Light Emission Properties of CVD Grown 2D monolayer WS2 for Optoelectronic Applications

By M Bakhtiar Azim

BSc in Materials & Metallurgical Engineering, BUET, 2017

Thesis Submitted in partial fulfillment of the

requirements for the degree of Master of Applied Science

in the

School of Engineering Science

Faculty of Applied Science

Copyright © M Bakhtiar Azim

SIMON FRASER UNIVERSITY

2020

Copyright in this work rests with the author. Please ensure that any reproduction

or re-use is done in accordance with the relevant national copyright legislation.

i

Approval

Name: M Bakhtiar Azim

Degree: Master of Applied Science

Title: Light Emission Properties of CVD Grown 2D monolayer WS2 for Optoelectronic Applications

Examining Committee: Chair: Michael Sjoerdsma

Senior Lecturer, School of Engineering Science

Michael M. Adachi Senior Supervisor, School of Engineering Science Assistant Professor _______________________________________

Bonnie Gray Supervisor, School of Engineering Science Professor

Ash Parameswaran Internal Examiner, School of Engineering Science Professor

_______________________________________

Date Defended/Approved:

[July 6th, 2020]

ii

Abstract

Two-dimensional Transitional Metal Dichalcogenides (TMDs) such as MX2 (M=

Mo, W; X= S, Se) have gained tremendous attention for use in optoelectronic

applications because of their high carrier mobility and indirect-direct band gap

transition for thin layers resulting in light emission. Moreover, monolayer TMDs

have exceptional other properties such as piezoelectricity, gate-induced

superconductivity, and tunable band structure. Mechanical exfoliation,

hydrothermal method, electrochemical exfoliation, chemical vapor deposition

(CVD) etc. are the most widely used methods for preparing monolayer TMDs.

Among these methods, CVD is regarded as the most promising approach because

it can produce large area crystal growth and uniform monolayers. The challenges

associated with other methods are either small flake size or low quality with lower

carrier mobility limiting performance in electronic devices. CVD grown TMDs tend

to show weak, non-uniform photoluminescence. If we want to use pristine TMDs

for optoelectectronics applications, we can use different chemical reagents such

as strong acid vapor for passivating surface of pristine TMDs which eventually

leads to enhanced photoluminescence. In this study, we first demonstrate growth

of monolayer triangular WS2 crystals using a 3-heating zone furnace using a

bottom-up CVD process. The average lateral crystal size is ~20-25 µm and the

largest crystal size is ~75 µm. Although, several research groups have reported

WS2 growth using WO3 and S precursors, specific parameters such as precursor

amount, growth substrate, growth pressure and flow rate, temperature, use of

gases (e.g. N2, Ar, Ar+H2), growth time, use of promoter (e.g. PTAS, NaCl, KBr),

pre-surface treatment of substrate etc. can vary widely from lab to lab, affecting

the growth morphology, mechanism, light emission, Raman spectra. Atomic Force

Microscopy (AFM) measurements indicate that the thickness of the monolayer

WS2 is ~1 nm. We also performed SEM imaging to investigate surface morphology

of monolayer WS2 and EDX to perform elemental analysis of monolayer WS2. X-

ray Photoelectron Spectroscopy (XPS) has been performed for pristine WS2 to

reveal its chemical states. Photoluminescence spectroscopy revealed a sharp

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emission peak at ~626 nm confirming indirect (bulk) to direct band-gap

(monolayer) transition in the monolayer. On the other hand, the PL intensity for

bi/tri-layer is relatively weak compared to monolayer. Moreover, we investigate the

effect of surface passivation using chemical reagents such as H2SO4-vapor for

modifying light emission property of pristine WS2 for using in next generation

optoelectronics. After H2SO4-vapor treatment, we achieved light emission at ~634

nm corresponding to red-shift with enhanced trion emission. Edges of H2SO4-

vapor treated sample shows enhanced biexciton compared to pristine-WS2. We

are able to achieve maximum 10-fold PL enhancement from our H2SO4-vapor

treated sample and, on an average, we got ~5 fold enhancement. H2SO4-vapor

treatment has not been used before for surface passivation. We also studied the

laser power dependence PL of pristine and H2SO4-vapor treated monolayer WS2

where it shows that with increasing laser power, pristine and H2SO4-vapor treated

monolayer WS2 shows enhanced PL specially at the crystal edges. In addition, we

also focused on investigating photoemission from pristine and H2SO4-vapor

treated monolayer WS2 along certain lines which eventually shows PL distribution

within a specific flake.

Keywords: TMDs; Optoelectronics; Laser Power; Photoluminescence; CVD;

WS2.

iv

Dedication

I would like to dedicate this thesis to my parents because of their

unconditional support!

v

Acknowledgements

After writing and editing all other parts, finally, I have started writing this

section. When I was writing Acknowledgement part, there were thousands of

things going through my mind. I have been recently diagnosed with a kidney tumor,

I don’t know whether it is benign or malignant, waiting for doing an MRI for last one

month which will decide the next step. It’s hard when someone has genuine heath

issue and simultaneously, he/she has to continue and focus on all other activities

overcoming all sorts of anxiety that is in my head for last four months. The effort I

put during this master’s is quite impossible to describe in just few words and I really

don’t want to drop without finishing it, all my efforts will go in vein. When I was

diagnosed with this health issue, I told myself, “I have to finish my thesis writing

somehow” that gave me courage to move forward day by day. With Almighty

Allah’s blessings and family support, I managed to gather all the data and

information that I believe makes this thesis complete.

Firstly, I would like to thank Almighty Allah to give me strength and patience

throughout these years to attain my goals for successful completion of graduate

studies. Then, I would like to express my gratitude towards my supervisor Dr.

Michael Adachi for his constant support and guidance and accepting me in his

group. I feel lucky and blessed to have a supervisor like him who is energetic,

helpful and caring at the same time. I was really motivated by my supervisor

because of his dedication and passion towards research. At the beginning, when I

started experimental works related to my thesis, I found it very difficult because as

a new group there was none to help except my supervisor. None had prior

experience related to my project that I was working on. I had a tough time initially

finding my way for appropriate research project. At one point due to my health

issues I thought that I wouldn’t be able to complete my studies. Now, because of

the blessings of Almighty Allah and moral support from my supervisor, I am writing

Acknowledgement after finishing other parts. During the second year of my MASc,

I spent quite a few months on experiments starting from purchasing equipments

and building up the measurement setup. We continued with experimental

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measurements taking lots of data. I was so excited when we observed expected

Chemical vapor Deposition (CVD) growth of monolayer WS2 after ~6 months’ hard

work, we performed more trial and error experiments and even organized a few

discussion sessions with lab mates and group members relating to this project. I

can also remember working in 4D Labs, LASIR for ~6-7 hours per day continuously

without any lunch break to get data on Photoluminescence. My supervisor always

motivated me with his usual encouraging smile. I made progress gradually

according to my supervisor’s suggestions and guidelines. I am out of words how

to express my gratitude towards my support system- my parents; without their

support, I won’t be here-living approx. 8000 miles away from home! I always feel

that the reason I am here because of the blessings of my parents.

I am also thankful to my lovely wife who is also doing her graduate studies

in Canada for motivating me towards my passion.

My lab colleagues who are MSc and PhD students helped me with growing

more samples and verifying the data again and again and suggested me what I

am supposed to try next. I would like to acknowledge CMC Microsystems, National

Sciences and Engineering Research Council of Canada (NSERC), Queen

Elizabeth Scholars, and Simon Fraser University (SFU). I am also grateful to Bud

Yarrow (BASc student, ENSC, SFU), Amin Abnavi (PhD student, ENSC, SFU),

Askar Abdelrahman (PhD student, ENSC, SFU), Thusani De Silva (MASc student,

ENSC, SFU) and Sofia Pineda (BASc student, Chemistry, SFU) for their valuable

suggestion and Professor Dr. Karen Kavanaugh (Physics, SFU) and Mirette Fawzy

(PhD student, Physics, SFU) for their help in performing TEM characterization. I

am also thankful to Dr. Saeid Kamal (LASIR), Dennis Hsiao, Rana Faryad Ali (PhD

student, Chemistry, SFU) and 4D LABS, SFU for providing constant support,

valuable suggestion and giving excess for using their facilities to successfully

complete the project. At the end, I want to thank from the bottom of my heart to all

the people that support and accompany me during my graduate studies at SFU.

vii

Table of Contents

Approval ........................................................................................................................... i

Abstract ........................................................................................................................... ii

Dedication ...................................................................................................................... iv

Acknowledgements ....................................................................................................... v

List of Figures................................................................................................................. ix

List of Tables ................................................................................................................ xiv

List of Acronyms ............................................................................................................ xv

Chapter 1: Introduction ................................................................................................. 1

1.1. Opportunities Beyond Silicon ................................................................................ 1

1.2. Project Goal .......................................................................................................... 1

1.3. Motivation of Thesis .............................................................................................. 2

1.4. The Structure of Thesis ........................................................................................ 3

Chapter 2: Background & Literature Review of 2D Materials ..................................... 4

2.1. Discovery of 2D Materials ..................................................................................... 4

2.2. Fundamentals of Semiconductor Materials ........................................................... 5

2.2.1. Electron, Hole and Exciton ................................................................................. 5

2.2.2. Direct and Indirect bandgap ........................................................................... 7

2.2.3. Carrier Recombination and Photoluminescence (PL) ......................................... 7

2.3. Crystal Lattice Band Structure .............................................................................. 9

2.4. Properties of TMDs ................................................................................................ 10

2.4.1. Electrical and Electronic Properties .................................................................. 11

2.4.2. Thermal Properties .......................................................................................... 12

2.4.3. Chemical Properties ......................................................................................... 12

2.4.4. Mechanical Properties ..................................................................................... 13

2.4.5. Young`s Modulus ............................................................................................. 13

2.4.6. Light-Emitting properties of 2D TMDs .............................................................. 14

2.5. Generation of Defects in 2D TMDs ..................................................................... 15

Chapter 3: Background & Literature Review of CVD growth of 2D monolayer, Characterization & PL Enhancement ......................................................................... 16

3.1. Production Methods of 2D TMDs Crystals .......................................................... 17

3.1.1. Exfoliation ....................................................................................................... 17

3.2. Overview of Characterization Methods of CVD Grown Monolayer WS2 .................. 27

3.2.1. Optical Imaging ................................................................................................ 28

3.2.2. Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) ......................................................................................................................... 28

3.2.3. Transmission Electron Microscopy (TEM) ........................................................ 29

3.2.4. Raman Spectroscopy ....................................................................................... 29

3.2.5. Atomic Force Microscopy (AFM) ...................................................................... 32

3.2.6. X-ray Photoelectron Spectroscopy (XPS)......................................................... 33

3.2.7. Photoluminescence (PL) .................................................................................. 35

viii

3.3. Overview of PL Enhancement based on Literature ................................................. 37

Chapter 4: Experimental Details ................................................................................. 57

4.1. Materials................................................................................................................. 57

4.2. Experiment ............................................................................................................. 58

4.2.1. CVD Growth of monolayer WS2 on SiO2/Si substrate ....................................... 58

4.3. Experimental Results & Discussions ...................................................................... 64

4.3.1. Optical ............................................................................................................. 64

4.3.2. SEM &EDS ...................................................................................................... 66

4.3.3. TEM ................................................................................................................. 67

4.3.4. Raman Spectroscopy ....................................................................................... 68

4.3.5. AFM ................................................................................................................. 70

4.3.6. XPS ................................................................................................................. 71

4.3.7. PL .................................................................................................................... 73

Chapter 5: PL Enhancement of Monolayer WS2 ........................................................ 84

5.1. Purpose of PL Enhancement .................................................................................. 84

5.2. Methodology ........................................................................................................... 85

5.3. Results and Discussion .......................................................................................... 86

5.3.1. PL Enhancement of H2SO4-Vapor Treated Monolayer WS2 ............................. 86

5.3.2. Room Temperature Laser Power Dependent PL of H2SO4-Vapor Treated monolayer WS2 .......................................................................................................... 91

5.3.4. XPS Study of H2SO4- Vapor Treated Monolayer WS2 ...................................... 96

Chapter 6: Future Projects & Conclusion .................................................................. 99

6.1. Limitation ................................................................................................................ 99

6.2. Contribution ............................................................................................................ 99

6.3. Future Work ........................................................................................................... 99

6.4. Conclusion ........................................................................................................... 100

Bibliography ................................................................................................................ 100

Appendix A. ............................................................................................................... 106

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List of Figures

Figure 1: Amplitude AFM images showing shape evolutions of CVD-grown WSe2 flakes at different growth temperatures of (a) 900⁰C, (b) 950⁰C, (c and d) 1025⁰C, (e and f) 1050⁰C. Unusual, non-triangle shapes are gradually found as the growth temperature increases. (a and b) Monolayer triangles with different sizes; (c and d) thin few layer truncated triangle and hexagon with curve edges; (e and f) thick few layer triangle and hexagon with straight edges. Reprinted (adapted) with permission from Liu, B., Fathi, M., Chen, L., Abbas, A., Ma, Y., & Zhou, C. (2015). Chemical vapor deposition growth of monolayer WSe2 with tunable device characteristics and growth mechanism study. ACS Nano, 9(6), 6119–6127. Copyright 2015) American Chemical Society.......................................................... 21

Figure 2: Effect of growth temperatures on the sizes and layer numbers of CVD-grown WSe2. Optical microscopy images of WSe2 flakes grown at (a) 850⁰C, (b) 900⁰C, and (c) 1050⁰C. The growth durations are 15 min for all cases. (d) The correlation of average WSe2 flake sizes and layer numbers with growth temperatures. The vertical error bars indicate standard deviations of the flake sizes in statistical analysis. Reprinted (adapted) with permission from Liu, B., Fathi, M., Chen, L., Abbas, A., Ma, Y., & Zhou, C. (2015). Chemical vapor deposition growth of monolayer WSe2 with tunable device characteristics and growth mechanism study. ACS Nano, 9(6), 6119–6127. Copyright (2015) American Chemical Society. ........... 21

Figure 3: Effect of growth durations on the sizes of CVD-grown monolayer WSe2. Optical microscopy images of WSe2 grown for (a) 1 min, (b) 5 min, and (c) 5 h. The growth temperatures are 950⁰C for all cases. (d) Plot of average flake sizes versus growth durations of 1 min, 3 min, 5 min, 10 min, 15 min, 30 min, 60 min, and 5 h. The vertical error bars are standard deviations in statistical analysis. Reprinted (adapted) with permission from Liu, B., Fathi, M., Chen, L., Abbas, A., Ma, Y., & Zhou, C. (2015). Chemical vapor deposition growth of monolayer WSe2 with tunable device characteristics and growth mechanism study. ACS Nano, 9(6), 6119–6127. Copyright 2015) American Chemical Society.......................................................... 22

Figure 4: Shape Evolution of CVD WSe2 with increased Temperature. Reprinted (adapted) with permission from Liu, B., Fathi, M., Chen, L., Abbas, A., Ma, Y., & Zhou, C. (2015). Chemical vapor deposition growth of monolayer WSe2 with tunable device characteristics and growth mechanism study. ACS Nano, 9(6), 6119–6127. Copyright 2015) American Chemical Society. .................................................................. 23

Figure 6:Raman Spectra at different excitation wavelength (a) 488 nm, (b) 514 nm, (c) 647 nm. Reprinted (adapted) with permission from Berkdemir, A., Gutiérrez, H. R., Botello-Méndez, A. R., Perea-López, N., Elías, A. L., Chia, C.-I., Wang, B., Crespi, V. H., López-Urías, F., & Charlier, J.-C. (2013). Identification of individual and few layers of WS 2 using Raman spectroscopy. Scientific Reports, 3(1), 1–8. Copyright © 2013, Springer Nature. ................................................................................................... 31

Figure 7: (a) Peak Frequency vs Number of Layers, (b) Intensity ratio vs Number of Layers[73]. Reprinted (adapted) with permission from Berkdemir, A.,

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Gutiérrez, H. R., Botello-Méndez, A. R., Perea-López, N., Elías, A. L., Chia, C.-I., Wang, B., Crespi, V. H., López-Urías, F., & Charlier, J.-C. (2013). Identification of individual and few layers of WS2 using Raman spectroscopy. Scientific Reports, 3(1), 1–8. Copyright © 2013, Springer Nature. ................................................................................................... 32

Figure 8: Absorption and related radiative and non-radiative processes involved during the whole procedure of Photoluminescence. .......................................... 36

Figure 9: (a) Raman spectra and (b) Raman intensity maps of a monolayer MoSe2 flake before and after HBr treatment. Reprinted (adapted) with permission from Han, H.-V., Lu, A.-Y., Lu, L.-S., Huang, J.-K., Li, H., Hsu, C.-L., Lin, Y.-C., Chiu, M.-H., Suenaga, K., & Chu, C.-W. (2016). Photoluminescence enhancement and structure repairing of monolayer MoSe2 by hydrohalic acid treatment. ACS Nano, 10(1), 1454–1461. Copyright (2016), American Chemical Society. .................................................................. 41

Figure 10: (a) PL intensity mappings of an individual MoSe2 flake before and after HBr treatment. Profiles in (b) and (c) show the PL intensity and photon energy modulation as a function of surface location along the solid line indicated in (a) Reprinted (adapted) with permission from Han, H.-V., Lu, A.-Y., Lu, L.-S., Huang, J.-K., Li, H., Hsu, C.-L., Lin, Y.-C., Chiu, M.-H., Suenaga, K., & Chu, C.-W. (2016). Photoluminescence enhancement and structure repairing of monolayer MoSe2 by hydrohalic acid treatment. ACS Nano, 10(1), 1454–1461. Copyright (2016), American Chemical Society.......... 42

Figure 11: (a) Photoluminescence of the as-grown and HBr-treated monolayer MoSe2 at 10 K. (b) Temperature dependence of PL for the MoSe2 after HBr treatment. (c) Trion and exciton peak energies. (d) Intensity of trion to exciton peak as a function of temperature. Reprinted (adapted) with permission from Han, H.-V., Lu, A.-Y., Lu, L.-S., Huang, J.-K., Li, H., Hsu, C.-L., Lin, Y.-C., Chiu, M.-H., Suenaga, K., & Chu, C.-W. (2016). Photoluminescence enhancement and structure repairing of monolayer MoSe2 by hydrohalic acid treatment. ACS Nano, 10(1), 1454–1461. Copyright (2016), American Chemical Society. ...................................... 43

Figure 12: (a) Optical images of as-prepared 1L-, 2L-, and 3L-MoS2 on SiO2/Si substrates. (b) Raman spectra of the as-prepared 1L-, 2L-, and 3L-MoS2

measured at room temperature. (c) PL spectra of the as prepared 1L-, 2L-, and 3L-MoS2. The PL peak due to the indirect band gap transition is denoted as I, and those due to the direct band gap transition are denoted as peaks A and B[36](Mouri et al., 2013). Reprinted (adapted) with permission from Mouri, S., Miyauchi, Y., & Matsuda, K. (2013). Tunable photoluminescence of monolayer MoS2 via chemical doping. Nano Letters, 13(12), 5944–5948. Copyright (2016) American Chemical Society ............................................................................................................... 46

Figure 13: (a) PL spectra of 1L-MoS2 before and after F4TCNQ doping. (b) PL spectra of 1L-MoS2 obtained at each doping step (0, 1, 2, 4, 6, 10, 13, and 16 steps). The inset shows the normalized PL spectra of 1L-MoS2 at each doping step. (c) Analysis of the PL spectral shapes for as-prepared and F4TCNQ-doped 1L-MoS2. The A peaks in the PL spectra were reproduced by assuming two peaks with Lorentzian functions, corresponding to the trion (X−) and the exciton (X) peaks, were overlapped. (d) Integrated PL intensity of the negative trion Ix−, exciton Ix,

xi

and the sum (Itotal) of Ix and Ix−as functions of the number of F4TCNQ doping steps. Solid lines show the calculated PL intensity curves calculated by solving the rate equations in the three-level model. Reprinted (adapted) with permission from Mouri, S., Miyauchi, Y., & Matsuda, K. (2013). Tunable photoluminescence of monolayer MoS2 via chemical doping. Nano Letters, 13(12), 5944–5948. Copyright (2013) American Chemical Society. .................................................................. 47

Figure 14: (a) PL spectra of 1L-MoS2 before and after being doped with p-type molecules (TCNQ and F4TCNQ). (b) PL spectra of 1L-MoS2 before and after being doped with an n-type dopant (NADH). Reprinted (adapted) with permission from Mouri, S., Miyauchi, Y., & Matsuda, K. (2013). Tunable photoluminescence of monolayer MoS2 via chemical doping. Nano Letters, 13(12), 5944–5948. Copyright (2013) American Chemical Society ................................................................................................... 48

Figure 15: PL spectra for both the as-exfoliated and TFSI treated (a) WS2, (b) MoS2, (c) WSe2, and (d) MoSe2 monolayers measured at an incident power density of 1 × 10−2 Wcm−2. The inset shows normalized spectra for each material. Absorption spectra of both as-exfoliated (dashed lines) and chemically treated (solid lines) WS2, MoS2, WSe2, and MoSe2 monolayers (e). Reprinted (adapted) with permission from Amani, M., Taheri, P., Addou, R., Ahn, G. H., Kiriya, D., Lien, D.-H., Ager III, J. W., Wallace, R. M., & Javey, A. (2016). Recombination kinetics and effects of superacid treatment in sulfur-and selenium-based transition metal dichalcogenides. Nano Letters, 16(4), 2786–2791. Copyright (2016) American Chemical Society. .................................................................................................. 51

Figure 16: Radiative decay of as-exfoliated (a) and chemically treated (b) WS2 at various initial carrier concentrations (n0) as well as the instrument response function (IRF). Reprinted (adapted) with permission from Amani, M., Taheri, P., Addou, R., Ahn, G. H., Kiriya, D., Lien, D.-H., Ager III, J. W., Wallace, R. M., & Javey, A. (2016). Recombination kinetics and effects of superacid treatment in sulfur-and selenium-based transition metal dichalcogenides. Nano Letters, 16(4), 2786–2791. Copyright (2016) American Chemical Society. .................................................................. 51

Figure 17: (a) Configuration of the growth setup utilized to prepare the MoS2 samples for this study. The temperature of the substrate and molybdenum precursor (in the furnace hot zone) and the sulfur precursor (surrounded by heating tape) is controlled and measured independently. (b and c) Schematic illustrating the two primary sample preparation routes investigated in this study. As-grown MoS2 triangular domains and films, which show tensile strain after growth were either (b) treated by TFSI directly, resulting in a small reduction in the PL QY, or (c) transferred from the growth substrate using a PMMA-mediated transfer process, releasing the strain, and subsequently treated by TFSI, resulting in a final PL QY of approximately 30%. Reprinted (adapted) with permission from Amani, M., Burke, R. A., Ji, X., Zhao, P., Lien, D.-H., Taheri, P., Ahn, G. H., Kirya, D., Ager III, J. W., & Yablonovitch, E. (2016). High luminescence efficiency in MoS2 grown by chemical vapor deposition. ACS Nano, 10(7), 6535–6541. Copyright (2016) American Chemical Society. ....................................... 52

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Figure 18: (a) Raman spectra measured on as-grown and transferred MoS2 single domains. (b) PL spectra of the MoS2 single domains measured before and after transfer at a laser power of 50 W/cm2. Reprinted (adapted) with permission from Amani, M., Burke, R. A., Ji, X., Zhao, P., Lien, D.-H., Taheri, P., Ahn, G. H., Kirya, D., Ager III, J. W., & Yablonovitch, E. (2016). High luminescence efficiency in MoS2 grown by chemical vapor deposition. ACS Nano, 10(7), 6535–6541. Copyright (2016) American Chemical Society. .................................................................................. 53

Figure 19: (a) Raman spectra measured on transferred MoS2 single domains before and after treatment by TFSI. (b) PL spectra obtained at a pump power of 0.1 W/cm2 for transferred MoS2 single domains both before and after chemical treatment by TFSI. (c) Radiative decay of transferred MoS2 single domains obtained at a pump fluence of 5 × 10−2μJ/cm2 both before and after chemical treatment by TFSI, as well as the instrument response function (IRF). Reprinted (adapted) with permission from Amani, M., Burke, R. A., Ji, X., Zhao, P., Lien, D.-H., Taheri, P., Ahn, G. H., Kirya, D., Ager III, J. W., & Yablonovitch, E. (2016). High luminescence efficiency in MoS2 grown by chemical vapor deposition. ACS Nano, 10(7), 6535–6541. Copyright (2016) American Chemical Society. ............................. 53

Figure 20: (a) Optical image of a transferred MoS2 single domain and log-scale luminescence images from the same area obtained (b) before and (c) after chemical treatment by TFSI. (d) Optical image of a transferred continuous MoS2 film and log scale luminescence images from the same area obtained (e) before and (f) after chemical treatment by TFSI[84]. Reprinted (adapted) with permission from Amani, M., Burke, R. A., Ji, X., Zhao, P., Lien, D.-H., Taheri, P., Ahn, G. H., Kirya, D., Ager III, J. W., & Yablonovitch, E. (2016). High luminescence efficiency in MoS2 grown by chemical vapor deposition. ACS Nano, 10(7), 6535–6541. Copyright (2016) American Chemical Society. ....................................................... 54

Figure 21: (a) CVD Setup in the ENSC Cleanroom, (b) Image representing reaction at High Temperature in CVD Furnace ........................................................ 58

Figure 22: Schematic of Small Quartz Tube and Boat/Holder ........................................ 58

Figure 23: (a) Schematic Experimental Set-Up of CVD growth, (b) 3D view of CVD Growth of monolayer WS2 on SiO2/Si ..................................................... 61

Figure 24: Temperature profile as a function of Time for CVD growth of monolayer WS2

............................................................................................................... 61

Figure 25: (a) and (b) Monolayer WS2 on SiO2/Si substrate (c) and (d) Multilayer WS2 on SiO2/Si substrate .................................................................................... 65

Figure 26: (a) Optical Image under Bright Field where monolayer is visible and (b) Optical Image under Dark Field where monolayer is not visible .............. 66

Figure 27: SEM of CVD grown Pristine monolayer WS2 ................................................ 67

Figure 28: EDS of Pristine monolayer WS2 (a) before CVD deposition and (b) after CVD deposition............................................................................................... 67

Figure 29: TEM of Pristine monolayer WS2 directly grown on TEM grids (a) TEM Image and (b) SAED Pattern ............................................................................ 68

Figure 30: (a) Raman Spectra of monolayer WS2, (b) Intensity ratio vs Number of layers ............................................................................................................... 69

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Figure 31: (a) AFM mapping of monolayer WS2 (b) Height profile along Blue Line, ....... 71

Figure 32: XPS Spectra of monolayer WS2 (a) S 2p, (b) Core level W 4f ...................... 73

Figure 33: (a,b) Fluorescence Images of monolayer WS2, (c) PL intensity map of pristine monolayer WS2, (d) 2D surface plot of Pristine WS2, (e) 3D Surface Plot of Pristine WS2and (f) PL spectra of grown monolayer WS2 ................... 76

Figure 34: (a) Laser Power Dependent PL study of Pristine WS2 in terms of Wavelength (nm) and (b) Laser Power Dependent PL study of Pristine WS2 in terms of Photon Energy (eV) ................................................................................ 79

Figure 35: PL Intensity variation with Laser Power in log scale ..................................... 79

Figure 36: PL Intensity variation along line with distance (a) PL Image of Pristine WS2 with Line 1, (b) PL variation along Line 1, (c) PL Image of Pristine WS2 with Line 2, (d) PL variation along Line 2, (e) PL Image of Pristine WS2 with Line 3, (f) PL variation along Line 3, (g) PL Image of Pristine WS2 with Line 4, (h) PL variation along Line 4, (i) PL Image of Pristine WS2 with Line 5, (j) PL variation along Line 5 and (k) PL Image of Pristine WS2

with Line 6, (l) PL variation along Line 6 ................................................. 82

Figure 37: (a) Setup of H2SO4 Vapor Treatment in Yellow Room, (b) Schematic Setup of H2SO4-Vapor Treatment ......................................................................... 86

Figure 38: PL Intensity mapping of Pristine-WS2 (a) before and (b) after H2SO4-vapor Treatment............................................................................................... 89

Figure 39: PL Intensity vs Wavelength (nm) before and after H2SO4-vapor treatment ... 90

Figure 40: Normalized PL Intensity vs Wavelength (nm) before and after H2SO4-vapor treatment ................................................................................................ 90

Figure 41: (a) Laser Power Dependent PL study of H2SO4-Vapor Treated Monolayer WS2 in terms of Wavelength (nm) and (b) Laser Power Dependent PL study of H2SO4-Vapor Treated Monolayer WS2 in terms of Photon Energy (eV) ........................................................................................................ 93

Figure 42: PL Intensity variation with Laser Power in log scale ..................................... 93

Figure 43: PL Intensity variation along certain line (a) PL Image of H2SO4-Vapor Treated Monolayer WS2 with Line 1, (b) PL variation along Line 1, (c) PL Image of H2SO4-Vapor Treated Monolayer WS2 with Line 2, (d) PL variation along Line 2, (e) PL Image of H2SO4-Vapor Treated Monolayer WS2 with Line 3, (f) PL variation along Line 3, (g) PL Image of H2SO4-Vapor Treated Monolayer WS2 with Line 4, (h) PL variation along Line 4, (i) PL Image of H2SO4-Vapor Treated Monolayer WS2 with Line 5, (j) PL variation along Line 5 and (k) PL Image of H2SO4-Vapor Treated Monolayer WS2 with Line 6, (l) PL variation along Line 6 ........................................................ 96

Figure 44: XPS Spectra of monolayer WS2 before and after H2SO4 vapor treatment (a) S 2p and (b) Core Level W 4f .................................................................... 98

xiv

List of Tables

Table 1: CVD process parameters for monolayer WS2 growth……………………………93

xv

List of Acronyms

CVD Chemical Vapor Deposition

WS2 Tungsten Disulphide

TMDs Transitional Metal Dichalcogenides

RT Room Temperature

LT Low Temperature

PL Photoluminescence

SEM Scanning Electron Microscopy

EDS Energy Dispersive Spectroscopy

TEM Transmission Electron Microscopy

AFM Atomic Force Microscopy

XPS X-ray Photoelectron Microscopy

1

Chapter 1: Introduction

1.1. Opportunities Beyond Silicon

Two-dimensional (2D) materials are a class of materials possessing

ultimate limit of thinness in vertical dimension and representing the thinnest

artificial materials in the universe, have demonstrated potential for discovering

interesting phenomena in condensed matter physics and as a promising platform

to push the frontier of semiconductor technology beyond the Moore’s law. When I

was taking a course “ENSC 893: Principles of Nanoengineering” in 1st year of my

masters, I got to know about Richard Feynman who raised up this question in his

famous lecture in 1959 “What could we do with layered structures with just the right

layers?”, “There’s plenty of room at the bottom”. Among these ‘right’ layers, 2D

materials at atomic scale are particularly interesting and have attracted lots of

attention in recent years.

1.2. Project Goal

The master’s project aims to achieve room temperature (RT) enhanced light

emission from pristine-WS2 using surface passivation. At first, monolayer WS2 was

deposited on SiO2/Si substrate using Chemical Vapor Deposition (CVD) and

different characterizations such as Raman, Photoluminescence (PL), Atomic

Force Microscopy (AFM), X-ray Photoelectron Spectroscopy (XPS), Transmission

Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and Energy

Dispersive Spectroscopy (EDS) of pristine-WS2 have been performed. Finally, PL

enhancement study has been performed using 50 ml 2.24 M H2SO4-Vapor

treatment for pristine-WS2 which has been done for the first time to achieve room

temperature enhanced light emission. In the field of photoluminescence, this is

new advancement regarding surface passivation with H2SO4-vapor treatment

which shows maximum 10-fold enhancement at room temperature. Surface

passivation supposed to passivate the point defects and surface vacancy/Sulphur

2

vacancy which ultimately reduces the non-radiative recombination sites and show

enhanced exciton peak. Suppression of Phonons usually takes place significantly

at Low Temperature (LT) and hence enhanced PL is observed at 77K or below for

2DTMDs, but we performed experiment at Room Temperature (RT); carriers at RT

can have enough energy to get to non-radiative recombination centers, so, in

general, a strong reduction of the intensity of the PL signal is observed which is

not seen in our samples at RT; excitonic effects are more efficient at LT but for our

samples exciton peak is sharp at RT-for pristine-WS2 and H2SO4-vapor treated

WS2. Most of the optoelectronics operating temperature is ~298k not 77k;

therefore, it is important to focus on achieving enhanced PL at RT. Furthermore,

laser power dependent PL, variation of PL of pristine-WS2 and H2SO4-vapor

treated WS2 along certain lines has been studied as well.

1.3. Motivation of Thesis

In 2004, two scientist Novoselov and Geim experimentally found unique

properties of 2D material ‘Graphene’ which was exfoliated using scotch-tape

method at University of Manchester. Although Graphene shows impressive

properties such as, but it has one limiting factor which is zero-band gap. Then,

researchers and scientists started to explore other 2D materials such as TMDs (i.e.

MoS2 ,WS2). TMDs show indirect to direct band gap transition and has wide band

gap which make them potential candidate for optoelectronic applications. The

weak van der wall force between each layer of TMDs enables tuning of properties.

Researchers and Scientists are working day and night to figure out why the

pristine-TMDs show weak, non-uniform PL and how they can enhance it. So far,

different chemical reagents have been used such as acetone, p-type and n-type

dopants, TFSI, HI etc. and still more work is needed.

3

1.4. The Structure of Thesis

This thesis includes the research findings over the time span of two years

of Master’s degree at the Department of Engineering Science, Simon Fraser

University.

It starts with a literature review in Chapter 2 that provides an overview of

Discovery of 2D materials, Fundamentals of Semiconductor Materials, Crystal

Lattice Band Structure, Properties of 2D TMDs, Structural Defects in 2D TMDs.

Chapter 3 covers the overview of 2D TMDs production, overview of

characterization techniques and Literature Review of Photoluminescence

Enhancement of TMDs.

Chapter 4 summarizes my work on CVD growth of monolayer WS2 and

different characterization techniques, Room Temperature PL of Pristine WS2,

Room Temperature Laser Power Dependent PL of Pristine WS2, Room

Temperature PL Variation of Pristine WS2 along certain lines that have been

performed throughout these 2 years.

Chapter 5 shows Purpose of PL Enhancement, Methodology, PL

Enhancement of H2SO4 Vapor Treated Monolayer WS2, PL dependence on Laser

Power, Room Temperature Laser Power Dependent PL of H2SO4-Vapor Treated

monolayer WS2, Room Temperature PL Variation of H2SO4-Vapor Treated WS2

along certain lines.

Chapter 6 includes the other experimental works that have been done

simultaneously during this 2 years, future work and conclusion of the thesis finally.

4

Chapter 2: Background & Literature Review of 2D Materials

Chapter 2 discusses about fundamental concepts such as exciton, trion,

biexciton, carrier recombination, indirect and direct band gap, quantum yield (QY),

light emission depending on TMDs crystal structure, role of defects in a crystal that

are very important for understanding 2D based semiconductors properties.

Fundamental concepts help to explain results properly that are being achieved in

this thesis and without clear explanation the goal of this thesis will not be achieved.

Chapter 2 focuses on 2D Materials’ Discovery, Fundamentals of Semiconductors,

TMDs Crystal Band Structure, TMDs Properties, and Defects in TMDs.

2.1. Discovery of 2D Materials

Although it has been studied for long time as a theoretical model [1]–[6],

atomically thick 2D crystals such as graphene was predicted to not exist because

of thermodynamic instability [7] and was described as pure ‘academic’ material.

However, people’s perception changed in 2004 when graphene was isolated by

scotch-tape method from graphite in the lab[8]–[10]. The discovery of graphene

not only brought Andre Geim and Konstantin Novoselov the 2010 Nobel prize in

physics, it also opened the door to an exciting world of 2D materials.

When thinned down to atomic layer, graphene shows quite different and

distinguished characteristics compared to graphite, it even got a nickname of

‘miracle material’ due to its superior properties. In terms of applied science, the

amazing characteristics graphene possesses rise up new opportunities for a wide

range of applications. These includes but not are not limited to optical absorption

of exactly πα = 2.3% (α is the fine structure constant), super-high intrinsic strength,

ultrahigh thermal conductivity, amazing room-temperature electron mobility etc.

Graphene has shown its potential to be used in various areas, such as flexible

electronics, photonics, energy generation and storage, sensors, bio applications,

paints and coating and so on [11]–[13]. It is almost impossible to mention all the

5

potential applications and new physical phenomena graphene has brought to us.

This atomically thin material has been an obsession for researchers around the

world since its birth in the lab and new things are still coming out every day.

On the other hand, the message we could take from graphene is that the

2D materials have extraordinary properties compared to their bulk forms and hold

huge potential for lots of applications, which is not fully explored at all. This inspired

people to start looking for other graphene-like materials, such as boron nitride

(hBN), transition metal dichalcogenides (TMDs), black phorsphone, silicene and

germanene. TMDs are semiconductors and have shown many superior properties

for applications in photonics. In this thesis, we will focus on TMDs. To better serve

for our topics, we start reviewing some basic concepts in semiconductor materials

in the following section.

2.2. Fundamentals of Semiconductor Materials

The core of electronic technology is to control the flow of electrons and

photonics is the technology to control the flow of photons. Semiconductor

optoelectronics connect these two technologies: photons create mobile charge

carriers and charge carriers in turn control the flow of photons. Semiconductor

based optoelectronic devices such as laser, light-emitting diodes (LED) have

changed our life a lot and this field is still moving on quickly. Thus, studying optical

properties of semiconductors, which is in the domain of semiconductor optics, is

essential for fabricating advanced optoelectronic devices[14].

2.2.1. Electron, Hole and Exciton

Semiconductor is a crystalline or amorphous solid with electrical

conductivity between conductor and insulator, typical examples include Si and

GaAs. The conductivity of semiconductors could be altered by changing the

temperature, doping with carriers or illumination with light. Atoms consisting of

solid-state semiconductor could not be treated as single entity like hydrogen

atoms, because they interact strongly with other nearby atoms. Thus, the

6

conduction electrons in semiconductor are not bound to single atom, they

collectively belong to all atoms as a whole. In addition, atoms in lattice structure

apply periodic potential on the electrons, the solutions to the Schr¨odinger

equations for the electron energy form energy bands. In each band, a great deal

of discrete energy levels are densely packed together, which could be well

approximated as continuum. The conduction and valence band are separated by

a bandgap energy Eg. The bandgap energy is an important parameter when

describing the electronic and optical properties of materials, and the value depends

on material. For example, the Eg is 1.2 and 1.42 eV (electron volts) for Si and

GaAs at room temperature, respectively[14].

The electrons in the semiconductor obey the Pauli exclusion principle, this

principle says that two or more electrons could not occupy the same quantum state

and electrons fill up the lowest available energy level first. At absolute zero

temperature, the valence band is fully occupied while the conduction band is

empty, thus material is not conductive at all. However, with increasing temperature,

some electrons will be thermally excited to transit from valence band onto

conduction band leaving behind some unoccupied quantum states called holes.

The electrons in the conduction bands act as mobile carriers and the unoccupied

states in valence band allow other electrons to exchange places with applied

electric field. Thus, the holes left in the valence band could be regarded as carriers

with positive charge. The overall effect is that every electron excitation creates

mobile carriers in both conduction and valence bands, free electron and hole,

respectively. The conductivity of semiconductor materials increases sharply with

temperature as more and more charge carriers are generated[14].

Under certain excitation condition such as light illumination, exciton might

be formed. Exciton is a bound electron-hole pair, the electron and hole interact

with each other through Coulomb forces, similar to hydrogen atoms. There are two

basic types of excitons, free excitons and tightly bound excitons. The free excitons

have large radius and are delocalized states, thus they can move freely throughout

crystal. In contrast, tightly bound excitons have small radius and are bound to

specific atom or molecule. Excitons can only exist in stable form when their

7

attractive potential is large enough to protect them from collisions with phonons,

the energy of these bound states is called binding energy. Excitons play an

important role in determining the electronic and optical properties of

semiconductors, especially for low dimensional ones [15].Other hybrid particle

such as trion (bound states of two electrons and one hole, or one electron and two

holes) or biexciton (bound states of two exciton) might be formed as well in some

semiconductor systems[14].

2.2.2. Direct and Indirect bandgap

Based on the band structure, semiconductor materials could be categorized

into two groups: direct- and indirect-bandgap materials. Direct-bandgap materials

refer to semiconductors that have the same wave number k (momentum) for the

conduction-band minimum and the valence-band maximum energy. Materials that

do not satisfy this condition are indirect bandgap. GaAs has indirect bandgap while

Si does not. Having a direct bandgap or not makes a significant difference for

semiconductors, especially when used as emitters. This is because electron

transition from the conduction to valence band in indirect-bandgap materials must

involve substantial momentum change of electrons, which requires much more

efforts compared to direct-bandgap ones. For example, GaAs is good light emitter,

while Si is not[14].

2.2.3. Carrier Recombination and Photoluminescence (PL)

PL is the light emitted by a system following the absorption of photons. In a

semiconductor, different mechanisms could lead to absorption and emission of

light. The main ones are listed below:

• Inter band transition: An absorbed photon could enable electrons to transit

from the valence band to conduction band, creating an electron-hole pair.

The combination of electrons and holes will be accommodated with photon

emission[14].

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• Impurity-to-band transition: This process usually happens in doped

materials. Absorption of photon could enable transition between a dopant

and bands. The recombination process might be accompanied with

radiative photon emission[14].

• Excitonic transition: The absorption of photon could enable the formation of

exciton. The recombination of the electron and hole might result in photon

emission, called exciton annihilation. Recombination of hybrid particles

such as trion and biexciton might be involved in radiative emission too[14].

The above processes might also involve non-radiative processes, for

example, inter band transition might be assisted by one or a few phonons. There

are also other non-radiative processes such as intra-band transition (transition

inside bands) and phonon transition. The internal quantum efficiency ηi for photon

emission of a semiconductor material is defined as the ratio between the radiative

electron-hole recombination rate and total recombination rate. The internal

quantum efficiency is an important parameter to describe the light emission

efficiency of a material. Usually, it is expressed in the form:

ηi=𝑟𝑟

𝑟=

𝑟𝑟

𝑟𝑟+𝑟𝑛𝑟 -------------------------------------(1)

where, r = rr + rnr is the total recombination rate, rr and rnr are the radiative and

nonradiative recombination rate, respectively[14].

The total probability of recombination is given by the sum of the radiative

and non-radiative Probabilities where r and nr are radiative and non-radiative

lifetime. The relative probability of radiative recombination is given by radiative

probability over the total probability of recombination.

During the non-radiative recombination, the electron energy is converted to

vibrational energy of lattice atoms, i.e. phonons. Thus, the electron energy is

converted to heat. Most common cause for non-radiative recombination events are

defects in the crystal structure. These effects include unwanted foreign atoms,

native defects, dislocations. All such defects have energy level structure that are

different from substantial semiconductor atoms and it’s quite common for such

defects to form one or several energy levels within the forbidden gap of the

9

semiconductor. Energy levels within the gap of the semiconductors are efficient

recombination centers, in particular if the energy level is close to the middle of the

gap. Trap-assisted recombination occurs when an electron falls into a “trap”, this

is an energy level within the band-gap caused by the presence of a foreign atom

or a structural defect. Once the trap is filled it cannot accept another electron. The

electron occupying the trap, in a second step, falls into an empty valence band

state, thereby completing the recombination process.

Atoms at the surface cannot have the same bonding structure as bulks

atoms due to the lack of neighboring atoms. Thus, some of the valence orbitals do

not form a chemical bond. These partially filled electron orbitals, or dangling bonds,

are electronic states that can be located in the forbidden gap of the semiconductor

where they act as recombination center. Surface recombination leads to a reduced

luminescence efficiency and also to a heating of the surface due to non-radiative

recombination at the surface. Both effects are unwanted in electro luminescent devices.

Surface recombination can occur only when both type of carrier are present. It is

important in the design of LEDs that the carrier-injected active region, in which

both type of carriers are presented, be far removed from any surface. Just as for

surface recombination, non-radiative bulk recombination and Auger recombination

can never be totally avoided. Any semiconductor crystal will have some native

defects. It is also difficult to fabricate materials with impurity levels lower than the

parts per billion range (ppb). Thus, even the purest semiconductors contain

impurities in the 1012cm-3. The internal quantum efficiency gives the ratio of the

number of light quanta emitted inside the semiconductor to the number of charge

quanta undergoing recombination. Not all photons emitted internally may escape

from the semiconductor due to the light escape problem, re-absorption in the

substrate, or after re-absorption mechanism.

2.3. Crystal Lattice Band Structure

TMDs appear in the form of MX2, with transition metal M from group IV (Ti,

Zr, Hf), group V (V, Nb, Ta) or group VI (Mo, W) covalently bonded with chalcogen

10

X (S, Se, Te). TMD layers are weakly bounded by vdW interactions, each

consisting transition metal atoms sandwiched between two layers of chalcogenide

atoms, forming a X-M-X structure in vertical direction [16]. Bulk TMDs exhibit

diverse electronic properties, ranging from metals to semiconductors to insulators,

depending on the metal type. For example, MoX2 and WX2 compounds are

semiconducting while NbX2 and TaX2are metallic [17]. The diversity in TMDs

properties arises from the differently filled non-bonding d bands of the transition

metals [16], [17]. Monolayer TMDs usually exhibit only two polymorphs, trigonal

prismatic (2H) or octahedral (1T) phase[17]. MoS2 and WS2, which are the main

focus of this thesis, are most commonly found in 2H phase.

One prominent feature of TMDs is the layer dependence of their band

structure. The band structures calculated from density functional theory (DFT) of

both MoS2 and WS2 [18]. Whilst in bulk form MoS2 and WS2 have indirect band

gaps of ~1.2 eV, in monolayer form they exhibit direct band gaps of 1.9 eV and 2.1

eV respectively. According to theoretical calculations, the valence band maximum

(VBM) for bulk TMDs is at Γ point and gradually shifts to K point in monolayer

TMDs, while the conduction band minimum (CMB) shifts from mid-way between Γ

and K points to K point [18] A direct band gap allows the electron-hole pair

recombination process to occur without the involvement of phonons. Therefore, a

direct band gap results in a greater efficiency in photon generation from an excited

state. For this reason, monolayer MoS2 and WS2 are observed to have strong

photoluminescence effect upon optical excitation [19], [20].

So far, we have introduced some fundamental concepts in semiconductor

optics including bandgap, exciton, internal quantum efficiency and so on. In the

following section, we will focus on discussing the properties of 2D TMDs.

2.4. Properties of TMDs

TMDs have extraordinary properties which make them attractive materials

for numerous studies and applications. Basically, all exceptional features of

graphene are based on perfect honeycomb structure with hybridization of sp2[21].

11

While two-dimensional TMD crystal’s properties are differing from graphene’s and

drastically depend on their thicknesses. Some of these important properties of

TMDs’ are summarized below.

2.4.1. Electrical and Electronic Properties

Among all properties of two-dimensional TMD crystals, the most intriguing

properties are electronic, which mainly depends on the thickness of the material

[22]. Semiconducting property of TMDs is arising from the band-gaps, which

ranges from 500 meV to 2 eV depending on the layer number [22]. Thus, band-

gap of bulk MoS2 and WS2are 1.2 eV and 1.3 eV, while band-gaps for the single

layers of the same crystals shift to 1.9 eV and 2.1 eV, respectively [22]. In addition,

with the reduction of material thickness, the indirect band-gap changes to the direct

band-gap [22]. As an example, monolayer WS2 has a direct bandgap of 1.9 eV,

while bulk WS2 possesses an indirect bandgap at 1.3 eV. The direct band gap

results in a sharp photoluminescence emission at ~ 1.9 eV from monolayer WS2.

Single-layer WS2 exhibits a much stronger PL emission than that of bilayer WS2

and the WS2 bandgap decreases as its thickness increases.

Electron mobilities of single-layer MoS2 and WS2 are in the range of 11000

cm2 V-1 s-1 and 40-200 cm2 V-1s-1, respectively [23]–[26]. Unfortunately, TMDs have

lower mobilities compared to other conventional semiconductors with a similar

band-gaps like InP and GaAs. It should be noted that there is no significant

difference between carrier mobility of CVD grown and exfoliated TMDs. Besides

the grain boundaries in TMDs have less influence on charge transport compared

to point defects. Consequently, it was suggested that CVD-grown TMDs have a

comparable quality to mechanically exfoliated samples. Also, electronic structure

of TMDs can be modified by applying the external electric field[22]. Another

interesting property is, compared to bulk counterparts, 2D TMDs are highly

sensitive to environmental perturbations, because of the high surface-to-volume

ratio and exposed bonds [22]. This feature allows playing with its electronic

properties by the surface modification by chemical functionalization [22].

12

The influence of layer number on band structure is due to the quantum

confinement effect and the change in orbital hybridization between f orbitals of W

atoms and pz orbitals of S atoms. Unlike graphene that the lattice is all occupied

by carbon atoms, the A and B sub-lattices of in WS2 lattice structure are occupied

by W atoms and a pair of S atoms [18]. The difference between A and B sub-

lattices results in the lift of the decency at K (K)’ points in the Brillouin zone and

creates a desirable bandgap in WS2.

In addition, tunability of photoluminescence (PL) is another important

property of TMDs therefore Monolayer TMDs are prospective candidates for

optical emitters, optoelectronic and photovoltaic devices owing to the high PL

intensities [22]. The PL intensities of bulk TMDs are lower than that for monolayer

samples and can be tuned by surface modification[22], [27].

2.4.2. Thermal Properties

As it was mentioned before, TMDs have many applications and electronic

devices are one of them. Generally, electronic devices need components with

good thermal management for better performance. Heat generated during the

operation of the device must be dissipated.

Factors such as defects, edges and isotropic doping can affect the thermal

conductivity of TMDs due to phonon scattering. For this reason, the thermal

conductivity of TMDs is very sensitive to the presence of vacancies and Stone-

Wales (SW) defects [21], [28], [29]. The thermal conductivity of TMDs shows high

structure dependence. Muratore et al. demonstrated that in layered TMDs thermal

transport characteristics along a cross-plane direction is influenced by phonon

scattering at domain boundaries[28], [30].

2.4.3. Chemical Properties

TMDs have versatile chemistry. Defects increase the reactivity and different

groups (oxygen, carboxyl, hydroxyl, hydrogen) can be attached to vacancies with

dangling bonds. Hence it has many potential applications in the field of catalysis,

13

energy storage, water-splitting and electrocatalytic hydrogen evolution reaction

(HER)[17], [28]. The absence of dangling bond makes layer stable against

reaction with surrounding species. The chemistry of material depends mainly on

the edge termination (coordination bond type), which can be either by M or X [17],

[28]. It was calculated by DFT, that TMDs sheets have an active edge, which can

be used in HER [17], [28].

2.4.4. Mechanical Properties

Mechanical properties of 2D materials play a significant role for their

applications. In recent years flexible electronic devices have received a great

interest and 2D materials are the most promising candidates. However, applied

strain and other external forces can modify the structure of crystalline TMDs, hence

affect the performance and lifetime of devices [28], [29], [31]. Consequently, the

mechanical properties of these materials must be well studied.

The mechanical and electrical properties of representative TMDs materials

WS2 and MoS2, have shown a high dependence on the applied tensile strain [32].

The PL and band-gap of monolayer WS2 crystals decrease with a strain but band-

gap remains direct, whereas in multilayer WS2 and monolayer MoS2 crystals, a

transition of direct band-gap to the indirect occurs [28], [32]. In addition, when the

strain was applied to monolayer MoS2 relatively rapid drop in PL and band-gap

were observed[32]. Thus, WS2 is more attractive for the flexible devices. In spite

of the intensive studies on 2D materials, the experimental measurements of the

mechanical properties of TMDs still remain few. Several groups have measured

Elastic Modules of WS2 and MoS2 by nano-indentation method [28], [32].

2.4.5. Young`s Modulus

A modulus can be defined as the numerical value (constant) representing a

physical property of the material or reaction of material to the external forces.

Modulus of elasticity or Young`s Modulus (E) is the mechanical property of a

material, which shows how stiff is the material and given as a ratio of stress (σ) to

14

strain (δ).Stress is defined as force (F) per unit area (A), while strain is a ratio of

elongation (ΔL) of material to its original length (L), respectively (Adilbekova, 2017;

Callister &Rethwisch, 2011).

Liu et al. have measured the 2D moduli of WS2 and MoS2 by AFM

nanoindentation as 177±12 and 171±11N/m, respectively [32]. Since 2D Young`s

Modulus of graphene is ~340 N/m, values for WS2 and MoS2 are about the half of

graphene’s[32]. Elastic properties of the heterostructures of graphene, WS2, and

MoS2 also were measured, it appeared that values are lower than the summed

modulus of the hetero-layers [28], [32].

2.4.6. Light-Emitting properties of 2D TMDs

2.4.6.1. PL properties

As discussed above, the monolayer TMDs cross over to become direct

bandgap semiconductors and show strong excitonic PL emission at the atomic

level [20], [34]. According to [34]the PL spectra from a monolayer and a bilayer

MoS2 at room temperature, where we could observe that the single layer exhibits

PL orders of magnitude stronger than that of the bilayer. There is more, the PL

spectral position and intensity from 2D TMDs could be tuned by electrical

gating[35], chemical doping[36], temperature [37], composition[38] and so on.

Light-emitting diodes (LED) based on 2D TMDs have shown great potential to be

used as excitonic emitters, which are based on electron hole recombination.

Different types of LEDs such as Schottky junctions [39], p-n junctions [40]–[42] and

vertical tunnel junctions have all been demonstrated. Low threshold down to a few

nano amps [42], [43] and external quantum efficiency up to 10% [44] make these

TMDs-based LEDs suitable for future optoelectronic applications.

2.4.6.2. Challenges facing 2D TMDs for Photonic Applications

Though 2D TMDs possess extraordinary optoelectronic properties and

show great potential for applications in photonics, such as light-emitting device, a

number of challenges still remain. Firstly, the PL quantum yield (QY) from

15

monolayer TMDs measured so far is much lower than the expected value for a

direct-gap semiconductor. For example, the value reported from monolayer MoS2

is only around 0.004 [34]. Secondly, the atomic thickness of such 2D TMDs

restricts their interaction length with light, which limits some applications and the

efficiency. Besides this, controlled large-scale growth is also one of the main

challenges.

2.5. Generation of Defects in 2D TMDs

Defects in 2D materials can appear during three processes given below:

1. During the TMDs growth

2. During irradiation with energetic particles (electrons or ions)

3. Chemical treatment

Structural Defect-dependent Properties Defects in TMD layered materials can be

classified as zero-dimensional, one-dimensional and two dimensional defects

[45],. Zero-dimensional defects are the most abundant defects in TMDs, including

point defects, dopants, or non-hexagonal rings. One-dimensional defects contain

grain boundaries, edges, and in-plane heterostructures. Layer stacking of different

TMDs, wrinkling, folding, and scrolling are assigned to two dimensional defects.

Structural defects in the crystal lattices of TMDs can significantly change their

physical and chemical properties. For example, sulfur vacancies, the most

common defects in chemically synthetic and mechanically exfoliated TMDs

monolayers due to the lowest formation energy of these defects, introduce

unpaired electrons into the lattice, resulting in a n-doping effect on the material.

These sulfur vacancies create additional density of states within the band gap W.

Zhou et al., 2013), and further alter the electrical transport properties of TMDs. As

1D defects in TMDs, visible light emissions from the edges of CVD-grown WS2

single-crystalline domain show similar or higher intensities compared to the interior

regions [46]–[48].

16

Chapter 3: Background & Literature Review of CVD growth of 2D monolayer, Characterization & PL Enhancement

Chapter 3 gives us an overall view of existing techniques for synthesizing and

growing 2D TMDs. Chapter 3 is divided into following sections:

3.1. Different production methods of 2D TMDs including CVD which has been

focused broadly in this chapter

Techniques that are available in our lab is mechanical exfoliation and CVD. Before

diving into any approach, the background study gives us good understanding of

pros and cons of different techniques. Moreover, CVD growth mechanism and

lateral size of crystals depend on following factors:

➢ Precursor amount

➢ Growth Temperature

➢ Ramp rate

➢ Holding Time

➢ Gas Type

➢ Pressure and Flow rate

➢ Separation between top-bottom substrate

➢ Distance between precursors WO3 and Sulphur

Section 3.1 is important for realizing how these factors are playing important role

regarding CVD. Furthermore, this section primary help to construct and design

experimental details.

3.2. Different characterization techniques for 2D TMDs

After growing monolayer TMDs, the first thing we are supposed to perform is

characterization to see if the material we are focusing on has been grown

successfully or not. This section discusses about how each characterization works

and why we need to perform these characterizations. It also focuses on different

17

characterization results that have been achieved so far and we can also compare

how our results are consistent with literature.

3.3. PL enhancement of TMDs

Different groups of different parts of the world are working experimentally on

enhancing PL of TMDs due to its weak, inhomogeneous, non-uniform light

emission. This section helps to figure out unique and novel reagents that can be

used considering how safe the reagent is. Acetone, TFSI, HI, Na2S etc. are all

being used to manipulate light emission properties of 2D TMDs.

3.1. Production Methods of 2D TMDs Crystals

2D materials are structurally planar materials that display highly anisotropic

properties, having different in-plane and out-of-plane characteristics. Atomically

thin layers, are known to exhibit novel properties that differ from their bulk.

Individual layer of TMDs can be obtained using top-down or bottom-up

approaches. The top-down strategy commonly involves exfoliation from the bulk

layered crystals such as mechanical exfoliation, chemical exfoliation, solution

based exfoliation, laser thinning while bottom-up approach grows the crystals

through vapor deposition such as chemical vapor deposition (CVD). In this section,

an overview of the techniques within these two categories will be given.

3.1.1. Exfoliation

3.1.1.1. Mechanical Exfoliation

Graphene was first exfoliated by the “scotch-tape” method, which involves

peeling off layers of carbon atoms from graphite using an adhesive tape. The

cleaving process is repeated over again until all that remains are one or several

layer(s) of graphite. Mechanical exfoliation is now generally applied to produce 2D

TMD crystals beyond grapheme[49]–[51]This method yields high quality layers

free from dopants that being introduced from chemical processes. Therefore,

mechanical exfoliation is ideal for studies on intrinsic physical properties of 2D

18

materials and the fabrication of proof-of-concept (PoC) devices. However, the

disadvantages of this method are its improbable industrial scaling and limited

crystal sizes.

3.1.1.2. Solution-based and Chemical Exfoliation

Sonicating bulk layered materials in liquids provides a promising route to

obtain large quantities of exfoliated nanosheets. The nanosheets dispersed in

solution can be easily applied in material coating, inkjet printing and the making of

composites or hybrids using a mix of dispersed materials. Direct sonication of

layered materials in common solvents such as N-methylpyrrolidone (NMP) and

dimethylformamide (DMF) have been reported to obtain few-layer graphene, BN,

MoS2 and WS2[52]–[54]. These direct sonication methods require solvents with

high surface energies to overcome the cohesive energy between the adjacent

crystal layers and face difficulty for high-yield production of monolayer flakes.

One of the most effective ways to improve yield is through intercalation of

the crystals using ions. The typical procedure involves soaking bulk TMD powder

in lithium-containing solution, followed by exposing the intercalated material in

water [55], [56]. A vigorous reaction of water with lithium between the layered

material produces H2 gas and separates the layers more easily. Further

optimization has been made for faster and more controllable lithium intercalation

by using a lithium foil anode and TMD-containing cathode[57]. The use of Li-ion

exfoliation gives a higher yield to quality monolayer nanosheets [56]. However, the

flammability of Li compounds, as well as the increasing price of Li resources look

for alternative intercalants. In general, liquid exfoliation methods can be useful for

solution-based or printable electronics, yet it may unavoidably introduce extrinsic

defects or alter crystal structure of thin TMDs and require additional post-treatment

steps for lattice reconstruction [56].

3.1.1.3. Chemical Vapor Deposition (CVD)

As discussed previously, the mechanical exfoliation of TMDs materials may

produce high quality crystals, but it is difficult to scale up due to its labour-

19

intensiveness and low-yield nature. Liquid-exfoliation may lead to a higher yield,

but it still faces limitations in its solution-based processes, smaller crystal sizes and

varying qualities. For these reasons, bottom-up approach using chemical vapor

deposition (CVD) may be the only scalable route in obtaining high-quality, large-

area and continuous 2D crystals necessary for wafer-scale device fabrications

[48]. All 2D crystals used within the scope of this thesis were made with CVD and

the detailed processes for growing monolayer WS2 will be described in Chapter 4.

3.1.1.3.1. CVD Growth of MoS2 and WS2

CVD methods have also been utilized for obtaining large-area ultrathin TMD

crystals with controllable thickness and domain sizes. Monolayer TMDs usually

come in a typical triangular morphology, with side lengths of the triangular domains

reaching over 100 μm[58]. The reported methods can be classified into three main

categories based on their growth techniques:

Vaporization and decomposition of metal oxide and chalcogen precursors and

deposition of TMD on a substrate: [59]–[61].

Lee et al. in 2011 for MoS2 growth. MoO3 and S powders were used as

reactants to synthesize MoS2 directly on SiO2 substrates [90]. Graphene-like

molecules such as reduced graphene oxide (rGO), perylene-3,4,9,10-

tetracarboxylic acid tetrapotassium salt (PTAS) and perylene-3,4,9,10-

tetracarboxylic dianhydride (PTCDA) were used to pre-treat the SiO2 substrate for

promoting growth of MoS2[59]. Further progress was made when Lee et al. applied

a similar technique to other substrates including quartz and sapphire [60]. Around

the same time, Zande et al. developed a method to grow MoS2 on ultraclean SiO2

substrate without seeding pre-treatment, revealing the critical roles that surface

cleanliness and smoothness of substrate play in CVD-growth of TMD crystals[61].

Direct Sulfurization of metal films:

In 2012, Zhan et al. accomplished the growth of MoS2 thin film by direct

transformation of Mo thin layer into MoS2 by reacting with sulphur under elevated

temperature [62]. In this way, the obtained MoS2 film is determined by the

20

thickness of the deposited Mo film, which is precisely controlled by an E-beam

evaporator. This provides a way for preparing large-area high-quality MoS2 films

with controllable thicknesses [62].

Conversion of metal oxide to metal disulphide through sulfurization:

Synthesis of thin-layer MoS2 was achieved on sapphire substrate with

thermally deposited MoO3 thin films with desired thickness. Similar to the previous

approaches, sulphur was introduced into the furnace under high temperature over

1000ºC for sulfurization which results in a few-layer MoS2 film [63].

3.1.1.3.1.1. Effect of Different Parameters and Growth Mechanism

In 2015, Bilu Liu et al. demonstrated evolution of different growth features

of WSe2 such as triangular, few layer truncated triangle and hexagon with curved

edges[64]. Other features are as important as monolayer triangular considering

their electronic, magnetic and catalytic properties. Growth temperature could affect

WSe2 growth in several manners, for example, the sublimation speed and

therefore the concentrations of WO3-x and Se sources, mobility and therefore

diffusion rate of atoms and active species on substrates during WSe2 growth,

potential shift between kinetic controlled and thermodynamic controlled growth

behavior during WSe2 and other TMDs growth etc. According to their study, at very

low temperatures, the amount of source materials sublimated would be very few

and thus, the concentrations of reactants would be low. Moreover, low temperature

would lead to less mobile active reactants, which made them difficult to diffuse

overgrowth substrate and difficult to add at the growing edges of 2D flakes.

Instead, it was energetically preferable to grow into three-dimensional structures

to compensate the low mobile nature of the active species. At a fixed growth

temperature and amount of source materials, increasing growth time would not

change the layer number and shapes of WSe2, instead, it would increase their

lateral sizes within certain period. The layer numbers and shapes of 2D flakes were

mainly related to the concentrations of the source materials and growth kinetics of

WSe2 flakes, which were sensitive to the growth temperatures and mass of source

materials, not the growth durations.

21

Figure 1: Amplitude AFM images showing shape evolutions of CVD-grown WSe2 flakes at different growth temperatures of (a) 900⁰C, (b) 950⁰C, (c and

d) 1025⁰C, (e and f) 1050⁰C. Unusual, non-triangle shapes are gradually found as the growth temperature increases. (a and b) Monolayer triangles

with different sizes; (c and d) thin few layer truncated triangle and hexagon with curve edges; (e and f) thick few layer triangle and hexagon with

straight edges. Reprinted (adapted) with permission from Liu, B., Fathi, M., Chen, L., Abbas, A., Ma, Y., & Zhou, C. (2015). Chemical vapor deposition

growth of monolayer WSe2 with tunable device characteristics and growth mechanism study. ACS Nano, 9(6), 6119–6127. Copyright 2015) American

Chemical Society.

Figure 2: Effect of growth temperatures on the sizes and layer numbers of CVD-grown WSe2. Optical microscopy images of WSe2 flakes grown at (a)

22

850⁰C, (b) 900⁰C, and (c) 1050⁰C. The growth durations are 15 min for all cases. (d) The correlation of average WSe2 flake sizes and layer numbers

with growth temperatures. The vertical error bars indicate standard deviations of the flake sizes in statistical analysis. Reprinted (adapted) with

permission from Liu, B., Fathi, M., Chen, L., Abbas, A., Ma, Y., & Zhou, C. (2015). Chemical vapor deposition growth of monolayer WSe2 with tunable device characteristics and growth mechanism study. ACS Nano, 9(6), 6119–

6127. Copyright (2015) American Chemical Society.

Figure 3: Effect of growth durations on the sizes of CVD-grown monolayer WSe2. Optical microscopy images of WSe2 grown for (a) 1 min, (b) 5 min, and (c) 5 h. The growth temperatures are 950⁰C for all cases. (d) Plot of

average flake sizes versus growth durations of 1 min, 3 min, 5 min, 10 min, 15 min, 30 min, 60 min, and 5 h. The vertical error bars are standard

deviations in statistical analysis. Reprinted (adapted) with permission from Liu, B., Fathi, M., Chen, L., Abbas, A., Ma, Y., & Zhou, C. (2015). Chemical

vapor deposition growth of monolayer WSe2 with tunable device characteristics and growth mechanism study. ACS Nano, 9(6), 6119–6127.

Copyright 2015) American Chemical Society.

23

Figure 4: Shape Evolution of CVD WSe2 with increased Temperature. Reprinted (adapted) with permission from Liu, B., Fathi, M., Chen, L.,

Abbas, A., Ma, Y., & Zhou, C. (2015). Chemical vapor deposition growth of monolayer WSe2 with tunable device characteristics and growth

mechanism study. ACS Nano, 9(6), 6119–6127. Copyright 2015) American Chemical Society.

Qingliang Feng et al. (2018) reported NaCl-assistant method for controlled

growth of single crystal monolayer WSe2 with a domain size up to 0.57 mm on

SiO2/Si substrate [65]. The growth thermodynamic of the NaCl-assistant synthesis

of monolayer WSe2 was investigated by tuning the growth temperature from 720

to 1000 °C. The loading mass of NaCl powder was preferred to be more little (10%

NaCl) for a suitable vapor of WO3 precursor and gas flow of H2 was optimized to 5

sccm. The thermodynamic and morphology evolution were both investigated and

showed a similar mechanism and evolution process as monolayer MoS2(1−x)Se2x.

Sang Yoon Yang et al. (2017) reported an effective method for achieving a

broad range of shape evolution in CVD-grown monolayer MoS2 flakes(Yang et al.,

2017). By controlling the S and MoO3 temperatures, the shape of the monolayer

MoS2 flakes was varied from hexagonal to triangular via intermediate shapes such

as truncated and multi-apex triangles. In their study, the shape evolution of the

MoS2 flakes could be explained by introducing the term “nominal Mo:S ratio”, which

referred to the amount of loaded MoO3 and evaporated S powders. By using the

nominal Mo:S ratio, they predicted the potential reaction atmosphere and

effectively controlled the actual proportion of MoO3−x with respect to S in the growth

region, along with the growth temperature. From their systematic investigation of

the behavior of the shape evolution, they developed a shape-evolution diagram,

which could be used as a practical guide for producing CVD-grown MoS2 flakes

with desired shapes.

[66]showed a schematic of the shape evolution of the MoS2 flakes with

respect to the nominal Mo:S ratio and growth temperature. This diagram provided

a guide for setting a starting point for the growth conditions by using the nominal

Mo:S ratio and growth temperature and adjusting these conditions to achieve MoS2

flakes with a desired shape. For example, the triangular MoS2 flakes with a

continuous film indicated that the growth conditions correspond to the point where

24

the nominal S-rich conditions and high growth temperature, which in turn reflected

the actual conditions with sufficient fluxes of both the precursors and the S-rich

atmosphere in the growth region. Based on their study, starting from this point, the

shape and film state of MoS2 could be tuned to isolated hexagonal flakes (or

isolated flakes with a three-point star shape) by decreasing the nominal Mo:S ratio

(or growth temperature) while keeping the growth temperature (or nominal Mo:S

ratio) constant.

This diagram could also be utilized for the formation of triangular MoS2 flakes.

The triangle was the most reported shape in two-element TMDs. They concluded

that:

(i) If a three point star shape is obtained by the current round of growth

(region I), one can reduce the nominal proportion of S with respect to

Mo (vertical shift from region I to region II) or increase the growth

temperature (horizontal shift from region I to region II).

(ii) If the current round of growth produces truncated triangular/hexagonal

flakes (region III), the transition to a triangular shape can be achieved

either by increasing the nominal proportion of S with respect to Mo

(vertical shift from region III to region II) or by reducing the growth

temperature (horizontal shift from region III to region II).

(iii) The transition from isolated triangular flakes to their merged state can

be controlled by increasing both the nominal Mo:S ratio and the growth

temperature (diagonal shift within region II). At a low growth temperature

and nominal Mo:S ratio, a low substrate temperature can enhance the

adsorption and nucleation process, as the growth is limited by a low

precursor flux, resulting in small, isolated MoS2 flakes with a relatively

high nucleation density. As the growth temperature and nominal Mo:S

ratio increases, the chance of desorption on the substrate increases

(negative effect on nucleation). On the other hand, the nucleation can

be enhanced by the increased precursor flux (positive effect on

nucleation). Therefore, it appears that competition occurred between

those two effects, resulting in isolated, larger flakes with a low nucleation

25

density. Despite the low nucleation density, an increased precursor flux

can contribute to the formation of a (semi) continuous MoS2 film by

facilitating the growth of isolated flakes and the merging process

between them.

Youmin Rong et al. (2014) showed that controlling the introduction time and

the amount of sulphur (S) vapor relative to the WO3 precursor during the CVD

growth of WS2 was critical to achieving large crystal domains on the surface of

silicon wafers with a 300 nm oxide layer[67]. They used a two furnace system that

enabled the S precursor to be separately heated from the WO3 precursor and

growth substrate. Accurate control of the S introduction time enabled the formation

of triangular WS2 domains with edges up to 370 mm which were visible to the

naked eye. They mentioned that one major challenge for growing continuous

sheets of monolayer WS2 using the current approach is that the S vapor reacts

with the bulk WO3 precursor and turns it into WS2 bulk powder. This resulted in the

quenching of the WO3 precursor and the CVD growth of WS2 domains stop, limiting

their size. The key to moving this forward would be the ability to introduce S and

WO3precursors into the growth chamber separately and therefore the WO3 bulk

powder would not quench and this should lead to the continuous growth.

Pengyu Liu et al. (2017) reported about High-quality WS2 film with the single

domain size up to 400 μm was grown on Si/SiO2 wafer by atmospheric pressure

chemical vapor deposition[68]. The effects of some important fabrication

parameters on the controlled growth of WS2 film have been investigated in detail,

including the choice of precursors, tube pressure, growing temperature, holding

time, the amount of sulfur powder, substrate position and gas flow rate. By

optimizing the growth conditions at one atmospheric pressure, they obtained

tungsten disulfide single domains with an average size over 100 μm.

Dong Zhou et al. (2017) presented a systematic spectroscopic study of

CVD-grown MoS2 and two types of MoS2 flakes have been identified: one type of

flake contains a central nanoparticle with the multilayer MoS2 structure, and the

other is dominated by triangular flakes with monolayer or bilayer structures(D.

Zhou et al., 2018). Their results demonstrated that two types of flakes could be

26

tuned by changing the growth temperature and carrier-gas flux, which originates

from their different nucleation mechanisms that essentially depends on the

concentration of MoO3−x and S vapor precursors: a lower reactant concentration

facilitates the 2D planar nucleation that leads to the monolayer/bilayer MoS2 and

a higher reactant concentration induces the self-seeding nucleation which easily

produces few-layer and multilayer MoS2. The reactant-concentration dependence

of nucleation could be used to control the growth of MoS2 and understand the

growth mechanism of other TMDs. The deep understanding of nucleation and

growth mechanisms is fundamental for the precise control of the size, layer number

and crystal quality of two-dimensional (2D) transition-metal dichalcogenides

(TMDs) with the chemical vapor deposition (CVD) method.

Kyung Nam Kang et al. (2015) demonstrated CVD growth of continuous

monolayer WS2 films with up to 433μm2 single crystals and mm2 size continuous

polycrystalline films and have furthermore elucidated the effect of the

concentration of H2during the reduction and sulfurization process[70]. They have

shown that controlling H2 concentration was crucial for large area WS2 deposition.

In the presence of an Ar carrier gas, increasing the local pressure, the crystal size

varied relatively little (a few micrometers) between low and high flow rates. In a H2

only environment, the flow rate had a dramatic effect on growth. In addition, in the

conditions of a high concentration of H2 and a low concentration of sulfur gas, the

grown WS2 was etched. Several different growth formations (in-plane shapes)

were observed depending on the concentration of H2.

[71] proposed the mechanism of such CVD process for growth of large-

scale WS2 monolayer. Firstly, flakes of WOyS2-y were formed. Then, the further

sulfurization produces triangular shape thick WS2+x flakes with the mixture of WIV

and WVI. As the apex of the triangles could be very active sites for nucleation,

series of triangles formed and merged into a big triangle. With the continuous

heating, the thick WS2+x flakes started expanding and thinning, and eventually WS2

monolayers were fabricated. The formation of thin layers of WS2 started at the

center of the thick triangles since there has less overlapped small triangles and

was exposed to sulfur for longest duration. It should be noticed that such center

27

areas are also the most exposed regions after the sulfur source is exhausted

during the final heating and cooling stages, which may cause the loss of the sulfur

in the monolayers. It was well known that WVI couldn’t be directly sulfurized by S

unless some intermediate wereformed. Therefore, they thought the transferring of

WVI to WIV at the initial growth stage facilitates the growth of large-scale single

crystals of WS2 monolayer under a relatively relaxed condition.

Mei Er Pam et. al (2019) reported effects of stoichiometry of transition metal

oxide precursors on the growth of TMD monolayers have not been studied yet.

[72]. They reported the critical role of the WO3 precursor pre-annealing process on

the growth of WS2 monolayers. Besides, several WO3 precursors with different

types of oxygen vacancies have also been prepared and investigated by X-ray

powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and density

functional theory calculation. Among all the non-stoichiometric WO3 precursors,

thermally annealed WO3 powder exhibits the highest oxygen vacancy

concentration and produces WS2 monolayers with significantly improved quality in

terms of lateral size, density and crystallinity.

3.2. Overview of Characterization Methods of CVD Grown Monolayer WS2

Generally, several methods are used for characterizing 2D materials such

as optical microscopy, scanning electron microscopy (SEM), atomic force

microscopy (AFM), Raman spectroscopy, Transmission electron microscopy

(TEM). Generally, a combination of few techniques is used for characterization of

2D TMDs. These particular techniques outlined below are used for pristine WS2

etc. in chapter 4 and 5 in order to understand the results presented in this thesis,

the characterization methods are discussed because for successfully confirming

growth of monolayer WS2, characterizations are important in interpreting the

results that have been achieved in chapter 4 and 5 as well.

28

3.2.1. Optical Imaging

Optical microscopy is the cheapest and easiest non-destructive methods for

imaging samples. However, for TMDs samples, it is very important to choose

suitable substrate for better contrast. Silicon covered with dielectric SiO2 and Si3N4

is the most commonly used substrate, because of enhanced contrast. The

wavelength of incident light is another key factor for enhancing the contrast. As a

result, suitable selection of incident light and substrate makes it possible to image

monolayer TMDs in optical microscopy. If transparent or semi-transparent

substrates like sapphire (double-side polished), GaN (single side polished), quartz

is being used for CVD growth of monolayer TMDs, then for optical imaging, it is

useful to consider dark-field imaging option in optical microscope as well.

Moreover, to distinguish between a monolayer and multilayer TMDs, dark-field

imaging can be very effective. In general, nano-meter thick film of any material is

transparent to visible light. However, monolayers of 2D TMDs despite their

thickness of less than 1 nm, are visible optically, when placed on top of SiO2/Si

substrate with right thickness (90nm, 290-300nm). Observed by K.S. Novoselov

and his colleagues in Graphene for the first time, the change in optical path due to

these monolayers is enough to change the interference color with respect to the

bare substrates. Later on techniques like measuring the optical contrast as a

function of layer number has been established to determine the number of layers

present in TMDs. Similarly, the change in intensity (brightness value) in grey scale

image can also be used to differentiate the number of layers present in the sample.

3.2.2. Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS)

Another characterization technique is SEM, which gives an opportunity to

obtain more detailed images of samples at the nanoscale. Details like folds, tears,

wrinkles, patches, twisted edges and contamination can be seen in high-resolution

SEM images. SEM is mainly performed to see the morphology of a sample. It is

also very useful to study growth mechanism on different features. SEM is widely

29

used for imaging suspended TMDs and also TMDs on substrates like Si, copper

(Cu) foil, Si3N4/Si or SiO2/Si. In SEM, an electron beam systematically scans

across the surface of a sample. Electron interactions produce a variety of signals

which allow us to map out nanoscale surface structures. The primary mode of

imaging used was Secondary Electron. The EDS study involves elemental analysis

of a sample where X-ray is focused into the sample being studied. An X-ray

detector is attached with the SEM.

3.2.3. Transmission Electron Microscopy (TEM)

Generally, TEM is used for imaging samples with atomic resolution. In TEM

electron beam is transmitted through the suspended thin sample and collected in

detector. This method makes it possible to obtain the atomic structure of material

only at low operating voltages, since a high voltage, like 80 eV, is sufficient to

introduce defects and damage monolayer TMDs. Besides, TEM gives information

about the crystallinity such as single crystalline, polycrystalline and amorphous of

material and contamination can be easily distinguished. For performing TEM

characterization, one option is to transfer the grown flakes on TEM grids using

PDMS or PMMA. But the process itself is difficult to perform because of surface

interaction between grown monolayer TMDs and SiO2/Si substrate is strong and

the flakes tends to get broken due to strain when such transfer method is

performed. Another option is to directly grow TMDs on the TEM grids but such

TEM grids need to have Si-oxide/nitride support, such as SiO2 or Si3N4 and should

be resistant to high temperature degradation.

3.2.4. Raman Spectroscopy

Raman spectroscopy is the nondestructive and simple method, which

allows obtaining valuable information about the vibrations of crystal lattices and

accurately determines the layer number of TMDs. When a laser is incident on

matter, it interacts with molecular vibrations, phonons or other excitations in the

system and it scatters either elastically or in-elastically. The first process where the

30

scattering is elastic and the signal is very intense is called Rayleigh scattering.

Such a signal is always removed using a notch filter or a band pass filter and the

remaining light is dispersed into the detector for Raman analysis. The second

process of inelastic scattering which provides information about the vibrational

modes present in the system is called Raman scattering and comprises two

components. The Stokes Raman scattering, where energy of the scattered beam

is increased as it absorbs a phonon and Anti-Stokes Raman Scattering, where

energy of the scattered beam is decreased due to the excitation of phonon. As

general energy-level diagram showing the states involved in Raman spectra is

presented as in Figure 5. Raman spectra shows the intensity of the scattered light

as a function of the energy shift from the incident light, commonly known ‘Raman

shift’ with units of cm-1. The typical accuracy of measurements of Raman spectra

is 1 cm-1 which corresponds to ~0.1 meV, sufficient to determine the number of

layers present in the sample of TMDs. Apart from being used to identify the number

of layers, Raman spectra has been widely used in studying the stacking

sequences, crystal orientation, edge orientation, molecular doping, strain effects,

electrical doping effects and thermal effects. In short while performing Raman

Spectroscopy, an incident light (laser) sent to the crystal emit or absorb phonons

and scattered in materials. Hence, Stokes (photon loss) and Anti-stokes (photon

gains) are detected and analyzed.

Raman spectroscopy is principally based on the inelastic scattering of

electromagnetic waves caused by the photon-phonon interaction within the

materials. In typical set-up, a laser beam is irradiated on the specimen and the

scatter photons would be collected to measure the shifts in wavelength caused by

the inelastic scattering interactions.

31

Figure 5: Energy-level diagram showing involved states in Raman spectra.

Figure 6:Raman Spectra at different excitation wavelength (a) 488 nm, (b) 514 nm, (c) 647 nm. Reprinted (adapted) with permission from Berkdemir,

A., Gutiérrez, H. R., Botello-Méndez, A. R., Perea-López, N., Elías, A. L., Chia, C.-I., Wang, B., Crespi, V. H., López-Urías, F., & Charlier, J.-C. (2013).

Identification of individual and few layers of WS 2 using Raman

32

spectroscopy. Scientific Reports, 3(1), 1–8. Copyright © 2013, Springer Nature.

Figure 7: (a) Peak Frequency vs Number of Layers, (b) Intensity ratio vs Number of Layers[73]. Reprinted (adapted) with permission from

Berkdemir, A., Gutiérrez, H. R., Botello-Méndez, A. R., Perea-López, N., Elías, A. L., Chia, C.-I., Wang, B., Crespi, V. H., López-Urías, F., & Charlier,

J.-C. (2013). Identification of individual and few layers of WS2 using Raman spectroscopy. Scientific Reports, 3(1), 1–8. Copyright © 2013, Springer

Nature.

3.2.5. Atomic Force Microscopy (AFM)

The Atomic Force Microscope (AFM) is a very high-resolution scanning

probe microscopy with resolution capable of fractions of nanometer. AFM gives a

very clear imaging of the surface of the sample. The AFM uses a very sharp

cantilever tip to “feel” the surface and thus gives a map of topography of the sample

surface, in contrast to conventional microscopy where samples are imaged by

“looking”. It gives information on the texture or material characteristic; soft or hard,

springy or compliant, smooth or rough. The working principle of AFM is based on

changes in attraction and repulsion forces between material and tip caused by van

der Waals interactions. The tip (tip having radius of curvature on the order of few

nanometers) is the element that interacts with the sample and it is a micro-

fabricated, extremely sharp spike that is mounted on the end of a cantilever. The

cantilever on which the tip is mounted on allows it to move up and down as it tracks

the sample. The cantilever has a very low spring constant allowing the AFM to

33

control the force to a great precision. Both the tip and cantilever is usually made of

silicon or silicon nitride, as both materials are hard, resistant to wear. The imaging

mode used in this experiment is tapping mode, where the cantilever is made to

oscillate and the tip “taps” across the surface giving information on the topography.

When the cantilever is scanned over a sample surface, placed on top of a

piezoelectric holder, changes in the force between the tip and the sample leads to

a deflection in the cantilever. This deflection is detected using a laser source and

a photodetector. Signal gathered is then fed into the feedback electronics and is

processed into images. Such images are analyzed using special software, which

gives the actual height profile of the sample.

Apart from these, because of high sensitivity to small forces caused by

deformation of materials, AFM can be used for mechanical characterization of 2D

materials. Other modes of AFM allow determining magnetic, frictional, electrical

and elastic properties of samples.

3.2.6. X-ray Photoelectron Spectroscopy (XPS)

To characterize the chemical composition of CVD-grown TMDs, X-ray

photoelectron spectroscopy (XPS) is performed. XPS is a surface analysis

technique that is used to determine the quantitative atomic composition and

chemistry. It has a sampling volume that extends from the specimen surface to a

depth of approximately 50 – 70 Aº depending on the nature of the specimen. In

XPS analysis, the binding energy of the core level electrons can be estimated by

measuring the kinetic energy of the electrons emitted by the absorption of the x-

ray photons. Hence, the elemental identification can be done since core level

electrons' binding energy hardly shifts regardless of the chemical bonding.

In addition, by probing the binding energy of the outer shell electrons, XPS

is sensitive enough to determine the shifts of the energy level due to differences in

the chemical composition and hence detect the chemical stoichiometry of the

specimen. It is important to be able to determine the chemical states of the

specimen since some elements like Mo/W or S can have different chemical state.

The stoichiometry of the as-grown WS2 monolayer can be calculated by: [47].

34

[𝑾] / [𝑺] = 𝛌𝑺𝟐𝒑/𝝀𝑾𝟒𝒇× 𝝈𝑺𝟐(𝒉𝝊)/𝝈𝑾𝟒𝒇(𝒉𝝊) × 𝑰𝑾𝟒𝒇/𝑰𝑺𝟐𝒑------------------(2)

where σS2p(hν) and σW4f(hν) are photo-ionization cross sections of the 2p and 4f

core level of S and W, respectively, and λS2p and λW4f are inelastic mean free paths

of the photoelectrons with kinetic energies corresponding to the S and W core

levels, respectively. The values of these abovementioned parameters can be

obtained from literatures. Accordingly, the [W]/[S] ratio is estimated to be ~ 0.6,

suggesting ~ 20% sulfur vacancies in the CVD-grown monolayer of WS2. Since

the existence of sulfur vacancies, we expect that our grown WS2 monolayer is a n-

type semiconducting material.

Guru P. Neupaneet. Al. (2017) showed that for pristine monolayer MoS2

sample, binding energy peaks associated with the S 2p and S 2s core levels were

found at 162.3, 163.5 and 226.5 eV specifically corresponding to S 2p3/2, S 2p1/2,

and S 2s1/2, respectively and two peaks associated with Mo 3d core levels were

found at 229.5 and 232.5 eV, corresponding to Mo 3d5/2 and Mo 3d3/2,

respectively[74]. All of these peaks were upshifted by 0.55 eV after the methanol

treatment. Because the binding energy value derived from an XPS spectrum is

referenced to the Fermi level in the material, the upshift of the XPS spectrum of

MoS2 after methanol treatment was attributed to the shift of the Fermi level toward

the conduction band, indicating the existence of n-type doping.

Huizhen Yao et. al. (2018) performed XPS to reveal the change of the

elemental composition and the surface stoichiometry ratio[75].. The binding

energies of S 2p1/2 and S 2p3/2 locate at around 162.3 and 163.6 eV, corresponding

to S2−and S2

2−species. The binding energies at around 32.8 and 34.9 eV reveal

the +IV chemical states of W corresponding to WS2 monolayers. It wasnoteworthy

that the FWHM of W4+ decreased distinctly after chemical treatment with Na2S

solution they performed, suggesting a more uniform chemical environment for W

species. The peaks at 36.1 eV and 38.2 eV were referred to as +VI chemical state

compounds, such as WO3 which was the by-product induced in the CVD process

and depends on sample preparation and cleanliness. After Na2S solution

treatment, the relative intensity of binding energy for the +VI chemical state was

depressed, suggesting the removal of WO3−X species during the chemical

35

treatment process. Meanwhile, the binding energies of W 4f and S 2p have a

significant upshift, which could serve as an indicator of n-doping in WS2

monolayers by Na2S solution treatment. The relative shift was found to be about

0.64 eV. This core-level shift toward a higher binding energy proved a relative shift

of the Fermi level toward the conduction band edge. The surface S/W ratio

increases from 1.52 to 1.90 during the Na2S chemical treatment as a result of the

possible absorption of the sulphur element on the WO3−X complex sites or

structural defect sites such as sulphur vacancies. The doping mechanism on WS2

monolayers by the Na2S treatment could be explained in a way for WS2

monolayers: tungsten with a valence electronic configuration of 6s25d4 possesses

an electropositive property and acts as an electron acceptor. When electronegative

S2−(electron donor) ions in the chemical solution are incorporated into WS2

monolayers, they occupied the location of sulfur vacancies or absorbed by WO3−x

species and electrons could effectively be injected into the WS2 monolayers.

Furthermore, the absorption of the sulphur element could effectively passivate the

structural defects and decreased the nonradiative recombination centres.

3.2.7. Photoluminescence (PL)

Whenever a direct band gap semiconductor is illuminated with energy (hγ)

greater than its band gap, an electron-hole pair is created in conduction and

valance bands respectively. Instant thermalization of energy leads to the energy

separation between the electron and hole to be almost equal to the band gap of

the semiconductor. This results in direct transition which manifests as an emission

of characteristic energy that is less than the band gap. Such a light matter

interaction which results spontaneous emission of light under optical excitation is

called photoluminescence and its general mechanism is represented as in Figure

8. Features of these emission spectra can be used to identify surface, interface

and impurity levels and to gauge alloy disorder and interface roughness [76]. For

example, the intensity of the PL signal gives information about the surface and

interface quality, transient PL intensity under pulsed excitation yields information

about the life-time of carrier and the variation of PL intensity under an external bias

36

maps the electric field on the surface of sample. As the whole process of PL relies

on radiative events, it gets difficult to relay on PL analysis when the sample is a

low- quality direct band gap semiconductor or indirect band gap semiconductor.

Figure 8: Absorption and related radiative and non-radiative processes involved during the whole procedure of Photoluminescence.

For bulk WS2, there are two direct transitions at the K point in the Brillouin

zones due to the splitting of the valence band. These two transitions are assigned

to A (1.95 eV) and B (2.36 eV) excitons, respectively and have been experimentally

detected by absorption spectroscopy. On the other hand, this splitting of the

valence band for a monolayer WS2 is absent, which means only one direct

electronic transition is expected to be observed. Also, PL FWHM can be an

indicator of sample quality. A smaller FWHM in principle suggests a higher quality.

PL spectroscopy is a non-contact, non-destructive technique used to probe

the electronic structure of the specimen. Laser light is irradiated on to the sample

and light is absorbed and the excess energy is used in photo-excitation within the

specimen. The photo-excitation causes the electrons to promote into available

excited states. The electrons in these excited states would then eventually relax

into a lower equilibrium state and the excess energy is released which may result

in the emission of light (radiative process) or a nonradiative process. Thus, the

37

energy of the emitted light released during the relaxation of the excited electron is

the difference between the energy level of the excited state and the equilibrium

state

3.3. Overview of PL Enhancement based on Literature

Anand P. S. Gaur et. al. (2019) demonstrated the PL spectra of 1L-WS2

grown via chemical vapor deposition and were analyzed by maneuvering the

interplay among free exciton, bound exciton and trion concentration through the

polar solvents treatment[77].The polar solvent introduced excess negative charge

(n-type doping) through the surface charge transfer by adsorbed molecules,

resulting in a substantial increase in trionic spectral weight, was observed in the

PL spectrum of chemically treated 1L-WS2. Besides, the FWHM of free exciton PL

band became narrower leading to the fact that defect/surface states were

suppressed significantly after the chemical treatment. The negative electron

doping was confirmed by Raman and PL respectively. Furthermore, in the

temperature dependent PL spectrum, an extra feature associated with bound

exciton along with trion emission evolved at the expense of free exciton emission

at the lowest temperature. In their study, the temperature dependent behavior of

excitonic and trionic peak was simulated by a model using the law of mass action

for trion formation[77].

Min Su Kim et. al. (2016) reported of a comprehensive nanoscale PL and

Raman spectroscopy investigation of triangular CVD-grown WS2 monolayers.

They visually identified distinct patterns of PL emissions in single WS2 grains,

which showed strong PL emissions from the edges and grain boundaries and they

found that these regions with strong PL emissions were very efficient in generating

biexcitons at high excitation power[78]. They showed strong PL emission and the

favored formation of biexcitons in the edge region to the larger local population of

charge carriers available for the formation of various exciton complexes. They

observed that preferential formation of trions and biexcitons could result in

enhanced PL emission[78].

38

Samantha Matthews et. al. (2019) reported about PL-based response from

single-and few-layer WS2 arising from three excitons (neutral, A0; biexciton, AA;

and the trion, A−)[79]. The A0 exciton PL emission was the most strongly quenched

by acetone whereas the A− PL emission exhibited a unique enhancement.

Moreover, PL response from the WS2 flake was exciton-type and layer number

dependent. They determined the acetone-induced changes in the A0, AA, and A−

exciton band amplitude, peak energy and energy distribution across individual WS2

flakes consisting of single and few-layer regions by using co-localized, confocal

Raman and PL emission mapping experiments. PL maps were used to determine

the WS2 layer thickness and map the A0, AA and A−. It had been shown that the

exciton amplitude, energy and FWHM were all affected by acetone vapor. Acetone

also induced detectable shifts in the exciton emission band energies. The exciton

band FWHM also changed under acetone vapor[79].

In their study, data inspection showed that the PL emission intensity:

(i) Heterogeneous across the WS2 flake when it was under air or acetone

vapor,

(ii) Generally quenched by acetone vapor and

(iii) Quenching was heterogeneous across the WS2 flake.

The largest extent of PL quenching was observed at the flake’s middle and

upper and lower edges, where the flake was mostly composed of few-layer

WS2.Thus, there was a strong spatial dependence in the acetone-induced PL

emission quenching from a WS2 flake and shifting in the exciton emission band

energies[79].

They showed a model to summarize the single- and few-layer WS2 exciton

PL emission behavior in the presence of acetone. Acetone caused an overall

decrease in the A0 and AA exciton band PL emission for single-and few-layer WS2.

A decrease in PL emission was also produced in the A− exciton band PL emission

in few-layer WS2, but there was an overall increase in the A− exciton band PL

emission from single-layer WS2[79].

Hyungjin Kim et. al. (2019) demonstrated synthetic tungsten diselenide

(WSe2) monolayers with PL QY exceeding that of exfoliated crystals by over an

39

order of magnitude. According to the study, PL QY of ~60% was obtained in

monolayer films grown by CVD, which was the highest reported value to date for

WSe2 prepared by any technique[80]. The high optoelectronic quality in their study

was enabled by the combination of optimizing growth conditions via tuning the

halide promoter ratio and introducing a simple substrate decoupling method via

solvent evaporation, which also mechanically relaxed the grown films. WSe2

monolayers grown via CVD have strong interactions with the substrate. Strong

coupling to the substrate of as-grown monolayers inhibited probing the intrinsic

properties. Using a solvent evaporation–mediated decoupling (SEMD) process,

they demonstrated reduced nonradioactive recombination and higher PL QY in the

grown monolayers by decoupling from the substrate. They highlighted the role of

the halide promoter (KBr) on the PL QY of the monolayer films varying the KBr-to-

WO3 precursor weight ratio and samples prepared with 1:2 = KBr: WO3[80].

They also discussed how SEMD process works which began by placing a

droplet of solvent with high vapor pressure on an as-grown WSe2 monolayer. As

the solvent evaporated, the surface tension pulled on the grown material and

decoupled the material from the substrate. Subsequently, the emission remained

stable over time. At the onset of solvent evaporation from the monolayer, they

started to observe strong emission at 1.65 eV at the edge of the crystal. Once the

solvent was fully evaporated, the emission became uniform over the full sample

domain. The post-SEMD emission peak position closely matched that of

unstrained CVD WSe2 monolayers and that of micromechanically exfoliated

samples indicating the full release of the built-in strain and thus, completing

decoupling of the synthetic monolayer from the substrate. It showed that the PL

intensity at 1.65 eV started to increase from the edge and becomes uniformly

enhanced over the SEMD process indicating that the substrate decoupling was

mediated by the solvent evaporation and was initiated from the edges of the

monolayer. The effect of SEMD was characterized by PL spectroscopy showing

emission peak blue shifts by ~80 meV from 1.57 eV for the as-grown sample to

1.65 eV after SEMD. SEMD released the biaxial tensile strain and the results

showed that acetone did not chemically modify the monolayers and did not affect

40

the recombination processes. Instead, acetone evaporation induced surface

tension–mediated decoupling of the monolayer. For grown WSe2 monolayers after

SEMD, they observed a lifetime of 4.1 ns, while exfoliated and as-grown samples

showed lifetimes of 1 or sub-1 ns, respectively. They performed absorption

measurements on as-exfoliated and as-grown samples and after SEMDand

observed a shift in the A and B exciton resonances for the as-grown samples with

the biaxial tensile strain, while no measurable shift is measured in the C exciton

resonance[80].

Hau-Vei Han et al. (2016) reported that in MoSe2 point defects (Se

vacancies) and oxidized Se defects could significantly trap free charge carriers and

localize excitons, leading to the smearing of free band-to-band exciton emission

and Hydrohalic acids were highly effcient agents to tune the exciton PL in MoSe2

monolayers grown by CVD[81]. Hydrohalic acid treatment such as HBr was able

to effciently suppress the trap state emission and promote the neutral exciton and

trion emission in defective MoSe2 monolayers through the p-doping process,

where the overall photoluminescence intensity at room temperature could be

enhanced by a factor of 30. They showed that HBr treatment was able to activate

distinctive trion and free exciton emissions even from highly defective MoSe2

layers. Their results suggested that the HBr treatment not only reduced the n-

doping in MoSe2 but also reduced the structural defects. Recently, a similar

phenomenon was observed on CVD-grown WS2 monolayer where they suggested

that the defects within the crystal act as nonradiative recombination sites and thus

quenched the intrinsic PL. Consistently, the edge enhanced PL emission has been

observed in CVD grown WS2 monolayer and the darkening of PL in the center of

TMDs island has been attributed to the charge defect-induced doping. [81].

Raman scattering is known to be sensitive to the doping level of 2D

materials and they used Raman spectroscopy to investigate the charge−phonon

interaction in MoSe2 layers. For a monolayer MoSe2 only one Raman active mode

(out-of-plane A1g) appears [81]

41

Figure 9: (a) Raman spectra and (b) Raman intensity maps of a monolayer MoSe2 flake before and after HBr treatment. Reprinted (adapted) with

permission from Han, H.-V., Lu, A.-Y., Lu, L.-S., Huang, J.-K., Li, H., Hsu, C.-L., Lin, Y.-C., Chiu, M.-H., Suenaga, K., & Chu, C.-W. (2016).

Photoluminescence enhancement and structure repairing of monolayer MoSe2 by hydrohalic acid treatment. ACS Nano, 10(1), 1454–1461.

Copyright (2016), American Chemical Society.

They performed temperature dependence PL measurements down to 10 K

for the as-grown and HBr-treated MoSe2 layers to further reveal the excitonic

nature of MoSe2 monolayer and the effect after HBr treatment. Figure 9(a)

compared the PL emission from as-grown and HBr-treated MoSe2 at 10 K.

Temperature-dependence measurement of MoSe2 PL suggested that the defects

within the as-grown MoSe2 crystals prohibited the intrinsic exciton emission and

the dominate PL peak was mostly from trapped exciton states, while for the HBr-

treated MoSe2 the trapped exciton state was greatly suppressed and both exiton

and trion peaks were detectable at a low temperature. Defects such as cation and

anion vacancies in TMDs could induce doping [81]

They concluded that point defects formed by Se vacancies could greatly

quench the PL of monolayer MoSe2 due to the trapping of free charge carriers and

non-radiative recombination. A low-temperature PL study showed that HBr could

effectively suppress the trapped exciton states and populate both exciton and trion

emission. Other hydrohalic acids such as HCl and HI also showed similar PL

enhancement effects. However, HBr was the most effective chemical with a

(a) (b)

42

suitable acidity and thus gave better controllability for tailoring the optical

properties of MoSe2. The drastic modulation of optical properties by HBr could be

attributed to various reasons, including removing impurities, p-doping to the MoSe2

and reducing the structure of MoSe2 [81]

Figure 10: (a) PL intensity mappings of an individual MoSe2 flake before and after HBr treatment. Profiles in (b) and (c) show the PL intensity and

photon energy modulation as a function of surface location along the solid line indicated in (a) Reprinted (adapted) with permission from Han, H.-V., Lu, A.-Y., Lu, L.-S., Huang, J.-K., Li, H., Hsu, C.-L., Lin, Y.-C., Chiu, M.-H., Suenaga, K., & Chu, C.-W. (2016). Photoluminescence enhancement and

(a)

(b) (c)

43

structure repairing of monolayer MoSe2 by hydrohalic acid treatment. ACS Nano, 10(1), 1454–1461. Copyright (2016), American Chemical Society.

Figure 11: (a) Photoluminescence of the as-grown and HBr-treated monolayer MoSe2 at 10 K. (b) Temperature dependence of PL for the MoSe2 after HBr treatment. (c) Trion and exciton peak energies. (d)

Intensity of trion to exciton peak as a function of temperature. Reprinted (adapted) with permission from Han, H.-V., Lu, A.-Y., Lu, L.-S., Huang, J.-K., Li, H., Hsu, C.-L., Lin, Y.-C., Chiu, M.-H., Suenaga, K., & Chu, C.-W. (2016). Photoluminescence enhancement and structure repairing of monolayer

MoSe2 by hydrohalic acid treatment. ACS Nano, 10(1), 1454–1461. Copyright (2016), American Chemical Society.

(a) (b)

(d) (c)

44

Guru P. Neupaneet. Al. (2017) showed that dipping 1Ls of MoS2, WS2 and

WSe2, whether exfoliated or grown by chemical vapor deposition, into methanol

for several hours can increase the electron density and could reduce the defects,

resulting in the enhancement of their photoluminescence, light absorption and the

carrier mobility[82]. This methanol treatment was effective for both n-and p-type

1L-TMDs, suggesting that the surface restructuring around structural defects by

methanol was responsible for the enhancement of optical and electrical

characteristics. Averaged PL spectra taken from the pristine state and from 16 h-

treated 1L-MoS2 indicated that an overall 2.2-fold enhancement was obtained from

this methanol treatment. Besides the increase of the PL intensity, a redshift of the

A exciton peak was also observed after the methanol treatment. PL intensity was

monotonically increased, and the peak wavelength of the A exciton peak up to 16

h were observed. The redshift of the PL peak of 1L-MoS2 has been attributed to

an increase in the trion spectral weight, indicating that the methanol treatment

caused an increase of the electron density in 1L-MoS2. Intensity ratio of trions (A−)

to neutral excitons (A0) was found to increase from 1.1 to 2.3 as the methanol

treatment time was increased from 0 to 16 h[74]

Averaged absorption spectra revealed that the increase in absorption was

specifically due to increases in the intensities of the A, B, and C exciton peaks,.

The methanol treatment seemed effective both for the exfoliated and CVD grown

samples. they suggested that the adsorption and dissociation of methanol

occurring on the surface of TMDs. Previously, a density function theory study

reported that methanol would favorably adsorb and dissociate on the edges of

MoS2 clusters through O•H dissociation and then the pathway of CH3O →CH3

→CH2. In the previous result of PL enhancement of 1L-MoS2 using organic

superacid of bis(trifluoro-methane) sulfonamide (TFSI), hydrogenation by TFSI

was regarded to be the most responsible for the fixation of the defects. Based on

such reports, they believed that consecutive hydrogen release occurring during

methanol dissociation at the defect sites of 1L-MoS2 and other TMD samples may

be responsible for the fixation of the defect states. Moreover, CH3O and CH3 are

45

known as electron-donating groups which may have induced the n-doping of our

1L-TMDs and the exfoliated MoS2 film samples(Neupane et al., 2017b).

Shinichiro Mouri et. al. (2013) demonstrated the tunability of the

photoluminescence (PL) properties of monolayer (1L)-MoS2 via chemical

doping[36]. The PL intensity of 1L-MoS2 was drastically enhanced by the

adsorption of p-type dopants with high electron affinity but reduced by the

adsorption of n-type dopants. This PL modulation resulted from switching between

exciton PL and trion PL depending on carrier density in 1L-MoS2. Extraction and

injection of carriers in 1L-MoS2 by this solution based chemical doping method

enabled convenient control of optical and electrical properties of atomically thin

MoS2. Optically generated electron−hole pairs in 1L-MoS2 form stable exciton

states even at room temperature because of the extremely large Coulomb

interactions in atomically thin two-dimensional materials. The stable exciton plays

an important role in the optical properties of 1L-MoS2. They mentioned that control

of the carrier density was one effective method to modulate the optical properties

of monolayer TMDs. The interplay between the exciton and charge carrier gave

rise to the formation of a many-body bound state such as a charged exciton (trion)

providing additional pathways for controlling the optical properties of 1L-MoS2. The

PL intensity of 1L-MoS2 was drastically enhanced when p-type dopants covered

its surface. This enhancement was understood because of switching the dominant

PL process from the recombination of the negative trion to the recombination of

the exciton under extraction of residual electrons in as-prepared 1L-MoS2. On the

other hand, the PL intensity was reduced when 1L-MoS2 was covered with n-type

dopants, which was due to the suppression of exciton PL by the injection of excess

electrons. They confirmed that bi-directional control of the Fermi level of 1L-MoS2

by chemical doping. The PL intensity of 1L-MoS2 was drastically enhanced by the

adsorption of p-type dopants (F4TCNQ and TCNQ). This intensity enhancement

was explained by the switching of the dominant PL process from the recombination

of negative trions to the recombination of excitons through extraction of the

unintentionally high doped electrons. Moreover, the PL intensity was reduced by

the adsorption of n-type dopants (NADH), which they referred to the suppression

46

of exciton PL through injection of the excess electrons. Their findings suggested

that both the extraction and the injection of electrons in 1L MoS2 could be realized

via the solution-based chemical doping technique, which provides a strong

advantage in tuning the optical and electrical properties of atomically thin TMDs

without the use of device structures. The PL intensity increases step by step with

increases in the F4TCNQ doping steps, approximately three times greater than that

of as prepared 1L-MoS2[36].

Figure 12: (a) Optical images of as-prepared 1L-, 2L-, and 3L-MoS2 on SiO2/Si substrates. (b) Raman spectra of the as-prepared 1L-, 2L-, and 3L-

MoS2 measured at room temperature. (c) PL spectra of the as prepared 1L-, 2L-, and 3L-MoS2. The PL peak due to the indirect band gap transition is denoted as I, and those due to the direct band gap transition are denoted

as peaks A and B[36](Mouri et al., 2013). Reprinted (adapted) with permission from Mouri, S., Miyauchi, Y., & Matsuda, K. (2013). Tunable

47

photoluminescence of monolayer MoS2 via chemical doping. Nano Letters, 13(12), 5944–5948. Copyright (2016) American Chemical Society

Figure 13: (a) PL spectra of 1L-MoS2 before and after F4TCNQ doping. (b) PL spectra of 1L-MoS2 obtained at each doping step (0, 1, 2, 4, 6, 10, 13, and

16 steps). The inset shows the normalized PL spectra of 1L-MoS2 at each doping step. (c) Analysis of the PL spectral shapes for as-prepared and

F4TCNQ-doped 1L-MoS2. The A peaks in the PL spectra were reproduced by assuming two peaks with Lorentzian functions, corresponding to the trion (X−) and the exciton (X) peaks, were overlapped. (d) Integrated PL intensity

of the negative trion Ix−, exciton Ix, and the sum (Itotal) of Ix and Ix−as functions of the number of F4TCNQ doping steps. Solid lines show the

calculated PL intensity curves calculated by solving the rate equations in the three-level model. Reprinted (adapted) with permission from Mouri, S.,

Miyauchi, Y., & Matsuda, K. (2013). Tunable photoluminescence of monolayer MoS2 via chemical doping. Nano Letters, 13(12), 5944–5948.

Copyright (2013) American Chemical Society.

48

Figure 14: (a) PL spectra of 1L-MoS2 before and after being doped with p-type molecules (TCNQ and F4TCNQ). (b) PL spectra of 1L-MoS2 before and after being doped with an n-type dopant (NADH). Reprinted (adapted) with

permission from Mouri, S., Miyauchi, Y., & Matsuda, K. (2013). Tunable photoluminescence of monolayer MoS2 via chemical doping. Nano Letters,

13(12), 5944–5948. Copyright (2013) American Chemical Society

Matin Amani et. al. (2016) developed a chemical treatment technique using

an organic non-oxidizing superacid, bis(trifluoro-methane) sulfonimide (TFSI),

which was shown to improve the quantum yield in MoS2 from less than 1% to over

95%[83]. They performed detailed steady-state and transient optical

characterization on some of the most heavily studied direct bandgap 2D materials,

specifically WS2, MoS2, WSe2 and MoSe2, over a large pump dynamic range to

study the recombination mechanisms present in these materials. Then they

explored the effects of TFSI treatment on the PL QY and recombination kinetics

and the results suggested that sulfur-based 2D materials were able to

repair/passivation by TFSI, while the mechanism was thus far ineffective on

selenium based systems. They also showed that biexcitonic recombination was

the dominant non-radiative pathway in these materials[83].

49

Their surface inspection revealed the following findings:

(i) high spatial variation even across the same material with the largest

imperfection density found on MoS2[83]

(ii) sulfide surfaces were dominated by structural defects and by acceptor

impurities causing local depressions[83] and

(iii) selenide surfaces were predominantly dominated by hillock-like

structures induced by donor impurities. This drastic difference in the

nature of defects may explain why sulfur-based TMD materials were

more responsive to the TFSI treatment. They showed dominant

recombination pathway at high pump-power for all these materials was

biexcitonic recombination[83].

TMDs have exhibited poor luminescence quantum yield (QY)—that is, the

number of photons the material radiates is much lower than the number of

generated electron-hole pairs. The prototypical 2D material molybdenum disulfide

(MoS2) was reported to have a maximum QY of 0.6%, which indicated a

considerable defect density[83]. QY values ranging from 0.01 to 6% have been

reported, indicating a high density of defect states and mediocre electronic quality.

The origin of the low quantum yield observed in these materials was attributed to

defect-mediated non-radiative recombination and biexcitonic recombination at

higher excitation powers surface passivation by chemical treatments. With the use

of this process, the photoluminescence (PL) in MoS2 monolayers increased by

more than two orders of magnitude, resulting in a QY > 95% and a characteristic

lifetime of 10.8±0.6 ns at low excitation densities. The treatment has eliminated

defect-mediated non-radiative recombination. Superacids are strong protonating

agents and have a Hammett acidity function (H0) that is lower than that of pure

sulfuric acid. The PL spectra of a MoS2 monolayer measured before and after TFSI

treatment in Figure 15 (b) showed a 190-fold increase in the PL peak intensity,

with no change in the overall spectral shape. The exact mechanism by which the

TFSI passivated surface defects was not fully understood in their study. Deep-level

traps—which contributed to defect-mediated non-radiative recombination,

50

resulting in a low QY—were observed for all these cases. The strong protonating

nature of the super acid could remove absorbed water, hydroxyl groups, oxygen

and other contaminants on the surface. Although these reactions would not

remove the contribution of defects to non-radiative recombination, they would open

the active defect sites to passivation by a second mechanism. One possibility was

the protonation of the three dangling bonds at each sulfur vacancy site[83].

51

Figure 15: PL spectra for both the as-exfoliated and TFSI treated (a) WS2, (b) MoS2, (c) WSe2, and (d) MoSe2 monolayers measured at an incident

power density of 1 × 10−2 Wcm−2. The inset shows normalized spectra for each material. Absorption spectra of both as-exfoliated (dashed lines) and chemically treated (solid lines) WS2, MoS2, WSe2, and MoSe2 monolayers

(e). Reprinted (adapted) with permission from Amani, M., Taheri, P., Addou, R., Ahn, G. H., Kiriya, D., Lien, D.-H., Ager III, J. W., Wallace, R. M., & Javey,

A. (2016). Recombination kinetics and effects of superacid treatment in sulfur-and selenium-based transition metal dichalcogenides. Nano Letters,

16(4), 2786–2791. Copyright (2016) American Chemical Society.

Figure 16: Radiative decay of as-exfoliated (a) and chemically treated (b) WS2 at various initial carrier concentrations (n0) as well as the instrument response function (IRF). Reprinted (adapted) with permission from Amani, M., Taheri, P., Addou, R., Ahn, G. H., Kiriya, D., Lien, D.-H., Ager III, J. W., Wallace, R. M., & Javey, A. (2016). Recombination kinetics and effects of

superacid treatment in sulfur-and selenium-based transition metal dichalcogenides. Nano Letters, 16(4), 2786–2791. Copyright (2016)

American Chemical Society.

Matin Amani et. al. (2016) have performed a thorough exploration of

chemical treatment on CVD-grown MoS2 samples[84]. They showed that the PL

QY of CVD-grown monolayers could be improved from ∼0.1% in the as-grown

case to ∼30% after treatment, with enhancement factors ranging from 100 to

1500X depending on the initial monolayer quality. Defects act as non-radiative

recombination centers and significantly quenched the emission. Previously, it was

demonstrated that treatment using the organic superacid bis(trifluoro-methane)

sulfonamide (TFSI) resulted in a PL QY near 100% in exfoliated MoS2 monolayers

and it was later demonstrated that this treatment mechanism was also effective on

52

exfoliated WS2 monolayers. They also studied the effect of the sulfur precursor

temperature during growth and showed that this also played a role in the ultimate

quantum yield, which could be achieved after treatment. They also found that after

TFSI treatment the PL emission from MoS2films was visible by eye despite the low

absorption (5−10%)[84].

Figure 17: (a) Configuration of the growth setup utilized to prepare the MoS2 samples for this study. The temperature of the substrate and

molybdenum precursor (in the furnace hot zone) and the sulfur precursor (surrounded by heating tape) is controlled and measured independently. (b

and c) Schematic illustrating the two primary sample preparation routes investigated in this study. As-grown MoS2 triangular domains and films,

which show tensile strain after growth were either (b) treated by TFSI directly, resulting in a small reduction in the PL QY, or (c) transferred from the growth substrate using a PMMA-mediated transfer process, releasing the strain, and subsequently treated by TFSI, resulting in a final PL QY of approximately 30%. Reprinted (adapted) with permission from Amani, M., Burke, R. A., Ji, X., Zhao, P., Lien, D.-H., Taheri, P., Ahn, G. H., Kirya, D.,

Ager III, J. W., & Yablonovitch, E. (2016). High luminescence efficiency in

53

MoS2 grown by chemical vapor deposition. ACS Nano, 10(7), 6535–6541. Copyright (2016) American Chemical Society.

Figure 18: (a) Raman spectra measured on as-grown and transferred MoS2 single domains. (b) PL spectra of the MoS2 single domains measured

before and after transfer at a laser power of 50 W/cm2. Reprinted (adapted) with permission from Amani, M., Burke, R. A., Ji, X., Zhao, P., Lien, D.-H., Taheri, P., Ahn, G. H., Kirya, D., Ager III, J. W., & Yablonovitch, E. (2016).

High luminescence efficiency in MoS2 grown by chemical vapor deposition. ACS Nano, 10(7), 6535–6541. Copyright (2016) American

Chemical Society.

Figure 19: (a) Raman spectra measured on transferred MoS2 single domains before and after treatment by TFSI. (b) PL spectra obtained at a

pump power of 0.1 W/cm2 for transferred MoS2 single domains both before and after chemical treatment by TFSI. (c) Radiative decay of transferred MoS2 single domains obtained at a pump fluence of 5 × 10−2μJ/cm2 both before and after chemical treatment by TFSI, as well as the instrument

response function (IRF). Reprinted (adapted) with permission from Amani, M., Burke, R. A., Ji, X., Zhao, P., Lien, D.-H., Taheri, P., Ahn, G. H., Kirya, D., Ager III, J. W., & Yablonovitch, E. (2016). High luminescence efficiency in

54

MoS2 grown by chemical vapor deposition. ACS Nano, 10(7), 6535–6541. Copyright (2016) American Chemical Society.

Figure 20: (a) Optical image of a transferred MoS2 single domain and log-scale luminescence images from the same area obtained (b) before and (c)

after chemical treatment by TFSI. (d) Optical image of a transferred continuous MoS2 film and log scale luminescence images from the same

area obtained (e) before and (f) after chemical treatment by TFSI[84]. Reprinted (adapted) with permission from Amani, M., Burke, R. A., Ji, X.,

Zhao, P., Lien, D.-H., Taheri, P., Ahn, G. H., Kirya, D., Ager III, J. W., & Yablonovitch, E. (2016). High luminescence efficiency in MoS2 grown by

chemical vapor deposition. ACS Nano, 10(7), 6535–6541. Copyright (2016) American Chemical Society.

Long Yuan et. al. (2015) systematically investigated the exciton dynamics

in monolayered, bilayered and trilayered WS2 using time-resolved PL under

conditions with and without exciton–exciton annihilation.They choose WS2 as a

model system because of the relatively low defect density in WS2 as shown by the

higher photoluminescence (PL) quantum yield (QY) than other 2D semiconductors

(∼6% in WS2, compared to ∼0.1% of MoS2). Exciton decays of the monolayer,

bilayer and tri layer all exhibit mono-exponential decay behavior. The PL lifetime

was measured to be 806 ± 37 ps, 401 ± 25 ps, and 332 ± 19 ps for WS2 monolayer,

bilayer and trilayer respectively, when free of exciton annihilation. The radiative

lifetime of excitons was determined to be ∼13 ns, ∼400 ns and ∼830 ns for the

monolayer, bilayer and trilayer, respectively. Furthermore, two orders of magnitude

enhancement of the exciton–exciton annihilation rate had been observed in the

55

monolayer compared to the bilayer and trilayer. They mentioned the strongly

enhanced annihilation in monolayered WS2 to enhance electron hole interactions

and to the transition to the direct semiconductor, which eliminated the need for

phonon assistance in exciton–exciton annihilation. Another hallmark of low-

dimensional electronic systems was the enhanced many-body interaction due to a

reduced dimensionality. Upon the generation of a high density of electrons and

holes, many-body scattering processes such as Auger recombination and exciton–

exciton annihilation could play an important role in nonradiative relaxation. These

nonradiative recombination processes defined the upper limit of excitation density

and ultimately the efficiency for applications such as semiconductor lasers and

light-emitting diodes. Exciton–exciton annihilation and Auger recombination have

been intensively investigated in quantum dots, carbon nanotubes and

semiconductor nanowires. While recent studies on MoS2, MoSe2 and WSe2

monolayers have shown the existence of exciton–exciton annihilation at high

excitation density.

Yumeng You et. al. (2015) demonstrated the existence of four-body,

biexciton states in monolayer WSe2. The biexciton is identified as a sharply defined

state in photoluminescence at high exciton density. Its binding energy of 52 meV

is more than an order of magnitude greater than that found inconventional

quantum-well structures. A variational calculation of the biexciton state reveals that

the high binding energy arises not only from strong carrier confinement, but also

from reduced and non-local dielectric screening[85]

Huizhen Yao et. al. (2018) reported that CVD-grown WS2 monolayers by a

simple immersion treatment method with an available sulphur based salt. After

chemical treatment with Na2S solution, the PL emission of triangular WS2

monolayers became homogeneous and was enhanced by 25-fold in the inner

region. The PL peak wavelength after Na2S treatment had an obvious red-shift,

which was attributed to the increase of trion and biexciton formation due to an

effective n-type doping[75]

They further investigated the PL enhancement by carefully adjusting the

Na2S solution to a lower concentration level of 0.02 M. Compared with 0.05 M

56

Na2S solution treatment, partial PL enhancement and a red shifted peak

wavelength around the edge regions of WS2 monolayers. It could be attributed to

the higher chemical reactivity of the WS2 edges which may adsorb chemical

species more easily. The results indicated that the chemical interaction between

the WS2 monolayers and S2−in the solution occurs slowly from the edge towards

the inner region and the electrons are gradually injected during the chemical

treatment process[75]

The result of XPS showed that the WO3−X has been effectively reduced after

the chemical treatment. The synergistic effect of charge doping and removed

impurities made remarkable modulation of the optical properties in WS2

monolayers [75].

Zhengyu He et al. (2016) studied biexciton emission in bilayer WS2 grown

by chemical vapor deposition as a function of temperature. A biexciton binding

energy of 36±4 meV is measured in the as-grown bilayer WS2 containing 0.4%

biaxial strain as determined by Raman spectroscopy. The biexciton emission was

difficult to detect when the WS2 was transferred to another substrate to release the

stain. Density functional theory calculations show that 0.4% of tensile strain lowers

the direct band gap by about 55 meV without significant change to the indirect

band gap, which can cause an increase in the quantum yield of direct exciton

transitions and the emission from biexcitons formed by two direct gap excitons.

They found that the biexciton emission decreases dramatically with increased

temperature due to the thermal dissociation, with an activation energy of 26 ± 5

meV[86].

From extensive literature studies on PL enhancement of different TMDs, we

can specify and select which novel chemical reagents we can use for surface

passivation eventually leading to achieve the goal of this thesis. Furthermore,

above studies also showed the probable mechanism behind each treatment that

also helps understanding possible mechanism related to H2SO4-vapor treatment

that has been done in this thesis.

57

Chapter 4: Experimental Details

Chapter 4 covers the experimental part of this master’s thesis. Chapter 4

discusses about Experimental Details and Results of CVD growth of monolayer

WS2 on SiO2 (300 nm)/Si substrate, Characterization of CVD growth of monolayer

WS2 on SiO2 (300 nm)/Si substrate and focuses on Laser Power Dependence PL

measurement of pristine WS2, PL variation along certain lines for pristine WS2.

4.1. Materials

Based on Chapter 3, the experimental design and details are being

achieved and after performing multiple trial and errors, I arrived to specific

experimental details that gives good and repetitive results. For the experiment, we

have used WO3 (Sigma Aldrich, >99.5% purity) and S (Sigma Aldrich, >99.5%

purity) as precursors. The SiO2 (300 nm)/Si wafers were brought from Wafer Pro

(Diameter: 100 mm, Orientation: <100>, Single Side Polished). The substrates

were cut using diamond cutter into following dimensions:

Bottom substrate at downstream that carries WO3 powders: 4.5 cm x 3 cm

Top substrate face down for growing monolayer WS2: 4 cm x 2 cm

Substrate at the upstream end carries S powder: 4.5 cm x 4.5 cm

(a) (b)

58

Figure 21: (a) CVD Setup in the ENSC Cleanroom, (b) Image representing reaction at High Temperature in CVD Furnace

The other things we have used are small quartz tube and boat that carries

the quartz tube. The dimension of small quartz tube is given below:

Small Quartz Tube: 30 cm length, 6 cm diameter, one side open and other side

closed.

Figure 22: Schematic of Small Quartz Tube and Boat/Holder

We have used TemPress 3-zone manual heating furnace for our CVD growth. We

used OHAUS as weight balance.

4.2. Experiment

4.2.1. CVD Growth of monolayer WS2 on SiO2/Si substrate

It has been proposed that metal oxide precursors in the gas phase undergo

a two-step reaction during CVD growth, where transition metal sub-oxides are

likely formed first and then the sulfurization of these sub-oxides leads to the

formation of TMDs. The experimental procedure is divided into two sections (1)

Growth of monolayer triangular WS2 and (2) Verification of successful growth of

2D-monolayer WS2 using multiple characterization techniques such as Raman,

AFM, PL, XPS, TEM, SEM and EDS, Optical Imaging. It is worth mentioning that

we were able to grow monolayer WS2 flakes directly on Si3N4 supported-TEM grid.

The experiment is specifically designed to grow monolayer 2D materials with

different temperature zones. At first, we have used a 3-heating zone furnace

59

(TemPress) to grow monolayer triangular WS2using a bottom-up CVD process.

We have performed multiple trial and error methods (~57 trial and error) for

confirmation of getting same results each time. Before performing the CVD

process the SiO2 (300 nm) / (500 µm) Si was cleaned properly using RCA and then

Acetone, IPA and water, then blow dried using N2 gas. We have used WO3 (500

mg, >99.5% Purity, Sigma Aldrich) and S (1g, >99.5% Purity, Sigma Aldrich) as

precursors and SiO2 (300 nm)/Si substrate (WaferPro) as bottom and growth

substrate-face down approach. These precursors are positioned at different

temperature zones, which are identified with respect to their melting temperatures

~800⁰C and ~180⁰C.The bottom substrate was used to carry one of the precursors

WO3 (mg). Here, we have used Ar gas at 2 SCFH flow rate to carry the S vapor

and facilitate the reaction and formation of 2D monolayer triangular WS2 on the

growth substrate. Specific parameters such as precursor amount, growth

substrate, growth pressure and flow rate, temperature, use of gases, growth time,

use of promoter, pre-surface treatment of substrate etc. play an important role the

growth morphology, mechanism, luminescence yield, Raman spectra and light

absorption/transmission. So far, the results we achieved is average lateral crystal

size is more than ~20-25 µm and the largest crystal size is ~75 µm. For addressing

the experiment properly, a schematic experimental set-up has been shown in

Figure 23. It is important here to mention that the side from which Ar gas is flowing

at atmospheric pressure is called Up-stream and the side which is connected to

the exhaust can be called Down-stream. Therefore, the substrate which is carrying

S precursor is close to Up-stream and substrate carrying WO3 precursor and

substrate for growth facing down is close to Down-stream.

60

(a)

(b)

61

Figure 23: (a) Schematic Experimental Set-Up of CVD growth, (b) 3D view of CVD Growth of monolayer WS2 on SiO2/Si

Figure 24: Temperature profile as a function of Time for CVD growth of monolayer WS2

Figure 24 shows the Temperature profile as a function of Time for CVD

growth of monolayer WS2. As we have mentioned before our furnace is manually

operated, it was difficult for us to control the temperature of Zone 2, because Zone

3 was at very high temperature, heat was dissipated to Zone 2 by convection.

Based on the facilities we have; we are able to grow by maintaining the Zone 2

temperature at moderate level by playing with recipe.

After performing the CVD process, the growth substrate was taken out

carefully to minimize contamination by the remaining WO3 powder on the bottom

substrate and checked under optical microscope (OLYMPUS MX40). The optical

images have been shown in Figure 25. Optical images play a vital role for initial

confirmation of triangular monolayer because from optical images we can

differentiate among WS2 continuous film, multilayer (bi/tri-layer) and triangular

monolayer (different color corresponds to different thickness of the 2D material).

After confirming deposition on the growth substrate under optical microscope, the

substrates have been kept in vacuum desiccator for prevention of O2 exposure that

affects the crystal quality.

62

Table 1: CVD process parameters for monolayer WS2 growth

Distance

(Ds)

Precursor

Amount

Temperature

(Ts)

Ramp Rate

(C/min)

Holding

Time

(ts)

Press

ure

Flow

Rate

(SCFH)

Position

of

Substrate

16 cm

between

two

precursors

WO3:

500mg

S:1 gm

Zone 3:

50⁰C to 100⁰C;

100⁰C to

550⁰C;

550⁰C to

800⁰C

Zone 2: 50⁰C

to 100⁰C;

100⁰C to

180⁰C

(reaches upto

320ºC)

Zone 1: 50⁰C

to 100⁰C

Zone 3:

20⁰C/min;

20⁰C/min;

3⁰C/min

Zone 2:

20⁰C/min;

20⁰C/min

Zone 1:

20⁰C/min

30 min Atmos

pheric

2 Face-

Down

Table 1 lists the process parameters optimized for 2D monolayer WS2. For

monolayer formation, radical vapor ratio is also crucial; therefore, substrate

temperature and their spatial locations are carefully adjusted. Ds indicates the

distance between WO3 and S precursors in face-down growth configuration and

TS indicates the temperature of the substrate. In brief, when the deposition

temperature is reached, S vapor reduces WO3 powder to volatile sub-oxides

producing intermediate products such as WO2 or WO3-x [53], [87] and the formed

radicals diffuse on the substrate reacting with sulphur. Monolayer formation is

realized by the equations (1 and 2).

WO3 + S →WO3-x + S ------------------------------(3)

WO3-x + S → WS2 + SO2----------------------------(4)

63

In general, precursor ratio (in fact, the dissociated radicals’ ratio) has been

one of the essential internal parameters determining the monolayer formation. The

optimal sulfurization process can be completed by controlling this ratio carefully.

As the deposition duration is finished, a rapid cooling down process needs to be

carried out to avoid other products such as multi-layer growth of WS2 where 2

SCFH of nitrogen gas (99.999%) is purged in the tube.

The whole CVD chamber has 3-zone but for this experiment, zone 3 and

zone 2 has been used whereas zone 1 is kept empty. Zone 3 has three

temperature profiles (1) 50⁰C to 100⁰C at ramping rate of 20⁰C/min, (2) 100⁰C to

550⁰C at ramping rate of 20⁰C/min and (3) finally, 550⁰C to 800⁰C at ramping rate

of 3⁰C/min and holding time of 30 minutes. On the other hand, The temperature of

zone 2 has two temperature profiles: (1) 50⁰C to 100⁰C at ramping rate of 20⁰C/min,

(2) 100⁰C to 180⁰C at ramping rate of 20⁰C/min and holding time of 30 minutes and

as zone 1 is not participating this process and kept empty, it has only one

temperature profile: (1) 50⁰C to 100⁰C at ramping rate of 20⁰C/min. Constant flow

of Ar gas was maintained at the beginning of the process. Moreover, before

performing each run, Ar gas is purged in the reaction chamber for 5 minutes to

reduce the possibility of any contamination. Regular cleaning of quartz tube is also

important (Wang, Feng et al. 2013) and has been done with Acetone, IPA and

water. The distance between the precursors of sulphur and WO3 is kept at 16 cm.

We performed micro Raman and PL spectral characterizations with

inViaTMQontor confocal Reinshaw system and Leica TCS SP5 II- The Broadband

Confocal system at room temperature where we used 514 nm and 488 nm

excitation wavelength respectively. We obtained the images of the flakes by

confocal optical microscopy (OLYMPUS MX40). Room Temperature Laser Power

Dependent (270 µW, 193 µW, 119 µW, 58 µW, 18 µW) PL and PL variation with

distance along certain lines at 193 µW Laser Power has been performed as well

using the same PL system.

64

4.3. Experimental Results & Discussions

4.3.1. Optical

As we have mentioned before, Optical imaging is very important to

differentiate between monolayers, bi/tri-layers and multilayers based on the

contrast. Two things here play important role: (1) types of substrate such as

SiO2/Si, Quartz and Sapphire and (2) if the substrate is SiO2/Si, then thickness of

SiO2. SiO2 with ~300 nm thickness has better visibility in nanoscale. The difference

in color contrast of each number of layers is clearly visible in SiO2/Si substrate as

it matches the right thickness. The magnification we have used is 80X.

Figure 25 (a) and (b) shows the images of the monolayer WS2 flake images

obtained by con-focal optical microscopy (Olympus MX40). According to reported

studies, our samples contain smaller flakes at the center of the substrate whereas

flakes get larger when we get closer to the edge of the substrate. We can assume

that the edge of SiO2/Si substrate have more dangling bonds possibly act as more

nucleation and further horizontal growth site. As a result of the described growth

procedure, the results we achieved are average lateral crystal size is ~20-25 µm

and the largest crystal size is ~75 µm. After performing multiple trial and error

methods by slightly changing the crucial parameters, we observed that flakes

touch, overlap or merge to form larger depending on which parameters have been

changed such as holding time (ts), growth temperature (Ts), Ds, precursor ratio,

rate of sulphurization etc. Continuous WS2 film formation occurs in TMDs by the

growth of individual flakes and their coalescence or merging[88], [89] .Therefore,

with greater holding time, the flakes usually grow larger till they merge to form a

continuous monolayer film or vertical growth of flake can dominate forming

multilayer which is not actually goal of this study.

It has been reported that grain-boundary structure, which is not visible by

optical microscopy but can be visible in the dark field optical images which

suggests that the flakes are not epitaxially connected but rather overlapped. This

can be attributed to the amorphous SiO2 layer that does not support Van Der Waals

65

epitaxy as lattice matched sapphire and GaN would provide [53], [87]. Because of

amorphous nature of SiO2, the distribution of monolayer WS2 either at the center

or at the edge is random. For the growth of 2D TMDs seeds are needed and with

greater holding time (ts), the seeding process continues after the initial growth of

monolayer WS2, which results in multi-layer film formation shown in Figure 25 (c)

and (d).

Figure 25: (a) and (b) Monolayer WS2 on SiO2/Si substrate (c) and (d) Multilayer WS2 on SiO2/Si substrate

According to reported data, boundaries inside the flakes confirm existence

of structural defects (line defects). Some of our samples also show butterfly like

flakes revealing line defects inside the flake [90]. These line defects are labeled as

grain-boundaries inside the butterfly shape flake due to the nature of growth where

more than one flake seem to merge [90]. According to some other studies, the line

defects demonstrated inside the single flake are hardly found to be the grain-

boundary but more likely to be the defects related to the stress effect (X. Wang et

al., 2013). These dark regions are suggested to be the seeds. In our lab, we look

(a) (b)

(c) (d)

20 µm 20 µm

10 µm 10 µm

66

for the change in color contrast in between the flake and the bare substrate to have

an idea about the number of layers present in the sample. Figure 26 shows a

bright field and dark field image of the same region taken by same con-focal

microscope distinguishing monolayer and multilayer effectively.

Figure 26: (a) Optical Image under Bright Field where monolayer is visible and (b) Optical Image under Dark Field where monolayer is not visible

4.3.2. SEM &EDS

SEM (NovaNanoSEM, 30 keV, 7000X) and EDS study show the

morphological and elemental analysis of monolayer WS2. The SEM images and

EDS Spectra have been shown in Figure 27 and Figure 28. SEM images clearly

show uniform deposition of WS2 on SiO2/Si substrate, there is no overlapping or

merging of crystal that form grain boundaries for this sample. EDX study further

confirms the presence of W and S element after CVD deposition.

(a) (b)

10 µm 10 µm

67

Figure 27: SEM of CVD grown Pristine monolayer WS2

Figure 28: EDS of Pristine monolayer WS2 (a) before CVD deposition and (b) after CVD deposition

4.3.3. TEM

TEM (Hitachi) has been performed on pristine monolayer WS2 grown

directly on Si3N4 TEM grids (Low Stress Nitride TEM grids with windows, Nitride

Thickness 50 mm, Window size: 0.5 mm x 0.5 mm)to investigate the crystal

structure and the Selected Area Diffraction Pattern (SAED) shows Hexagonal

single crystalline structure in trigonal prismatic co-ordination ((Figure 29 (b)).

Monolayer WS2 shows a three-fold symmetry in terms of the two sets of spots in

its diffraction pattern, the bright ka spots (indicated by green arrow) and dark kb

(a) (b)

68

spots (indicated by blue arrow). The asymmetry of the W and S sublattices

separates the [1̅100] diffraction spots into two families: ka= {(1100), (1010̅), (011̅0)}

and ka=-kb. We are able to grow WS2 directly on TEM grids which have not been

performed before.

Figure 29: TEM of Pristine monolayer WS2 directly grown on TEM grids (a) TEM Image and (b) SAED Pattern

4.3.4. Raman Spectroscopy

The next approach was to perform characterization techniques that can

further give us confirmation about successful growth of monolayer WS2 on SiO2/Si

substrate. At first, Raman Spectroscopy (ReinshawinVia System 2000 micro-

Raman in backscattering geometry, Excitation wavelength: 514 nm polarised

green laser light, 50X objective lens on an optical microscope attached with the

Raman Spectroscopy) has been done as a fingerprint characterization technique

to identify monolayer WS2. The Raman Spectra of monolayer WS2 has been shown

in Figure 30 (a) and how number of layers depend on the intensity ratio has been

shown in Figure 30 (b).

1120̅

2̅110

100

110

(a) (b)

1120̅

011̅0

1̅100

2̅110

ka

kb

100

110

69

Figure 30: (a) Raman Spectra of monolayer WS2, (b) Intensity ratio vs Number of layers

Figure 30 (a) shows the Raman scattering spectra between 355 and 417

cm-1 that matches exactly with the [73]. Monolayer WS2 shows two characteristic

Raman peaks corresponding to in-plane vibration of W and S atoms (E12g) at 355

cm-1and the out-of-plane vibration of S atoms (A1g) at 417 cm-1where the change

in difference between these two peaks and intensity ratio IE12g/IA1

gare used as an

indicator for the number of layers[81]. When the layers decrease, the mode at 355

cm-1 shifts to the lower frequencies and the mode at 417 cm-1 shifts to higher

(a)

(b)

70

frequencies. According to literature, WS2 has three main Raman active phonon

modes (2LA(M), ~350 cm−1; E12g, ~355 cm−1; and A1g, ~418 cm−1)[91].

The 350 cm-1 peak is attributed to the 2LA (longitudinal acoustic) mode

merged with the E12g modes. The LA phonon vibrational mode, as a function of

crystalline disorder, arises from in-plane collective movements of atoms in the

lattice, while the E12g is optical mode and originates from the in-plane vibration of

S and W atoms. On the other hand, the 417 cm-1 Raman peak is the out-of-plane

vibration A1g characteristic of WS2. It has been reported that not only the frequency

difference (∆) of E12g and A1g peaks, but also the peak intensity ratio of 2LA to A1g

of WS2 is highly sensitive to its thickness. For single-layer WS2 grown on SiO2 at

an excitation wavelength of 514 nm, the height of the 2LA (E12g) peak is roughly 2

times that of the A1g peak (I2LA(E12g)/IA1g ~ 0.9 for bilayer and smaller than 0.9 for

three or more layers). Our-grown WS2 shows I2LA(E12g)/IA1g ~ 2 with a frequency

difference ~ 62 cm-1 under 514 nm excitation, which evidences that our CVD-grown

material is monolayer WS2.

4.3.5. AFM

AFM is being considered as one of the important characterization

techniques for 2D materials for measuring thickness. AFM (Bruker) has been

performed in 4D Labs to measure the thickness of monolayer WS2. The thickness

of monolayer WS2 is ~1 nm indicating successful growth of monolayer WS2; on the

other hand; the thickness of bi/tri-layer is more than 1 nm (Figure 31).

(a)

71

Figure 31: (a) AFM mapping of monolayer WS2 (b) Height profile along Blue Line,

4.3.6. XPS

X-ray Photoelectron Spectroscopy (XPS) was performed to reveal the

change of the elemental composition. The binding energy peak position of a

specific element depends on the oxidation state. The standard C 1s peak (285 eV)

is used as a reference for correcting the shifts. Figure 32 (a) shows the high

resolution XPS spectra of S 2p doublet in the pristine WS2 monolayers confirming

successful deposition of WS2 by CVD. The binding energies of S 2p3/2 and S 2p1/2

located at around 162.4 and 163.7 eV, corresponding to S2−and S22−species [75],

[83], [84], [92]–[94]((Figure 32 (a)). Three characteristic XPS peaks of WS2 at

binding energies 32.6 eV, 34.7 eV, and 37.9 eV corresponding to W4f7/2, W4f5/2,

and W5p3/2 core energy levels, respectively, are observed for tungsten (W) atom.

The W4f7/2, which represents the 4+ valence state, showing a dominant

contribution and it indicates the WO3 (6+) precursor is sufficiently sulfurized even

without employing H2 in our experimental setup. The binding energies at around

32.6 and 34.7 eV reveal the +IV chemical states of W corresponding to WS2

monolayers as shown in Figure 32 (b). As for WS2 monolayers, tungsten with a

valence electronic configuration of 6s25d4 possesses an electropositive property

and acts as an electron acceptor [75]. When electronegative S2−(electron donor)

ions in the sulphur based acid, chemical solution or vapor are incorporated into

(b)

72

WS2 monolayers, they occupy the location of sulfur vacancies or absorbed by

WO3−x species and electrons can effectively be injected into the WS2 monolayers.

In addition, the absorption of the sulphur element can effectively passivate the

structural defects and decrease the non-radiative recombination centres [75], [83],

[84]. Binding energy with respect to the doping element cannot be detected which

indicates that no extra impurity is induced after the chemical treatment. These

results further clarify that S2−in the chemical solution plays a key role in strong PL

enhancement and the chemical interaction is subsistent between the solution and

the pristine WS2.In the case of modifying a sample by adding other elements, if the

electro negativity of the doping element is higher than the base element, the

electron density around the base element decreases and the binding energy

increases. Therefore, binding energy peak shifts positively. Conversely, if the

electro negativity of the doping element is lower than the base element, the

electron density around it increases and the binding energy decreases, leading a

red shift in BE peak position. The main cause of the peak shift in XPS spectra is

mostly related to chemical shifts due to the presence or absence of the chemical

states of the element having different formal oxidation state[75], [83], [84]. The

intensity may also be changed because it is directly linked to the number of atoms

in the respective chemical state. It has been reported [75], [83], [84] that peaks at

36.1 eV and 38.2 eV are referred to as +VI chemical state compounds, such as

WO3.The presence of such peaks depends on the preparation of the sample. In

our experiment, these peaks are absent in the XPS data, assuming that our

samples are prepared in a clean way confirming no presence of WO3 residue.

Sulphur based acid, chemical solution or vapor can be used to enhance PL

because of the presence of S2−and S22−species in pristine WS2 monolayers and

the chemical interaction is subsistent between the Sulphur based acid, chemical

solution or vapor and the pristine WS2 monolayers. It has been reported that the

chemical interaction between the WS2 monolayers and sulphur based acid,

chemical solution or vapor occurs slowly from the edge towards the inner region

and the electrons are gradually injected during the chemical treatment process

eventually leading to enhanced PL[75], [83], [84], [92]–[94].

73

Figure 32: XPS Spectra of monolayer WS2 (a) S 2p, (b) Core level W 4f

4.3.7. PL

4.3.7.1. Room Temperature PL of Pristine WS2

Photoluminescence spectroscopy (Leica TCS SP5 II- The Broadband

Confocal system, 488 nm excitation wavelength, Ar laser, 193 µW laser power, 5

mints, 20X magnification with Green Filter) revealed a sharp but weak emission

peak at ~626 nm confirming indirect (bulk) to direct band-gap (monolayer)

transition in the monolayer triangular WS2as shown in Figure 33 (f).

(a)

(b)

74

(a)

(c)

(b)

75

(d)

(e)

(f)

76

Figure 33: (a,b) Fluorescence Images of monolayer WS2, (c) PL intensity map of pristine monolayer WS2, (d) 2D surface plot of Pristine WS2, (e) 3D Surface Plot of Pristine WS2and (f) PL spectra of grown monolayer WS2

Figure 33 shows the photoluminescence properties of the grown WS2

monolayers are scanned with a 488 nm laser with 5 mints integration time. Figure

33 (c) demonstrates the total PL intensity map. Figure 33 (f) shows the PL spectra

with the exciton peak at ~626 nm, this result shows excitonic emission is more

profound at the interior regions of the flake whereas luminescence is due to trion

emission at the edges. In Figure 33 (c), maximum PL intensity map contains dark

regions where we do not observe PL. As we can see from photoluminescence

image, it shows non-uniform photoluminescence and darkening at the center of

monolayer triangular WS2. To understand such results based on ongoing research

and previous studies, point defects (S vacancies) in monolayer WS2can

significantly trap free charge carriers and localize excitons, leading to the

suppressing of free band-to-band exciton emission. In general, CVD-grown TMDs

could exhibit a unique PL property which is absent in their exfoliated TMDs

counterparts. Defects within the monolayer WS2 crystal act as non-radiative

recombination sites and thus quench the intrinsic PL. The darkening of PL in the

center of WS2 islands is attributed to the presence of structural defects such as

point defects and dislocations within the metastable nuclei and charge defect

induced doping. During the CVD growth procedure, the initial nucleation occurs at

the beginning of the growth process followed by the incoming WS2species

coalescing into the nuclei leading to an enlarged grain. Point defects formed by S

vacancies can greatly quench the PL of monolayer due to the trapping of free

charge carriers and non-radiative recombination. Non-uniform PL features are

caused by structural imperfection and n-doping induced by charged defects.

Uniform PL is found to be intrinsic, intense and non-blinking, which are attributed

to high crystalline quality. Variation in PL could be responsible, including external

electrostatic doping induced by the dielectric environment, strain, absorbates/

clusters and structural defects. Defects will give rise to gap states and will reduce

the device performance. For example, sulfur vacancies are seen to reduce the

77

photoluminescence efficiencies by typically 104.Crystals having more defects in

them, such as stacking faults, twins etc., which can act as non-radiative

recombination centers. Semiconductors are characterized by two types of mobile

carriers, electrons in the conduction band and holes in the valence band. Both

bands are separated by an energy gap. When an electron loses energy and falls

into the valance band, it gets neutralized by a hole which absorbs its energy. This

process is called recombination and the energy of recombination will emerge as a

photon. Most common cause for non-radiative recombination events are defects

in the crystal structure. This effect includes unwanted foreign atoms, native

defects, dislocations. All such defects have energy level structure that are different

from substantial semiconductor atoms. And it’s quite common for such defects to

form one or several energy levels within the forbidden gap of the semiconductor.

Energy levels within the gap of the semiconductors are efficient recombination

centers. Trap assisted recombination occurs when an electron falls into a “trap”,

this is an energy level within the bandgap caused by the presence of a foreign

atom or a structural defect. Once the trap is filled it cannot accept another electron.

The electron occupying the trap, in a second step, falls into an empty valence band

state, thereby completing the recombination process. Atoms at the surface cannot

have the same bonding structure as bulks atoms due to the lack of neighboring

atoms. Thus, some of the valence orbitals do not form a chemical bond. These

partially filled electron orbitals, or dangling bonds, are electronic states that can be

in the forbidden gap of the semiconductor where they act as recombination center

leading to a reduced luminescence efficiency. Understanding of indirect to direct

band gap transition is important because high-intensity PL has appealed special

attention to study optical properties of such large-area monolayers that can be

used for novel photonic devices[87].

4.3.7.2. Room Temperature Laser Power Dependent PL of Pristine WS2

As we know, when laser light is irradiated on to the sample, light is

absorbed, and the excess energy is used in photoexcitation within the specimen.

The photoexcitation causes the electrons to promote into available excited states.

78

The electrons in these excited states would then eventually relax into a lower

equilibrium state and the excess energy is released which may result in the

emission of light (radiative process) or a nonradiative process. Thus, the energy of

the emitted light released during the relaxation of the excited electron is the

difference between the energy level of the excited state and the equilibrium state.

Therefore, with increase in laser power, the photon emission would be much higher

eventually resulting in increase of exciton peak in pristine WS2.

(a)

(b)

79

Figure 34: (a) Laser Power Dependent PL study of Pristine WS2 in terms of Wavelength (nm) and (b) Laser Power Dependent PL study of Pristine WS2

in terms of Photon Energy (eV)

From Figure 34, we can see that with the decrease of laser power, the

exciton peak tends to decrease as well corresponding to lower photon emission

and weaker PL intensity. On the other hand, with the increase of laser power, the

exciton peak tends to increase as well corresponding to higher photon emission

and higher PL intensity compared to lower laser power. The system has limitation

of using it at higher laser power, so we were only able to use 50% Ar laser power

that is equal to 272 µW. We have used power meter to accurately measure power.

Figure 35: PL Intensity variation with Laser Power in log scale

Figure 35 shows how PL intensity varies with laser power in log scale. It

shows that the relation of PL intensity of pristine WS2with laser power is almost

linear where R2 value is 0.99548. It is important to interpret this data confirming

linear relationship of PL intensity with Laser Power.

80

4.3.7.3. Room Temperature PL Variation of Pristine WS2 along certain lines

From PL intensity map we see that the pristine WS2 shows non-uniform

nature whereas the edge shows strong PL emission (brighter) corresponding to

biexciton emission according to literature and when we move from edge to middle

it starts to show weak emission specially in the middle assuming presence of more

defects or thicker region considered as seed in the middle. Therefore, it is

important to show how PL varies along lines with distance for pristine WS2. Further

study is necessary for clearly explaining the data based on STM and HRTEM study

that could potentially show presence of defects in pristine WS2.

Line 1

(a) (b)

Line 2

(c) (d)

81

(h) (g)

Line 4

Point A

Point A

Line 3

(e) (f)

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Figure 36: PL Intensity variation along line with distance (a) PL Image of Pristine WS2 with Line 1, (b) PL variation along Line 1, (c) PL Image of Pristine WS2 with Line 2, (d) PL variation along Line 2, (e) PL Image of Pristine WS2 with Line 3, (f) PL variation along Line 3, (g) PL Image of Pristine WS2 with Line 4, (h) PL variation along Line 4, (i) PL Image of

Pristine WS2 with Line 5, (j) PL variation along Line 5 and (k) PL Image of Pristine WS2 with Line 6, (l) PL variation along Line 6

Figure 36 shows how PL intensity changes along a line within a particular flake.

Figure 36 (a), (b) and (c) shows almost uniform and higher PL intensity along Line

1, Line 2 and Line 3 whereas, along Line 5, we can see a dip in the PL intensity

corresponding to middle region of the flake probably due to presence of defect or

thicker region acting as seed for formation of pristine WS2. Higher and almost

(j) (i)

(l) (k)

Line 6

Line 5

Point B

Point B

Point C

Point C

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uniform PL intensity along Line 1, Line 2 and Line is due to biexciton formation,

with increasing laser power (272 µW) the formation of biexciton increases resulting

in higher photoemission corresponding to stronger PL signal.

84

Chapter 5: PL Enhancement of Monolayer WS2

Chapter 5 discusses the purpose of PL enhancement, experiment, results and

discussion.

5.1. Purpose of PL Enhancement

The purpose of PL enhancement is as follows:

1. According to literature, mechanically exfoliated and CVD grown monolayer

2D materials tend to show weak PL; which is not applicable for

optoelectronics such as LED. So, what’s the solution? Surface modification

is supposed to passivate the point defects and surface vacancy/Sulphur

vacancy which ultimately reduces the non-radiative recombination sites and

show enhanced exciton peak.

2. Suppression of Phonons takes place significantly at Low Temperature (LT)

and hence enhanced PL is observed at 77K or below for 2D materials, but

we performed experiment at Room Temperature (RT); carriers at RT can

have enough energy to get to non-radiative recombination centers, so, in

general, a strong reduction of the intensity of the PL signal is observed

which is not seen in our samples at RT; according to literature excitonic

effects are more efficient at LT but for our samples exciton peak is sharp at

RT. Most of the LEDs operating temperature is ~298k not 77k; therefore, it

is important to focus on achieving enhanced PL at RT.

According to literature that has been discussed in Chapter 3, different chemical

reagents have been used to treat pristine TMDs to achieved high QY with

enhanced light emission. Still, there is scope for exploration in this field and in this

chapter H2SO4-vapor has been used for the first time for surface passivation of

pristine-WS2. The methodology is discussed below.

85

5.2. Methodology

At first, monolayer WS2 was grown on SiO2/Si using 3-zone CVD furnace.

Then, 50 ml 2.24 M H2SO4 (equivalent to ~20%) has been used to treat the

samples with vapor on hotplate for 15 mints at 150ºC then blow dry without rinsing

it in water; then again put it on hotplate for 10 mints at 150ºC. At 337ºC, 98% pure

H2SO4 starts vaporizing, by diluting it we can use lower temperature such as 100-

150ºC. For this study, a CVD grown sample with very good coverage 1.5 cm x 1.5

cm was first cut into two species, then one of them was being used for PL

measurement without any surface treatement regarded as pristine and other part

was being used for H2SO4 vapor treatment and PL measurement was performed

after being treated with H2SO4 vapor for comparison. The experiment was

repeated thrice to verify and repeatibility of the process. Further trial and error

should be performed with the same PL system before making a final conclusion.

In addition, 1.0 M, 0.5 M and 0.05 M H2SO4 solution as vapor has been used to

see the PL enhancement effect but we didn’t notice any kind of enhancement.

Simultaneously, we performed dipping samples in 1.0 M, 0.5 M and 0.05 M H2SO4

solution but the samples got destroyed eventually.

(a)

86

Figure 37: (a) Setup of H2SO4 Vapor Treatment in Yellow Room, (b) Schematic Setup of H2SO4-Vapor Treatment

5.3. Results and Discussion

5.3.1. PL Enhancement of H2SO4-Vapor Treated Monolayer WS2

We have already discussed the purpose of PL enhancement before. Now

let’s see the possible mechanism of PL enhancement using strong protonated

acid. Here, we will try to discuss possible mechanism of PL enhancement using

an acid based on the reported studies. To further clarify we will need to perform

High Resolution Transmission Electron Microscopy (HRTEM), Scanning Tunneling

Microscopy (STM) that will give us more details in atomic level how acid treatment

modify the structure. Performing HRTEM, STM has not been done yet because it

is beyond the scope of the master’s thesis. It is important to mention of choosing

H2SO4 among all other acids. Firstly, it is important to select an acid which can

easily donate H+ in an aqueous solution. The criteria for choosing an acid are

Hammett Acidity Function (H0). The acid ionization constants typically used to

measure weak acids' acidity are only valid in dilute aqueous solutions. A more

general measure of acidity that in principle is valid for any acid is the H0. The

Hammett acidity function, H0 is analogous to the pH used to describe the acidity of

aqueous solutions but instead refers to the pure acid:

(b)

87

pH=−log(AH+) (for dilute aqueous solutions)

H0=−log(AH+) (for pure acids)

Where, AH+ is the activity of H+ , which in many dilute solutions is approximately

equal to the hydrogen ion concentration (that is why the pH is often defined in

terms of [H+].

At first glance it may seem that the Hammett acidity function is simply a

generalization of the pH concept for use in non-aqueous solutions. This is

especially so since in water the pH and H0 do refer to the same quantity. However,

the hydrogen ion of the Hammett acidity function is more than a generalization of

the pH concept. Its real genius lies in that AH+ does not necessarily represent an

actual chemical species of identity H+ but rather an acid's ability to protonate weak

indicator bases, B, specifically via the reaction:

H++B⇌BH+--------------------------------(5)

This reaction gives the weak acid BH+ which can ionize in the reaction that is the

reverse of that above:

BH+⇌B+H+---------------------------------(6)

The extent of this ionization will depend on AH+ according to

Kion=[BH+]/AH+[B]--------------------------(7)

Taking the negative logarithm of both sides and rearranging gives the Henderson-

Hasselbach equation for the indicator base, B:

−Kion=−logAH+−log[BH+]/[B]--------------(8)

which can be rearranged to give:

pKion= H0−log[BH+]/[B]--------------------(9)

or,

H0= pKion−log[BH+]/[B]--------------------(10)

From this it is apparent that Ho represents an acid's ability to donate a hydrogen

ion, as measured in terms of its ability to shift the equilibrium between B and BH+

towards BH+. More negative values of H0 correspond to stronger Brønsted acids

with a greater hydrogen ion transfer ability while less negative ones indicate

weaker Brønsted acidity. The value of H0 has been experimentally determined for

a number of strong acids by measuring the ratio of BH+ to B using weakly basic

88

aromatics like 2,4,6-trinitroaniline, various nitrotoluenes and trifluoromethyl-

benzene as the indicator base, B. Sulfuric Acid has a Hammett acidity of -11.3.

Since superacids are defined as acids with greater Brønsted acidity than pure

sulfuric acid this means that superacids have (H0<11.3]. A list of organic and

inorganic acid is given below based on H0 that can be used further for future studies

of PL enhancement:

1. Fluoroantimonic acid (H0 = −21 and −23 respectively)

2. Hydrogen fluoride (H0 = −15.1)

3. TFSI (H0 = −14) with 1,2-Dichloroethane (as solvent)

4. HClO4 (H0= -12)

5. Sulfuric Acid (H0 = −11.3)

From the above list, TFSI has already been used widely for PL

enhancement of TMDs. The function of strong acid is to supply a strongly acidic

ambient to push the equilibrium toward greater binding of hydrogen with the Mo/W

dangling bonds to passivate defect states in TMDs, the passivation method should

deactivate only the defect states without a permanent change in the intrinsic crystal

and electronic structure of TMDs. Moreover, the adsorbed molecules should be

chemically and thermally stable on TMDs; consequently, they should not

decompose or desorb during the fabrication processes or during operation under

ambient conditions.

(a) (b)

89

Figure 38: PL Intensity mapping of Pristine-WS2 (a) before and (b) after H2SO4-vapor Treatment

(a)

90

Figure 39: PL Intensity vs Wavelength (nm) before and after H2SO4-vapor treatment

Figure 40: Normalized PL Intensity vs Wavelength (nm) before and after H2SO4-vapor treatment

The PL measurement was performed using the same PL system with same

configuaration before and after surface treatment with H2SO4 vapor. The

experiment was repeated thrice but further trial and error should be performed

keeping all paramenters and configuaration constant. In chapter 4, we saw that

pristine WS2 showed PL peak at ~626 nm whereas after H2SO4 vapor treatment,

the peak shifts to ~634 nm. Figure 38 shows PL mapping of monolayer WS2 before

and after H2SO4 vapor treatment. From Figure 38, it is evident that H2SO4 vapor

has potential to passivate the surface defects of pristine WS2 that eventually

results in PL enhancement. Moreover, in Figure38 (a) the pristine WS2 shows

more PL emission from edge compared to other region, due to the formation of

biexciton. Figure 39 and Figure 40 shows PL enahncement before and after

H2SO4 Vapor treatment indicating maximum ~10 fold enhancement and on

average ~5 fold enhancement. Surface passivation by chemical treatments induce

defect-mediated non-radiative recombination and biexciton recombination. Deep-

91

level traps contributes to defect-mediated non-radiative recombination, resulting in

a low PL being observed in pristine WS2((Figure 38 (a)). The strong protonating

nature of the strong acid could remove absorbed water, hydroxyl groups, oxygen

and other contaminants on the surface of a sample. Although these reactions

would not remove the contribution of defects to non-radiative recombination, they

would open the active defect sites to passivation by a second mechanism. One

possibility of enhancement could be the protonation of the three dangling bonds at

each sulfur vacancy site.The primary focus should be on the enhancement of

exciton peak; which is being achieved in this investigation. After normalizing the

data, according to literature, acid treatment can promote sharp exciton as well as

trion emission (shifting towards right in terms of wavelength) from defective

monolayers which is relatable to the data. After H2SO4 Vapor Treatment, the PL

spectra is moving towards red-shift (in terms of wavelength) corresponding to

increase of trion emission and biexciton formation due to effective n-type doping

by vapor treatment. Based on literature search, it is possible to say with brief

explanation that the reason behind PL enhancement is because of exciton (major

peak), increase of trion emission and biexciton formation (the slight shift in peak

indicates trion emission).If the exciton encounters a defect, it may be trapped,

retaining some of its exciton character. Bound excitonic states are unstable and

will decay into stable states with less energy by emitting a photon.

5.3.2. Room Temperature Laser Power Dependent PL of H2SO4-Vapor Treated monolayer WS2

Room Temperature Laser Power Dependent (272 µW, 193 µW, 119 µW,

58 µW, 18 µW) PL study of H2SO4- vapor Treated monolayer WS2 has been

performed with the same PL system. Figure 41 shows stronger PL signal after

H2SO4- vapor treatment with increasing laser power. The reason we can predict

could be two folds: (1) surface passivation of the Sulphur vacancies due to vapor

treatment and (2) increase in laser power promotes more excitation which

eventually leads to more photoemission.

92

(a)

(b)

93

Figure 41: (a) Laser Power Dependent PL study of H2SO4-Vapor Treated Monolayer WS2 in terms of Wavelength (nm) and (b) Laser Power

Dependent PL study of H2SO4-Vapor Treated Monolayer WS2 in terms of Photon Energy (eV)

Figure 42: PL Intensity variation with Laser Power in log scale

Figure 42 shows how PL intensity varies with laser power in log scale. It

shows that the relation of PL intensity of H2SO4-vapor treated monolayer WS2 with

laser power is almost linear where R2 value is 0.93011.

5.5.3. Room Temperature PL Variation of H2SO4-Vapor Treated WS2 along

certain lines

We follow the same procedure that we showed in chapter 4, here we instead

tried to study the PL Variation of H2SO4-Vapor Treated WS2 along certain lines.

94

(f) (e)

Line 2

Line 1

Line 3

(a) (b)

(c) (d)

95

(h) (g)

(j) (i)

(l) (k)

Point A

Line 6

Point A

Line 5

Line 4

96

Figure 43: PL Intensity variation along certain line (a) PL Image of H2SO4-Vapor Treated Monolayer WS2 with Line 1, (b) PL variation along Line 1, (c)

PL Image of H2SO4-Vapor Treated Monolayer WS2 with Line 2, (d) PL variation along Line 2, (e) PL Image of H2SO4-Vapor Treated Monolayer WS2

with Line 3, (f) PL variation along Line 3, (g) PL Image of H2SO4-Vapor Treated Monolayer WS2 with Line 4, (h) PL variation along Line 4, (i) PL

Image of H2SO4-Vapor Treated Monolayer WS2 with Line 5, (j) PL variation along Line 5 and (k) PL Image of H2SO4-Vapor Treated Monolayer WS2 with

Line 6, (l) PL variation along Line 6

Figure 43 shows how PL intensity changes along a line within a particular flake

after H2SO4 vapor treatment. Figure 43 shows nearly uniform and comparatively

higher PL intensity along Line 1, Line 2 and Line 3 whereas in Figure 43 (a), (b)

and (c), along Line 6 in Figure 43 (f), we can see a dip like we saw in Chapter 4

for pristine WS2 but the PL signal is much more stronger. This study somehow

reveals that after H2SO4 vapor treatment of monolayer WS2; the PL intensity

enhances indicating nearly uniform surface passivation of Sulphur vacancies of

monolayer WS2.

5.3.4. XPS Study of H2SO4- Vapor Treated Monolayer WS2

XPS study has been done using same system in 4D labs with same

configuration that has been used for pristine WS2 in Chapter 4. Figure 108 shows

XPS spectra S 2p and core level W 4f before and after H2SO4 vapor treatment.

The slight shift in both XPS spectra indicates the surface modification has been

done by changing chemical composition. In Chapter 4, before vapor treatment, the

binding energies of S 2p3/2 and S 2p1/2 locate at around 162.4 and 163.7 eV,

corresponding to S2−and S22−species[75], [83], [84], [92]–[94] ((Figure 44 (a)).

Three characteristic XPS peaks of WS2 at binding energies 32.6 eV, 34.7 eV and

37.9 eV corresponding to W4f7/2, W4f5/2, and W5p3/2 core energy levels,

respectively in Figure 44 (b). After vapor treatment the binding energies of S 2p3/2

and S 2p1/2 locate at around 163.1 eV and 164.3 and three characteristic XPS

peaks of WS2 at binding energies 33.7 eV, 35.4 eV and 38.6 eV corresponding to

W4f7/2, W4f5/2 and W5p3/2 core energy levels, respectively[75], [83], [84], [92]–[94].

97

We can also see a relative up-shift about in both spectra in the binding energies of

W 4f and S 2p which can serve as an indicator of n-doping in WS2 monolayers by

H2SO4 vapor treatment[75], [83], [84], [92]–[94]. This core-level shift toward a

higher binding energy proved a relative shift of the Fermi level toward the

conduction band edge. A brief discussion of possible mechanisms of PL

enhancement in pristine WS2 using Sulphur based acid, chemical solution or vapor

is presented in Chapter 4. More experiments such as High-Resolution TEM and

Scanning Tunneling Microscopy (STM) are needed to further understand the

mechanism behind PL enhancement.

(a)

98

Figure 44: XPS Spectra of monolayer WS2 before and after H2SO4 vapor treatment (a) S 2p and (b) Core Level W 4f

(b)

99

Chapter 6: Future Projects & Conclusion

Chapter 6 discusses limitation, future work and conclusion.

6.1. Limitation

Further experiments should be performed with the same PL system keeping other

conditions same along with STM and HRTEM studies to verify the process. More

controllable furnace such as tube furnace is needed for controlling and introducing

Sulphur vapor more precisely.

6.2. Contribution

The main contribution of this thesis in the field of PL study, is to try surface

passivation with unique and novel chemical reagents H2SO4-vapor at room

temperature. When treated with H2SO4-vapor, pristine-WS2 shows maximum 10-

fold enhancement with enhanced exciton and trion emission at RT that has not

been studied before.

6.3. Future Work

The goal of this master’s thesis was to develop a CVD process where we can get

repeatable results, then further characterize it with different techniques and finally,

perform surface passivation using H2SO4-vapor. The goal has been achieved

successfully. Moreover, the results I have achieved can pave way for other future

projects as follows:

1. Monolayer WS2 can be used as piezo-sensors for harmful chemical and gas

detection

2. Monolayer WS2 functionalized with cytokines can be used as biosensor for

cancer detection

3. Surface passivated monolayer WS2 has potential applications in LED

100

6.4. Conclusion

In conclusion, our first approach was to get monolayer WS2 on SiO2/Si substrate

by performing CVD. We played with multiple factors to master the technique and

to get repeatable results. Then, we performed multiple characterization techniques

such as micro Raman and PL, AFM, TEM, SEM and EDX, XPS optical imaging for

confirming successful growth of monolayer WS2. The results we achieved where

the largest lateral crystal size is ~75 um and avg. lateral crystal size is more than

~20-25 um. Data regarding Raman and PL matched with the reported ones.

Photoluminescence from our pristine sample is relatively weak and the possible

reasons behind weak PL have been discussed based on previous and ongoing

studies. Then, to enhance PL we have used 50 ml 2.24 M H2SO4 vapor treatment,

showing maximum ~10 times PL enhancement and on average ~5 times PL

enhancement showing peak at ~634 nm corresponding to red-shift trion emission

compared to peak at ~626 nm for pristine-WS2. Finally, the results of some other

projects have been discussed in short that open doors for future projects with

potential applications.

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Appendix A.

Figure A1: Oxygen Plasma treatment before CVD growth for better

coverage of monolayer WS2

Results and Discussion:

Oxygen Plasma treatment has been done before CVD growth for better coverage

of monolayer WS2; but it didn’t work out, as we can see from figure all are multi-

layers.

50 µm

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Figure A2: (a) TEM of Multilayer WS2 transferred on TEM grids, (b) SAED

pattern of Multilayer WS2

Results and Discussion:

SAED pattern of Multilayer WS2 shows a polycrystalline crystal structure of

multilayer WS2 transferred on TEM grids.

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Figure A3: Biexcitonic Emission from Edge of Pristine WS2 with High Laser

Power