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Engineered Two-Dimensional Nanomaterials for Advanced Opto-electronic Applications

Ghidewon Arefe

Submitted in partial fulﬁllment of the requirements for the degree of Doctor of Philosophy in the Graduate School of Arts and Sciences

Columbia University

2018

2 Abstract

Engineered Two-Dimensional Nanomaterials for Advanced Opto-electronic Applications

Ghidewon Arefe

Two dimensional (2D) materials have unique properties that make them exciting candidates for various optical and electronic applications. Materials such as graphene and transition metal dichalcogenides (TMDCs) have been intensively studied recently with researchers rac- ing to show advances in 2D device performance while developing a better understanding of the material properties. Despite recent advances, there are still significant roadblocks facing the use of 2D materials for real-world applications. The ability to make reliable, low-resistance electrical contact to TMDCs such as molybdeum disulfide (MoS22)has been a challenge that many researchers have sought to overcome with novel solutions. The work laid out in this dissertation uses novel techniques for addressing these issues through the use of improved device fabrication and with a clean, and potentially scalable doping method to tune 2D material properties. A high-performance field-e↵ect transistor (FET) was fabricated using a new device platform that combined graphene leads with dielectric encapsulation leading to the highest reported value for electron mobility in MoS2.Devicefabricationtechniqueswerealsoinvestigatedand a new, commercially available lithography tool (NanoFrazor) was used to pattern contacts directly onto monolayer MoS2.Throughaseriesof control experiments with conventional lithography, a clear improvement in contact resistance was observed with the use of the NanoFrazor.

3 Plasma-doping, a dry and clean process, was investigated as an alternative to traditional wet-chemistry doping techniques. In addition to developing doping parameters with a chlorine plasma treatment of graphene, a series of experiments on doped graphene were conducted to study its e↵ect on optical properties. Whereas previous studies used electrostatic gating to modify graphene’s optical properties, this work with plasma-doped graphene showed the ability to tune absorbence and plasmon wavelength without the need for an applied bias opening the door to the potential for low-power applications. This work is a just small contribution to the larger body of research in this ﬁeld but hopefully represents a meaningful step towards a greater understanding of 2D materials and the realization of functional applications. Contents

List of Tables iv

List of Figures v

Acknowledgements xvi

Dedication xxi

1 Introduction and Background 1 1.1 Introduction ...... 1 1.2 Two-dimensionalmaterials ...... 2 1.3 Challenges in 2D materials ...... 4 1.4 Engineering of 2D materials ...... 4 1.5 Characterization Techniques ...... 5 1.5.1 Optical characterization ...... 5 1.5.2 Electrical measurements ...... 6 1.6 Thesis organization ...... 7

2 High mobility monolayer MoS2 12 2.1 Introduction ...... 12

2.2 Growth and characterization of CVD MoS2 ...... 14

2.3 CVD MoS2 heterostructure and device fabrication . . . . 16 2.4 Electricaltransportmeasurements...... 19

i 2.5 Hall E↵ectmeasurements...... 22 2.6 Conclusion ...... 23

3 Thermal Scanning Probe Lithography 25 3.1 Introduction ...... 25 3.2 Thermal Scanning Probe Lithography ...... 26 3.3 tSPL patterned electrical contact ...... 29 3.4 EBL vs tSPL comparison ...... 31 3.5 Metal workfunction ...... 34 3.6 Local doping with nanolithography ...... 37 3.7 Conclusion ...... 38

4PlasmaDoped2Dmaterials 44 4.1 Introduction ...... 44 4.2 Plasma processing ...... 45 4.3 Plasma doped graphene ...... 47 4.4 Raman spectroscopy of doped graphene ...... 50 4.5 XPSofdopedgraphene...... 52 4.6 Plasma doped TMDCs ...... 53 4.7 Optical spectroscopy of doped TMDCs ...... 55 4.8 Electrical transport of doped 2D materials ...... 56 4.9 Conclusion ...... 58

5 Chlorinated graphene spectroscopy 61 5.1 Introduction ...... 61 5.2 Electronicstructureofgraphene ...... 62 5.3 Optical properties of graphene ...... 62 5.4 Sample preparation ...... 64 5.5 Results ...... 65 5.5.1 Reﬂectance ...... 66 5.5.2 Transmission ...... 67

ii 5.6 Conclusions ...... 67

6 Plasmonic phenomena in chlorinated graphene 72 6.1 Introduction ...... 72 6.2 Graphene plasmons ...... 73 6.3 Infrared nano-imaging ...... 73 6.4 Sample preparation ...... 74 6.5 Results ...... 76 6.6 Conclusions ...... 77

7 Future Work 80 7.1 Proposed experiments ...... 80 7.2 Proposed tSPL experiments ...... 80

7.3 Proposed Cl2 plasma doping experiments ...... 81

Appendix A. Supplementary Information 95 A.1 Mechanicalexfoliation ...... 95 A.2 Heterostructurefabrication...... 95 A.3 CVDgraphenetransfer...... 96 A.4 ICPdopingconditioncalibration ...... 97

iii List of Tables

4.1 Plasma parameters for developing conditions to dope graphene with an Oxford ICP plasma etcher. Only data points of successful, i.e. not destroyed, doping samples are shown. The RF and ICP power settings were established by determining the lowest power settings for the ICP etcher that would still result in successful plasma strike...... 50 4.2 Plasma parameters for doping CVD TMDCs based on conditions established by Kim et al. This table only shows TMDC samples that were successfully (i.e. not

destroyed) treated with a Cl2 plasma. [76]...... 55 5.1 Chlorine plasma parameters to dope CVD graphene with an Oxford ICP etcher. Samples A-D were prepared on

SiO2 for reﬂectivity measurements and C and D were not doped in order to use them as controls. The reﬂectivity and transmission of sample J, on sapphire, was measured

before and after Cl2 plasma treatment...... 65 6.1 Plasmon wavelengths for CVD and exfoliated graphene samples before (initial) and after (doped) chlorine plasma treatment...... 76 A.1 Plasma parameters for developing conditions to dope graphene with an Oxford ICP plasma etcher...... 97

iv List of Figures

1.1 A single sheet of carbon atoms, graphene (top) is the basis for various forms of nanomaterials including (bottom row, from left) 0D ”buckyballs”, 1D carbon nanotubes, and 3D graphite (which also exists in macroscopic form) [1]...... 2 1.2 Images of early graphitic ﬁlm and devices fabricated by Novoselov et al [2]. a) Optical image of large mulitlayer graphene ﬂake of thickness 3 nm on top of an oxidized Si wafer. b) Atomic force microscope (AFM) image of 2

x2mmareofﬂakenearitsedge.DarkbrownistheSiO2 substrate surface, orange is 3 nm above substrate surface. c) AFM image of single-layer graphene. d) Scanning electron microscope (SEM) image of experimental device prepared from few-layer graphene (FLG). e) Schematic view of device in (d)...... 3 1.3 Resistance values of direct metal, top contacted 2D semiconductors. Compilation of contact resistance values from literature as a function of sheet resistance for graphene, black phosphorus and TMDCs. The dashed line represents a standard for resistance established by industry. PlotfromAllain,etal[5]...... 9

v 1.4 Optical characterization techniques. Image adapted from Semiconductor material and device characterization [7]. . 10 1.5 Optical images of mechanically exfoliated a) MoS2 b)

graphene and c) WSe2.Basedonthevisualcontrastof

these images, single-to-few layer MoS2 and graphene are

identifiable. The WSe2 flake in (c) is on the order 50+ nm. ImagescourtesyofNathanFinney...... 10 1.6 Raman spectra of pristine (top) and defected (bottom) graphene [9] ...... 11 1.7 a) Schematic of field e↵ect transistor for electrical transport measurements. b) Typical Hall bar structure device configuration...... 11

2.1 a) Brightﬁeld microscope image image CVD MoS2 grown

on SiO2. b) Large area, single-crystal CVD Mo2.c)Pho-

toluminescene spectra of CVD MoS2 provides conﬁrma- tion of single layer growth. inset: Optical microscope

image of CVD MoS2 on PDMS stamp prior to transfer. Scale bar: 10 mm. a, b from [41], c from [10] ...... 16

vi 2.2 i) CVD MoS2 grown directly onto a SiO2/Si substrate. ii) Sample coated with the transfer polymer (e.g cellulose acetate butyrate (CAB)). iii) Water is used to intercalate between the transfer polymer and substrate. The trans-

fer polymer is peeled o↵of substrate with MoS2 ﬂake(s)

attached to its underside. iv) MoS2 flake, still attached to polymer film, is aligned with and transferred onto hBN flake on device substrate. v) Sample is immersed in solvent (e.g. acetone) to dissolve polymer. vi) Af-

ter polymer is removed and sample dried with N2 gas,

MoS2/hBN sample is ready for device fabrication. Trans- fer process based on techniques developed by Schneider et al [43]...... 17

2.3 From Cui et al [10]. a) Exploded view of MoS2 het- erostructue device and close up look of one dimensional edge contact between metal and graphene electrode. b) Optical microscope image of completed hBN-encapsulated

MoS2 heterostructure. c) High-resolution cross-sectional TEM image of heterostructure with false-color close-up showing individual layers (TEM image of cross-section and false-color close-up courtesy of Pinshane Huang). . . 19 2.4 Data published in Cui et al [10]. a) Contact resistance as

afunctionofback-gatevoltage(Vbg)andtemperature for and b) Arrehnius plot for Schottky barrier height

extraction of monolayer CVD MoS2 device encapsulated withhBN...... 21

vii 2.5 Data published in Cui et al [10]. Gate-voltage-tunability

of the Schottky barrier (fB)heightingraphene-MoS2

contacts, shown for MoS2 devices of varying thickness

(1-4 layer). As the gate voltage increases, fB is reduced by the modulation of the graphene contact’s Fermi level (see inset). The Schottky barrier height reduces to approximately 0 at suciently high gate voltage...... 22 3.1 a) Scanning electron micorscope (SEM) image of NanoFra- zor probe; inset: transmission electron microscope (TEM) image of NanoFrazor tip. b) Diagram of NanoFrazor probe and tip showing components that are key to tSPL process (images courtesy of SwissLitho Ag)...... 27 3.2 Nanofrazor patterning process for electrical contact to

2D materials. a. 2D material on SiO2/Si wafer coated with PMGI and PPA. b. Heated tip used to write pattern in PPA layer. c. Undercut in PMGI layer produced by immersing exposed regions with developer. d. Metal deposited into exposed regions form electrical contact. . . 30

3.3 Optical microscope image of exfoliated multilayer MoS2 with metal contacts patterned with tSPL. The channel length (distance between the electrodes) in this device is 10 mm...... 31

3.4 Transfer curve of multilayer exfoliated MoS2 device with tSPL patterned metal contacts with an applied bias of +10 mV. Shown with y-axis on a log scale (lef t) and linear scale (right)...... 32

viii 3.5 Estimated ﬁeld-e↵ect mobility of exfoliated multilayer

MoS2 based on two-probe measurements as a function of temperature. With decreasing temperature, the ﬁeld ef- fect mobility increases due to fewer scattering losses due to thermal e↵ects. Mobility appears to peak around 100 Kbelowwhichitdecreasesduetoscatteringlossesdueto non-thermal contributions (charge impurities, substrate e↵ects, phonons)...... 33

3.6 Left: Optical image of Mo22 device after tSPL patterning. Center: Non-contact AFM image. Right: KPFM image...... 34

3.7 Left: Optical image of Mo22 device after tSPL patterning. Center: Non-contact AFM image. Right: KPFM image...... 34 3.8 Contact metal workfunctions relative to TMDC band diagrams. Adaptedfrom[55]...... 35

3.9 Two-probe measurement data for MoS2 devices with aluminum (Al) contacts. Clockwise from top left: Transfer curve while sweeping the gate voltage up; Transfer curve while sweeping the gate voltage up and down; Output curves of EBL patterned device with Al contacts at various gate voltages (from -60 to +60 V); and Output curves of Nanofrazor (tSPL) patterned device with Al contacts from -60 to 60 V. While the current output is slightly higher for EBL patterned contacts, the tSPL (Nanofrazor) patterned contacts shows far less hysteresis indicating the possibility of a cleaner contact interface. . 36

ix 3.10 Two-probe measurement data for MoS2 devices with palladium (Pd) contacts. Clockwise from top left: Trans- fer curve while sweeping the gate voltage up; Transfer curve while sweeping the gate voltage up and down; Out- put curves of EBL patterned device with Pd contacts at various gate voltages (from -60 to +60 V); and Out- put curves of Nanofrazor (tSPL) patterned device with Pd contacts from -60 to 60 V. In addition to having a higher current output then the EBL patterned devices, the Nanofrazor (tSPL) patterned device appears to have an Ohmic contact...... 40

3.11 Two-probe measurement data for MoS2 devices with platinum (Pt) contacts. Clockwise from top left: Trans- fer curve while sweeping the gate voltage up; Transfer curve while sweeping the gate voltage up and down; Out- put curves of EBL patterned device with Pt contacts at various gate voltages (from -60 to +60 V); and Out- put curves of Nanofrazor (tSPL) patterned device with Pd contacts from -60 to 60 V. In this comparison, the Nanofrazor (tSPL) patterned device had current output

100x greater at +60 Vbg as compared to the EBL patterned device with the same Pt contact metal...... 41

3.12 a) AFM and b) KPFM images of exfoliated MoS2 used for TCNL doping. Doped region highlighted in green. As TCNL operates in contact mode, note residue accumulation at edges of doped region in AFM image. c)Single

layer of MoS2 conﬁrmed with Raman spectra. d) Optical microscope image of ﬁnal device...... 42

x 3.13 Output curves of TCNL chlorine-doped monolayer MoS2 shown in Figure 3.12. Clockwise from top left: Undoped channel and one doped contact between contacts 1-2; undoped channel and one doped contact between contacts 6-5, undoped control device between contacts 3-4; and doped channel and doped contacts between contacts 6-3. While the control device (3-4), shown in the bottom right, appears to have Ohmic contact, the output curves for the other 3 variations of doping (either contact or channel with doping) shows signs of rectification and only turns on with positive applied source-drain voltage. Data was collected with fixed back gate voltages of 20 (black)40(red)and60(green)V...... 43 4.1 Aurora Borealis (Northern lights), a naturally occurring example of a plasma, as seen from Goodsell Observatory at Carleton College in Northfield, Minnesota. Photo by Drew Weitz...... 46 4.2 Cross-sectional diagram of inductively coupled plasma (ICP) system. (image courtesy of Oxford Instruments). . 47 4.3 Clockwise from top left: top left) Raman specta of CVD graphene before (black) and after (red) chlorine plasma treatment using conditions listed in Table A.1. Top right) Graphene D-peak near 1350 cm-1 is present before doping but does not increase signficantly after plasma treatmetn. Bottom row) the G’ (or 2D) and G peak of graphene is shifted after doping...... 51

xi 4.4 Left (right): Raman spectra of the G-peak (2D-peak) position for graphene as a function of doping. Black points are experimentally measured values from Das et al, solid blue lines are theoretical trends [75]. Figures adaptedfromDasetal[74]...... 52 4.5 XPS spectra of chlorinated graphne. Left: C-1s spectra shows oxidation of graphene in initial state. Right: Cl-2p peak around 201 eV is actually a split peak comprised of

2p3/2 at 200.6 eV and 2p1/2 at 202.2 eV. Measurements performed at SLAC, plots courtesy of Theanne Schiros (FIT/Columbia)...... 54

4.6 Left: Optical image of CVD MoS2,right:Ramanspectra

for pristine and Cl2 plasma-treated CVD MoS2 mono-

layer. CVD MoS2 samples provided by Robert Burke (ARL)...... 56

4.7 Left: Optical image of CVD WSe2,right:Ramanspectra

for pristine and Cl2 plasma-treated CVD WSe2 mono-

layer. CVD WSe2 samples provided by Kyung Kang and Eui-Hyeok Yang (Stevens Institute of Technology). . 57 4.8 Photoluminescence spectra of TMDCs before (black) and

after (red) Cl2 plasma-treatment. Left: CVD WSe2 mono-

layer. Right: CVD MoS2 monolayer. Despite identical doping conditions as described in Section 4.6 with iden- ticalplasmatreatmentconditions ...... 58 4.9 Peak resistivity of graphene, also refered to as the charge- neutrality-point (CNP). Dirac cones shown (from left-to- righ) for p-doped, intrinsic, and n-doped graphene. Note

position of the Fermi level (EF)inthevalenceband,at CNP, and conduction band, respectively. Image from Geim et al [1]...... 59

xii 4.10 Left: Optical image of exfoliated graphene device with gold electrodes. Right: Two-probe resistance measurement of device before (solid) and after (dashed) doping with a chlorine plasma treatment...... 60

4.11 Left: Optical image of CVD MoS2 monolayer device with gold electrodes. Right: Two-probe DC measurement of device before (black) and after (red) doping with a chlorine plasma treatment...... 60 5.1 Electronic structure of graphene. (a) Hexagonal lattice of graphene made up of 6 carbon atoms. Points A and B mark inequivalent points referred to as K and K’ in (b). (outlined in red) with corresponding orbital overlap. Image courtesy of Mak et al [84]...... 63 5.2 a,b) Raman spectra of CVD graphene sample A and B before (black) and after (red) chlorine plasma treatment. c,d) G-peak and e,f) G’ (2D) peak of CVD graphene before/after doping with peaks normalized to show positional shift...... 69

5.3 Reflectance of Cl2 plasma-doped CVD graphene versus control samples. Inset: Schematic of experimental set- up, incident light source, hv, is reflected o↵the surface. . 70 5.4 a) Diagram of reflectance and transmission measurements

on sapphire (Al2O3)substrates.b)Reﬂectanceandc)

transmission of CVD graphene on Al2O3 before (black) and after (red) chlorine plasma treatment. d) Percent change in reﬂectivity (black) and transmission (blue) after doping...... 71

xiii 6.1 a) Optical microscope image of CVD graphene, scale bar is 20 mm. b) Optical microscope image of exfoliated graphene, scale bar is 20 mm. c) Raman spectra of CVD graphene sample before (black) and after (red) chlorine plasma doping treatment. d) Raman spectra of exfoliated graphene sample before/after (black/red) chlorine plasma treatment...... 75 6.2 a) Diagram of IR nano-imaging experiment. b) AFM image of CVD graphene sample. c) Cross-sectional linecut of plasmon relative intensity before (black) and after (red) chlorine plasma doping treatment. d) Scanning plasmon image of CVD graphene sample, red line represents linecut for data plotted in (c)...... 77 6.3 a) Diagram of IR nano-imaging set-up (image courtesy of Basov et al (Fei-Basov:2011). b) AFM image of CVD graphene sample. c) Cross-sectional linecut of plasmon relative intensity before (black) and after (red) chlorine plasma doping treatment. d) Scanning plasmon image of CVD graphene sample, red line represents linecut for data plotted in (c)...... 78 6.4 Scanning plasmon image of exfoliated monolayer graphene before (a) and after (b) chlorine plasma treatment. Note the di↵erence in magnitude di↵erence in color scale in both images. .. b) Scanning plasmon image of exfoliated graphene sample...... 79 6.5 Linecut of plasmon intensity from exfoliated graphene sample measured before and after chlorine plasma treatment. Data data taken from cross-section highlighted withredlineinFigure6.4b...... 79

xiv A.1 a) Bulk crystal exfoliated onto Scotch tape b) SiO2 wafer treated with an oxygen plasma prior to the transfer of flakes onto the substrate c) Scotch tape with exfoliated flakes is brought into contact with plasma-treated wafer and placed on a hotplate at 100 C for 3-5 minutes d) Af- ter cooling, tape is peeled o↵substrate with flakes trans-

ferred onto SiO2 e) macroscopic view of flakes exfoliated onto wafer f) 100X magnification of few-layer graphene onwafer. ImagesfromHuangetal[99]...... 96 A.2 a) Device fabrication process: (i) Bottom-hBN flake exfoliated onto a SiO2/Si substrate. (ii) MoS2 on hBN. MoS2 was transferred onto bottom-hBN. (iii-vi) Four graphene flakes on hBN/MoS2 stack. Four graphene flakes were sequentially transferred onto the stack around the perimeter of the MoS2 channel. (vii) Top-hBN on graphene/MoS2/hBN stack. Top-hBN was transferred on top of the stack for encapsulation. (viii) Patterning of PMMA into Hall bar geometry. PMMA layer in the stack was patterned by e-beam lithography into a Hall bar configuration. (ix) Dry etching of the stack. The stack is etched using PMMA as an etch mask. (x) For- mation of metal leads. Metal leads are fabricated by e-beam lithography and deposition of metals. Scale bar: 50 m. b) Cross-section STEM image of the stack (from top to bottom: hBN (8 nm), MoS2 (3L), hBN (19 nm)). Scale bar: 50 nm. STEM image courtesy of Pinshane Huang[10]...... 98

xv Acknowledgements

Writing a dissertation is an exercise in solitude, yet this dissertation is the culmination of contributions from many people who helped me during my graduate research. There are countless individuals that have helped me along the way that I must acknowledge as important con- tributors to this body of work. The most important decision one can make in pursuing a doctoral degree that will not only influence your time in graduate school but also a career in your field, is selecting an advisor. I had the great fortune of working for Professor James Hone during my time at Columbia Uni- versity. Not only is he a widely respected expert in our field, he is also one of the best teachers I have ever had and I attribute my success in great part to his mentorship. His enthusiasm for new experiments and willingness to let his students explore new ideas allowed me to thrive in the lab and develop my own experimental expertise. Jim is also one of the most generous people I know and I will always be grateful for the opportunity to work in his lab and for his continuous support to ensure I succeeded. In addition to serving on my disseration committee, Professor Theanne Schiros was instrumental in guiding me towards a second act during my graduate research. Her work on chlorine doped 2D materials helped provide me with a new focus that became a large part of my research during my later years at Columbia. It was always a joy to work with

xvi Theanne and I will always be grateful for her mentorship. Thank you to the rest of my dissertation committee members Pro- fessor Cory Dean, Professor Je↵Kysar, and Professor Jim Schuck for assisting me with this process and providing great feedback during my defense. Additionally, I am grateful to Cory for allowing me to use equipment in his lab, for always being available to discuss results and providing guidance on research throughout my time at Columbia as well as during our measurements at the National High Magnetic Field Labo- ratory (Tallahassee, Florida). Despite spending most of my time in the North West Corner Building and being physically separated from the rest of the Mechanical Engineering department, Professor Kysar and Professor Vijay Modi were faculty members that were always helpful and I will always be thankful for their support. Iwasluckytohaveagreatgroupofcolleaguesthatwerealways around to help me in lab and made for a fun work environment. I will always be grateful to my fellow students and postdocs in the Hone Lab over the years: Daniel Chenet, Chris DiMarco, Nate Finney, Demi Ajayi, Nick Petrone, Erika Penzo, Adam Hurst, Sunwoo Lee, Alex Cui, Jenny Ardelean, Yibo Zhu, Cheng Tan, Bumho Kim, Yuanda Gao, Dongjea Seo, Smiti Bhattacharya, Julia Chang, Abhinandan Antony, Emanuil Yanev, Damien Chang, Daniel Rhodes, Gwan Hyoung Lee, Young Duck Kim, Arend van der Zande, Sanghoon Chae, Younghun Jung, Minsup Choi, Shanece Esdaille, Filippo Pizzocchero, and Bjarke Jessen. In addition to my colleagues in the Hone Lab, I was fortunate to work with many other labs at Columbia and must acknowledge their as- sistance through the years. Thank you to the following faculty members and research sta↵who allowed me to use their facilities and for providing guidance on experiments: Professor Collin Nuckolls, Professor Philip Kim, Professor Tony Heinz, Professor Ken Shepard, Professor

xvii Dmitri Basov, Dr. Jerry Dadap, Professor Richard Osgood, Professor John Kymissis, Professor Luis Campos, Professor Xavier Roy, and most importantly, Professor Xiaoyang Zhu. As a masters student at the Uni- versity of Minnesota many years ago, I brieﬂy worked with a student in Professor Zhu’s lab (while he was also at Minnesota) to study quantum dots with ultraviolet photoelectron spectroscopy. All these years later, it was nice to collaborate with his group once more. Thank you to all the other students, postdocs, and sta↵members that I was fortunate to work with at Columbia University: Rebeca Ribeiro, Brittany (Wright) Proctor, Bianca Howard, Megan Armstrong, Kai James, Kay Igwe, Yazmin Feliz, Derek Novo, Leo Li, Matt Yancowitz, Scott Diet- rich, Evan Telford, Da Wang, Archana Raja, Alexey Chernikov, Tavi Semonin, Keith Behrman, Diego Scarabelli, Roberto Cavallari, Yilei Li, Figo Yeh, En-Min Shih, Shaowen Chen, Sai Sunku, Kihong Lee, He- len Tran, Dennis Wang, Erik Young, Tarun Chari, Jaeeun Yu, Daniel Paley, James Vichiconti, Nirit Shamay, Nava Ariel Sternberg, Ti↵any Simon, Jessie Jones, Fred Sanzhez, Mel Francis, Sandra Morris, Jean Cadet, and Becca Chambers. While most of my research was conducted at Columbia University, a signiﬁcant portion of my work was conducted at other institutions. At City College (New York, New York) I worked closely with Inna Ko- rzhovska, Professor Lia Krusin, Professor Elisa Riedo, Annalissa Calo, Ray Zheng, and Milan Begliarbekov. I worked a lot with Inna and Milan on topological insulators and thank them for their friendship through the years. From MIT, Professor Tomas Palacios and Xu Zhang were instrumental in our work on chlorine doping and were always will- ing to provide samples whenever needed. I am also grateful to the sta↵ at the Center for Functional Nanomaterials at Brookhaven National Laboratory (Upton, New York), Aaron Stein, Fernando Camino, Ming Lu, Gwen Wright, and Grace Webster. At Army Research Lab, I had

xviii a great experience working with Madan Dubey, Matthew Chin, Robert Burke, Barbara Nichols, Eugene Zakar, Sina Najmei, Alex Mazzoni, Alin Chipara, Diane Green, Teresa Kipp, Nelson Mark, Paul Sunal, Nick Strnad, Jim Mulcahy, Glen Birdwell, and Von Cresce. While I did not conduct research for this dissertation during my master’s degree at the University of Minnesota, it is important to acknowledge those that helped guide me during my masters studies. Pro- fessor James Kakalios, my masters advisor, helped me regain my excitement for research and provided me with guidance and an opportunity to conduct research when others denied me such opportunities. Jim made me realize the importance of having a good advisor and provided guidance that I have carried throughout my career. Jim made research fun and I am happy that I now get to fulﬁll my goal of adding my dissertation to the shelf in his oce (and immediately get back to work...). Professor Steve Girshick was instrumental in my success early in my graduate career and actually helped recruit me as an undergraduate to pursue a graduate degree. I am thankful for his mentorship throughout the years. I must also acknowledge my colleagues and friends from the University of Minnesota who helped me along the way: Charlie Black- well, Lee Wienkes, Kent Bodurtha, Yves Adjallah, Rebecca Anthony, Rik Liptak, Lorenzo Mangolini, Anil Bika, Luke Franklin, Katie Glass, and Clara Mata. My time in industry prior to graduate school helped me learn some experimental techniques and taught me how to manage large projects. From Emerson Process Management, I would like to thank Lisa Nelson, Carl Ransen, Fred Sittler, Chad McGuire, John McIntire, Chis Eriksen, Tina Grunig, Dave Wehrs, and Gary Kokkonen. At 3M, I developed optical device fabrication and characterization skills that I was able to directly apply during my time at Columbia. Thank you to Sergey Lamansky, Jon Anim-Addo, Adam Haag, Bill Tolbert, Ha Le, and Pete

xix Grasse. Graduate school would not be possible without funding so I am especially grateful to the National Science Foundation for providing funding through the Integrative Graduate Educational and Research Traineeship (IGERT) for nanoparticle science and engineering in support of my masters studies at the University of Minnesota and to the GEM Consortium and 3M for providing funding for my doctoral studies at Columbia University. The GEM sta↵, Michelle Lezama, Marcus Huggins, and Michael Smith were all instrumental in helping me secure funding for my doctoral studies. IamdeeplyindebtedtoallthesupportIhavereceivedfromallthe aforementioned individuals as well as to those I may have forgotten. I hope to make the most out of my career and to always use science for good.

xx Dedication

This work is dedicated to my parents, Teblez and Arefe, for their constant support, and for instilling in me the value of hard work and dedication. To my siblings, Susie, Salem, and Luwam, thank you for keeping me humble. To my extended family, friends and everyone that gave me support throughout the years, thank you.

xxi Chapter 1

Introduction and Background

1.1 Introduction

Nano –fromtheLatinnanus, meaning dwarf, has become a ubiqui- tous prefix that implies advanced, cutting-edge technology, as in nanotechnology. While the potential for application of nanotechnology is real and could revolutionize our world, aside from a few products, nanotechnology has remained largely in the laboratory. Engineering on the nanoscale, where nano is the SI prefix for one billionth (10-9), has been enabled by advances in technologies used in the microelectronics and semiconductor industry and will be key to their continued success. As can be surmised from the prefix, nanomaterials are materials with a size dimension on the order of a nanometer. As shown in Figure 1.1 a single sheet of carbon atoms, also known as graphene,arranged in a honeycomb lattice, forms the basis for other forms of carbon nanomaterials. Zero-dimensional Buckyballs, one-dimensional carbon nanotubes and three-dimensional graphite can all be formed with graphene, a two-dimensional material, as the foundation (see Fig. 1.1) [1].

1 Figure 1.1: A single sheet of carbon atoms, graphene (top) is the basis for various forms of nanomaterials including (bottom row, from left) 0D ”buckyballs”, 1D carbon nanotubes, and 3D graphite (which also exists in macroscopic form) [1].

1.2 Two-dimensional materials

Two-dimensional (2D) materials are atomically thin materials that have long been theorized but were not experimentally produced until 2004 with the observation of monocrystalline graphitic films of thickness of afewatoms[2,3].Inthisearlywork,grapheneproducedwithme- chanical exfoliation was fabricated into a field-e↵ect transistor (FET) as shown in Figure 1.2 [2]. Since then, the field of research in 2D materials has been an interdisciplinary e↵ort that has seen great strides in understanding the fundamental properties of these materials [4].

2 Figure 1.2: Images of early graphitic film and devices fabricated by Novoselov et al [2]. a) Optical image of large mulitlayer graphene flake of thickness 3 nm on top of an oxidized Si wafer. b) Atomic force microscope (AFM) image of 2 x 2 mmareofflakenearitsedge.Dark brown is the SiO2 substrate surface, orange is 3 nm above substrate surface. c) AFM image of single-layer graphene. d) Scanning electron microscope (SEM) image of experimental device prepared from few- layer graphene (FLG). e) Schematic view of device in (d).

Materials used for this research were obtained using either mechanical exfoliation (details in Appendix A.1) or were grown using chemical vapor deposition (CVD) as will be discussed in more detail in Chap- ter 2. In addition to using multiple commercial suppliers for the bulk crystals used for mechanical exfoliation, the CVD materials used for this research was grown by collaborators at Columbia University (New York, New York), Stevens Institute of Technology (Hoboken, New Jer- sey), Army Research Lab (Adelphi, Maryland), Rice University (Hous- ton, Texas), and the Massachusetts Institute of Technology (Cambrdge, Massachusetts).

3 1.3 Challenges in 2D materials

Although there are significant barriers to producing 2D materials at a scale and cost that could enable manufacturing, there are still fundamental technical challenges associated with 2D device performance. A key challenge is making reliable, low-resistance contact to 2D materials. Researchers spanning engineering and physical science disciplines, both in academia and industry, have been investigating methods for reducing electrical contact resistance to 2D materials with a significant amount of e↵ort focused on graphene and transition metal dichalcogenides (TMDCs) such as molybdenum disulfide (MoS2)andtungsten diselenide (WSe2). Figure 1.3 is a compilation of resistance values of various 2D materials from literature [5] relative to a standard (dashed line) for contact resistance established by the semiconductor industry. While graphene is on the low-end of resistances reported in Figure 1.3, the lack of a bandgap in graphene limits it use as a channel material for applications where semiconductors are typically used. Semiconducing

TMDCs, such as MoS2, are ripe for improvement in contact resistance and with some clever engineering are poised to overcome these challenges.

1.4 Engineering of 2D materials

Despite the plethora of challenges standing in the way of the realization of applications based on 2D materials, the pursuit of solutions will continue. As the research community continues to develop a stronger understanding of the fundamental physics behind the properties of 2D materials [4], the engineering of this novel class of materials has enabled demonstrations of the viability of 2D materials such as grahpene and TMDCs for various opto-electronic applications [6].

4 1.5 Characterization Techniques

Several characterization techniques were used in this study of 2D materials. These techniques were used to better understand the optical and electronic properties as well as the chemical composition before and after processing. The following are brief descriptions of some of the key techniques used across this study.

1.5.1 Optical characterization

Of the various optical characterization techniques shown in Figure 1.4, this study used methods that involve emission, reflection and transmission of light. A time consuming component of working with 2D materials is the identification of single-to-few layer flakes by optical in- spection. In fact, often the first step in fabricating devices made from 2D materials is actually finding them. Despite having a thickness on the order of atoms, 2D materials can be visually identified with brightfield optical microscopy [2]. Images of various 2D flakes taken with an optical microscope are shown in Figure 1.5. AformofopticalemissionusedforcharacterizationinvolvesRaman scattering, a process that occurs when a photon incident on a lattice imparts (absorbs) energy in the form of a phonon and re-emerges as a lower (higher) energy photon; a frequency shift known as Stokes (anti- Stokes)scattering.Thisscatteringprocessprovidesinformationonthe structural and electronic properties of materials and has led Raman spectroscopy to become a standard tool in 2D material research [8]. In addition to the information provided by Raman spectroscopy, it is arelativelyfasttoolthatisnon-destructiveando↵ershighresolution measurements [9]. Raman spectra of pristine graphene shown in the top spectra of Figure 1.6 highlights characteristic peaks used for identification. The

5 bottom spectra in Figure 1.6 shows additional ’D’ peaks associated with defects in graphene. As will be shown in Chapter 4, the D peak will also appear in doped graphene.

1.5.2 Electrical measurements

Electrical measurements are key to understanding the transport properties of 2D materials. A myriad of device applications are being pursued with 2D materials but, one could argue, the most commonly fabricated device structure using 2D materials is the ﬁeld e↵ect transistor (FET). While nothing is simple when it comes to 2D materials, the relatively simple, and well understood, architecture of a FET makes it the ideal test-bed for studying the properties of 2D materials. Starting with Ohm’s Law, the resistance (R) of a 2D material can be calculated if the current and voltage across the device is known. With the dimensions shown in Figure 1.7a, W, L, and tOX which are channel width, channel length, and oxide (or dielectric) thickness, respectively, the resistivity, r,ofthedevice,assumingaconstantcurrentfromprobe1-4

(I1-4)isgivenby:

W V ⇢ = 2-3 (1.1) L I1-4

Using a silicon backgate, when a voltage is applied to the backgate of the FET, an electric ﬁeld is formed in the dielectric oxide layer and serves to modulate the Fermi level (EF)inthedevicechannel.

Information about the gate voltage (Vg)anddieletricpropertiesallows for the calculation of the charge carrier concentration, n,ofthedevice.

Knowing the capacitance, C, across the dielectric (typically SiO2)allows for the calculation of the ﬁeld-e↵ect mobility, mFE as shown in Equation

1.2. While mFE gives a good approximation of mobility, it is usually an

6 overestimation (by a factor of 2) of the more accurate Hall mobility

(mHall)whichisdependentonchargecarrierconcentration(n)asshown in Equation 1.3. In order to determine n, a four terminal measurement is required which allows for the resistance of the channel to be measured independently of the contacts.

1 d 1 d(1/⇢) µFE = = (1.2) C dV g C dV g

R 1 µ = Hall ; R = (1.3) Hall ⇢ Hall ne

1.6 Thesis organization

This work is a summary of experimental work focused on the engineering of 2D mateirals for opto-electronic applications. While this thesis is not an exhaustive document of all the experiments under taken on 2D materials by the author, it has been shaped to focus on two methods for engineering 2D materials. The ﬁrst half of this thesis revolves on the method of novel device fabrication to improve the performance of 2D material devices. Chapters 2 and 3 summarize e↵orts to improve device performance in molybdenum disulﬁde (MoS2), a semiconducting 2D material which has shown promise for a variety of electronic applications due its band gap which is lacking in graphene [1, 6]. Early work on

MoS2 focused on the construction of van der Waals (vdW) heterostructures with novel device architecture. This work led to record setting values for electron mobility and provided a device platform which has been replicated for other 2D materials [10]. Recent e↵orts have stepped back in terms of device architecture and have looked at new tools for device fabrication. Chapter 3 highlights the use of a new commercial tool for patterning contacts on 2D materials with heated probe tip as an alternative to conventional lithography

7 techniques. While this work is relatively new, early results are promising and are worth continued pursuit. The second half of this thesis looks at the use of plasma chemistry to modify optical and electronic properties of 2D materials. Chapter 4highlightstheuseofplasmadopingandthecharacterizationofthe plasma process to dope 2D materials. Chapter 5 focuses on the optical properties of graphene doped using a chlorine plasma process and highlights the ability to significantly modify reflectance and transmission in graphene. Chapter 6 continues the story of chlorine plasma-treated graphene but looks at the modification of plasmonic phenomena after doping. Chapter 7 provides a short synopsis of future experiments that will continue on these topics outlined in this work. Many of these experiments were planned but were not completed in time to include in this thesis due in-operable or faulty lab equipment but plans were passed on to new members of the Hone lab to continue these e↵orts.

8 Figure 1.3: Resistance values of direct metal, top contacted 2D semiconductors. Compilation of contact resistance values from literature as a function of sheet resistance for graphene, black phosphorus and TMDCs. The dashed line represents a standard for resistance established by industry. Plot from Allain, et al [5]

9 Figure 1.4: Optical characterization techniques. Image adapted from Semiconductor material and device characterization [7].

Figure 1.5: Optical images of mechanically exfoliated a) MoS2 b) graphene and c) WSe2.Basedonthevisualcontrastoftheseimages, single-to-few layer MoS2 and graphene are identiﬁable. The WSe2 ﬂake in (c) is on the order 50+ nm. Images courtesy of Nathan Finney.

10 Figure 1.6: Raman spectra of pristine (top) and defected (bottom) graphene [9]

Figure 1.7: a) Schematic of ﬁeld e↵ect transistor for electrical transport measurements. b) Typical Hall bar structure device conﬁguration.

11 Chapter 2

High mobility monolayer

MoS2

2.1 Introduction

Molybdenum disulphide (MoS2)hasattractedalotofattentionasa two-dimensional (2D) material with the potential for future electronic applications [11–13]. Previously studied for its low friction properties and use as a dry lubricant [14], the electronic and optical properties of

MoS2 have made it an attractive complement to graphene for various applications [15]. Whereas the more popular graphene is a gapless 2D material, monolayer MoS2 has a direct bandgap of 1.88 eV [11, 16, 17] the opens up the potential for uses in microelectronics. This bandgap provides an advantage over silicon as it minimizes the potential for source-to-drain tunneling that exists at the scaling limit of transistors

[15]. The size of the band gap in MoS2 decreases inversely with the number of layers to a bulk value of 1.29 eV [11] and, due to quantum conﬁnement, shifts from direct gap in monolayer form to an indirect gap in bulk [11, 18].

Despite the excitement around MoS2,aswithany2Dmaterial,the

12 electrical and optical properties of MoS2 are strongly a↵ected by impurities and its dielectric environment [12, 18–20], hindering the study of its intrinsic physics and limiting the design of devices based on such two-dimensional materials. In particular, the theoretical upper bound of the electron mobility of monolayer (1L) MoS2 is predicted to range from several tens to a few thousands at room temperature and exceed 105 cm2 V1 s1 at low temperature depending on the dielectric environment, impurity density, and charge carrier density [21–23]. In contrast, experimentally measured 1L MoS2 devices on silicon dioxide (SiO2) substrates have exhibited a room-temperature two-terminal ﬁeld-e↵ect mobility ranging from 0.1 to 55 cm2 V-1s-1 [12, 24, 25]. This value increases to 1560 cm2 V-1s-1 with encapsulation by high-k dielectric materials [12, 18] due to a more e↵ective screening of charged impurities [22]. Owing to the presence of large contact resistance from the metal-

MoS2 Schottky barrier, however, these two-terminal measurements un- derestimate the true channel mobility [13, 26, 27]. Multi-terminal Hall mobility measurements [18, 28] still show a mobility substantially below theoretical limits, particularly at low temperature, with best reported values of 174 cm2 V-1s-1 at 4 K for 1L [18] and 250 cm2 V-1s-1 and 375 cm2 V-1s-1 at 5 K for 1L and 2L devices, respectively [28]. Typi- cally, these thin samples exhibit a crossover to non-metallic behaviour at carrier densities below 1 x 1013 cm-2 [18, 28, 29] or at smaller carrier densities by the engineering of local defect states and improved interfacial quality [30]. The scattering and disorder that lead to this non-metallic behaviour can arise from multiple origins, such as lattice defects, charged impurities in the substrates and surface adsorbates, and it has been dicult to identify their separate contributions [12, 17, 18, 21, 28, 29, 31, 32]. We have previously demonstrated that the encapsulation of graphene within hexagonal boron nitride (hBN) reduces scattering from substrate

13 phonons and charged impurities, resulting in band transport behaviour that is near the ideal acoustic phonon limit at room temperature and ballistic over more than 15 mm at low temperature [33]. These results were realized using a novel technique to create one-dimensional edge contacts to graphene exposed by plasma-etching an hBN/graphene/hBN stack. Such an approach has not yet proved e↵ective with MoS2. How- ever, recent reports that graphene can create a high-quality electrical contact with MoS2 [17, 34] motivated a hybrid scheme, in which the channel MoS2 and multiple graphene leads are encapsulated in hBN and the stack is etched to form graphene-metal edge contacts. This new scheme is distinct from previous approaches in that the entire

MoS2 channel is fully encapsulated and protected by hBN to achieve multi-terminal graphene contacts without any contamination from the device fabrication process. This new scheme was employed for a comprehensive study on the transport properties of 1-6 layer MoS2 devices [10]. However, for this chapter, the focus will be on a monolayer system and will highlight advances in engineering MoS2 monolayers grown from chemical precursors and the the novel techniques developed to fabricate high-performance devices.

2.2 Growth and characterization of CVD

MoS2

Although much work has been done with mechanically exfoliated MoS2 [10, 12, 18, 19, 35, 36], this form of producing 2D materials is not scale- able for manufacturing purposes. Chemical vapor deposition (CVD) is an epitaxial growth technique that is commonly used for integrated circuit device fabrication [37]. CVD is a technique that is commonly

14 used to grow silicon ﬁlms for the microelectronics industry however interest in 2D materials has pushed researchers to develop methods for growing materials such as graphene and MoS2 via epitaxial methods [38–41].

Solid molybdenum trioxide (MoO33)andsulfurprecursorswere used to grow MoS2 monolayers, however, in contrast to previous ef- forts [39], the MoS2 used in these experiments were not grown with the use of a nucleation seed [41]. Silicon substrates with a top layer of silicon dioxide (oxide thickness varied from 90 to 300 nm) (SiO2/Si) to be used as growth substrates were thoroughly cleaned with solvents followed by annealing with forming gas. Growth substrates were then placed in a 2-inch diameter quartz tube inside a CVD furnace. Inside the quartz tube, below the growth substrates, was a crucible containing the solid precursors (14 mg of MoO3 with 120 mg of S). With nitrogen flowing through the furnace at atmospheric pressure, growth occurs over several hours over a temperature range of 105-700 with specific ramp and cool down intervals [41]. After growth, optical microscopy and photoluminescence (PL) were used to characterize the size and quality of the single crystal CVD MoS2 flakes as well provide confirmation of the number of layers. Brightfield microscope images in Figure 2.1a, b show the large scale size of the single-crystal MoS2 grown by CVD. The PL spectra in Figure 2.1c provides confirmation of single-layer growth and reveals a peak corresponding to the band-gap of monolayer MoS2 of approximately 1.9 eV.

In addition to the high quality of MoS2 crystals produced, this growth technique has resulted in CVD MoS2 monolayers with transport properties that are on par with those produced by mechanical exfoliation[10, 41].

15 Figure 2.1: a) Brightﬁeld microscope image image CVD MoS2 grown on SiO2. b) Large area, single-crystal CVD Mo2. c) Photoluminescene spectra of CVD MoS2 provides conﬁrmation of single layer growth. inset: Optical microscope image of CVD MoS2 on PDMS stamp prior to transfer. Scale bar: 10 mm. a, b from [41], c from [10]

2.3 CVD MoS2 heterostructure and device fabrication

Based on the enhancement of electron mobility observed in graphene with the use of a hexagonal boron nitride (hBN) substrate [42] and hBN encapsulation [33], in order to fabricate a high-performance CVD

MoS2 device, the atomicallly ﬂat hBN was chosen for a substrate.

In order to use CVD MoS2 in a heterostructure device, the ﬁrst step was to remove it from the growth substrate in order to stack it in the desired conﬁguration. Common methods of transferring CVD

16 Figure 2.2: i) CVD MoS2 grown directly onto a SiO2/Si substrate. ii) Sample coated with the transfer polymer (e.g cellulose acetate butyrate (CAB)). iii) Water is used to intercalate between the transfer polymer and substrate. The transfer polymer is peeled o↵of substrate with MoS2 flake(s) attached to its underside. iv) MoS2 flake, still attached to polymer film, is aligned with and transferred onto hBN flake on device substrate. v) Sample is immersed in solvent (e.g. acetone) to dissolve polymer. vi) After polymer is removed and sample dried with N2 gas, MoS2/hBN sample is ready for device fabrication. Transfer process based on techniques developed by Schneider et al [43]. grown materials, such as graphene, involve the use of reactive chemi- cals to etch away the substrate [38]. While the use of etching based transfer methods have been successfully used to fabricate high performance devices using CVD graphene on silicon dioxide substrates [40], this method of transfer does not allow for the careful alignment of layers into a heterostructure configuration. Instead, this work focused on using a wedging transfer technique whereby CVD MoS2 could be carefully removed from the growth substrate and then deliberately and carefully aligned before transferring it to the desired hBN substrate as shown schematically in Figure 2.2. The wedging transfer method is based on earlier work where Schneider et al [43] investigated methods for transferring nanostructures (e.g. 2D flakes, carbon nanotubes,

17 quantum dots, etc). The basis for this transfer method operates on the intercalation of water between the hydrophobic layer of MoS2 and the hydrophilic growth substrate, SiO2. In order to selectively pick up the desired MoS2 monolayer, the entire surface (MoS2 and SiO2)iscoated with a hydrophobic polymer that serves as the transfer agent [43]. The transfer occurs through the intercalation of a layer of water between a hydrophilic substrate and a hydrophobic nanostructure (for example, graphene flakes, carbon nanotubes, metallic nanostructures, quantum dots, etc.) locked within a hydrophobic polymer thin film. For this work, the polymer transfer film used was cellulose acetate butyrate (CAB). As a result, the film entrapping the nanostructure is lifted o↵and floats at the air-water interface. The nanostructure can subsequently be deposited onto a target substrate by the removal of the water and the dissolution of the polymeric film.

Once the CVD MoS2 monolayer is deposited on the bottom hBN substrate, the next step was to transfer the few-layer graphene flakes to be used for electrodes. Mechanically exfoliated graphene was deposited onto polydimethylsiloxane (PDMS) stamps using Scotch tape. Us- ing a micromanipulator aligned under an optical microscope, few-layer graphene flakes were aligned with and transfered onto the MoS2/hBN layers. After multiple transfers of graphene to define the MoS2 channel region, using the same PDMS transfer method, a top layer of multilayer hBN was transferred onto the stack resulting in a complete heterostructure stack. Step-by-step details of the device fabrication process from stacking to final electrode metal deposition are outlined in Appendix A.2. An exploded view of the completed heterostructure in Figure 2.3a shows the individual components of the stack and reveals how the graphene leads make contact with both the MoS2 channel and metal electrodes (Fig. 2.3a inset). While an optical microscope image of the

18 completed device looks fairly simple (Fig. 2.3)b, high resolution transmission electron microscopy (TEM) of the device’s cross-section reveals pristine interfaces as well as individual atomic layers of the heterostructure after stacking (Fig. 2.3c).

Figure 2.3: From Cui et al [10]. a) Exploded view of MoS2 het- erostructue device and close up look of one dimensional edge contact between metal and graphene electrode. b) Optical microscope image of completed hBN-encapsulated MoS2 heterostructure. c) High-resolution cross-sectional TEM image of heterostructure with false-color close-up showing individual layers (TEM image of cross-section and false-color close-up courtesy of Pinshane Huang).

2.4 Electrical transport measurements

In order to understand the electrical properties of MoS2,electrical transport measurements were conducted. A key metric is the contact resistance, RC,givenbytherelation:

19 1 L R = R R )(2.1) c 2 2P 4P l ⇣ where R2P is two-probe resistance, R4P is four-probe resistance, L is the two-probe length of the device channel and l is the four-probe length. The calculated contact resistance as a function of temperature and back gate voltage is shown in Figure 2.4. Due to the increase of the

MoS2 band gap with decreasing thickness from few-layers to monolayer [11], it is more dicult to form Ohmic contact, or, in other words, achieve a lower Schottky barrier, with thinner MoS2 [44]. In this regard, graphene has been proved to be one of potential candidates as electrode for MoS2 [35, 45]. A large gate-tunability of Fermi energy of graphene enables us to move graphene’s Fermi level close to conduction band of

MoS2 by increasing back gate voltage, resulting in reliable and stable

Ohmic contact even for monolayer MoS2, as shown in Figure 2.4. At reasonably high charge carrier concentration, contact resistance from approximately 0.7 k⌦ mmto10k⌦mmcanbereliablyachievedacross all samples at low temperature. The Schottky barrier height of the graphite-contacted device was determined using the 2D thermionic emission relation [35]:

3/2 qB qV ds Id = AT exp exp (2.2) kBT kBT ⇣ ⌘h ⇣ ⌘i where Id, A, T, kB, , fB,andVds are drain current, the e↵ective Richardson constant, temperature, the Boltzmann constant, electronic charge, the Schottky barrier height, and source-drain bias (50 mV), respectively. Here, is an ideality factor, which is related to the tunneling e↵ect contribution under high charge carrier concentration and at low temperature. To estimate the Schottky barrier height of graphene-MoS2 with di↵erent back gate voltage (Vbg), we employed

20 Figure 2.4: Data published in Cui et al [10]. a) Contact resistance as a function of back-gate voltage (Vbg)andtemperatureforandb) Arrehnius plot for Schottky barrier height extraction of monolayer CVD MoS2 device encapsulated with hBN.

3/2 the Arrhenius plot, ln(Id/T )asafunctionof(1/kBT) as shown in Figure 2.4b. From the slope of the Arrhenius plot, it is possible to extract the Schottky barrier heights for di↵erent MoS2 thickness. Here, we assume the ideality factor as 2 <<20 and we check the avail- ability of ideality factor from the Ohmic behavior in a 2-probe output curve at low temperature. Figure 2.5 exhibits the calculated Schottky barrier heights in hBN-encapsulated MoS2 devices with varying thick- nesses of MoS2.Largemodulationofgraphene’sFermienergyallows for the high tunability of the Schottky barrier height in graphene-MoS2 contact, resulting in the small Schottky barrier height with relatively high Vbg.TheSchottkybarrierheightbecomesclosetozeroatVbg of approximately 80 V even for monolayer MoS2,whichenableustoform the Ohmic contact at very low temperature.

21 Figure 2.5: Data published in Cui et al [10]. Gate-voltage-tunability of the Schottky barrier (fB)heightingraphene-MoS2 contacts, shown for MoS2 devices of varying thickness (1-4 layer). As the gate voltage increases, fB is reduced by the modulation of the graphene contact’s Fermi level (see inset). The Schottky barrier height reduces to approximately 0 at suciently high gate voltage.

2.5 Hall E↵ect measurements

The Hall E↵ect is a phenomena where the presence of a magnetic ﬁeld in material induces the formation of a transverse current [46]. This phenomena is important in the characterization of 2D materials as it provides an accurate measurement of the charge carrier concentration and mobility.

In an hBN-encapsulated MoS2 device, the Hall voltage and Hall mobility was measured with a standard lock-in technique. By applying acurrentacrossthedeviceataﬁxedbackgatevoltagewhilesweeping the magnetic ﬁeld (B), the total charge carrier concentration, n,of

MoS2 is obtained from:

1 dB nHall = (2.3) e dRxy ⇣ ⌘

22 where e and (dB/dRxy)areelectronchargeandmagnetic-ﬁeld-dependence of Hall resistance. With n known, the Hall mobility can be estimated with:

µ = (2.4) Hall ne where sis conductivity. From this relation, with n of 1.1 x 1013 cm-2, the hBN-encapsulated, CVD-grown, monolayer MoS2 device has a Hall mobility of approximately 1,000 cm2 V-1 s-2. This mobility is the highest reported value for monolayer MoS2 and highlights the key role that hBN encapsulation plays in revealing the intrinsic properties of 2D materials once they are protected from the environment.

2.6 Conclusion

Novel techniques have been developed to fabricate Van der Waals heterostructures. MoS2 produced with CVD have been shown to be of high quality and have beneﬁted from recent advances in growth techniques. The use of hBN encapsulation allowed for the measurement of near-intrinsic electrical transport of MoS2 and the achievement of high- mobility in device performance. The use of graphene contacts played akeyroleinallowingfortunableelectricalcontactsthatallowedfor Ohmic contacts even at low temperatures. This device platform also enabled a deeper investigation into magnetotransport where the observation of Shubnikov de Haas oscillationsin

MoS2 was made possible with hBN encapsulation. Additional details on magnetotransport measurements on these devices has been published in Cui et al. [10]. The advancements and findings of this study [10] have been beneficial to the field of 2D material research and have pro- liferated throughout the research community [47–49]. Despite the use

23 of CVD-grown MoS2 for this investigation, challenges remain in scaling this technology but this provides a strong foundation for future im- provements towards the actualization of 2D-material-based electronic devices.

24 Chapter 3

Thermal Scanning Probe Lithography

3.1 Introduction

Making direct metal electrical contact to two-dimensional (2D) transition metal dichalcogenides (TMDCs) has proven to be one of the great- est challenges in the ﬁeld of 2D electronics [10, 50]. The formation of alargeSchottkybarrierbetweenmetalcontactsandsemiconducting 2D materials has limited the ability to investigate novel physics and hindered the development of advanced opto-electronic applications. As will be discussed in Chapter 4, various methods of doping 2D materials have been investigated to overcome this obstacle. While the use of doping has proven to reveal unique properties, there are still methods for engineering electrical contacts to 2D TMDCs that can result in reduced contact resistance. This chapter highlights a novel lithographic technique for patterning contacts on 2D materials which have resulted in one of the lower reported values for Schottky barrier and total resistance based on two-probe measurements with direct-metal contact single-layer molybdenum dioxide (MoS2) produced via chemical vapor

25 deposition (CVD) growth. -Electronbeamlithography(EBL)andopticallithography(OL)are two techniques that have enabled advances in nanofabrication. Both techniques use masks with a desired pattern and use a beam source (electron beam ranging in energy from 30 keV to 100 keV for EBL and an optical light source, typically ultraviotlet, for OL) for patterning. EBL and OL have proven to be powerful tools that have their bene- ﬁts (scalability, reliability, adaptation by semiconductor industry, etc), however, below 30 nm, even the higher resolution EBL starts to hit limitations due to proximity e↵ects. With EBL, at small scale size (approximately 30 nm), the proximity e↵ect causes patterns to be written larger than desired due to the interaction between the electron beam writing the pattern and the substrate and resist of the subject [51]. While EBL and OL continues to be widely used in 2D material research, advances in scanning probe technologies such as atomic force microscopy (AFM) and scanning tunnelling microscopy (STM) has led to the use of tip-based lithography for patterning nanomaterials as a cleaner alternative without the potential for damage due to high energy electron beams.

3.2 Thermal Scanning Probe Lithography

While STM and AFM are well known techniques for characterizing and manipulating materials on the nanoscale [52–54], thermal scanning probe lithography (tSPL) is a relatively new technique that has been investigated to pattern thin ﬁlms. TSPL is a subtractive patterning process where a heated AFM tip is used to pattern features in a polymer coated sample. The origin of tSPL stems from the Millipede Project at IBM (Zurich, Switzerland) from the early 1990s where an AFM tip was used for the writing of topographic features, which could then also

26 be read by the tip as a potential method of data storage [53]. Ulti- mately, this technology did not come to market due to the superiorty of ﬂash memory developed by Toshiba in the 1980s. Regardless, tSPL technology has now emerged as a potential lithographic technique for 2D materials.

Figure 3.1: a) Scanning electron micorscope (SEM) image of NanoFra- zor probe; inset: transmission electron microscope (TEM) image of NanoFrazor tip. b) Diagram of NanoFrazor probe and tip showing components that are key to tSPL process (images courtesy of SwissLitho Ag).

Sample preparation

Prior to coating MoS2 with resist for patterning, all samples were placed in a closed petri dish with a few drops of Bis(trimethylsilyl)amine

(C6H19NSi2, commonly known as hexamethyldisilazane, or HMDS) for 90 seconds to promote polymer adhesion immediately before coating. While the samples are kept dry and not immersed or directly in contact with HMDS, the vapor produced by HMDS enhances surface wetting in order to promote polymer adhesion onto 2D ﬂakes on silicon substrates. Samples are exposed to HMDS in order to chemically bond its Si atom to the oxygen of oxidized surfaces. This results in the re- lease of ammonia (NH3) and allows the methyl groups of the HMDS

27 to fragment and form a hydrophobic surface. This ultimately improves surface wetting and enhances adhesion of polymers used as a resist for lithography (Microchemicals.com). In order to pattern electrical contacts using tSPL, a bilayer resist is used (Fig. 3.2a). The first layer is a solution of pure polymethyl glutarimide (PMGI) (Sigma-Aldrich, St. Louis, Missouri) which is spin- coated on the sample surface at a speed of 2000 rpm for 35 seconds and cured in air on a hotplate at 200 C for 1 minute. After curing PMGI, the second layer, polyphthalaldehyde (PPA) (Sigma-Aldrich), a solution of 1.3 wt % in Anisole (Sigma-Aldrich), is spin-coated at 3000 rpm for 35 seconds followed by curing in air on a hotplate at 90 C for 3 minutes. These spin-coating parameters result in a PMGI layer thickness of 210 nm and a 20 nm thick PPA layer which were confirmed by a Bruker AFM (Billerica, Massachusetts). Patterning with tSPL is conducted with a NanoFrazor Explore by SwissLitho AG (Zürich, Switzerland), a commercial tSPL tool. The NanoFrazor uses a heated probe to locally heat a surface with nanoscale resolution. For these experiments, the probe tip is set to a temperature of at least 1000 Cusingaresistiveheatingelementwithanapplied writing force voltage of at least 9 V and a writing depth of 20 nm. This depth allows for pattering exclusively in the top PPA layer by ablating the polymer as the tip is brought into contact. This process is extremely fast and happens on the order of 1 ms (Fig. 3.2b). A single pixel is on the order of 50 nm however, for larger features (up to 100 mmby100mm) a stitching function is used. Once the PPA layer has been patterned, the underlying PMGI layer which is still protecting the 2D material is now exposed. In order to pattern the 2D material through the PMGI with PPA acting as a mask at this stage, samples are then immersed in a solution of tetramethylammonium hydroxide (TMHA) in deionized (DI) water (0.17 mol/L) for approximately 6-7

28 minutes followed by a gentle rinse in DI water for 30 seconds and then in isopropyl alcohol for 30 seconds. After rinsing, excess water and solvents are removed by gently blowing the samples with dry nitrogen

(N2). This chemical etching step removes the exposed PMGI layer leaving behind the desired exposure pattern with an undercut that is critical to making metal contact to the sample (Fig. 3.2c). Metal contacts can then be deposited directly onto the chip; metal deposited onto remaining PPA layers will be removed during lifto↵whereas the metal deposited into the exposed areas will remain and form electrical contacts (Fig. 3.2d) to the 2D material. Metal deposition is performed using an AJA (Scituate, Massachusetts) Orion 8E e-beam evaporator (Pressure: 10-8 Torr; evaporation rate: 1 Angstrom/s). Finally, metal/resist lift-o↵is performed by dipping samples in hot (100 C) Microchem Remover PG (Newton, Massachusetts), aproprietaryresistremover,forafewhours,thenrinsinginisopropyl alcohol and drying with nitrogen (N2). Three devices, one on CVD

1L MoS2 and two on exfoliated 1L MoS2 ﬂakes, are presented in this paper by using t-SPL. They have been fabricated using di↵erent metals: chrome/gold (Cr/Au) for CVD MoS2 (10 nm/20 nm, as shown in Figure 2c-d), palladium/gold (Pd/Au) (10 nm/10 nm, as shown in Figure 2a-b), and aluminum/gold (Al/Au) (10 nm/10 nm, as shown in

Figure 4a-c)for exfoliated MoS2. EBL devices with Pd/Au (10 nm/10 nm, as shown in Figure S2) are fabricated using convential lithography procedures as a control.

3.3 tSPL patterned electrical contact

As a ﬁrst attempt at using tSPL to pattern electrical contacts on 2D materials, exfoliated multilayer MoS2 was prepared on a SiO2 wafer. Electrical contacts were designed and patterned by tSPL as described

29 Figure 3.2: Nanofrazor patterning process for electrical contact to 2D materials. a. 2D material on SiO2/Si wafer coated with PMGI and PPA. b. Heated tip used to write pattern in PPA layer. c. Undercut in PMGI layer produced by immersing exposed regions with developer. d. Metal deposited into exposed regions form electrical contact. in Section 3.2 and shown in Figure 3.2. After patterning a two-probe electrical contact conﬁguration, metal was deposited using an AJA In- ternational (Scituate, MA) Orion 8E evaporator where 10 nm of chrome and 10 nm of gold were deposited, in sequence. After deposition, the device was immersed in Microchem Remover PG (Newton, MA), a commercially available resist remover comprised of 99% N-methyl pyrro- lidinone, to remove the excess resist and metal layer leaving behind well-deﬁned electrodes (Fig. 3.3.) Two-probe electrical measurements were conducted using a Keith- ley (Cleveland, OH) model 2400 Multimeter for the back gate voltage source, a Yokogawa (Tokyo, Japan) GS200 DC voltage source to apply a bias across the electrodes, and an Agilent (Santa Clara, CA) model 34401A multimeter to measure output. With an applied bias of +10 mV across the 10 mm channel, resistance on the order of 10 kWwas observed at room temperature (Fig. ??. While this is a two probe measurement

30 Figure 3.3: Optical microscope image of exfoliated multilayer MoS2 with metal contacts patterned with tSPL. The channel length (distance between the electrodes) in this device is 10 mm. that is not able to distinguish between channel and contact e↵ects, this value of resistance for MoS2 is on the lower end of values reported in literature as discussed earlier in Section 1.3 (Fig. 1.3). As shown in Figures ?? and 3.5, a metal-to-insulator (MIT) transition is observed at gate voltage of approximately -90 V [18]. This transition manifests itself as a crossing-over between conductance versus gate voltage curves acquired at di↵erent temperatures, indicating two di↵erent regimes [5]. With this ﬁrst successful test of using tSPL to pattern electrical contacts on 2D materials, further studies were conducted with compar- isons to typical lithography methods and an investigation on the e↵ect of Fermi level pinning.

3.4 EBL vs tSPL comparison

In order to investigate the quality of electrical contact made using tSPL, devices were fabricated to compare traditional electron beam lithography (EBL) with tSPL. For this experiment, monolayer CVD MoS2

31 Figure 3.4: Transfer curve of multilayer exfoliated MoS2 device with tSPL patterned metal contacts with an applied bias of +10 mV. Shown with y-axis on a log scale (lef t) and linear scale (right). supplied by 2DLayer (Raleigh, North Carolina) was purchased. With a single batch of MoS2 monolayers grown on SiO2/Si, the wafer was split and two devices were patterned; one with EBL, the other with tSPL. The sample patterned with EBL was spin coated with polymethylmethacrylate (PMMA) 950A6 at 2500 rpm and cured at 180 Cfor5 minutes. Using a Nanobeam (Cambridge, United Kingdom) NB4 EBL system, a series of electrodes were patterned into the PMMA resist which was subsequently developed in MIBK:IPA (1:3) for 70 seconds opening up exposed areas of MoS2 to make electrical contact. After development, the sample was gently dried with nitrogen to remove excess developer. The device patterned by tSPL was prepared as described in Section 3.2 and shown in Figure 3.3. After patterning the devices, metal contacts consisting of chromium and gold were deposited with an Angstrom Engineering (Kitchener, Ontario) EvoVac electron beam deposition system. The ﬁrst metal layer, used for both adhesion to the substrate and as the contact metal to MoS2 was 10 nm of chromium and the second layer was 10 nm of gold. Deposition took place at a pressure of approximately 10-7 Torr as

32 Figure 3.5: Estimated ﬁeld-e↵ect mobility of exfoliated multilayer MoS2 based on two-probe measurements as a function of temperature. With decreasing temperature, the ﬁeld e↵ect mobility increases due to fewer scattering losses due to thermal e↵ects. Mobility appears to peak around 100 K below which it decreases due to scattering losses due to non-thermal contributions (charge impurities, substrate e↵ects, phonons). lower pressures typically result in better (i.e. lower resistance) contacts to 2D materials [33]. Given the clear distinction in conductivity and threshold voltage shown in Figure 3.6, despite holding the MoS2 growth and metal deposition as constants, it is evident that the use of tSPL to pattern electrodes on 2D materials is worthy of further investigation as a means of improving electrical contact.

33 Figure 3.6: Left: Optical image of Mo22 device after tSPL patterning. Center: Non-contact AFM image. Right: KPFM image.

Figure 3.7: Left: Optical image of Mo22 device after tSPL patterning. Center: Non-contact AFM image. Right: KPFM image.

3.5 Metal workfunction

After showing a clear improvement in contact resistance by using tSPL patterned contacts instead of the traditional EBL method, the e↵ect of contact metal on contact resistance was investigated to see if this improvement is consistent regardless of metal workfunction. Two-probe

FET devices were prepared using exfoliated MoS2 monolayers and the number of layers was conﬁrmed by Raman spectroscopy and photoluminescence [35]]. The metals chosen for this study were aluminum (4.08 eV), platinum (6.35 eV) and palladium (5.22eV) [55]. From Figure 3.8,

34 adapted from Liu et al [55], aluminum provides access to the conduction band of MoS2 while both platinum and palladium are in the band gap yet near the valence band edge. Due to this alignment, aluminum is predicted to result in n-type behavior since its work function is ac- cessible by the conduction band of MoS2,PdandPtshouldintheory allow access to the valence band of MoS2 resulting in p-type transport.

Figure 3.8: Contact metal workfunctions relative to TMDC band diagrams. Adapted from [55]

Devices were patterned with either EBL or tSPL as described in Section 3.2 with a pair of one of each lithography type in each metal deposition. However, due to the use of mechanically exfoliated MoS2 for this experiment, ﬂakes were soaked for 2 hours in acetone to remove residue from the Scotch brand tape (3M, Saint Paul, MN) and then gently rinsed in isopropyl alcohol and dried with nitrogen. Two probe measurements were conducted at Sungkyunkwan Uni- versity (SKKU) in a vacuum probe station. At room temperature with an applied bias of 0.1 V, the current output is larger for devices patterned with tSPL with both Pt and Pd contacts as shown in Figures

35 3.11 and 3.10. This is also true with Al contacts but only at suciently high gate voltage is this improvement apparent (Fig. 3.9).

Figure 3.9: Two-probe measurement data for MoS2 devices with aluminum (Al) contacts. Clockwise from top left: Transfer curve while sweeping the gate voltage up; Transfer curve while sweeping the gate voltage up and down; Output curves of EBL patterned device with Al contacts at various gate voltages (from -60 to +60 V); and Output curves of Nanofrazor (tSPL) patterned device with Al contacts from -60 to 60 V. While the current output is slightly higher for EBL patterned contacts, the tSPL (Nanofrazor) patterned contacts shows far less hysteresis indicating the possibility of a cleaner contact interface.

36 3.6 Local doping with nanolithography

The ability to locally dope 2D materials on the nanoscale has been realized with thermochemical nanolithographic (TCNL) doping. In this process, a heated AFM tip is brought directly into contact with a material and scanned over the area to be doped. As will be discussed in Chapter 4, there are several methods for doping 2D materials, all with various beneﬁts and challenges, however, TCNL provides the potential for customized doping patterns that could enable unique device architectures. This work highlights early attempts at locally doping 2D

MoS2 with what appears to be a promising technique. The operational basis for TCNL is similar to tSPL as discussed in Section 3.2. Whereas tSPL used a heated tip to ablate polymer resist to form a pattern, with TCNL, the AFM tip is operated in contact mode and locally heats the material as it scans the surface. In addition to chemically modifying the material in the presence of dopant molecules, the TCNL process cleans the surface of any particulates and residue as has been previously discovered with contact mode AFM [56, 57]. This unitentional ”cleaning” of the 2D ﬂake is shown in the AFM image shown in Figure 3.12a which shows a build-up of residue just outside the region scanned with the probe tip (highlighted in green).

During the doping process, the MoS2 sample was exposed to hy- drochloric (HCL) acid vapor which reacts with the regions of the flake that are scanned with the heated tip to induce chlorine-doping. Fig- ure 3.12b shows a Kelvin probe force microscopy (KPFM) image of this same flake where a contrast in surface potential of approximately 0.08V is observed between regions of the doped and undoped MoS2 flake. After local doping with TCNL, a series of two-terminal devices were fabricated on this exfoliated monolayer MoS2 flake. Raman spectroscopy was used to confirm the exfoliated flake was a single layer of

37 MoS2 prior to doping (Fig. 3.12c), and multi-terminal contacts were fabricated with EBL patterning and chrome and gold contacts as shown in Figure 3.12d after doping. Figure 3.13 shows output curves of the TCNL chlorine-doped monolayer MoS2 device shown in Figure 3.12. Clockwise from top left: Un- doped channel and one doped contact between contacts 1-2; undoped channel and one doped contact between contacts 6-5, undoped control device between contacts 3-4; and doped channel and doped contacts between contacts 6-3. While the control device (3-4), shown in the bottom right, appears to have Ohmic contact, the output curves for the other 3 variations of doping (either contact or channel with doping) shows signs of rectiﬁcation and only turns on with positive applied source-drain voltage. Data was collected with ﬁxed back gate voltages of 20 (black) 40 (red) and 60 (green) V.

3.7 Conclusion

Nanolithographic patterning and local doping using tSPL and TCNL have been shown to be viable techniques for nanodevice fabrication. The use of tSPL to pattern two-probe FETs in a controlled experiment against similar devices patterend with EBL resulted in clear reduction in device resistance with tSPL. TCNL shows promise as a technique for locally modifying 2D material properties but much work is to be done. Future e↵orts on this technique needs to focus on process optimization and repeatability. A device architecture of interest is the realization of an in-plane P-N junction whereby TCNL could locally dope half of a channel. The next step in this investigation is two-fold: (1) An investigation into the surface properties of the contact region after lithography will conducted to determine if the MoS2 is being unintentionally doped or

38 if remaining residue is playing a role in the di↵erent electrical properties observed in Figure 3.6 between tSPL and EBL patterned devices. (2) Electrical transport studies with four-point probe and Hall e↵ect measurements will be conducted to isolate true contact resistance and determine the level of doping in the MoS2.

39 Figure 3.10: Two-probe measurement data for MoS2 devices with palladium (Pd) contacts. Clockwise from top left: Transfer curve while sweeping the gate voltage up; Transfer curve while sweeping the gate voltage up and down; Output curves of EBL patterned device with Pd contacts at various gate voltages (from -60 to +60 V); and Output curves of Nanofrazor (tSPL) patterned device with Pd contacts from -60 to 60 V. In addition to having a higher current output then the EBL patterned devices, the Nanofrazor (tSPL) patterned device appears to have an Ohmic contact.

40 Figure 3.11: Two-probe measurement data for MoS2 devices with platinum (Pt) contacts. Clockwise from top left: Transfer curve while sweeping the gate voltage up; Transfer curve while sweeping the gate voltage up and down; Output curves of EBL patterned device with Pt contacts at various gate voltages (from -60 to +60 V); and Output curves of Nanofrazor (tSPL) patterned device with Pd contacts from -60 to 60 V. In this comparison, the Nanofrazor (tSPL) patterned device had current output 100x greater at +60 Vbg as compared to the EBL patterned device with the same Pt contact metal.

41 Figure 3.12: a) AFM and b) KPFM images of exfoliated MoS2 used for TCNL doping. Doped region highlighted in green. As TCNL operates in contact mode, note residue accumulation at edges of doped region in AFM image. c)Single layer of MoS2 conﬁrmed with Raman spectra. d) Optical microscope image of ﬁnal device.

42 Figure 3.13: Output curves of TCNL chlorine-doped monolayer MoS2 shown in Figure 3.12. Clockwise from top left: Undoped channel and one doped contact between contacts 1-2; undoped channel and one doped contact between contacts 6-5, undoped control device between contacts 3-4; and doped channel and doped contacts between contacts 6-3. While the control device (3-4), shown in the bottom right, appears to have Ohmic contact, the output curves for the other 3 variations of doping (either contact or channel with doping) shows signs of rectiﬁca- tion and only turns on with positive applied source-drain voltage. Data was collected with ﬁxed back gate voltages of 20 (black) 40 (red) and 60 (green) V.

43 Chapter 4

Plasma Doped 2D materials

4.1 Introduction

Despite the inherent properties of two-dimensional (2D) materials, the ability to further tune the optical and electrical properties of this class of materials will be key to seeing applications come to fruition. As with traditional semiconductor technology, doping processes are used to intentionally add positive charge carriers (holes) or negative charge carriers (electrons) to a material in order to modify optical and electronic properties for a speciﬁc purpose. There are several methods for intentionally doping 2D materials which include solvent-based chemical doping [58, 59], in-situ doping during growth [48, 60, 61], and plasma based doping processes [62]. In addition to the new thermochemical nanolithography doping technique discussed in Section 3.6, one of the more novel studies on the doping of 2D materials were based on the chemical reaction of graphene with hydrogen atoms present in hydrogen silsesquioxane (HSQ), a commercially available resist that is used in the fabrication of devices. When exposed to an electron beam during lithography (see Section 3.1), the hydrogen in HSQ that is coating graphene ends up reacting and forming bonds

44 with the graphene. Following a subsequent thermal annealing process, hydrogen would detach leaving ’dehydrogenated’ graphene that would subsequently react with oxygen resulting in hole doping [63]. Of the doping techniques available for 2D materials, plasma doping shows promise as it is a clean and dry process that relies on a standard tool commonly used in the semiconductor industry. This chapter provides an overview of plasma processing with a discussion on the background on earlier studies on plasma-doped 2D materials and the use of inductively coupled plasma (ICP) systems to dope graphene and transition metal dichalcogenides (TMDCs). The subsequent results and characterization of these doped 2D materials will also be presented.

4.2 Plasma processing

Plasma, known as the fourth state of matter, is an ionized gas. Prime examples of a plasma in nature are the Aurora Borealis (shown in Fig. 4.1) and Australius (Northern and Southern Lights). These brilliant light shows in the night sky start with the ejection of electrons from the sun, also known as solar wind, which then travel through the earths magnetosphere. Since charged particles tend to follow electric field lines, solar wind electrons are driven towards the earths ionosphere and in turn create a charged negative layer. An opposing positive charged layer will form allowing for charged particle acceleration and subsequent light emission. Engineers have harnessed plasmas in the laboratory for everything from complex satellite propulsion systems to simple fluorescent light bulbs. One field heavily reliant on plasma processing is the semiconductor industry where plasma systems are used to selectively etch fine features on the order of nanometers. This dry etching process can be

45 Figure 4.1: Aurora Borealis (Northern lights), a naturally occurring example of a plasma, as seen from Goodsell Observatory at Carleton College in Northfield, Minnesota. Photo by Drew Weitz. purely chemical if the gas species generated in the plasma are responsi- ble for the desired removal of material, or, the etching can be physical if material is removed due to ion bombardment [64]. Commercial plasma systems typically employ a combination of chemical and physical etching, a process known as reactive ion etching (RIE). An inductively coupled plasma (ICP) process operates by coupling energy from a radio frequency (RF) source (typically operating at 13.56 MHz) to an ionized gas. In addition achieving higher etch rates due to ahigheriondensity(1011 cm3), etch profiles from ICP etching tends to be more isotropic than in the more reactive RIE systems. As shown in Figure 4.2, separate controls for the RF (table power) source and ICP (coil power) source provides the ability to tune ion energy and ion density, respectively. This tunability allows for the ability to run a plasma process with minimal damage to a 2D material sample due to ion bombardment. The plasma generated in an ICP system is typically centralized immediately above the sample stage as shown in Figure 4.2,meaning the risk of damage to the 2D flake lattice by ion bombardment is high and plasma conditions like power, gas flow,

46 and pressure must be carefully selected to minimze damage.

Figure 4.2: Cross-sectional diagram of inductively coupled plasma (ICP) system. (image courtesy of Oxford Instruments).

4.3 Plasma doped graphene

Given the ubiquity of plasma processing in the semiconductor industry, it was only natural that scientists would attempt to use plasma to intentionally dope 2D materials. Despite graphene’s unique electrical properties, it’s lack of a band-gap makes it an ideal candidate to start the study of plasma-based doping of 2D materials. Early attempts at using plasma to dope graphene looked at modifying its optical properties with the use of an oxygen plasma resulting in bright photoluminescene that is otherwise not observed due to the lack of a bandgap [65]. The use of hydrogen plasmas were also investigated

47 and resulted in turning the highly conductive graphene into its hydrogenated derivative, graphane,whichisaninsulator[66].Attemptsat intentionally electron (n-type) doping graphene used ammonia (NH3) plasma where carbon atoms were substituted by NH radicals formed from the dissociation of NH3 [67]. Several groups have looked at the use of hydrogen plasma to dope graphene. Results have ranged from looking at the basic e↵ects of hydrogen plasma on graphene [68], to observing the emergence of pro- nounced negative magnetoresistance [69], as well as enhanced photoresponse [70]. Additionally, the use of fluorine and chlorine plasma have been investigated as methods for doping graphene. In a controlled experiment where exfoliated graphene samples were doped with Cl2,CF4, and H2 plasmas, the chlorine plasma treatment showed promise as the most controllable method [62]. A significant challenge in all the aforementioned plasma processes for doping graphene, or other 2D materials, is the potential for damage to the material. By using Raman spectroscopy to study the disorder introduced into graphene with plasma doping, Wu et al showed that chlorine plasma treatments exhibited slower kinetics and introduced less disorder than hydrogen or fluorine treatments [62]. The unique nature of graphene chlorination was further bolstered by theoretical calculations and experimental verification showing high coverage of chlorine atoms on graphene [71, 72]. The ability to dope graphene with chlorine plasma with minimal damage was also observed with the first successful case of doping graphene with a chlorine plasma while still retaining high mobility was by Zhang et al [73]. Using a electron cyclotron resonance (ECR) downstream plasma system, graphene was exposed to ionized chlorine atoms in the plasma with minimal disorder introduced by bombardment. One advantage of using an ECR plasma system is having a sample

48 stage that is downstream from the region of plasma generation allowing the opportunity to dope samples via plasma treatment while minimizing direct bombardment of energetic ions and thus minimizing damage to the graphene. Since downstream chlorine plasma etchers are not readily available, this work focused on using the Oxford (Cambridge, United Kingdom) model Plasmalab 100 ICP etcher, a readily available plasma system found in most clean room facilities at institutions across the U.S. This work used this same model of plasma etcher at Columbia University (New York, New York), Brookhaven National Lab Center for Func- tional Nanomaterials (Upton, New York), Army Research Lab (Adel- phi, Maryland), and City University of New York - Advanced Science Research Center (New York, New York). In order to determine the plasma conditions needed to dope graphene, a parametric study was conducted using CVD graphene grown in a single batch and treated with various plasma conditions. Given the energetic and destructive nature of plasma processing, especially with an ICP system where the sample stage is directly immersed in the plasma region, several runs were conducted to ﬁrst establish a set of conditions that minimized RF and ICP power but still allowed for the striking of a plasma. Once that was achieved, several CVD graphene samples were treated with various chlorine plasma conditions and studied with Ra- man spectroscopy to narrow down a set of parameters where the power was suciently low to minimize destruction of the graphene while still doping it with chlorine atoms. Details on this ICP calibration for doping are provided in Appendix A.4.

49 sample time (s) Cl2 ﬂow (sccm) P(mTorr) RF (W) ICP (W) 12 5 80 20 8 10 13 5 100 25 8 10 14 10 80 40 8 10 A 20 80 20 8 10

Table 4.1: Plasma parameters for developing conditions to dope graphene with an Oxford ICP plasma etcher. Only data points of successful, i.e. not destroyed, doping samples are shown. The RF and ICP power settings were established by determining the lowest power settings for the ICP etcher that would still result in successful plasma strike. 4.4 Raman spectroscopy of doped graphene

After establishing a range of operational parameters for the ICP, conditions were fine tuned to establish the ability to dope graphene with minimal damage. As seen in Table A.1, settings for time, gas flow, and pressure were varied while plasma power conditions were fixed based on values established during calibration (see Appendix A.4). In order to verify the success of the plasma conditions developed for doping, after visual observation with microscopy to make sure the graphene was not etched away completely, Raman spectroscopy was used to determine the amount of disorder introduced during doping. As shown in Figure 4.3, sample 12 maintained characteristic Raman peaks before and after doping. In addition to the shifts observed for the G and G’ (or 2D) peaks, the D peak, which is indicative of the amount of disorder in graphene, is relatively small. As the sample was grown with CVD, some disorder and the presence of a D peak before plasma treatment was expected due to the growth or transfer process as is shown in Figure 4.3. In order to quantify doping levels in graphene, previous e↵orts have developed a correlation between charge carrier concentration based on

50 Figure 4.3: Clockwise from top left: top left) Raman specta of CVD graphene before (black) and after (red) chlorine plasma treatment using conditions listed in Table A.1. Top right) Graphene D-peak near 1350 cm-1 is present before doping but does not increase signﬁcantly after plasma treatmetn. Bottom row) the G’ (or 2D) and G peak of graphene is shifted after doping. transport measurements and corresponding shifts in Raman spectra [74]. The data points (black dots) shown in the Raman spectra in Figure 4.4 represent experimentally measured values (from [74]) which deviate from, but still follow the trends of, the solid blue lines which represent the theoretically predicted relationship from Pisana et al [75]. From the Raman spectra of the G-peak position of graphene Sample 12 (Figure 4.3), a peak shift of approximately 10 cm-1,from1590to 1600 cm-1,wasobservedafterchlorineplasmadoping(conditionslisted in Table A.1). In addition to showing that the initial condition of this

51 Figure 4.4: Left (right): Raman spectra of the G-peak (2D-peak) position for graphene as a function of doping. Black points are experimentally measured values from Das et al, solid blue lines are theoretical trends [75]. Figures adapted from Das et al [74]. sample was slightly doped, using the relationship between the position of the G-peak and Fermi energy in Figure 4.4, the Fermi energy of this sample increased from approximately 0.2 eV to 0.57 eV after chlorine plasma treatment indicating successful positive (hole) doping.

4.5 XPS of doped graphene

X-ray photoelectron spectroscopy (XPS) is a characterization technique that probes the surface properties of solids and provides quantitative information on elemental composition. For chlorinated graphene, XPS can be used to identify the presence of chlorine-carbon bonding as well as determination of chlorine coverage on graphene [72]. The operation of XPS is based on the photoelectric e↵ect whereby a photon incident on a material causes the removal of an electron from the surface. XPS is considered a low-energy phenomena as it uses photon sources such as x-ray and ultraviolet photons as opposed to higher energy phenomena like Compton scattering which uses a gamma ray photon source. In this XPS investigation of chlorinated graphene, a CVD graphene

52 sample was transferred onto a SiO2 substrate and treated with a chlorine plasma using an ICP etcher as described in Section 4.2. XPS measurements were conducted at the SLAC National Accelerator Labora- tory (Menlo Park, California). Samples were measured in an ultra-high vacuum (approximately 10-9 Torr) chamber with a wiggler source on the 10-1 beamline of the Stanford Synchrotron Radiation Lightrource (SSRL). In Figure 4.5, carbon (C) 1s (left) and chlorine (Cl) 2p (right) peaks are plotted for both pristine graphene (black line) and after Cl treatment (red line). From the C 1s spectra, it is evident that the CVD graphene sample was oxidized in its initial state prior to doping (black line). A change in the C 1s spectra is observed after doping (red line), however, it is hard to resolve C-Cl bonding due to the oxidized state of the graphene prior to doping. In the Cl 2p spectra, the appearance of apeakaround201eVisindicativeofthepresenceofchlorineonthe surface of the graphene. The Cl 2p peak is actually comprised of peaks at 200.6 eV (2p3/2) and 202.2 eV (2p1/2) neither of which is present in the pristine graphene sample. Unfortunately, samples used in this measurement were slighlty oxidized, however, the presence of C-Cl bonds are still observable indicating successful chlorination of graphene using an Oxford ICP plasma etcher.

4.6 Plasma doped TMDCs

Monolayers of molybdenum disulﬁde (MoS2)andtungstendiselenide

(WSe2) were grown directly onto a silicon dioxide (SiO2)substrates using a chemical vapor deposition (CVD) process and subsequently doped using a plasma process similar to the work done by Kim et al

[76]. In this earlier study, monolayers of MoS2)werepreparedonSiO2 substrates using mechanical exfoliation and treated with both chlorine

53 Figure 4.5: XPS spectra of chlorinated graphne. Left: C-1s spectra shows oxidation of graphene in initial state. Right: Cl-2p peak around 201 eV is actually a split peak comprised of 2p3/2 at 200.6 eV and 2p1/2 at 202.2 eV. Measurements performed at SLAC, plots courtesy of Theanne Schiros (FIT/Columbia).

(Cl2)andhydrogen(H2)plasma.Thisgroupobservedanenhancement of photoluminescence (PL) with Cl2 treatment resulting in p-type doping and n-type doping with subsequent H2 plasma treatment. For this thesis, a similar experiment was conducted using an Oxford Plasmalab

ICP 100 plasma etcher was used to treat these TMDCs with a Cl2 plasma for the ﬁrst time on CVD grown MoS2 monolayers to see if a similar enhancement in PL occurred as observed by Kim et al [76] as well as to what e↵ect this plasma chemistry would have on an intrinsi- cally p-type TMDC in WSe2. Starting with previously establised ICP plasma conditions for TMDCs

[76], samples were treated with a Cl2 plasma with the following conditions: 80 sccm Cl2 ﬂowrate, 20 mTorr, 5 WRF,and100WICP for 24 seconds.

54 sample time (s) Cl2 ﬂow (sccm) P(mTorr) RF (W) ICP (W) MoS2-1 14 80 20 5 100 MoS2-3 14 80 25 5 25 WSe2-2 21 80 20 5 100 Table 4.2: Plasma parameters for doping CVD TMDCs based on conditions established by Kim et al. This table only shows TMDC samples that were successfully (i.e. not destroyed) treated with a Cl2 plasma. [76].

4.7 Optical spectroscopy of doped TMDCs

After treating CVD TMDCs with a chlorine plasma process, Raman spectroscopy and photoluminescence spectroscopy was conducted to characterize the doped materials. The Raman active modes in TMDCs, shown in Figure 4.6 only show minimal if any peak shift, unlike the shifts observed in graphene (see Fig. 4.3). Using a Renishaw inVia micro-Raman with the following conditions: laser wavelength = 532nm edge, grating = 2400 l/mm, 0.09 mW laser power (1 percent power at 100X magniﬁcation), 30 second exposure, and 3 accumulations, the resulting Raman spectra for MoS2 is shown in

Figure 4.6 and for WSe2 in Figure 4.7. For both TMDCs, a negligible shift in Raman peaks is observed between the pristine (black line) and

Cl2 plasma treated (red line) samples shown in Figures 4.6 and 4.7. Despite the negligble changes in the Raman spectra, signiﬁcant changes are observed in PL spectra taken on these same TMDC samples as shown in Figure 4.6 for the doped MoS2 and Figure 4.7 for the doped WSe2. After Cl2 plasma doping, the MoS2 PL spectra decreases by 3X wheras the WSe2 PL spectra increases by a factor of 3X. In addition to verifying the quenching of PL signal observed by Kim et al [76] in MoS2, this work shows this technique works for CVD grown materials and provides new insight on Cl2 doping of p-type materials like WSe2.

55 Figure 4.6: Left: Optical image of CVD MoS2, right: Raman spectra for pristine and Cl2 plasma-treated CVD MoS2 monolayer. CVD MoS2 samples provided by Robert Burke (ARL)

4.8 Electrical transport of doped 2D materials

In addition to evaluating doped 2D materials with various spectroscopic techniques to understand the e↵ect of doping, samples were prepared and doped with chlorine plasma for electrical transport measurements. The goal of this portion of the larger study was to see if a 2D material could survive plasma doping as a functional electrical device. For this experiment, graphene was mechanically exfoliated onto

SiO2 substrates and a monolayer was identiﬁed using optical microscopy (see left image in Fig. 4.10). A ﬁeld-e↵ect device was fabricated on the graphene using conventional electron-beam lithography (EBL) to pattern electrodes and electron beam deposition to deposit gold contacts with chrome adhesive layer between the graphene and gold. A plot showing 2-probe resistance as a function of gate voltage for a monolayer graphene device before (solid line) and after (dashed line) Cl2 plasma doping is shown on the right in Figure 4.10. Since this measurement took place in air, the applied back gate voltage was limited to

56 Figure 4.7: Left: Optical image of CVD WSe2,right:Ramanspec- tra for pristine and Cl2 plasma-treated CVD WSe2 monolayer. CVD WSe2 samples provided by Kyung Kang and Eui-Hyeok Yang (Stevens Institute of Technology). a range of -60 to +60 V to minimize the potential for dielectric breakdown. While the initial, undoped, sample shows a charge neutrality point (CNP), or Dirac point, of approximately +10 V, indicating mild, p-type doping, the CNP shifts dramatically after the CVD graphene device has been treated with a Cl2 plasma. For ideal, intrinsic graphene, the CNP would appear at 0 V as shown in Figure 4.9. Shifts to the left of the CNP (negative gate voltage) indicate n-type doping and to the right of the CNP indicate p-type toping. Given the limitation of back gate voltage, the CNP for the doped graphene sample is not observed in the plot shown in Figure 4.10 however it is clearly well above +60 V indicating signiﬁcant p-type doping and the ability to treat a fully functional device with a plasma doping process without killing the device. Given the successful doping of a CVD graphene device, the next step was to try a similar process with a CVD MoS2 device. CVD MoS2 grown at Army Research Laboratory (Adelphi, Maryland) was made into a device using EBL and deposition similar to the process described above for graphene (shown in Fig. 4.11, left side). The only successful

57 Figure 4.8: Photoluminescence spectra of TMDCs before (black) and after (red) Cl2 plasma-treatment. Left: CVD WSe2 monolayer. Right: CVD MoS2 monolayer. Despite identical doping conditions as described in Section 4.6 with identical plasma treatment conditions

2-probe measurement of this CVD MoS2 device does not provide a clear indication of doping but is noteable for a decrease in the current output after doping as shown in the transfer curve in Figure 4.11.

4.9 Conclusion

Plasma processing has been shown as a method for doping 2D materials. The use of plasma etchers to dope materials leverages a technology that is readily available in most clean room facilities. As 2D materials continue to be studied by researchers eager to exploit their novel properties for various applications, understanding how to dope materials such as graphene and TMDCs with a controllable method will be key to realizing applications.

In this series of experiments, graphene, MoS2 and WSe2 samples were prepared and doped with a chlorine plasma process. These experiments are a continuation of the work by Schiros, Zhang et al [72, 73] and are the basis for the optical measurements presented in Chapters 5and6.

58 Figure 4.9: Peak resistivity of graphene, also refered to as the charge- neutrality-point (CNP). Dirac cones shown (from left-to-righ) for p- doped, intrinsic, and n-doped graphene. Note position of the Fermi level (EF) in the valence band, at CNP, and conduction band, respectively. Image from Geim et al [1].

The preliminary electrical transport results presented in Section 4.8 conﬁrmed the ability to signiﬁcatnly p-dope graphene using an ICP plasma etcher as was previously shown with an RIE/ECR etcher by Zhang et al [73]. Future e↵orts will attempt to boost both conductivity and mobility of graphene with the construction of FETs on hBN substrates and subsequent treatment with a chlorine plasma.

59 Figure 4.10: Left: Optical image of exfoliated graphene device with gold electrodes. Right: Two-probe resistance measurement of device before (solid) and after (dashed) doping with a chlorine plasma treatment.

Figure 4.11: Left: Optical image of CVD MoS2 monolayer device with gold electrodes. Right: Two-probe DC measurement of device before (black) and after (red) doping with a chlorine plasma treatment.

60 Chapter 5

Chlorinated graphene spectroscopy

5.1 Introduction

The underlying physics that govern the optical properties of graphene reveal a unique material with the potential for revolutionary advances in opto-electronic applications [77–82]. The most obvious unique optical property of graphene is its strong optical absorbance of approximately 2.3 percent [78, 83–86] despite being only a single atom in thickness. It was this optical property in particular which led to the earliest observa- tions of mechanically exfoliated graphene using an optical microscope [2, 3]. While graphene is widely heralded for its mechanical strength and electrical properties [87], the lack of a band-gap requires graphene to be doped in order to reveal the novel physics governing its optical characteristics. While many studies have used electrostatic gating to e↵ectively dope graphene [49, 78, 84], in this study, graphene was doped using a chlorine plasma process. The novel physics behind doped graphene were further investigated using optical spectroscopy

61 techniques.

5.2 Electronic structure of graphene

In order to understand the optical properties of graphene, it is necessary to ﬁrst provide an explanation of the electronic structure which governs the optical response. As shown in Figure 5.1a, the hexagonal lattice of graphene, with carbon atoms at each vertex, relies on sp2 bonding to form this stable structure, where points A and B. Due to the degeneracy that arises from these inequivalent points (K and K’ in momentum space, as shown in Figure 5.1b), a linear energy-momentum dispersion arises – referred to as the Dirac cones. Trigonal warping e↵ects away from the two inequivalent points (K and K’) are shown in the electronic dispersion with a saddle-point singularity at M in Figure 5.1c along with the equi-energy contour lines of the upper (conduction) band (from Mak et al [84]).

5.3 Optical properties of graphene

The optical transitions that take place within the Dirac cone of graphene depends on the spectral range under investigation. In the far infrared (IR) range, intraband transitions take place, and in the mid- to near-IR range, interband optical transitions dominate which is the focus of this investigation. Interband transitions are direct optical transitions between the valence and conduction bands in graphene. From Mak, Heinz et al [84] ”within the tight-binding model, the optical sheet conductivity from interband transitions can be readily calculated. For pristine graphene at zero temperature, the optical conductivity in the linear dispersion regime of graphene is found to be independent of frequency. The associated ”universal” conductance of graphene is determined solely by

62 Figure 5.1: Electronic structure of graphene. (a) Hexagonal lattice of graphene made up of 6 carbon atoms. Points A and B mark inequivalent points referred to as K and K’ in (b). (outlined in red) with corresponding orbital overlap. Image courtesy of Mak et al [84]. fundamental constants and assumes the value of s(w)=pe2/2h (cite) This conductivity corresponds to an absorbance of A(w)=(4p/c)s(w) = pa 2.29% ⇡ In order to experimentally determine the change in absorbance in chlorine plasma doped graphene, absorbance must be determined from reflectance and transmission measurements by R R 4 = g+s s = A (5.1) R R n 2 1 s s where A is absorbance, Rg+s is the reflectance of graphene on a substrate, Rs is the reflectance of the substrate, and ns is the refraction

63 index of the substrate. A similar relation holds for transmission measurements by T g+s T s 2 T = = A (5.2) T s ns +1 where Tg+s is the transmission of graphene on a substrate and Ts is the transmission of the substrate. The optical conductivity of graphene is calculated by

2 ⇡e ~! +2EF ~! 2EF (!,T)= tanh + tanh (5.3) 4h 4kBT 4kBT  ⇣ ⌘ ⇣ ⌘ 5.4 Sample preparation

Since the signal-to-noise ratio of Fourier Transform Infrared (FTIR) measurements is proportional to the measurement spot size (e.g. larger signal-to-noise ratio with a larger spot size), e↵ort was taken to use large graphene monolayer samples for this experiment. For this reason, only CVD graphene samples were prepared as this provided a repeatable way to produce multiple graphene monolayer samples with dimensions on the order of 50 mm. Monolayer sheets of graphene were grown by chemical vapor deposition (CVD) onto copper foil and transferred onto silicon dioxide SiO2 substrates as well as sapphire (Al2O3)substratesusingapreviouslyde- veloped novel dry-transfer method [40, 87] discussed in Appendix A.3.

For this experiment, CVD graphene was prepared on both SiO2 and sapphire (Al2O3)substratestoenablebothreﬂetivityandtransmission measurements. As with previous studies, graphene was characterized with Raman spectroscopy before and after Cl2 plasma doping. Samples were doped using a chlorine plasma process as described in Section 4.3 using an Oxford ICP plasma etcher. Samples prepared on SiO2 were all grown in the same batch and split into four individual samples: two for doping (A,B), and two as controls (C,D). The sample

64 prepared on Al2O3 was measured as a control and then re-measured after chlorine plasma treatment. Plasma conditions to treat these samples are listed in Table 5.1. sample time (s) Cl2 ﬂow (sccm) P(mTorr) RF (W) ICP (W) A 20 80 20 8 10 B 10 80 10 8 10 C - - - - - D - - - - - J 20 80 20 8 10

Table 5.1: Chlorine plasma parameters to dope CVD graphene with an Oxford ICP etcher. Samples A-D were prepared on SiO2 for reﬂectivity measurements and C and D were not doped in order to use them as controls. The reﬂectivity and transmission of sample J, on sapphire, was measured before and after Cl2 plasma treatment.

5.5 Results

FTIR was used to experimentally determine absorbance by measuring reﬂectance and transmission. FTIR operates based on the use of a monochromatic light source incident on a solid sample (also works for some gases and liquids) and measuring the amount of light absorbed before and after passing through the sample at distinct wavelengths across a desired range. After CVD graphene samples were prepared, samples were char- acteized with both Raman and FTIR spectroscopy before and after chlorine plasma treatment to develop an understanding of how the optical properties of graphene would change with doping. While Raman spectroscopic results of chlorine plasma treated graphene were investigated in Section 4.4, it was important to measure Raman spectra in this experiment in order to develop a correlation of shifts in Raman peaks due to doping with changes in optical properties like reﬂectivity. Figure

65 5.2 shows the Raman spectra for Samples A and B that were treated with di↵erent plasma conditions as listed in Table 5.1. Of particular note is the signiﬁcant increase in the D-peak (approximately 1350 cm-1) after chlorine plasma treatment in Sample B (Figure 5.2b). Sample B was exposed to plasma for a shorter duration (10 seconds) compared to Sample A (20 seconds), however, the chamber pressure was much lower (10 versus 20 mTorr) for Sample B. While not quantiﬁed, the lower chamber pressure during chlorine plasma treatment appears to increase the occurrence of ion bombardment on the sample leading to more disorder in graphene as shown by the large increase in the D-peak.

5.5.1 Reﬂectance

Reflectance measurements were performed at room temperature in an optical cryostat under vacuum in order to minimize the oxidation of the sampel over time. Since the graphene samples were on a reflective substrate, reflectance of graphene was normalized with respect to the the reflectance of the SiO2 substrate. In Figure 5.3 the normalized reflectance of graphene is plotted as a function of incident photon energy. As the photon energy moves from the mid-to-near IR range, an increase in reflectance was observed for CVD graphene samples treated by chlorine plasma (Samples A, B). When measured at 0.1 eV, the reflectance of the doped samples is approximately 4 percent larger than the control. While this number may seem small, compared to similar measurements in literature, this is a significant change in reflenctance. Previous work used electrostatic gating of graphene on a SiO2/Si whereby a voltage was applied to the Si substrate to increase the number of charge carries in graphene [77, 88]. For example, in work by Li et al [88], 71 V was applied to the SiO2/Si backgate of a graphene sample in order achieve an increase in reflectance of approximately 3 percent. While this voltage

66 is well within the dielectric breakdown voltage of high quality SiO2/Si substrates, it is still a substational energy input that would could limit the performance of future applications. This comparison points to an advantage of chlorine-plasma doped graphene samples should they be pursued for opto-electronic applications.

5.5.2 Transmission

Samples for transmission measurements were prepared on optically transparent sapphire substrates and measurements were performed across ahigherrangeofphotonenergyascomparedtothereflectancemea- surements. As shown in Figure 5.4b and c, both the reflectance and transmission is higher for Cl2 plasma doped CVD graphene on sapphire substrates. Whereas the biggest increase (approximately 1 percent) in reflectivity, as shown in Figure 5.4b, takes place at lower photon energy (around 0.4 eV), the largest increase in transmission (of approximately 0.25 percent), as shown in Figure 5.4c, occurs at the higher end of the photon energy spectrum (around 1 eV). These di↵erences are shown in Figure 5.4d. These are signifant changes in optical properties due to doping as compared to previous results shown in [11, 77, 84].

5.6 Conclusions

AnovelmethodfordopinggraphenewithaCl2 plasma process has been shown to modify its optical properties. Based on the development of a process for doping graphene with an ICP plasma etcher discussed in detail in Chapter 4, this work turned towards developing an understanding of the optical properties of chlorine doped graphene. Whereas previous e↵orts used electrostatic gating [77, 88] to ob- serve an increase in reﬂectivity with doping, the work outlined in this

67 chapter revealed a larger increase in reflectivity in graphene via chlorine plasma doping. The ability to achieve similar levels of modification of the optical properties of graphene but without an applied bias may have a significant impact on the use of graphene for opto-electornic applications. In particular, the use of graphene for optical switches or anti-reflective coatings could benefit from this ability to modify reflectivity without the need for an active input. This unique method provides a template for doped graphene devices that are able to operate at lower gate voltages opening the door for low-power opto-electronic applications.

68 Figure 5.2: a,b) Raman spectra of CVD graphene sample A and B before (black) and after (red) chlorine plasma treatment. c,d) G-peak and e,f) G’ (2D) peak of CVD graphene before/after doping with peaks normalized to show positional shift.

69 Figure 5.3: Reﬂectance of Cl2 plasma-doped CVD graphene versus control samples. Inset: Schematic of experimental set-up, incident light source, hv, is reﬂected o↵the surface.

70 Figure 5.4: a) Diagram of reflectance and transmission measurements on sapphire (Al2O3) substrates. b) Reflectance and c) transmission of CVD graphene on Al2O3 before (black) and after (red) chlorine plasma treatment. d) Percent change in reflectivity (black) and transmission (blue) after doping.

71 Chapter 6

Plasmonic phenomena in chlorinated graphene

6.1 Introduction

The collective oscillations of electron clouds in a conductive material are known as plasmons.Thefieldofplasmonics,aspecializationwithin the realm of nanophotonics, is concerned with the interaction between photons at electrons in a conductive material [89]. At the nanoscale, plasmons are essentially confined electromagnetic field systems that are on the same order of, or smaller than, the wavelength of light. Significant research on surface plasmons, the electromagnetic waves coupled to charge excitations at the surface of a metal, has been geared towards potential applications as varied as optical biosensing, quantum information processing, and optical nanoantennas to just name a few [90]. While metals are widely available for use for plasmonic systems, the lack of tunability and large Ohmic losses limit their applicability. As an alternative, graphene has garnered recent interest as a unique plasmonic material, not only for its mechanical strength and electronic properties but, as was discussed in Chpater 5, also for its unique optical

72 properties. This chapter builds upon previous experiments that focused on the doping of graphene with a chlorine (Cl2)plasmaprocessandthe investigation of its modiﬁed optical properties after doping.

6.2 Graphene plasmons

The ability to tune the optical and electronic properties of graphene make it an ideal focus for a study on plasmonic material. As discussed in chapters 4 and 5, the optical properties of graphene can be reliably tuned with various doping processes. Signiﬁcant work on graphene plasmons has been performed using electrostatic doping [90, 91] as well as with iron-chloride (FeCl3)intercalationdoping[92].Thisworkcon- tinues the use of Cl2 plasma doping to modify the optical properties of graphene and is the ﬁrst known study of plasmons in this system. While the tunability of optical properties is in itself an ideal property of graphene, for the study of graphene plasmons, it is advantageous to work with highly doped material as that pushes plasmon wavelengths into the infrared (IR) range as it scales with the fourth root of the Fermi energy (Ef)[92].Forthisreason,thisstudyongrapheneplasmonsfo- cuses on doped systems using IR spectroscopy techniques.

6.3 Infrared nano-imaging

In order to probe the plasmonic properties of graphene, apertureless or scattering-type scanning near-ﬁeld optical microscope (SNOM) was used to access high momentum plasmons. With a spatial resolution on the order of 20 nm that is also independent of light source wavelength, scattering-type SNOM is highly capable technique that operates across visible, infrared, and microwave frequencies [93]. The key to the wavelength-independent resolution of scattering-type

73 SNOM is the optical field enhancement at the apex of a sharp, laser illuminated metal tip scanning the sample surface. The sharp tip, with an incident light source, launches circular surface plasmons in the sample under measurement in all directions while simultaneously detecting the near field plasmonic singal under it [91]. This creates an complex interference pattern between the plasmons moving through the sample and their reflection at the sample edges (as shown in Figure 6.2a). In addition to this versatility, the use of a scattering-type SNOM also enables the ability to perform infrared nano-imaging of graphene as produced without the need for intricate periodic structures [91]. Scat- tering amplitude and surface topography is measured simultaneously by pseudoheterodyne interferometric detection [93, 94].

6.4 Sample preparation

Monolayer graphene samples were prepared using two methods: mechanical exfoliation and chemical vapor deposition (CVD) growth, as previously described in Chapter 4. After graphene samples were produced and transferred onto SiO2/Si substrates, chlorination was performed with a Plasma Quest (Hook, England, UK) model 145 electron cyclotron resonance reactive ion etcher (ECR/RIE). The ECR power and dc bias in the chamber were carefully optimized to ﬁnely control the chlorination parameters. Before each run, oxygen plasma was ac- tivated to clean the chamber for 10 min. Then the desired chlorine plasma recipe was run for another 10 min to properly condition the chamber, before treating the real samples with plasma. During the chlorine plasma treatment of both CVD and exfoliated graphene samples, both the pressure and ﬂow rate of chlorine gas were kept at constant values of 20 m Torr, and 80 sccm, respectively, with ECR power at 100W, a DC bias of 12V, and a substrate temperature of 30 C.

74 Both before and after chlorine plasma treatment, Raman spectra of graphene was collected using a Renishaw (Wotton-under-Edge, United Kingdom) inVia confocal microscope with a 532 nm wavelength laser source. In addition to using Raman spectroscopy to identifty single layers of exfoliated graphene [8, 95, 96], this technique also provides information on graphene doping [62, 67, 74, 96] as previously discussed in detail in Chapter 4.

Figure 6.1: a) Optical microscope image of CVD graphene, scale bar is 20 mm. b) Optical microscope image of exfoliated graphene, scale bar is 20 mm. c) Raman spectra of CVD graphene sample before (black) and after (red) chlorine plasma doping treatment. d) Raman spectra of exfoliated graphene sample before/after (black/red) chlorine plasma treatment.

Figure 6.1 shows optical microscope images of (a) CVD and (b) exfoliated graphene samples used for plasmon measurements. Raman

75 spectra in Figure 6.1 (c), and (d) show spectra for CVD graphene sample and exfoliated graphene sample, respectively, both before (black line) and after (red line) after chlorine plasma treatment. While the D peak (near 1350 cm-1 either appears or increases in amplitude after doping, the characteristic peaks of graphene near 1600 cm-1 (G peak) and near 2650 cm-1 (G’, or, 2D peak) persist after doping. In cases where plasma power is too high and the graphene ethces away during chlorine plasma treatment, both the G and 2D peaks become signiﬁcantly reduced in amplitude or disappear completely [62].

6.5 Results

Graphene plasmons were directly probed using IR nano-imaging with -1 aCO2 laser source operating at a wavelength of lIR =10.5mm or 950 cm-1.Inthisregime,thesurfaceopticalphononfromthegraphene/substrate interacation does not a↵ect the surface plasmons [91, 94]. Figure 6.2 b,d show AFM and scanning plasmon images, respectively, of a CVD graphene sample treated chlorine plasma. The initial doping of graphene samples varied between those grown with CVD and those prepared by mechanical exfoliation, resulting in various responses to chlorine plasma treatment.

sample lp, initial (nm) lp, doped (nm) Dl(nm) CVD-A 290 410 +120 Exfol. 2b 230 300 +70 Exfol. 3 0 310 +310

Table 6.1: Plasmon wavelengths for CVD and exfoliated graphene samples before (initial) and after (doped) chlorine plasma treatment.

76 Figure 6.2: a) Diagram of IR nano-imaging experiment. b) AFM image of CVD graphene sample. c) Cross-sectional linecut of plasmon relative intensity before (black) and after (red) chlorine plasma doping treatment. d) Scanning plasmon image of CVD graphene sample, red line represents linecut for data plotted in (c).

6.6 Conclusions

Plasmon measurements of chlorine plasma treated graphene samples were conducted with IR nano-imaging. Previous attempts to measure plasmons in doped graphene were conducted with electrostatic gating [91], ferroelectric substrates [97], and solvent-based chemical treatments [92] to modify the charge carrier concentration and plasmon properties. As shown in this chapter, the use of chlorine plasma to dope graphene has lead to the observation of large increases in plasmon wavelength.

77 Figure 6.3: a) Diagram of IR nano-imaging set-up (image courtesy of Basov et al (Fei-Basov:2011). b) AFM image of CVD graphene sample. c) Cross-sectional linecut of plasmon relative intensity before (black) and after (red) chlorine plasma doping treatment. d) Scanning plasmon image of CVD graphene sample, red line represents linecut for data plotted in (c).

78 Figure 6.4: Scanning plasmon image of exfoliated monolayer graphene before (a) and after (b) chlorine plasma treatment. Note the di↵erence in magnitude di↵erence in color scale in both images. .. b) Scanning plasmon image of exfoliated graphene sample.

Figure 6.5: Linecut of plasmon intensity from exfoliated graphene sample measured before and after chlorine plasma treatment. Data data taken from cross-section highlighted with red line in Figure 6.4b.

79 Chapter 7

Future Work

7.1 Proposed experiments

The results outlined in previous chapters is the culmination of several years of often frustrating experiments. Despite those challenges, there is still more work to be done, in particular with thermal scanning probe lithography (tSPL) and chlorine plasma doping. While the work laid out in this thesis will be tightened up for future publication, these proposed experiments have already been shared with colleagues to enable them to continue this work.

7.2 Proposed tSPL experiments

As discussed in Chapter 3, tSPL is a novel technique for patterning contacts on 2D materials. While this work is a good ﬁrst attempt at fabricating devices usign tSPL, more work needs to be done to verify the quality of the electrical contacts. In particular, at the four-point probe measurements must be taken to get a true idea of the contact resistance in devices patterned with tSPL. At the time of this writing, valiant attempts were being made to measure four-point resistance in

80 devices with contacts patterned using tSPL, however, due to faulty wiring in the cryostat used for this measurement, reliable data was not collected. In addition to further electrical characterization, a careful study of the chemical composition of contacts patterned with tSPL will be required to understand why contacts seem to perform better with this novel patterning technique. Some work using time-of-ﬂight secondary ion mass spectroscopy (ToF-SIMS) was started with collaborators at the National Physical Laboratory (NPL) in Teddington, England to study the interface of 2D heterostructures. NPL’s Surface and Nanoanal- ysis Group has a high resolution ToF-SIMS system that I proposed we use to study the chemical composition of regions of 2D materials pat- terened with both tSPL and tradition electron beam lithography. This comparison will reveal di↵erences in chemical composition due to the di↵erences in pattering processes. I believe this will be a value tool to properly evaluate tSPL as a fabrication technique for 2D materials.

7.3 Proposed Cl2 plasma doping experiments

The Cl2 plasma doping work in this thesis has built upon the novel ﬁndings of Schiros, Zhang et al [72, 73] and added an understanding of the optical properties of chlorine-plasma doped graphene. Of particular interest is continuing this work is with electrical transport characterization of chlorinated graphene with hexagonal boron nitride (hBN) substrate and encapsulation. As discussed in Section 4.8, a graphene

ﬁeld e↵ect transistor (FET) treated with Cl2 plasma was successfully p-doped as indicated by a signﬁcant shift in the Dirac peak (see Fig.

81 4.10. While this early work only showed 2-probe results, our collaborators at MIT, Zhang, et al, have shown that this plasma treatment process of graphene also increases conductivity by a factor of approximately 2X [73]. Earlier work done by Dean et al which shows a boost in mobility of graphene with the use of hBN [42] leads us to believe that it might be possible to both increase (or at least maintain) conductivity in highly doped graphene with high mobility by combining the use of

Cl2 plasma treatment with an hBN substrate. There are a myriad of other experiments I had hoped to condcut with Cl2 plasma doped materials including using this technique with graphene waveguides. Following from the work of Phare et al [98], electro-optic graphene modulators have the potential to be used for high speed applications. Typically, graphene in these systems need to be electrostatically doped with an applied gate voltage to modify its optical properties. As discussed in Chapter 5, the observed increase in graphene’s transparency after Cl2 plasma treatment (see Fig. 5.4 indi- cates that this technique could lead to a higher-eciency in graphene electro-optic modulators. My hope is that the work carried laid out in this thesis and the continuation of these e↵orts by colleagues in the future will lead to the eventual application of 2D materials for advanced opto-electronic applications.

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94 Appendix A

Supplementary Information

A.1 Mechanical exfoliation

Two dimensional (2D) materials can be produced with mechanical exfoliation; a ”top-down” method for producing nanoscale materials from macroscopic quantities. Starting with a bulk crystal, 2D layers are peeled o↵using an adhesive ﬁlm, in most cases, Scotch brand (3M, Saint Paul, MN) tape. After repeated peeling of layers from the bulk crystal,

ﬂakes are transferred onto a substrate, typically SiO2,viatheappli- cation of heat or direct rubbing. Figure A.1 shows the steps involved with mechanical exfoliation. A detailed description of this process is highlighted in work by Huag et al [99].

A.2 Heterostructure fabrication

A high-mobility, CVD-grown, monolayer MoS2 device was fabricated with few-layer graphene leads and hBN encapsulation. Figure A.2 out- lines the step-by-step process of device fabrication from stacking to electrode deposition.

95 Figure A.1: a) Bulk crystal exfoliated onto Scotch tape b) SiO2 wafer treated with an oxygen plasma prior to the transfer of flakes onto the substrate c) Scotch tape with exfoliated flakes is brought into contact with plasma-treated wafer and placed on a hotplate at 100 C for 3- 5 minutes d) After cooling, tape is peeled o↵substrate with flakes transferred onto SiO2 e) macroscopic view of flakes exfoliated onto wafer f) 100X magnification of few-layer graphene on wafer. Images from Huang et al [99].

A.3 CVD graphene transfer

Graphene grown directly onto think copper foil is coated with a layer of polymethylmethacrylate (PMMA) and placed into a petri dish of ammonium persulfate (APS-100, commercial vendor name), a copper etchant, which dissolves the copper foil over a period of hours leaving a ﬁlm of PMMA coated graphene ﬂoating on the surface. After multiple rinses in dionized water, the graphene is scooped onto a substrate and gently dried with nitrogen. Finally, the PMMA layer is removed by annealing at 350 C in in forming gas (5% H2 in Ar)

96 A.4 ICP doping condition calibration

In order to develop conditions to for doping graphene with a chlorine

(Cl2)plasma,severalplasmarunswereconductedtodetermineappro- priate plasma conditions. Table A.1 highlights some of the conditions tested to determine doping conditions. Since each plasma system has it’s own unique quirks, these plasma conditions are a good starting point for attempting to replicate this doping process. sample time (s) Cl2 ﬂow (sccm) P(mTorr) RF (W) ICP (W) 4 5 20 5 8 10 5 10 20 5 4 6 6 15 20 5 8 20 7 5 80 20 8 10 8 10 80 20 8 10 9 5 20 5 8 20 10 5 20 5 8 40 11 5 20 5 20 10

Table A.1: Plasma parameters for developing conditions to dope graphene with an Oxford ICP plasma etcher.

97 Figure A.2: a) Device fabrication process: (i) Bottom-hBN flake exfoliated onto a SiO2/Si substrate. (ii) MoS2 on hBN. MoS2 was transferred onto bottom-hBN. (iii-vi) Four graphene flakes on hBN/MoS2 stack. Four graphene flakes were sequentially transferred onto the stack around the perimeter of the MoS2 channel. (vii) Top-hBN on graphene/MoS2/hBN stack. Top-hBN was transferred on top of the stack for encapsulation. (viii) Patterning of PMMA into Hall bar geometry. PMMA layer in the stack was patterned by e-beam lithography into a Hall bar configuration. (ix) Dry etching of the stack. The stack is etched using PMMA as an etch mask. (x) Formation of metal leads. Metal leads are fabricated by e-beam lithography and deposition of metals. Scale bar: 50 m. b) Cross-section STEM image of the stack (from top to bottom: hBN (8 nm), MoS2 (3L), hBN (19 nm)). Scale bar: 50 nm. STEM image courtesy of Pinshane Huang [10].