfoliation. As synthesized GO with a concentration of 1 mg/ml was used as a starting material and was reduced in

o ethylene glycol for 3-5 hours at 80 C. Molybdenum disulfide (MoS2) bulk powder was taken in a separate round bottom flask as shown in Figure 3- and was exfoliated at 50oC using ethylene glycol as an exfoliation agent. As the surface tension of ethylene glycol was ~42, which is the closest solvent for exfoliation of 2D materials (cite). This solution acts as an exfoliation agent and the bulk MoS2 was reduced to few layer flakes in the solution after 16-18 hours. Later, the rGO in ethylene glycol was added

o to this exfoliated MoS2 and was reduced further for another 24 hours in the same beaker at 50-70 C, which resulted in the formation of the composite rGO/MoS2 mixture. This was further dried using a vacuum filter as shown in Figure 3- 10 (c) & (d), and washed several times with ethanol and DI water to remove the ethylene glycol. It was also noted that the filtration when done with hot ethylene glycol proved much more effective in remove all of the solvent when doing the washing stage. Later the final product was collected with the help of a Teflon filter and later dried to give a film like material on the

Teflon filter.

ethylene glycol free composite on Teflon lined filter and (d) Dried composite on Teflon filter which forms like a paper.

Structural and chemical characterization of the MoS2/rGO composite was performed using SEM and

Raman microscopy. As observed in Figure 3- 11 (a) and (b), the SEM images of the MoS2/rGO composite revealed that most of the flakes are non-uniformly distributed throughout the rGO sheets and the rGO sheets formed a matrix to encapsulate the MoS2 flakes. Because of the use of ethylene glycol solvent, some of large flakes of MoS2 were broken down into smaller flakes, which was common from this type of synthesis. Raman spectra was collected on the sample and it can be observed from Figure 3- 11 (d), the

-1 MoS2 peaks appear at 387 and 408 cm corresponding to E2g and A1g vibration modes and rGO peaks

-1 -1 appear at 1350 and 1600 cm . As it is evident from the Raman spectra that the FWHM of the 1350 cm and 1600 cm-1 peak reduced from the original GO, confirming the reduction of the GO. This was further confirmed by looking at the high resolution XPS of the composite (Figure 3- 11 (e)). As it can be observed in C 1s peak, the functional groups C=O, C-O are reduced leaving only the C-C bonds which is located at 284.9 eV. Also, looking at the Mo 3d and S 2p high resolution spectra shows that the material is still stoichiometric MoS2, similar to that of the starting material, although exfoliated. There is no

6+ significant presence of the Mo (corresponding to MoOx) state which usually occurs at 236 eV, showing the relative inert nature of the solvent during the exfoliation process. 81

(d) Raman spectra corresponding to the MoS2/rGO composite. The peaks of MoS2 were observed at 387 and 408 cm-1 and for rGO at 1350 and 1600 cm-1. (e) High resolution XPS spectra for C 1s showing the reduction of the graphene oxide evident from reduced C=O and C-O peaks and Mo 3d, S 2p peaks remain unchanged and there is no oxidation of Mo or S.

Chapter 4

HYBRID SYNTHESIS OF 2D HETEROSTRUCTURES

Introduction

Carbon based materials have been used as substrates for applications like lithium ion batteries, hydrogen evolution reactions, methanation reactions etc., because of their inert nature and stability. Metal supported carbon based materials have been studied for over a decade in these fields, which advanced the understanding of nanoparticle effects on catalysis, and helped in improving the efficiency of the catalysts.

Graphene based materials have been used recently for LiB’s and HERs as substrates as they provide an inert surface and does not participate in the chemical reaction. In the case of LIB’s, careful assembly of graphene sheets to form graphite has shown improved stability of the battery performance and increased the cyclic capacity to ~300 mAh/g, and hence it has been used as a base material for current state of the art LIBs. As graphene is semimetal, it can provide an excellent conduction and this specific property has been utilized in electronics and energy conversion applications such as HERs, where it could be used as a substrate.

Another class of layered materials, the transition metal dichalcogenides (TMDs) have been of interest because of their semiconducting nature and have a band gap ranging from 1.2-1.8 eV depending on the number of layers. As these are also layered materials, it sparks the interest for catalysis because of its exceptional surface area, similar to graphene. Out of the various available TMD’s, molybdenum disulfide and tungsten diselenide have been widely used for various different applications. Beyond electronic applications, inorder to use these materials for energy applications, high aspect ratio materials need to be synthesized. Typical synthesis techniques for these applications involve the use of hydrothermal synthesis or liquid exfoliation techniques, which usually take over 24 hours for the growth to happen, and require further processing inorder to integrate this solution synthesized material into forming composites. 83 Although the scalability is not an issue in hydrothermal synthesis, control over the growth morphology after the synthesis in making heterostructures is very difficult. As integration involves further processing of this liquid material, direct synthesis techniques needs to be explored which will be impurity free.

Chemical vapor deposition (CVD) techniques have been developed for this particular growth using different precursors such as transition metal (VI) oxide, transition metal chloride and transition metal carbonyls etc., as transition metal sources and chalcogen powders as chalcogen sources. As these processes are performed at much higher temperatures (>500oC), there is a higher probability to obtain a much cleaner material. Typical CVD synthesis materials yield a lot of control over growth in both morphology and thickness. Conditions can be tuned to promote more vertical growth, which would further improve the amount of active surface area available per unit area of material. Hence it is important to explore a controllable way to grow uniform large flakes of TMDs across a large area.

In the current work, controlled large area syntheses of TMDs/graphite specifically MoS2/graphite and

WSe2/graphite have been explored for specific applications such as hydrogen evolution reactions and lithium ion batteries. Control over the orientation of growth by tuning the CVD conditions will give us the opportunity to directly use these materials for these applications.

Integration of Graphene and Graphene Oxide (GO) as substrates

Monolayer graphene has been used as a substrate to grow hBN via CVD, which showed us that it can be an excellent substrate material. Beyond electronic applications, stacks of graphene have to be made inorder to use them for catalysis or Lithium ion battery applications. Growing TMDs on top of these bulk graphene or graphite materials can be used directly as catalysts and electrodes for these energy conversion and storage applications.

Previously, chemical exfoliation was used to stack GO and hBN to form BCON, which showed some unique properties and higher sensitivity for radiation sensing. Similar strategies can be used to stack

TMDs with GO. As a first approach, GO with concentrations of 1-5 mg/ml were used to form the 84 composites with TMDs. In order to use the GO as a substrate, highly concentrated solution of GO was made where concentration >10 mg/ml. This solution was then dispersed on a teflon substrate either drop casted or spin coated, and spread uniformly. The resultant dried film has a thickness of ~10-25 microns, which could be used as a substrate for TMD growth.

Graphite Paper (GP) Characterization

The graphite paper as received from global graphene and graphite co. was used as the substrate for the

MoS2 growth. The as received graphite paper was ~10 μm in thickness and flexible (compared with the other substrates such as SiO2, sapphire etc.,) which can be observed in Figure 4- 1(a). This was further characterized using FESEM to observe the surface morphology. As it can be observed in Figure 4- 1(b), the graphite surface looks clean with steps of graphene sheets assembled together. Further characterization using Raman and XPS was performed to observe the quality of the graphite paper. From

Figure 4- 1 (c) we can see that the G/D ratio of graphite is pretty uniform with very little D peak at ~1370 cm-1, indicating the high purity of the assembly process of the graphene sheets. Further x-ray photoelectron spectroscopy (XPS) analysis was used to identify the surface chemistry of the graphite paper. As it can be observed in Figure 4- 1(d), the surface is mostly clean and free of any impurities. The slight nitrogen content in the graphene surface comes from the assembly process of the graphene sheets. It is also observed that there is very little amount of oxygen (~2.5%) present on the graphene sheets.

Growth of MoS2 via Powder Vaporization

Powder vaporization was used to grow the transition metal dichalcogenides specifically Molybdenum disulfide (MoS2) on substrates such as graphite, silicon, silicon dioxide and sapphire. As powder vaporization involves the use of precursors based on Molybdenum and sulfur were used to grow MoS2.

For Molybdenum, MoO3 obtained from sigma Aldrich (99.9% purity) was used as a precursor powder and pure sulfur (Alfa aesar, 99.99%) was used as the sulfur source. A specially designed crucible was used to support the substrates as shown in Figure 4- 2. The crucible was obtained from robocasting and was made of alumina. The crucible was specially designed to hold 5 samples of 1x1 cm2 and was elevated at a height of 1 cm where the precursor MoO3 can be placed. The dimensions of the crucible were made from previous observations of the growth. A horizontal tube furnace was used for the growth where the temperature was controlled between 750-800oC and a pressure of 10-500 torr was maintained throughout the growth process.

Precursor control

Sulfur powder was used upstream and an Ar carrier gas was used throughout the growth run. The sulfur powder was heated to 160-250oC depending on the amount of sulfur needed for the growth. The amount of the precursor was varied depending on the type of growth morphology that was needed. Typically, vertical flower like structures were grown using this process irrespective of the substrate that was used.

Several growth optimization runs were carried out by varying the pressure, temperature and precursor amounts. Molybdenum oxide (MoO3) decomposes into MoO2 at higher temperatures, and the MoO2 typically converts into MoS2 in the presence of sulfur. When the pressure of the system is <500 torr, most of the material that grows is vertical flakes or vertical flower like structures of MoS2. At higher pressures, more film like growth was observed.

Figure 4-2. Schematic of the MoS2 obtained using blender inc., (b) Image showing before and after growth of MoS2 on silicon substrates on the 3D printed crucible. (Add the heating and cooling cycle ramp diagram).

As an attempt to grow controllable vertical flakes of MoS2, Molybdenum film was deposited on graphite paper. As from the literature, it was shown that a thickness of >5 nm will yield in a more continuous vertical flakes. As we can see from Figure 4- (a), Mo of about 5 nm was deposited on graphite and this

o sample was sulfurized at 750 C. As we can see from Figure 4- 3(b), there is vertical growth of MoS2, with a flake size ranging from 10-50 nm. Thicker films of Mo have been tried which did not yield the same efficient growth of MoS2 vertical flakes. This process of Mo deposition and sulfurization was scalable although, most of the flakes that were grown were nano-crystalline with very small domain sizes (10-50 nm). In order to improve the flake size and crystallinity, modifications had to be made in order to grow large area vertical flowers. By using the MoO3 and sulfur, larger flakes with higher density was achieved, in this case mostly vertical flakes on the substrate.

Figure 4-3. (a) 5 nm Mo film deposited on graphite paper, sulfurized Mo film on graphite and its corresponding SEM images, which is similar to the observations from literature (cite judy cha’s paper),

(b) Powder vaporization of MoO3 and S on graphite and Silicon substrates forming MoS2 across the edges and center using a special 3D printed crucible and SEM image corresponding to this growth process.

Initial observations using SEM microscopy showed that most of the vertical flakes were concentrated only towards the edges of the substrate. As we move towards the center of the substrate, we can see that there is large deposition of MoO2 particles and reduced density of MoS2 flakes on the center, which could be observed in Figure 4- 4. These flakes typically ranged between 200-500 nm at the center, whereas towards the edge, the flakes can grow as tall as 1-5 μm. This edge growth could be related to the non- uniform formation of steps on the edges from cutting the graphite paper. This cutting gave a step, where the surface energy was lowered and hence we could see the formation of the vertical flakes. Along with the surface energy, these corners were in direct contact with the gas flow and hence most of the reactive sulfur species were reacted at the edges first. Although the growth was only concentrated at the edges, 89 this method of powder vaporization provided a strong evidence of vertical flake growth, which could potentially be used for applications where high density and high surface area materials are needed such as for hydrogen evolution reactions, lithium ion batteries.,

In order to improve the coverage of the MoS2 vertical structures on graphite paper, the substrate were treated with UV-Ozone treatment for various times ranging from 10-60 minutes. This UV-Ozone treatment created defects on the surface of graphite, replicating the defects created on the edges in the previous case. As observed in Figure 4- 5 (a), the Raman spectra was collected for all the different UV-

Ozone treatments and as we can see that the defect formation starts from 20 minutes, which is evident 90 from the D-peak at 1348 cm-1. It is also clear that the D-band gets saturated after 30 minutes and no further increase in the D-peak was observed. After the growth of MoS2 by the process as describe above, the coverage of the vertical flakes translated to the entire substrate, forming more flower like structures which are oriented vertically. As it can be observed that the growth was mostly uniform across the edges and center and it was continuous throughout the substrate. There was no evidence of particles of MoO2 as there was complete conversion and growth of MoS2, which is further confirmed by SEM (Figure 4-

5(b),(d)). Beyond the 20 minute, the growth looked similar to that of the 30 minute growth, as shown in

Figure 4- 5(c),(e). There was no further enhancement in the flower like growth beyond 20 minute treatment on the substrate. The thickness of the vertical flakes ranged from 2-8 nm throughout the substrate and this was consistent with the surface treatment higher than 20 minutes on the substrate.

Figure 4-5. (a) Raman spectra of the graphene paper treated with different oxidation times using UV-

Ozone. As we can see that at 20 minutes, we can see the formation of D-band in graphene, (b), (d) SEM images showing the growth on the 20 minute UV-Ozone treated sample, (c) and (e) SEM images showing the growth on 30 minute treated UV-Ozone sample. 92 After optimizing the substrate conditions, the effect of time on the growth was studied. The time of evaporation of the precursors in the system was changed by keeping the MoO3 and sulfur in the ratio of

60:1, a constant temperature of 800oC at a pressure of150 torr. The samples were taken out of the system and immediately looked at the SEM. As it can be observed from the Figure 4- 6, the formation and deposition of MoO2 was observed at the initial deposition stage and tiny flakes of MoS2 started to grow on the surface of the MoO2 chunks simultaneously. At about 15 minutes, the vertical structures seem to form because of the availability of more sulfur and hence more conversion of MoS2. After 20 minutes, most of the flakes were converted into MoS2 and everything was oriented vertically. As higher ratio of

Mo:S was used, most of the flakes that were deposited and converted were all vertical. Hence, this would imply that improving the time of reaction between the sulfur and MoO2 at low pressures aided the growth of vertical structures of MoS2. It was also noted that a sulfur overpressure was maintained throughout the growth process and was turned off only during the cooling cycle when the temperature reached 400oC.

Figure 4-6. Growth of MoS2 at various times the MoO3 was introduced in the system. The effect of precursor times can be observed for 5 minutes, 10 minutes and 15 minutes.

93 Effect of Pressure:

The effect of pressure on the growth was studied by fixing the temperature and precursor ratio in the

o system to 800 C and MoO3:S::1:60. The pressure in the system was varied from 10 torr – 700 torr. As it can be observed in Figure 4- 7, pressures less than 250 torr had more vertical flower like growth whereas going higher than 250 torr upto 500 torr, there was only partial coverage of vertical flakes on the substrate with a mix of the lateral flakes. At higher pressures of >500 torr, the growth was mostly lateral and film like instead of vertical flakes. This could be because of the amount of material available for reaction. At lower pressures, more of the MoO2 can be volatilized, which increases the growth kinetics and hence most of the flakes orient vertically. At higher pressures, the reaction kinetics is slowed down and hence the growth that is favored is more lateral than vertical.

Growth of MoS2 film like material at higher pressures (>500 torr).

Growth on Various substrates:

Similar pattern was observed when the growth substrate was changed. As it can be observed in Figure 4-, the growth looked nearly the same irrespective of the substrate that was used. As it can be observed in

Figure 4- 8, the growth had similar vertical flower like structures which are continuous throughout the graphite paper, and had nearly the same thickness of the flowers. All the conditions were kept constant for 94

o the growth (800 C, 200 torr, 30 minutes, 1:60:: MoO3:S). This shows that the growth did not depend on the substrate of choice and was repeatable across multiple substrates.

Hence, we can conclude that the growth was mostly dependent on the amount of precursor radicals that are available in the system during the time of growth, and hence at lower pressures, the amount of MoO2 present in the local environment around the substrate was much higher. As the sulfur amount was constant throughout the process, this presence of higher concentration of MoO2 aided in growing more vertical structures.

Characterization of MoS2 Flowers on Graphite Paper

Beyond the structural characterization using SEM, other characterization techniques such as Raman microscopy, Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) were used to understand the chemical composition, whereas cross-sectional transmission electron spectroscopy (CS-

TEM) was used to understand the high resolution structural information of these flower like structures.

Raman Study of MoS2 flowers/Graphite paper

Raman spectra were used to confirm the growth of MoS2 flowers on graphite paper. Typically, MoS2 has

-1 -1 vibrational modes at ~384 cm (in-plane vibrations) for the E2g and 404-410 cm for A1g (out of plane vibration modes). The most prominent peaks for MoO2 are 739 cm-1 and 200.5 cm-1. As there is no overlap with the frequency modes in Raman microscopy, it can be used to detect the presence of MoS2 vs

MoO2 in the sample. From Figure 4- 9, we can see the evolution of MoS2 from MoO2 as for 5 minute growth, we can see a lot of MoO2 signal and it decreases considerably as we increase the growth time in the system forming MoS2. A small amount of signal for MoO2 can still be seen in Figure 4- 9(c), indicating that a small amount might be left unconverted. All the Raman spectra was collected using 488 nm Ar ion laser with a power of ~1 mW and an integration time of 15 seconds. As the spot size for the

488 nm was ~250 nm at this power, an average signal over 0.25 μm was obtained and hence we could pick up all the flakes which are in that region. 96

Figure 4-9. Raman spectra showing the evolution of MoS2 flowers from MoO2 particles on graphite paper. (a) Raman spectra showing the presence of large flakes of MoO2 as observed in section, (b) 97

Reduced intensity of MoO2 spectra indicates the increased conversion to MoS2 and (c) After 30 minute growth, majority of the flakes are converted to MoS2.

AES and XPS analysis of MoS2 flowers/Graphite Paper:

Auger electron spectroscopy and x-ray photoelectron spectroscopy was used to study the chemical nature of the MoS2/graphite. As both of these techniques are surface sensitive, the information that will be collected is mostly obtained from the top 10 nm of the surface. As we can see in Figure 4- 10 (a) and

10(b), the elemental maps using AES reveals that most of the surface is MoS2. There was no presence of oxygen on the surface, indicating a complete conversion of MoS2 on the surface. The XPS results further confirm the presence of Mo and S and absence of any other impurities, showing the relative clean approach of synthesis. From Figure 4- 10(e), we can see that there is a minimal amount of oxygen and carbon contamination, which is occurred during the transfer-measurement stage. As carbon and oxygen species are present everywhere in the atmosphere, it is safe to assume that ~2.5% carbon and 0.8% oxygen was from environment. From high resolution spectra of Mo 3d, it can be noted that there is no

6+ presence of Mo states (236 eV), showing the complete conversion of MoO3 to MoS2. The Mo peaks for

MoS2 occur at 229.1 eV for Mo 3d5/2 and 232.3 eV for Mo 3d3/2, which are at an optimal separation of

3.125 eV, which is consistent for MoS2. The full width at half maximum for Mo 3d5/2 obtained from peak fitting was 0.7 eV. Typically, the FWHM window for Mo 3d ranges from 0.65-0.74 eV, depending on the quality, thickness of the sample. This can be further confirmed by looking at the S high resolution spectra, where S 2p has a peak to peak separation of 1.2 eV, consistent for MoS2. There is no evidence of elemental S or S-O bonding in the S 2p spectra, which suggests that most of the sample that was grown was mostly MoS2. The peak at 226.8 eV in Mo high resolution corresponds to S 2s. For all of the XPS peaks, a Shirley background fit was used.

Figure 4-10. (a),(b) Mo and S elemental maps as collected using Auger electron spectroscopy, (c), (d)

High resolution Mo spectra as collected in XPS showing the Mo 3d5/2 an Mo 3d3/2 with a peak-peak separation of 3.135, an optimal separation for MoS2, (d) high resolution S spectra showing the S 2p3/2 and S 2p ½ with a peak to peak separation of 1.2 eV and (e) Survey spectra as collected in XPS showing the presence of Mo and S only and no presence of impurities. 99

Cross-sectional TEM study of MoS2 flowers/Graphite Paper

Cross sectional TEM was used to understand the presence of different layers across the section. Inorder to section the sample, focus ion beam was used to cut the sample into small sections. Gallium ions were used to cut the sample into different cross sections. Multiple cross sections were collected from different spots on the sample to check the uniformity of the sample. As it can be seen in Figure 4- 11 (a),(b) and (c) shows the sectioning locations and Figure 4- 11 (d),(e) and (f) shows the sections that were collected after the FIB. These sections were then placed on a Cu TEM grid and imaged in TEM.

Figure 4-11. (a), (b), (c) FIB images showing the sample locations where the sections were made, (d), (e) and (f) Sample sections corresponding to (a), (b) and (c) which is used for TEM imaging.

Cross sectional TEM imaging and EDS mapping was performed on these samples after the FIB sectioning. Annular dark field imaging was used to look at the sample cross section. From Figure 4- 12 it can be clearly observed that the interface between the graphite and MoS2 was nearly pristine and there was no inter-diffusion of Mo inside the graphite layers. It is also clear that most of the sample was uniform with Mo and S maps and carbon whereas oxygen appeared everywhere where the sample slicing 100 was made. By looking at the amount of oxygen present from the analysis (0.2-0.8 at%), it can be safely ignored.

Growth of WSe2/ Graphite based substrates via MOCVD 101 Metal organic chemical vapor deposition (MOCVD) is a much cleaner process than powder vaporization and has a lot of control over the growth dynamics. In this case, tungsten diselenide (WSe2) was grown via

MOCVD on graphite in a vertical furnace which is heated using inductive coils. The copper RF induction coils are used to maintain the temperature where the graphite crucible sits on a glass holder with a thermocouple which measures the temperature. As metal organic precursors are used in this case which are flown using a bubbler and carrier gas, it provides us with a much more control over the precursors and hence the growth dynamics. Typical carrier gas that is used in this case is hydrogen, as it is needed to reduce the metal organics to individual sources of transition metal and chalcogen. Tungsten hexacarbonyl

(W(CO)6) and dimethyl selenium (Se(CH3)2) were used as precursor materials, reacted at temperatures

o 600-900 C and pressures of 1-700 torr, for the synthesis of WSe2 (Figure 4- 13). Flexible graphite paper

(WSe2). As graphite paper is very stable under hydrogen environment, it would act as an ideal surface for the growth of transition metal dichalcogenides.

102

Figure 4-13. MOCVD growth of tungsten diselenide (WSe2) in an RF induced heating system.

Effect of Pressure on the growth

Similar to powder vaporization, effect of pressure on the growth of WSe2 was also studied. As it can be observed in Figure 4- 14, the growth morphology looked quite similar to MoS2 growth where lower pressures yielded more vertical flakes. Inorder to understand the effect of pressure on the growth process, the Se:W ratio was fixed at 10,800, temperature was fixed at 800oC and substrate annealing temperature of 500oC. As we go higher in pressure (P>100 torr), the growth morphology was more lateral and the individual monolayer flake size increased from ~500 nm at 200 torr to 3 μm at 700 torr. This could be because of the increase in the breakdown of the precursor, more availability of the tungsten source was present, which increased the nucleation density on the substrate which could be visible from the particulate deposition on the substrate. This ultimately grew to form WSe2 vertical flakes at lower 103 pressures. As we increase the pressure, the reactivity of tungsten and selenium to form WSe2 reduces, producing more lateral flakes. Also, the amount of particulates that were present on the sample surface decreased as we increase the pressure. These particulates are mainly WSe2 nucleation sites which did not completely grow into WSe2 flakes.

Figure 4-14. SEM images showing the effect of pressure on the WSe2 growth. Representative images for the growth were taken for the different pressure ranges at (a) P<100 torr, (b) 100

P~400-700 torr.

Effect of precursor residence time

Precursor introduction time was also studied in the system. As we can see from the Figure 4- 15, growth progressed linearly as we increase the time for the precursors in the system. Prior studies on sapphire indicated that 30 minutes of Se and W precursors in the system was sufficient to make uniform monolayer triangles in the system. Similar observations were made on the graphite paper as well. The growth was nearly constant with a larger domains were observed at 30 minutes. The domain size increased considerably when compared to smaller times of the precursors in the system. Beyond 30 minutes, there wasn’t significant increase in the domain size of 2-4 μm.

104

105

EFFECT OF Se:W ratio on the growth of WSe2

Following substrate characterization, the effect of Se:W ratio on the growth of WSe2 was investigated.

Evident from Figure 4- 16, Se:W ratio has a significant impact on the growth. At higher ratio’s of Se

(Figure 4- 16 (a)), there is only particulate growth. When the ratio was lowered to 10,800 the WSe2 grows laterally to form a nearly continuous film, which could be observed in Figure 4- 2 (b). Further characterization of the material using PL and Raman showed the presence of bi-layer material on the GP.

As it can be observed in Figure 4- 16 (d), (e), the PL map of bilayer WSe2 could be observed at 1.61 eV, and the map in Figure 4- 16 (d) shows that it is pretty much uniform corresponding to the SEM images.

Similar morphology could be observed throughout the sample, showing the scalability of this process.

When the ratio was lowered even further (Figure 16(c)), the material that was grown was tungsten rich

WSe2. PL signal was not obtained indicating the material could be a few layers. Further understanding on the chemistry of the material needs to be made using XPS.

106

Figure 4-16. SEM images showing the surface morphology of the growth with various ratios of Se: W for

(a) 21,670, (b) 10,800 and (c) 500. It is clearly observed that at higher ratios of Se:W, there is no formation of WSe2 on graphite, when the ratio was lowered, the growth is much prominent, (d) Raman intensity map showing the presence of bilayer WSe2 growth on Graphite paper and (e) PL intensity peak at the WSe2 regions in Figure 4- 2(d).

Effect of Substrate defects on WSe2 Growth

Substrate plays an important role in the quality of WSe2. In order to understand the impact of the substrate, GO was used as it has the similar structure as GP. The thickness of GO was tailored to 10 µm, matching the thickness of the GP. GO was initially reduced in hydrogen for 15 minutes at 500oC forming 107

o reduced GO (RGO) which was used for the growth of WSe2 at 800 C. The SEM image of the surface morphology of GO is shown in Figure 4- 17(a). The XPS spectra of the C 1s peak after the reduction process can be seen in Figure 4- 17(b) which has a very small concentration of the C-O, C=O peaks indicating a near complete reduction of the GO into RGO. The growth of WSe2 under similar conditions to that of GP results in defective WSe2 and a reduction in the size of the flakes obtained on the RGO when compared with GP which can be observed in Figure 4- 17(c) and 17 (f). This could because of the basal plane functional groups in the RGO and the surface roughness of the RGO which could contribute to this type of growth.

Figure 4-17. (a) SEM image of Graphene Oxide (GO) which was used as initial substrate, (b) High resolution XPS spectra showing the reduction of GO into RGO in the reactor before the growth of WSe2,

(c) image showing the impact of the substrate defects on the growth, (d) GP before the growth, (e) High resolution XPS showing that no change occurred in the GP before the growth and (f) SEM image showing the growth of WSe2 on the GP. 108

Improved Stability of WSe2 on GP

Stability of the WSe2 is important when understanding the new material system. XPS was used to understand the impact of sample when stored under nitrogen. High resolution scans were collected every

30 days. From the high resolution scans of W 4f, Se 3d and O 1s it is clear that there is little change in the peak structure with time, indicating excellent stability of material. From the high resolution tungsten spectra (Figure 4- 18(a)) it can be seen that there is presence of elemental tungsten (Wele in Figure 4-

18(a)). The W 4f peaks corresponding to WSe2 are located at 32.2 and 34.1 eV resp. separated by 1.9 eV.

This was consistent with the shifts seen in Se 3d. The oxygen peak that was observed was small, and is hypothesized to be from the environmental effects and not from the sample. 109

Chapter 5

RADIATION INTERACTION OF GRAPHENE HETEROSTRUCTURE

Introduction Previous understanding on the radiation interaction was performed on graphene on silicon carbide and also on GO. Where high energy radiation was exposed to GO, it is immediately reduced rGO, which ultimately had the same properties as that of graphene. This reduction to rGO was termed as the most cleanest and cheapest way to reduce GO as it doesn’t involve any chemicals and no post processing steps were needed. Typically, electromagnetic radiations with high frequency like γ-rays with high energy states in the nuclei (high γ-decay rates) are used for this reduction. Although the reduction was successful, there was no complete understanding of the radiation interaction and the chemical changes that occur in the material system. Predictions about the water molecule decomposition and alcohol decomposition were made as an attempt to understand the radiation interaction, although no in-situ measurements were made to completely understand the findings. Understanding the radiation effects on graphene based materials is necessary as they could be used for potential radiation shielding. This study will aim to bridge the gap between the radiation interactions with matter targeting the specific radiation response for BCON with x-rays, alpha particles, beta particles, gamma particles and heavy ions. Further, this study will lay foundation for the future in-situ observations of radiation interactions with matter especially for the 2D materials and understanding the radiation hardness of the materials.

111 In-Situ XPS for Understanding the radiation interaction

To study the radiation properties of BCON, environmental effects should be removed as they might interfere with the measurements and understanding about the interaction mechanism. Hence, an in-situ

XPS was used in this case to minimize the environmental impact on BCON. As the XPS is equipped with

Al kα x-rays, it can be used directly as our first radiation source. For understanding other radiation effects, radioisotope sources were obtained from spectrum techniques. These isotopes as observed in

Figure 5- 1 (a) were exempt point sources which could be handled without special handling procedures.

The activity of these sources was very low and sufficient to create changes in the material of choice.

Depending on the type of radiation study, different sources were used. Polonium 210 (Po210) provided the alpha particles, Strontium 90 (Sr 90) provided the beta particles and Cobalt 60 provided the gamma particles. The energies and half-life information is listed in the table along with Figure 5- 1.

Figure 5-1. Image showing the set of radioisotopes obtained from spectrum techniques which included

Cs-137, Co-60, Sr-90, Tl-204 and Po-210. Each disc is 1” in diameter and all radioisotopes are completely sealed in the epoxy except for the Po-210 source, table showing the activity, half-life and energies of the different radiation sources used in the current work.

Sample Preparation: 112 In order to understand the radiation effects, the samples were mounted onto an Aluminum dropdown bar as shown in Figure 5- 2 where the radiation nuclide was mounted. Free standing BCON film was placed on top of the nuclide and aluminum foil was used to cover the entire film, exposing 1cm X 1cm area, which is the interaction area with radiation nuclide and x-rays. The sample was pumped down in the load lock chamber as shown in Figure 5- 2 until a pressure <5 e-7 was reached where most of the outgassing from the sample is removed after which the sample was transferred into the analysis chamber for further analysis.

In between XPS measurements the setup was held in the load lock chamber rather than the analysis chamber to avoid any damage to the charge neutralizer system, detector, or x-ray gun. The BCON 113 material was irradiated by the in-situ gamma source for 3.5 hours, and the total absorbed dose from gamma irradiation was 0.1 Gy for the total time of 3.5 hours. The spectra was taken at these intervals in the analysis chamber and then placed back into the load lock chamber.

Interaction mechanism simulation using Stopping Range in Matter (SRIM)

Stopping range in matter (SRIM) simulations were used to identify the depth of travel of the radiation particles in BCON. Several considerations were made when modelling BCON through SRIM. Model 1 was based on complete uniform distribution of GO and hBN in one single layer, which was the ideal scenario for BCON structure. As it can be observed in Figure 5- 3(a), the travel depth for alpha’s was about 27.5 μm and most of the energy deposition occurs at the last 1.5 μm i.e., 26-27.5 μm. Similarly, another model based on a non-uniform distribution of GO and hBN sheets was considered, which presented the most accurate resemblance for the actual BCON. The top few layers were considered to be a uniform distribution of hBN and GO and further down, individual aggregation of sheets were taken in this model. A beam of 10,000 particles were used for the simulation and as we can see Figure 5- 3(a), most of the particles travel into the structure of BCON, mostly radial into the sample. There is very less number of particles that were deviated and hence most of the particles travel a distance of about 25.5 μm into the

BCON sample and stop. Hence the sample thickness for the experiments was simulated to 25-26 μm as most of the energy deposition happens at this thickness from the models that were considered. 114

Figure 5-3. (a),(b) Travel depth for Alpha particles in BCON for model 1 (uniform BCON sheet) and model 2 (heterogeneous distribution of hBN and GO). Helium ions were used to simulate alpha particles as they resemble very closely with the energy of the alpha particles with an energy of 5304 keV, (b),(c) 115 Energy loss as simulated in SRIM for the two models showing the energy deposition in BCON structure at various thicknesses and (e),(f) Simulated image showing the complete stopping range for the alpha particles in BCON at 27.5 μm for model 1 and 25.5 μm for model 2.

Al k-α X-Ray Interaction with BCON

In order to understand the effect of x-rays on BCON, an Al K-α monochromatic x-ray source was used for the in-situ study (1486 ev). As most of the radiation is a function of photon number, the number of K- alpha photons produced by an electron of energy Eo incident on the Al anode (Z=13) into a solid angle of

4π sr is given by equation 2.7

휔푅 Eq. 1 푄 = 3.48퐸4 (푈 − 1)1.67 (퐸표,푍,ℎ푣) 퐴푐 표

Where ω is the fluorescent yield, R is the backscattering of electrons, A is the atomic weight of the target x-ray anode element, c is a constant (2.7E4 keV m2/kg).7 The overvoltage, Uo, is a function of the critical excitation energy for the K shell of the target element (Ec) and the incident electron energy (Eo). Briggs

7 et.al., calculated the quantum efficiency for Al K-α with an incident electron energy of Eo = 15 keV and an electron beam spot size of ~2 mm diameter, to be 3.6E13 photons/cm2/s. The energy of the Al K-α x- ray source during the XPS measurements presented here was 1486 eV and the critical activation energy

8 휔푅 for aluminum Ec = 1.559 keV. All other parameters ( ) were assumed to be the same for the quantum 퐴푐 efficiency calculation for Al K-alpha in the Kratos Axis Ultra were identical to the calculations for Al K-α

7 2 source in Briggs . Therefore the flux of Al K-α photons at an Eo = 1486 eV is 3.2 E13 photons/cm /s.

The energy-absorption coefficient for BCON was calculated based on the average atomic mass absorption coefficient data as obtained from NIST for the elements of C,B,N and O. By taking the average of the 116 different elements, the final absorption co-efficient was calculated to be 1.09E3 cm2/g. Using both the flux of x-rays and the total energy-absorption coefficient at 1486 eV, the rate of energy absorbed per unit mass by an irradiated material was calculated from Eq. 3 to be 8.3E3 Gy s-1(or J kg-1 s-1). This value is

9 known as the absorbed dose rate (DAR).

휇푎 퐷 = 휑 ∗ 퐸 ∗ ( ) Eq. 2 퐴푅 푝 푝 휌

Where μ/ρ is the mass attenuation coefficient, 휑푝 is the particle flux of the radiation source incident on the GO sample and Ep is the photon energy.315 As the same x-ray source was used for analysis, spectra could be continuously measured during the entire irradiation process. The C 1s spectra scan width was 12 eV with a step size of 0.1 eV and a dwell time of 2000 ms. The only different parameter for the O 1s spectra scan was that the width was 10 eV. It took 205 seconds to scan the C 1s spectra, and 171 seconds to scan the O 1s spectra. The time between each C 1s and O 1s spectra was 53 seconds and the time for the detector channels to reset and commence the second run of each spectra took 57 sec. During this entire period the x-ray is on and is irradiating the GO sample. Therefore the time resolution for each spectral measurement during the x-ray exposure was 486 seconds, which is a total dose absorbed of 4

MGy. The x-ray gun was turned on for almost 19 hours continuously giving a total radiation dose of 560

MGy. The degradation rate of the BCON was then studied for every 20 minutes, allowing us to find the rate of oxygen removal and any changes in hBN in the material. The initial survey spectrum revealed that there was very little amount of sulfur present in the sample (< 0.1%) and no presence of any other contaminants. 117

Figure 5-4. XPS survey spectra showing the composition of BCON with a very little amount of sulfur contamination.

The goal of the x-ray irradiation was to determine the rate at which the degradation of BCON takes place and compare this with already studied irradiation of GO. After the 19 hour irradiation, the degradation rate that was obtained was found to be non-linear exponential (Figure 5- 5 (b)), similar to that of GO

(Figure 5- 5(a)). The presence of hBN in the BCON caused no changes to the output of the reduction.

This behavior can be a combination of different species decay constants which occur at the same time.

Similar to GO, the double exponential decay fit was matched where the decay constants were calculated to be -0.92 and -.02. These have a baseline offset which indicates that complete oxygen removal is difficult to obtain during this type of irradiation. The various functional groups that are present in the

BCON would contribute to the reduction and from previous analysis using thermal reduction, it was identified that carbonyl groups would reform and limit the oxygen content to 63%. 118

Figure 5-5. The degradation rate of (a) GO and (b) BCON as a function of absorbed x-ray irradiation.

The value of X=O/C ratio, where Xo is the initial O/C and Xa is the O/C ratio after “a” amount of radiation. (c) and (d) High resolution scans from XPS showing the reduction of the phenolic, carbonyl species without a change in the O=C-NH species.

The high resolution spectral analysis of individual components can be observed in Figure 5- 6 and this would determine the species that were targeted during the x-ray reduction. From the high resolution spectra of B1s and N1s (Figure 5- 6(c),(d)), we can see that there is no comprehensive change in the peak shape or peak structure except for a slight reduction in the peak intensity. This clearly suggests that BN was not reduced and it did not participate in the reduction process. The overall stoichiometry for boron nitride (B:N) remained unchanged through the irradiation process. Although, the C 1s and O 1s had a lot of changes similar to that of the GO. The peak position regions for each potential carbon compound are boxed in both spectra. The boxed regions for C 1s have multiple peaks in each region: R1 = carbon sp2 + 119 carbon sp3 + covalent sulfate (C-SO2), R2 = carbonyl (C=O) + epoxide (C-O-C) + hydroxyl (C-OH), and

R3 = carboxylic acid (O=C-OH) + (O=C-NH). The boxed regions for O 1s include R4 = epoxide + hydroxyl + water and R5 = carboxylic acid + carbonyl+ covalent sulfate. The peaks corresponding to carboxylic acid (HO-C=O) and (HN-O=C) which is pointed as R3 in the Figure 5- did not change during this irradiation and this matches the peak shift in the oxygen high resolution spectra. There is a considerable reduction in the R2 and R3 which include the changes in the carbon SP2, SP3, covalently

bonded O2 species, the epoxide, carbonyl and hydroxyl groups. The contributions of core photoelectrons for each of these compounds to both R2 and R4 are:

퐶 1푠: 푅2 = 2푃푒푎푘푒푝표푥𝑖푑푒 + 푃푒푎푘푐푎푟푏표푛푦푙 + 푃푒푎푘ℎ푦푑푟표푥푦푙Eq., 3 and 푂1푠: 푅4 = 푃푒푎푘푐푎푟푏표푛푦푙 + 푃푒푎푘ℎ푦푑푟표푥푦푙 + 푃푒푎푘푒푝표푥𝑖푑푒 + 푃푒푎푘푤푎푡푒푟 Eq. 4

120

(b)

(a)

As the carbonyl species are found at the higher binding energies, this could represent a reduction in the carbonyl species more than the hydroxyl peaks first. The overall changes from the high resolution analysis were found to be about 7% or greater in the oxygen reduction for the C-OH: C-O-C species.

Effect of The Gamma Particles On BCON

121 A laminated disc with a 2 mm diameter disc source of Co-60 was one of the radioisotopes from a set of sources purchased from Spectrum Techniques was used for the in-situ gamma irradiation XPS measurements. The energy of the Co-60 photon was 1.25 MeV and the energy-absorption coefficient for

BCON at this energy value was 2.69E-2 cm2/g. This source had had an activity of 1 μCi (3.7E4 Bq). The absorbed dose rate calculation from this disc source is 0.8 μGy s-1, as calculated from Equation 5.5

Equation 5 퐶퐸푝 휇 퐷 = ∗ ( 푎) 퐴푅 2휋푟 휌

Where C is the activity in Bq, Ep is the photon energy in eV, and μa/ρ is the energy-absorption coefficient.5 The 1 μCi Co-60 radioisotope source was mounted onto the stage holder in the XPS and was completely sealed in a laminate plastic disc. It was initially tested in the analysis chamber without the x- ray gun on and no secondary excitations were picked up from the detector. The BCON sample was mounted on top of the Co-60 source, 1 mm away from the source with the plastic laminate acting as a barrier. Because the BCON samples are partially insulating, the BCON and the radioisotope disc were wrapped in aluminum foil with a hole cut out to expose the area which had the source and the paper

(Figure 5- 2). Therefore, the ionizing radiation attenuates through the entire BCON paper and only the backside (relative to the radioactive source) is measured by the XPS system. The attenuation of gamma rays is minimal relative to the thickness of the BCON paper, therefore there was no limit to the thickness of the paper but the thickness was still recorded to be 26 µm, in order to account for any attenuation that

6 occured. The in-situ radionuclide Co-60 source had a very low DAR, which was 9 orders of magnitude smaller than the x-ray source in the XPS. Therefore, it was important to limit the x-ray exposure during

XPS measurements to an absolute minimum without losing too much information from the analyzed spectra.

122

R3 component remained unchanged.

As shown in Figure 5- 7, oxygen removal is much more significant while exposing the material to gamma radiation for an interval of 3.5 hours for the BCON material when compared with the GO. This could be because of the interaction of the gamma radiation with the sample and Compton scattering effect from the

1.25 MeV gamma photons. As BCON still contains the oxygen radicals even at ultra-high vacuum conditions, this oxygen species can cause an interaction and might form a weak water molecule on the

− surface of GO in BCON. Water radiolysis results in both reducing radicals (푒푎푞 and H) and oxidizing

10 products (OH, HO2, and H2O2). These radicals from water can form as a result of the high energy photoelectrons from x-ray irradiation, but become more prominent in gamma and charged particle radiation (alpha and Ar+ ion). Another reason there is a more noticeable change in composition after this small absorbed dosage of gamma radiation is related to the setup for this in-situ study, because the BCON surface that is analyzed is opposite to the side of the radionuclide gamma source. The particle track of the (b)

123 gamma photon through the 24 µm BCON interacts with the material via photoelectric absorption and pair production, both of which often result in the formation of free radicals or electrons. As the gamma beam attenuates through the BCON this cascade of electrons spreads and energy is transferred over a larger area. Therefore, both the presence of water and the attenuation of gamma particles through the BCON could be the cause for the increased change in composition.

Figure 5-8. Image showing the XPS plot of BCON, (b) Image showing the Oxygen 1s spectra.

124 Alpha Irradiation of BCON (Boro-carbon-Oxy-nitride)

Similar to the graphene oxide study, a point source Po-210 was used as the alpha source to irradiate the

BCON material for 36 hours under vacuum in the XPS chamber. This was then analyzed in-situ in the

XPS with an additional x-ray exposure time was 670 seconds for each run. The O/Cx670_s = 0.91, which is exceeded by the measured change from the alpha irradiation O/Cαf = 0.935. These additional changes of

X-ray were then removed to determine the actual changes induced by the alpha particles. The change in the O/C as a result of the various dosage of alpha irradiation is shown in Figure 5- 9. After 36 hours of alpha irradiation the total absorbed dose was 0.42 Gy. As the photon flux of the radionuclide was very low, it couldn’t be exactly estimated if the ratio of the (O/C) final for the alphas is truly 0.964 times the initial (O/C) ratio. In order to determine more accurately, a higher flux alpha source was required and an elaborated setup had to be made in the XPS instrument in order to the in-situ analysis. The trends in composition change are similar to the (GO) alpha study, however the total change is slightly greater as it was exposed for an extended period of time. Also, from Figure 5- 9 (b), it can be noticed that the ratio of

B/N follow the similar trend as before except that the change is much higher in the B/N content. This could be because there was no measurement made for hBN alone inorder to observe the changes, as hBN tends to stay in powder form and preparation of hBN samples suitable for irradiation was difficult.

125

(

Figure 5-9. In-situ XPS measurements of alpha irradiated BCON sample, comparing the change in (a)

O/C and (b) the changes in B/N as a function of alpha irradiation time.

One reason why there is a greater change in composition as a function of absorbed dose is that the attenuation of alpha particles through matter is much greater than any of the analyzed radiation sources.

All alpha particles are completely absorbed in BCON after traveling through 25 µm of material. The 126 average energy deposition as a function of distance (or linear energy transfer, LET) is 0.026 dMeV/dµm.

In order to obtain the best result, the thickness of the BCON material was tuned based on the energy deposition plot as obtained from SRIM. As the alpha particle attenuates through the BCON it loses velocity and the energy deposition increases (Figure 5- 10). The thickness of the BCON where the greatest amount of energy is transferred was found to be about 25 microns and this was likely where the greater changes in the alphas occurred even though the absorption dose was very low.

From the SRIM simulations, the energy deposited by the alphas before travelling into the material in vacuum was calculated. The model for BCON was chosen as a composition of BN & GO in one layer followed by alternating layer of GO & BN. This would give a structure which is approximately true for the heterogeneous BCON material used for the irradiation study. The stopping ranges of alphas are shorter and deposit most of its energy after travelling a distance of 26.2 um into the structure. This was slightly more than that of GO as the contribution from BN increases the depth of travel for alphas. Similar conditions were used to simulate the energy loss in BN and it was found that alphas travel nearly identical distance to that of GO (25 um), although the decomposition of the BN was much greater when compared with that of GO in the BCON structure which can be observed in Figure 5- 4. As the XPS measures only the top 10 nm in the material structure, the complete understanding of the alphas in the structure can be obtained only by tailoring the thickness accurately to the energy deposition in the material. Hence 26 microns of BCON thickness was used for this study and this is why a greater change was seen for alpha particles at lower doses of radiation. 127

(

Figure 5-10. (a) Energy loss as a function of attenuation distance for a Po-210 alpha particle in BCON.

The alpha particle is completely absorbed after traveling through 26 μm of BCON. As the particles travels through the material, the maximum energy deposition occurred at 25-26 um. (b) Energy loss of alphas in hBN. The total energy is deposited at 24.2 um into the material. This information was acquired from 128 running simulation using SRIM (stopping range for ions in matter). As there are two different material systems in BCON, the model that was used for this study was a mixture of GO& hBN in the same layer.

In-Situ Measurements With Beta Source

The in-situ radionuclide Sr-90 source was used as a source for Beta particles which had energy of 0.546

MeV with an activity of 0.1 µCi. Similar to the gamma source, it had a very low DAR. The dose of absorption was calculated similar to that of the gamma particles. The BCON material was irradiated by the in-situ beta source for 3.5 hours, and the total absorbed dose from beta particles as calculated was about 0.01 Gy. All the XPS measurement parameters were kept similar to the gamma radiation study and the sample was again transferred to the load lock chamber when not exposed to x-rays, while still being irradiated with the beta source.

Figure 5-11. Plot showing the changes in the C/O ratio with the total energy absorbed from beta radiation.

As we can see from the Figure 5- 11, the exposure to beta radiation definitely increased the change in C/O ratio. This reduction in the species can further be confirmed by looking at the C 1s high resolution 129 spectra. As we can see that most of the oxygen species were completely removed from the sample surface, along with the reduction of some of the C-C peaks. This is because of the possible interactions of the beta particles with the electronic species in the functional groups of the molecule i.e, the oxygen functional groups. These oxygen functional groups that are attached to the carbon atoms, when impacted with a beta particle produce radicals11. These radicals could be from the splitting of the functional groups into free ions in the system. Some of these free ions might combine back like the OH radicals with low dose absorption of the Beta radiation from the Sr-90. The possible splitting/ recombination of the water molecules could occur at a relative high absorption rate12 (107 Gy) which could produce a major change in the functionality of the materials. Although the radiation absorption was low, the possible other functional groups might definitely interact with the radiation and undergo the radical splitting which could possibly be studied further.

Neutron Interaction with BCON

Neutron interaction with BCON was performed at the Breazeale Nuclear Reactor. The BCON material with a thickness of 25 um was exposed to the thermal neutrons with a fluence 1.3 x 109 n/cm2-s at 800 kW. The sample was exposed to 4 different exposure time (1 minute, 2.5 minutes, 5 minutes and 10 minutes). As the neutron irradiation was carried out at an outside facility, the samples had to be transported in vacuum bags sealed from UV light in order to reduce the impact of the surroundings while transportation. In order to calibrate the measurements, a control sample was also placed along with the other samples for irradiation.

The interaction mechanisms of a neutron particle in GO is not well known and therefore the attenuation coefficient and energy absorption coefficient were not defined. Similar to the GO, the total absorption of

BCON was negligible during the transmission analysis. With the addition of hBN, we expected to see a lot of changes in the BCON as the boron could decompose into a lithium ion and an alpha particle. In- order to see these changes, the number of events that occur in the material should be much higher. From 130 the neutron transmission data, the total flux obtained with and without the sample was nearly identical, indicating that the number of events where a neutron interacted with a boron atom occurrence was less.

Hence, we did not see a lot of change in the material.

131 The total x-ray exposure time for the XPS measurements of BCON pre-neutron irradiation and GO post- neutron irradiation was 600 seconds, which yields a O/Cx600_s =0.93. Both the BCON control sample and the BCON neutron radiated sample are shown in Figure 5- 6 with the x-ray corrected value. The small change in the O/C of controlled sample could be ignored as there was no statistically significant change observed by analyzing the high resolution spectra of C1s and O1s. The difference in the O/C change between the control sample and the neutron irradiated sample is 0.012.

Although there is a slight change in the B1s spectra, further investigation needs to perform by increasing the BN concentration in the material. This could provide a way to improve the number of collision events that could occur with the thermal neutrons, as the cross section for the thermal neutrons is quite low.

Further analysis of the neutron irradiation with varying concentration of boron nitride in the BCON structure needs to be synthesized in order to provide the degradation rate as a function of energy absorbed.

Heavy Ion (Ar ion) interaction with BCON

Although not radiation, heavy ions such as Ar and Kr were used to understand their impact when exposed to BCON. Typically, Ar ions are used in the XPS to clean the surface for adventitious carbon on the surface. Although in this case, Ar ion were used as heavy ions where a potential gas sensing capability could be explored in BCON. The Ar ion interaction for BCON is measured by irradiating the BCON with

Ar ions in-situ in the XPS. The dosages of the Ar ions were controlled by switching the ion gun for different times (30, 60 & 120 seconds). As observed from the high resolution peaks of C1s and B 1s from

Figure 5- 14, Ar ions etched the C1s and removed most of the functional groups (carbonyl, carboxyl and hydroxyl) just with a few seconds of etching. Interestingly, the B1s spectra did not show a lot of changes with the peak counts. There was a peak shift of 0.2 eV to the higher side and the B 1s peak broadened than the initial B 1s peak. This might be because of reduction of species present on the BN that are bonded to GO. The peak broadening at 187 eV might be because of the formation of a temporary bond

(B-C) during the etching process. 133

Figure 5-14. High resolution spectra of (a) C1s & (b) B1s showing the changes in the peak structure over

Ar ion time.

Further analysis of the survey spectrum was made which could be observed in Figure 5- 15. After the Ar ion exposure, the Ar ions were trapped in the structure of BCON. This was further repeated and the measurement was performed after 3 hours. It was observed that even after 3 hours of resting time, the Ar ions still remain trapped in the BCON structure, which can be observed in Figure 5- 15. This suggests that

BCON could be used as a potential gas trapping material or a heavy iron trapping material.

134

Environmental Impacts on X-ray interaction with GO

Most of the radiation sensing was performed under ultra-high vacuum system away from the environment. To understand the environmental effects, GO sample was studied under x-ray irradiation of the XRD. The x-ray irradiation was performed in X-Ray diffraction (XRD) tool MPD xpert pro. The total dosage that was obtained was calculated using a giger counter and was estimated to be 155 R/min. Similar to the XPS, the sample was placed for atleast 16 hours to see if there is any notable difference in the sample before and after irradiation. As the changes will be observed on the surface, XPS was used in the analysis. A minimum time delay was used before introducing the sample in XPS and XRD tool. Pre and post analysis using XPS indicated that there was no change in the peaks for C 1s and O 1s of GO. As the dosage was very less (~104 times) when compared to that of the XPS, minimal change occurred in the 135 sample, which could be seen in Figure 5- 16. As it can be observed in Figure 5-, C1s and O 1s high resolution scans showed almost no change in the intensity or peak position. In order to completely understand the radiation effect, the sample had to be exposed to at least 2 days in order to see a small change as calculated from the x-ray study in XPS. 136

Figure 5-16. Before and after comparison of x-ray exposure in ambient atmosphere showing that there is no significant change in the sample after 16 hours of exposure.

Chapter 6

Applications of 2D-TMD heterostructures

MoS2/Graphite as Superhydrophobic surface

Surface Wettability Study

Vertical structures of MoS2 were predicted to have a higher contact angle, as the vertical structure minimizes the surface energy. Hence, In order to understand the surface wettability, static contact angle measurements were performed on the synthesized samples. Surface characteristics typically cause an impact on the free energy of any surface when it interacts with a liquid or a vapor. As reported in the literature, the contact angle varied with the growth temperature316 (550-900oC) from 23o to 98o indicating an improvement of minimization of surface free energy of MoS2 and exposure of the highly crystalline

(001) planes (edges). It also varied as a function of the number of layers. Although, surface roughness did not play a major role since it was an atomically smooth and layered sample317,318. In this case, these were only a few layers thick (2-10 nm), the surface roughness over a large region for the water contact droplet is high, thus minimizing the surface free energy required to stabilize the water droplet and hence we see an improvement in the contact angle. Furthermore, this surface energy was also minimized by increasing the extent of edge sites exposed also increased the total active sites in a given surface area when compared with horizontally aligned flakes. As water is a polar solvent with the highest surface tension at room temperature (~72 mJ/m2), its reactivity towards the edge sites is higher. Hence, when having so many reactive sites on the surface, templating the surface with a higher surface area improved the contact angle, thus indicating the importance of the flower shaped architecture on the surface.

138

Figure 6-1. Contact angle measurement for (a) Bare graphite paper, (b) As grown MoS2 flowers on GP,

(c) As grown MoS2 flowers after 2 minutes of gentle UV-Ozone treatment, (d) 10 minute UV-Ozone treatment showing a significant reduction in contact angle showing an increase in the surface free energy,

(e) As grown partially covered MoS2 flowers have a contact angle less than the UV-Ozone treated one and (f) complete wettability using a lower surface tension liquid (ethanol).

319 o The typical contact angle for high purity graphite (HOPG) is ~90 , which is considered hydrophobic. In this case, the graphite paper used as our substrate exhibits a contact angle of ~35o (Figure 6- 1(a)), showing the effect of impurities and the assembly process that was used. Typically, graphite paper had much higher surface energy320 and this is highly dependent on the sheet assembly process and the surface

o impurities on graphite. After the growth of MoS2 flowers, the contact angle varied from 35 to 156.16 ± 5

(Figure 6- 1(b)), indicating that the surface is now super-hydrophobic after the MoS2 flower growth. The super-hydrophobic MoS2 followed a perfect case of Wensel state where the liquid droplet spreads into the porous structure beyond the drop (Figure 6- 2(a) and Figure 6- 2(b)), and the transition from Cassie- 139 Baxter state to Wensel state was only a fraction of second321 which was evident from the slight wetting that observed after the water drop was dried following the measurement. As the surface energy minimization is directly linked to the contact angle, this higher contact angle indicates that the surface

2 energy for the MoS2 flower system is much lower than that was reported i.e., 46 mJ/m . Partially covered

MoS2 flowers on the surface shows a lower contact angle, although the water droplet covers most of the surface, having a mixture of MoO2 and MoS2 on the surface yielded a lower contact angle as observed in

Figure 6- 1(e). Further, the contact was ~0 when using any alcohol or other solvents with a surface tension <50 mJ/m2.

Figure 6-2. Wetting transition from (a) Cassie-Baxter state to (b) Wensel state for water contacting on

MoS2 flowers and (c) image showing the reduction of contact angle with the addition of the S-O functional groups on the surface.

Further, these flowers were treated with UV-Ozone, which imparted a partial oxidation on the surface of the MoS2 flowers, without altering the structure. As observed in Figure 6- 1(c) and 1(d), we can see that just a simple treatment of 2 minutes lowered the contact angle by ~ 30o and almost by 60o for a 10 minute treatment. As observed previously (cite), surface treatment of MoS2 results in the formation of MoOx-

MoS2 on the surface, which reduces the band gap and hence increasing the conductivity of the surface.

Further, this MoOx is hydrophilic, hence reducing the contact angle on the surface. Further, most of these surface features form a S-O bonds on the surface along with MoOx formation. This was confirmed by 140 looking at the high resolution XPS spectra. As it can be observed in Figure 6- 3(a) and 3(b), as synthesized MoS2 has no MoOx and S-O bonds on the surface. As we increase the UV-Ozone time from 2 minutes to 10 minutes, we can see the start of the formation of S-O and Mo-Ox bonding on the surface.

The Mo-O bonding is more prominent at 10 minutes along with the S-O bonds although, the S-O bonds starts forming more readily at 2 minute treatment of UV-Ozone.

141 Electrochemical impedance spectroscopy (EIS)

Electrochemical impedance spectroscopy (EIS) was used to understand the behavior of the MoS2 and uv- ozone treated MoS2 samples. EIS provides a way to understand the conductivity and electron transfer resistance in a sample. As we can see from the resistance plots (Figure 6- 4) that are collected for the as grown samples vs 10 minute UV-Ozone sample, the resistance reduced by a factor of 4x from the as grown sample vs the treated sample. The resistance for the as grown sample was 2000 1/ohm, whereas for the 10 minute UV-Ozone treatment goes down to 500 1/ohm. This further confirms the observation seen with the XPS study, which is the formation of the conductive MoOx layer on top of the MoS2. Hence, having less resistance to electron transfer on the surface promotes HERs.

Figure 6-4. Impedance measurements of as synthesized MoS2 vs UV-Treated MoS2 showing the reduction of ~ 4 x in the resistance of the MoS2 heterostructure after the 10 minute treatment. 142 The contact angle study was further extended to observe the changes of the surface energy on the flowers grown on different substrates. As we can observe in Figure 6- 5(a), the as cleaned SiO2 surface had a

o o contact angle of 41 , which further increased to 106 after the growth of MoS2 flowers. This was subsequently reduced to 81o and 54o after treatment with UV-Ozone for 2 minutes and 10 minutes, respectively. Although we start with similar surfaces on graphite paper (Figure 6- 5(a)) vs SiO2 (Figure 6-

5(a)), there is a broad range in surface energies for the same material after the growth of MoS2 (Figure 6-

5(b) versus Figure 6- 5(b)) indicating the importance of substrate for the growth of TMDs. Even though the MoS2 flowers are thick, with a density that covers the entire surface, the surface termination energy after the growth of the flowers was quite different. Also, the UV-Ozone treatment on the flowers brought the contact angle close to bare substrate unlike that of the growth as observed on graphite paper. This clearly indicates that the surface where the MoS2 flowers were grown is important for its reactivity and stability. MoS2 having a near similar lattice mismatch to graphite when compared to that of SiO2 tends to be form more stable structures, although oriented differently. It forms a more reactive surface on SiO2 than that of graphite, hence having a lower contact angle. This reactivity further increases after treatment with UV-Ozone and has a significant reduction in contact angle (~50%). Even before the UV-Ozone treatment, the presence of oxygen in the substrate had an impact in forming a more reactive surface.

143

Hydrogen Evolution Reaction (HERs) for MoS2/Graphite heterostructure

HER Setup:

HER measurements were carried out using a 3-electrode system with a graphite rod as counter electrode, and a saturated calomel electrode (SCE) as reference electrode. The reference electrode was calibrated with respect to reversible hydrogen electrode (RHE) by using high purity H2 saturated 0.5 M H2SO4 electrolyte which is the electrolyte for HER measurement. In order to perform the calibration, a platinum wire was used as the working electrode to run the cyclic voltammetry (CV) at a scan rate of 1 mV/s, and the average of the two potentials at which the current is zero was considered as the thermodynamic potential. Herein, E (RHE) = E (SCE) + 0.274 V. For HER measurement, the linear sweep voltammetry 144 was conducted at a scan rate of 1 mV/s using a Versa STAT 4 potentiostat. The working electrodes are the self-standing films on the graphite paper. To evaluate the inherent activity of the catalysts, the Tafel plots were linearly fitted.

HER Measurements for as grown MoS2/graphite was measured and was found that it required an onset potential of 373 mV with a linear Tefal slope of 177.4 mV/Dec. This was comparable to the as grown

MoS2 vertical structures using hydrothermal synthesis. These MoS2/graphite heterostructures were further treated with tributyllithium (TBL) inorder to have a phase transition for MoS2 from semiconducting 2H to conducting 1T phase. This conversion was performed at Navel Research Laboratories with the help of Dr.

Jeremy T. Robinson. The samples were tested immediately after the lithium induced phase transfer and we can see from Figure 6- 6 that the onset potential decreased to 230 mV, with a reduction in the Tefal slope to 99 mV/Dec. This reduction in the Tafel slope was accounted to the increased conduction of the

MoS2, and hence we see a drop in onset potential as well. Although this phase conversion yielded better

Tafel slope for MoS2/graphite, this material was not stable as literature suggested that the phase transition was not a permanent. 145

Figure 6-6. Plot showing the HER performance for as grown 2H-MoS2/graphite vs 1T-MoS2/Graphite.

As we can clearly notice from the Figure 6- 6, the onset potential reduced by 150 mV and Tafel slope reduced by 78 mV/Dec.

HER Performance for UV-Ozone treated samples

HER performance was tested for the as grown samples which were treated with UV-Ozone. As it can be observed in Figure 6-, the as grown sample had a similar onset potential of ~323 mV as before whereas the UV-Ozone treated samples have a slightly reduced onset potential of ~270 mV. Although the onset potential did not have a big change, the Tafel slope was decreased considerably and the maximum current density achieved by the samples were increased as we increase the UV-Ozone treatment time for up to 10 minutes. The Tafel slope for these curves was linearly fitted and was found to correspondingly decrease as we increase the UV-Ozone treatment, which can be observed Figure 6- 7 (a) & (b). This study showed 146 that formation of the S-O bonding on the surface and Mo-O bonding on the surface increased the conduction, and this conduction was increased to a maximum at 10 minute UV-Ozone treatment, which gave the best Tafel slope of 54 mV/Dec. This was by far the best Tafel slope reported to date for the vertical structures of MoS2 on any substrate. This direct growth method of MoS2 on graphite eliminated the transfer process, which is typically involved in this kind of testing.

Figure 6-7. (a) Current density vs voltage plot for as grown MoS2 flowers on graphite and 1,5,10 minute

UV-Ozone treated samples and (b) Tafel slope corresponding to the HER plots. As it can be observed that the Tafel slope was reduced by 3.5x from pure MoS2/graphite to 10 minute UV-Ozone treated sample.

Along with the transition metal, sulfur was also hypothesized to be an effective catalyst for HERs. As the sample was treated with UV-Ozone, some of the bonded sulfur was left free, which further aided in increasing the performance of HERs. This enhancement that was observed for the HERs was accounted to the increased conduction by forming the MoOx species and the free sulfur species available during the catalytic reaction. In order to understand the stability of these UV-Ozone treated samples, cyclic performance was measured. As it can be observed in Figure 6- 8, the performance did not degrade after 147 1000 cycles which were performed in just over a day. Hence we can say that this simple treatment to the sample will significantly enhance the performance of the MoS2.

Figure 6-8. Cycling performance for 10 minute UV-Ozone treated sample. As it can be observed that the sample is very stable even after 24 hours in solution for over 1000 cycles.

Beyond the 10 minute UV-Ozone treatment, the MoS2/graphite did not produce considerable results. As it can be observed in Figure 6- 9, the contact angle was 0 for all the samples beyond 15 minute UV-Ozone treatment. As we can see from the Figure 6- 10 (a) & (b), the 15 minute sample had a similar performance as that of 10 minute treated sample with ~323 mV as onset potential and a Tafel slope of 56 mV/Dec.

Beyond 15 minutes, the onset potential increased and the maximum current density reached by the sample also decreased to 60 mA/cm2 for the 20 minute sample and to 20 mA/cm2 for the 25 minute sample. The

Tafel slope for the 25 minute sample went up to 209 mV/Dec, indicating that the sample had a performance which was lower than that of the as grown MoS2 sample. Beyond the 15 minute UV-Ozone 148 treatment, the MoS2 on the surface got converted to MoO3, which has a higher band gap than that of

MoS2, and hence has a poor HER performance. The surface was dominated by the oxide species and hence the conduction was decreased for the sample. This could be the reason why we see an increase in the onset potential of 452 mV and an increased Tafel slope of 209 mV/Dec.

149

Hydrogen Evolution Reactions (HERs) for WSe2 based materials

Similar HERs measurements were performed on the MOCVD grown WSe2 samples. As it can be observed in Figure 6- 11 (a) and (b) the HERs performance for the vertical flakes which are ~100-200 nm tall have a lower Tafel slope of 76.5 mV/Dec when compared to that of the lateral flakes which was 199.8 mV/Dec. Further, the HERs performance increases as we increase the number of layers as mono-bilayer material Tafel slope was higher than that of the 5nm film which was 152.2 mV/Dec. When the sample had partial vertical and partial lateral flakes i.e., a hybrid orientation on the surface, the Tafel slope reduces to 129.3 mV/Dec. SEM images in Figure 6- 11(c), (d) and (e) shows the different surface features that were used for this HER study. As it can be observed in Figure 6- 11 (c), most of the flakes are lateral, while Figure 6- 11(d) has partial coverage with the vertical flakes along with lateral flakes and Figure 6-

11(e) sample shows that most of the region (>90%) on the surface was vertically oriented flakes. It has to 150 be noted that for this study, only fully covered surfaces with WSe2 were used. This study suggests that most of the vertical flakes will have a lower Tafel slope and hence higher activity for acid catalysis. The current WSe2/graphite vertical flakes were very stable in the solution and this is one of the lowest Tafel slope reported for the WSe2 based materials.

Tafel slope

151

UV-Ozone Treatment on WSe2/Graphite samples

Further, UV-Ozone treatment was performed on the WSe2/graphite samples and unlike MoS2/graphite, the WSe2 performance degraded with the presence of oxygen. This could because of the formation of

WO3 species directly, which has a higher band gap. This lack of conductivity increased the Tafel slope and onset potential from 273 mV to 373 mV and the Tafel slope from 76 mV/Dec to 220 mV/Dec for the

10 minute UV-Ozone treated sample as observed in Figure 6- 12. Further understanding of the degradation was performed by looking at the high resolution XPS spectra for W 4f and Se 3d and it was observed in Figure 6- 13(a) that W forms +6 state very easily than Mo, which ultimately results in the reducing the conductivity of the samples. Figure 6- 13(b) confirms that the Se also bonds with the oxygen forming the Se-O bonding state. It can also be observed that because of lack of conductivity, the 10 minute UV-Ozone treated sample was initially reduced which could be confirmed by looking at the HER curve. As the sample stabilizes, the evolution of hydrogen starts from the surface at a higher potential. 152

Ozone time.

153

Figure 6-13. High resolution spectra for (a) W 4f showing the formation of W+ 6 states and W+4 corresponding to WSe2 and (b) Se 3d shows the formation of Se-O with increasing UV-Ozone times.

Alloying effect on HERs

As previously noted, oxygen defects in the MoS2 aided the performance where as it degraded the performance for WSe2. We can hence say that the performance is mostly chalcogen dependent and right chalcogen substitution should be selected in order to improve the performance for HERs. Most of the alloying effect was only studied for the chalcogen element and hence understanding needs to be made when we have a substituted transition metal instead of a chalcogen. The main advantage of having a CVD system is the amount of control that we can get when using the precursors. In order to make chalcogen alloys, WS2 films with film thickness ~10 nm were used for this study. Similar thickness material was grown by replacing the WO3 precursor with a mixture of WO3 and MoO3 in the system, and the resultant films clearly had doping from the Mo in the sulfur films. From XPS, it was confirmed that one of the sample with a WO3: MoO3 ratio of 9:1 gave a film with Mo0.1W0.9S2 and another film with 50:50 mix of

WO3 and MoO3 precursor gave a film of Mo0.5W0.5S2. Further, the vertical MoS2 films were treated in Se in MOCVD to form MoSe2. HERs measurements were performed on all these alloyed samples and it was 154 observed that alloying with Chalcogen atoms in the TMD structure will improve the HERs performance.

As it can be observed in Figure 6- 14(b), the Tafel slope for the 50% Mo substitution reduced the Tafel slope by 24% which brought it down from 100.5 mV/Dec for WS2 to 76.2 mV/Dec for the alloy, whereas complete substitution of the MoS2 vertical flowers into MoSe2 reduced the Tafel slope to 64 mV/Dec.

CHAPTER 7 155

Summary and Future Work

Summary

This thesis addresses large scale liquid phase exfoliation and functionalization of two dimensional materials. Given the extensive research on chemical exfoliation of graphene, these processes were used as a starting point to establish a base for the future materials exfoliation such as hexagonal boron nitride, transition metal dichalcogenides like MoS2, WSe2, etc. Initial work began with chemical exfoliation of boron nitride using the modified Hummers process or a process developed by James Tour71 , with the conditions of the exfoliation modified to suit for boron nitride. In short, bulk boron nitride powder was mixed with a nitric and sulfuric acid and this was added to potassium permanganate. This mixture was then heated to 75oC and stirred using a magnetic stirrer at 450 rpm for 16-24 hours. This was then let to cool naturally to room temperature, and later added to a water bath where 3 ml hydrogen peroxide was added. This changed the color of the solution from dark brown to white. This exfoliation mixture was then transferred into individual beakers and later cleaned using a sonication/centrifugation process. This sonication/centrifugation process involved centrifuging the mixture at 6500 rpm for 1 hour and later the supernatant was decanted. A series of ethanol, HCl and water was added in the simultaneous runs and sonicated for 30 minutes before centrifuging again. Finally, the supernatant which had the exfoliated hexagonal boron nitride was separated and dried on various substrates for further characterization. This sonication/centrifugation process to separate out mono-few layer hBN is necessary in order to achieve large exfoliation efficiency for the exfoliated flakes. Structural information was obtained using SEM and

XRD, which showed that most of the flakes are electron transparent, and most of the exfoliated flakes are

(002) oriented. Further layer information was obtained using AFM, Raman and TEM. AFM suggested that monolayer hBN was stacked on top of the other layers of hBN. Raman microscopy showed the reduction in intensity as the layer number decreased and the Raman peak position for monolayer was identified at 1368.2 cm-1. Further, TEM confirmed the presence of monolayer by looking at the diffraction 156 pattern. XPS was further used to identify the bonding characteristics of the exfoliated flakes. By looking at the high-resolution peak for B 1s, it was observed that hBN had B-O and B-S bonding in the material.

This was further confirmed by UV-Vis spectroscopy, where a new optical bandgap was opened at 3.9 eV which corresponds to the defects created by the B-O. This was further confirmed by using FTIR spectroscopy which showed a small peak at 1054 cm-1. Finally, this exfoliation process showed that the exfoliated flakes had a maximum size equal to that of the starting bulk hBN flakes and the exfoliated flakes are highly stable in various solvents including water, IPA, acetone. The current process that was developed for obtaining high percentage of large scale exfoliation is significant in understanding the stability and properties of hBN in solvents. As hBN is highly inert, its stability in solutions is limited for a short time, which minimizes the reaction time for hBN to be used as filler material for composites especially for polymers. It was shown previously that hBN can form a very good dielectric material for polymer composites. This advancement in exfoliation process will drastically effect the interaction time that the polymers can have when using hBN to make composites. Further, GO has been studied extensively using Modified Hummers process for over a decade. As the current process is adapted from

Modified hummers process, further understanding in the exfoliation and property study of hBN can be made by utilizing the tools that are already available in the literature to explore new properties. Further, this exfoliated hBN was utilized to make a composite with GO in the current work. As there are additional bonding sites available on the hBN lattice i.e., B-O and B-S sites, these can act as a linkage sites to make stable composite with GO. This formation of heterostructures via liquid phase would be an easier and cheaper route towards making large area composites and exploring new properties.

After the exfoliation of hBN and graphene oxide (GO), they are mixed together and sonicated for 2 hours at 75oC, resulting in forming a new heterostructure material called BCON. pH study was made to look at the behavior of interaction of hBN and GO. It was found that at a pH of 4-8, the bonding between the hBN and GO occurs, whereas at higher or lower pH, both the materials tend to separate out. After a stable 157 pH was set to 4, parent concentrations were changed and it was observed that GO rich materials tend to separate out as films where as hBN rich materials separate out as ribbon like materials when dried. This heterostructure that was formed was observed for the first time which was stable and free standing. As previously shown by Vinod et.al., addition of hBN in GO to make 3D foam like structures improved the mechanical strength of the entire structure as hBN tend to form as anchor sites in the structure. Although, in their study the hBN was not bonded to GO. This bonding of hBN in GO will further improve the structural stability and mechanical properties of the heterostructure. As observed by Mahabir et.al.,

BCON heterostructure had 20% higher mechanical properties (yield strength and Young’s Modulus) than

GO alone, suggesting the effect of hBN loading in the films. Further characterization using XRD, TEM,

XPS was performed on the BCON to identify the crystallinity and functionalization of BCON. XRD and

TEM suggested that most of the flakes after forming BCON tend to settle down as (002) and the hBN flakes heterogeneously distribute in the GO sheets. Further looking at the XPS suggests that hBN and GO form new bonds, O=C-NH, which was evident by looking at the nitrogen and the carbon high resolution peaks. Previous studies indicated that GO has a very good response to radiation such as X-rays, alpha particles and gamma particles. As hBN naturally has ~20% B10, which is highly responsive to thermal neutrons, this BCON material was tested to be used as one material for all radiation sensing. Radiation testing was performed on BCON using X-rays, alphas, betas, gamma and thermal neutrons. X-ray response to BCON showed that it followed a double exponential decay, whereas alpha, beta and gamma response showed that the decay is much more sensitive than GO. Finally, thermal neutron response showed that there is a slight decay in B 1s as observed from XPS after a 10 minute exposure. This response to thermal neutrons was observed for the first time with these ultra-thin structures suggesting their potential use as a material for all radiation detection. This work can be further extended to make future devices to detect all kinds of radiation with just one material system. As the response for different radiation sources is different, the plots can be used to distinguish the response thereby identifying the type 158 of radiation. As it was also shown that it is highly responsive to very short dosages of radiation, this material could potentially be used as a sensitive detector.

Liquid phase exfoliation was one of the many ways to make heterostructures in large scale. Another way to synthesize graphene based heterostructures is using chemical vapor deposition (CVD). Two techniques were explored to synthesize transition metal dichalcogenides (TMDs) on graphite, mainly powder vaporization and metal organic chemical vapor deposition. As in CVD type techniques, there is an immense amount of control over the various growth properties, which could help us in controlling the morphology and the growth dynamics. Another important factor compared to liquid exfoliation is the quality of material that is obtained via CVD. As liquid exfoliation involves the use of chemicals, residues of the chemicals and solvent-material interaction can cause an interference with the properties. For example, in the current work it was shown that MoS2 can be exfoliated with a better efficiency using ethylene glycol as a solvent and removal of this ethylene glycol from the material required a lot of post washing using various solvents like ethanol and DI water. As these are van der Waal’s materials, they tend to stack on top of each other while drying and could cause a lack of surface exposure which is important for using these materials for various applications like hydrogen evolition reactions (HERs) where the surface area of the material is very important. In the current work, large area high density vertical structures of TMD’s specifically MoS2 and WSe2 were grown on graphite separately. Further characterization showed that the TMD’s that were grown were high quality and uniform across a large area (1cm x 1cm). This direct growth of high quality films on graphite substrate will avoid the need to transfer the material to test for applications such as HERs or lithium ion batteries (LiBs). Previous studies in the literature suggested that transfer of materials can have deposits of carbon residue left over on the material as it involves the use of polymer route to transfer the films. Hence, this current process will minimize all the post processing steps needed to transfer and direct testing of the materials can be made as this is grown on graphite. Cross sectional TEM imaging suggested that there was no oxide present and 159 hence most of the oxide was converted to their respective sulfide and selenides. These heterostructures were then tested for hydrogen evolution reactions (HERs) in an acid environment and was found that the

CVD grown material perform comparatively with respect to the materials grown via liquid phase exfoliation or hydrothermal synthesis. As the TMD’s grown were mostly semiconducting materials, lithium intercalation was performed to convert them to metallic and it was observed that the HERs performance can be enhanced and Tafel slope can be reduced from 180 mV/Dec to 99 mV/Dec, and onset potential can be reduced from 373 mV to 23 mV. Further, defects were introduced in the system by controlled treatment using UV-Ozone and it was observed that for the case of MoS2/Graphite, the Tafel slope was reduced as much as 54 mV/Dec, whereas contrasting behavior was observed for

WSe2/Graphite, where the Tafel slope went up to 230 mV/Dec. The Tafel slope for vertical flakes of

WSe2/Graphite was 65 mV/Dec, which is one the lowest reported for this material system. As there is a lot of effort made in improving the catalysts performance for HERs and achieving a performance close to platinum (32 mV/Dec), this reduction in the Tafel slope for MoS2 and WSe2 would help in using this material as a large scale catalyst for HERs. As Pt is very expensive, alternate cheaper catalyst materials are currently being explored and achieving a lower Tafel slope for these TMDs will aid in utilizing these materials as a cheaper alternative for HERs catalysts. In order to understand the oxygen defect effect on the improvement in HERs for MoS2/graphite, contact angle measurements and impedance spectroscopy was performed on the samples. It was noted that by increasing the UV-Ozone treatment time on the sample, the contact went from 160o to 90o, and forming Mo-O and S-O species on the surface. This formation of these Mo and S species on the surface improve the conductivity, and made free sulfur available for catalysis, which then improved the performance of HERs. In the case of WSe2, there was a formation of WO3 species, which have a higher band gap and hence lower conduction. Hence, we see a reduction in performance for WSe2/graphite.

160

Future Work

Radiation study of 2D materials

As the current research on radiation impact of 2D materials provided a benchmark to study the future 2D materials. Semiconducting two-dimensional layered materials (2DLMs) such as the transition metal

322 dichalcogenides (TMDs), especially MoS2 and WSe2, have gained much recent interest driven by their unique electrical properties and the promise of novel devices. As these 2DLMs become nominally all- surface materials once reduced to monolayer thicknesses of less than 1 nm, they become highly sensitive to extrinsic factors such as the underlying substrate279 and airborne dopants323,324. The chemical and electronic changes that occur upon exposure to radiation, unlike those that occur upon gas sensing, are often detrimental325 and must be understood to open up these materials to applications in high radiation environments such as space. As such, the interactions between 2DLMs and incident radiation such as X- rays have also recently become of interest to develop high-performance radiation-hardened electronics.

Recent reports on multilayer epitaxial graphene showed that it is electronically stable under 10 keV X-ray bombardment in air despite that fact that the topmost layers oxidize and peel off326. This is because the bottom-most layers are the ones most responsible for charge transport, and they were found to be unaffected by the X-rays. The interaction with the substrate was also proposed to stabilize the epitaxial graphene and reduce X-ray damage327. At the nanoscale limit, interactions between incident radiation and the substrate will also affect 2DLMs. As radiation effects are not well known in TMDs, the techniques used in here can be extended to study the effects of various radiation sources on TMDs. As recent surge in the interest of TMDs for various applications mainly in electronics, understanding of the radiation effects needs to be made. As TMDs can be grown on various substrates, the effect of substrates-TMD interaction can also be studied. As a start, TMD’s grown on various substrates like SiC, sapphire and graphite were studied to understand their degradation under X-rays. Al K-α x-rays were used similar to the BCON study, and in-situ observations were made to understand the changes in the material. As it can be 161 observed in Figure 7- 1, the binding energy shift is different for the same material when tested on different substrates. This difference in the shifts will provide a way to understand the interaction of the

TMDs with substrates under radiation.

Figure 7-1. Shifting in XPS spectra near the valence band maximum (VBM) and for the major tungsten

(W 4f) and selenium (Se 3d) peaks, for WSe2 on: a) graphite paper, b) sapphire and c) silicon carbide. The valence band maximum was determined for each sample by applying a linear fit to both the valence band edge and the background noise inside the valence band. The after spectrum shown here corresponds to the maximum X-ray dose of ≈5.3 MGy.

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Alloying Effects of TMDs on HERs

Understanding of the alloying effects in HERs can be studied by substituting the chalcogen atom in the already grown TMDs on graphite paper. This understanding of the effect of chalcogen atoms will provide an optimum value of doping, which will improve the stability of the TMDs and improve the performance for HERs, which could ultimately lead to the use of this catalyst commercially. From the observations made in the current work, the type of chalcogen that is attached to the transition metal has a big effect on improving the Tafel slope and hydrogen production. As it was observed that S, when bonded with O, improved the Tafel slope because of reducing the band gap and freeing up the amount of available sulfur for catalysis. Although, this was not the case for WSe2, as it turned into WO3, which has a higher bandgap than the TMD itself. Hence, it can be predicted that having an alloy with S will improve the HERs performance, as WS2 has a lower band gap than WSe2, and the presence of S in the structure will improve the stability and availability of the active catalyst material i.e., S, Se and W. Understanding the effect of the alloys will further help us in stabilizing the structures and improving the catalyst performance for

HERs in acid medium, which will ultimately produce a catalyst which is highly cheaper and efficient to use. Also, this work has also shown that a few nanometer thick materials over a large area will have significant improvement in the Tafel slope and performs in comparison with the current catalysts. Hence, by controlling the alloying effects, it can be possible to make an ultra-thin highly stable catalyst for hydrogen evolution, which could also be used as a photo-catalyst.

163

Lithium ion battery performance of Vertical MoS2/Graphite

Theoretically, MoS2 has a higher capacity than graphene (683 mAh/g vs 300 mAh/g), hence MoS2 can be a good catalyst material for future battery applications. Typically graphite based anode materials are used in current LiBs. For example, nickel-graphite anode is used in current Tesla batteries. As the cost of graphite has not changed in the past decade, making anode materials based on graphite would allow us to keep the similar base cost for the anode material. Hence, developing an anode material around graphite would be very useful as it can be directly integrated into the current market.

Figure 7- 2 summarizes the most common materials that are theoretically being tested as anode materials for LiBs. As we can see that a large pool of materials has been shown to have a theoretical capacity 164 higher than that of graphite working at various operating potential. Out of all these materials, chalcogenides have a much higher theoretical capacity. Out of the chalcogenides, sulfur based systems have a much higher capacity than the rest of the chalcogens and pure S has the highest theoretical capacity (~1680 mAh/g). Pure S is not very stable and hence cannot be used directly as an anode material.

Hence, amongst all the sulfur based materials for Anodes, most stable and higher amount of sulfur per monolayer of material is with TMDs. One single layer of TMDs have 2 monolayers of sulfur attached to one layer of transition metal, which stabilizes the sulfur from oxidizing. Hence, these materials are highly studied for LiBs. Amongst all the TMDs, MoS2 received a lot of attention because of its higher theoretical capacity. Although MoS2 has a higher capacity, the stability of the material and formation of polysulfurs will cause degradation in performance over a large cycle number and hence cannot be used. Hence, in order to address these issues, heterostructures have been made to solve the stability problems and delay the formation of the irreversible polysulfurs. One such method was to form composite with graphene or reduced graphene oxide. As MoS2/graphite and rGO were synthesized in the current work, these materials can further be tested to see its performance for LiBs.

As synthesized exfoliated MoS2/rGO was used as a benchmark, as it was a standard procedure. Typical battery assembly process can be divided into three major steps. The catalyst preparation, anode preparation and anode assembly. For the catalyst preparation, rGO/MoS2 composite was mixed with polymer binder, carbon black to improve the conductivity and this mixture was dried over a copper sheet.

This was dried in vacuum oven at 50oC for at least 24 hours to completely remove any traces of water that is present in the material. This was then later punched into 1 inch circular electrodes as shown in Figure

7-3, which will be used in coin cell architecture to test the material performance. After making these electrodes, these electrodes are subjected to another drying stage before the assembly can take place.

Careful assembly was made in a nitrogen glovebox to avoid any exposure to oxygen. LiPF6 mixed with 165 ethylene carbonate/ dimethyl carbonate was used as an electrolyte and a lithium foil was used as a cathode, and the as prepared heterostructure was used as an anode material.

Figure 7-3. Step by Step procedure for lithium ion battery assembly for liquid phase synthesis materials.

Cycling performance was performed for the exfoliated MoS2/rGO heterostructure at a cyclic current density of 100 mA. As it can be observed in Figure 7- 4, initial performance indicated that this material is stable up to 30 cycles with a reversible capacity of 600 mAh/g. The first discharge current was 680 mAh/g, which indicated a very less loss throughout the entire charging cycles. This reduction in loss is common in LiBs as the structure rearrangement takes place when lithium intercalates into the heterostructure and reduction of the available of the oxygen in rGO.

166

Figure 7-4. Cyclic performance for MoS2/rGO showing a uniform charge/discharge performance, (b)

Image showing the voltage vs specific capacity for the first two cycles

This performance was further compared with the vertical flowers of MoS2/Graphite and it was found that the CVD material outperformed the exfoliated material with a much higher first discharge current of 960 mAh/g and a cyclic performance of 750 mAh/g. This reduction in the current from first discharge to the cyclic performance can be because of the conversion of semiconducting MoS2 to semi-metal MoS2 (1T phase). The CVD material was stable for over 30 cycles with a cyclic capacity of 750 mAh/g. As this heterostructure does not involve any post growth processing steps, this could be used as a direct binder- free material for LiBs. 167

Figure 7-5. Plot showing the higher cyclic performance for CVD MoS2/graphite heterostructure with a cyclic capacity of 750 mAh/g.

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VITA Ganesh Rahul Bhimanapati  PhD in Material Science and Engineering, 2017  Master of Science (MS) in Energy and Mineral Engineering, option in Fuel Science, 2013  Bachelor of Technology (B.Tech) in Chemical Engineering, Osmania University, 2009 Publications (For more information: please visit https://scholar.google.com/citations?user=xDCqIN0AAAAJ&hl=en&oi=ao)

 Ganesh R Bhimanapati, Xiaotian Zhang, Yu Lei, Mauricio Terrones,Joan M. Redwing, Joshua A. Robinson, “Growth and stability studies of WSe2 on flexible graphite and graphene oxide substrates”,inpreparation.  Ganesh R Bhimanapati*, Roger C. Walker*, Sarah Eichfeld, Kehao Zhang, Joshua A. Robinson “ X-Ray effects on transition metal dichalcogenides grown via chemical vapor deposition”, Submitted.  Kehao Zhang, Nicholas J. Boyrs, Ganesh R Bhimanapati, Baoming Wang, Teague Williams, Ke Wang, Aman Haque, P. James Schuck, Joshua A Robinson, “ Controlling Morphology and Excited State Dynamics of Monolayer MoS2 via Substrate Engineering”, Submitted.  Sarah M. Eichfeld, Brian M. Bersch, Yu-chuan Lin, Ganesh R. Bhimanapati, Aleksander F. Piasecki, Kehao Zhang, Nick Glavin, Andrey Vevodin and Joshua A. Robinson, “Selected area growth of two dimensional materials”, Submitted.  Ganesh R Bhimanapati, Trevor Hankins, Rafael A. Vilá, Joshua A Robinson, “Growth and Tunable Surface Wettability of Vertical MoS2 layers for Improved Hydrogen Evolution Reactions”, ACS Applied Materials & Interfaces,2016, 8(34),22190-22195.  Ganesh R Bhimanapati, Nicholas R. Glavin, Joshua A. Robinson, “2D Boron nitride: Synthesis and applications”, Semiconductors and Semimetals,2016, 95,101-147.  N Briggs, S Subramanian, YC Lin, S Eichfeld, B Jariwala, G Bhimanapati, Kehao Zhang, Joshua A Robinson, “ Realizing 2D Materials Via MOCVD”, ECS Transactions, 75,8,725-731.  Ganesh R Bhimanapati, Maxwell Wetherington, Joshua A Robinson, “Synthesis and radiation response of Boro-carbon-oxy-nitride (BCON): A Graphene oxide and hexagonal boron nitride hybrid”, 2D Materials,2016,3,025028.  D Ruzmetov, K Zhang, G Stan, B Kalanyan, G R Bhimanapati, SM Eichfeld, Robert Burke, Pankaj B Shah, Terrance P O’Regan, Frank J Crowne, A Glen Birdwell, Joshua A Robinson, Albert V Davydov, Tony G Ivanov, “ Vertical 2D/3D semiconductor heterostructures based on epitaxial molybdenum disulfide and gallium nitride” ,ACS nano, 10,3580-3588  Ismail Alperen Ayhan, Qi Li, Praveen Meduri, Hyukkeun Oh, Ganesh R Bhimanapati, Guang Yang, Joshua A Robinson, Qing Wang,” Effect of Mn3O4 nanoparticle composition and distribution on graphene as a potential hybrid anode material for lithium-ion batteries”,2016, RSC Advances,6,33022-33030.  Ganesh R Bhimanapati, Zhong Lin, Youngwoo Son, Judy Cha, Vincent Meunier, Valentino Cooper, Liangbo Liang, Steven G. Louie, Emilie Ringe, Wu Zhou, Bobby G. Sumpter, Humberto Terrones, Fengnian Xia, Saptarshi Das, Jun Zhu, Deji Akinwande, Nasim Alem, John Schuller, Di Xiao, Mauricio Terrones, Joshua A. Robinson “Graphene and Beyond: Recent Advances and Looking Forward”, ACS Nano, 2015, 9 (12), 11509.  Ganesh R Bhimanapati, Daniel Kozuch, Joshua A Robinson, “Large-Scale Synthesis and Functionalization of Hexagonal Boron Nitride Nanosheets”, NanoScale, 2014,6,11671.  Michael S. Bresnehan, Ganesh R Bhimanapati, Ke Wang, David Snyder, Joshua A Robinson, “Impact of copper overpressure on the synthesis of hexagonal boron nitride atomic layers”, ACS Appl. Mater. Interfaces, 2014, 6(19), 11755.  Zakaria Y Al Balushi, Takahira Miyagi, Yu-Chuan Lin, Ke Wang, Lazaro Calderin, Ganesh Bhimanapati, Joan M Redwing, Joshua A Robinson, “The impact of graphene properties on GaN and AlN nucleation”, Surface Science,2015, 634,81.