BASED SEMICONDUCTOR HETEROSTRUCTURES AND ZnTe:O ALLOYS

OPTICAL, ELECTRONIC AND INTERFACE PROPERTIES OF 2D MoS

LAYERS

BASED SEMICONDUCTOR HETEROSTRUCTURES AND ZnTe:O ALLOYS

INTU SHARMA

DEPARTMENT OF PHYSICS

INDIAN INSTITUTE OF TECHNOLOGY DELHI

JULY 2017

© Indian Institute of Technology Delhi (IITD), New Delhi, 2017

OPTICAL, ELECTRONIC AND INTERFACE PROPERTIES OF 2D MoS

LAYERS

BASED SEMICONDUCTOR HETEROSTRUCTURES AND ZnTe:O ALLOYS

By

INTU SHARMA Department of Physics

Submitted

In fulfillment of the requirement of the degree of Doctor of Philosophy to the

INDIAN INSTITUTE OF TECHNOLOGY DELHI

JULY 2017

C e r t i f i c a t e

I am satisfied that the thesis entitled “Optical, electronic and interface properties of 2D MoS

record of original and bonafide research work carried out by her under my supervision. The results contained in it have not been submitted in part or in full to any other university or institute for the award of any degree/diploma.

Prof. B. R. Mehta Department of Physics

Indian Institute of Technology Delhi

New Delhi, India-110016

iii

A c k n o w l e d g e m e n t s

Along the journey of my Ph. D at Department of Physics, Indian Institute of Technology Delhi, I have been supported scientifically and emotionally by many people. Although few words are not enough to justify their contribution, still I am trying in my best possible way to thank all who contributed directly or indirectly to this thesis.

Every Ph. D is a challenging journey and successful completion of this journey requires a mentor, who in my case is Prof. B. R. Mehta. With an attitude of trying challenging research problems and keen observation of research progress, he made this thesis a reality. Therefore, first and foremost, I wish to express my sincere thanks and reverence to him for guiding me from beginning to the end of this challenging journey. While writing these lines, I can vividly recall few occasions when I almost broke down on the Ph. D path. It was the tremendous faith shown by him in my research abilities that helped me to overcome such situations. At this point of time, I can clearly say that stepping stone of this work lies in the inspiration given by him that “Never Give Up and Keep Working”. For me, besides the appreciation given by him, the level of patience and calmness that he maintained on failures made the things little easy when nothing seemed to be working. His perfection of analysing data and designing the manuscript helped me to learn the art of paper writing in a precise and concise manner. Working under his guidance not only enriched my existing knowledge about materials science but also gave me many lessons about rational thinking, great leadership and an organized way of carrying out tasks. I would also like to thank Dr. Poonam Mehta for keeping a track of my research progress and giving me motivation at numerous occasions.

I sincerely acknowledge our collaborators Prof. Juerggen Brugger and Dr.

Andreea Veronica from Ecole Polytechnique Fédérale de Lau-sanne, Lausanne, Switzerland from whom we got stencil masks.

My sincere gratitude goes to my SRC members Prof. Sujeet Chuadhary, Prof.

Viresh Dutta and Dr. M. C. Bhatnagatar, who could find time to evaluate my Ph. D progress and also for their valuable suggestions. Their suggestions helped me to improve the quality of the work carried out in this thesis.

I would like to thank Heads, Department of Physics, during my tenure Prof.

H. C. Gupta, Prof. K. Thyagarajan, Prof. B. R. Mehta and Prof. Anurag Sharma for the research oriented environment in the Department.

I would take this an opportunity to thank all my teachers from very beginning to until now who motivated and inspired me to excel in my life. This thesis would have not been completed without acknowledging Smt. Nilam Soni, Sh.

Prakash Chand, Sh. Devender Singh, Dr. Satinder Sharma, Dr. Chandan

Bhardwaj, Dr. Sanjay Gupta, Prof. Devender Mehta, Prof. C. N. Kumar, Prof.

iv

Viswamitter and Dr. Bhiwansh Bhera who helped in shaping my childhood and directing me towards my goal in my higher studies. Special thank goes to Prof. S. C.

Kashyap, who was the mentor of my independent study course in M. Tech and co- supervisor of M. Tech thesis. Because of him, I could find a wonderful and renowned supervisor Prof. Matthias Wuttig, RWTH Aachen for carrying out M. Tech thesis work and could realise my DAAD fellowship. During DAAD tenure, while working in international research team I learnt a lot. Besides academic gain, I also visited many countries of European Union where I dream to be in my childhood days.

I express my special thank and sincere gratitude to Prof. K. L. Chopra who is the inventor and the mentor of Thin Film Laboratory; I am blessed to be a part of it.

I would like to thank Dr. Deepak Varandani for his valuable comments and suggestions during all these years. I would also like to thank seniors, Dr. Aadesh P.

Singh, Dr. Yogita Batra, Dr. Mandeep Singh, Dr. Himani Sharma, Dr. B. Geetha Priyadarshini, Dr. Ashok Kumar, Dr. M. G. Sreenivasan, Dr. Bharti singh, Dr.

Saurabh Sengar, Dr. Rakesh kumar, Dr. Navnita Kumari, Dr. Shivani Dhall, Vinod Singh and Saurabh Singh for their help and support. I would also like to thank my colleagues Saatviki, Narayan, Khusboo, Rishabh, Akshay, Dipika and juniors Mehar, Manoj, Mujeeb, Nisha, Vishakha and Manan for their support and friendly behaviour in the laboratory. Especially I would like to thank Chanda Panwar with whom I spent many delightful moments in the laboratory during my initial years. I would like to acknowledge B. R Khatri Sir and Dixit Ji for providing technical help during the experimental part of the thesis.

I would like to thank all the people associated with Nanoscale Research Facility at IIT Delhi, especially, Dr. Raman Kapoor from whom I learnt basic of Atomic force microscope. I would like to thank Dr. J. P. Singh with whom support I was able to access the Raman/PL and AFM instruments whenever I needed.

Few friends who were supportive and helpful throughout both on personal and professional fronts were Suchitra Yadav and Deepti Chudhary. I enjoyed wonderful company of Deepika, Sajid and Nilmani during these years.

I would take this an opportunity to thank my Nana Ji who motivated me to excel during my childhood days by celebrating each of my achievement. I would express my sincere thanks and gratitude to my parents Smt. Rattni Devi and Sh.

Prakash Chand who form the back bone of my existence and all of my achievements. The lesson of hard work, honesty, simplicity and punctuality that they taught me helped me to achieve my dreams and goals. During these years, they visited me frequently as I was unable to visit them due to my busy schedule. I dedicate this thesis to them. My siblings Bindu, Meena and Ashu deserve a special thank for their immense love, care and moral support. They were always beside me whatever the situation was and helped me to come out of that.

I would like to thank my in laws Smt. Prabha Tiwari and Sh. Rajendra

Tiwari for their understanding and supporting nature which helped me to complete

v

my Ph. D work without any hurdle. I would like to thank my sister in law Smt.

Anjali Dwivedi and Sh. Adiya Dwivedi for their love and support.

From the bottom of my heart, I would like to thank Indian Institute of Technology, Delhi for introducing the most important person of my life “Dr.

Ashutosh Kumar. Starting from the friend, the relation evolves to become soul mates. We got married in year 2012 and a new chapter of my life begins. Life was amazing with him. Lot of delightful moments, trips and successes have been celebrated together. All the frustration and failures that came to me during Ph. D.

were always shared by Ashutosh. He was always besides me during those tough situations and always showed faith in me. It was his belief that I will come out of those situations with golden colors and finally I am writing the last few lines of my thesis. I am blessed to have Ashutosh as my husband. There are no words to express his contribution to this thesis. In brief, I would like to say “Materialization of this thesis is possible only due to the tremendous love, faith and affection given by him and the sacrifices that he has made during all these years". Finally, I want to say thank you Ashu for everything.

This work would have not been possible without the financial support of funding agencies such as Counsel of Scientific and Industrial Research, Department of Science and Technology India and Nanoscale Research Facility at IIT Delhi. I could manage my expenses and could make to attend some conferences outside India with their support.

Above all, I owe it all to Almighty God for granting me the wisdom, health and strength to undertake this research task and making me capable of completing it.

At last, I would like to mention that this thesis which comprises of about 175 pages only presents the successful work of my Ph.D., however share of failures was always greater than successes. The main reason behind getting successes was those failures which are not reported in this thesis. Those failures gave me a lesson that I want to convey by means of this famous quote “Failure is always temporary, only giving up makes it permanent”

Date: Intu Sharma

Place: New Delhi

vii

A b s t r a c t

Size and shape dependent properties of the materials provide a platform for utilizing them in various technological applications. Recently, MoS2, a two-dimensional transition metal dichalcogenide (2D TMD) has drawn the attention of a wide section of research community due to extreme flexibility, high mobility, nanoscale thickness and possibility of modulating its electronic structure by tuning number of layers.

Interfaces of MoS2 with other nD (where n=0,1,2,3) materials need to be investigated as they offer interesting physics different from conventional 3-dimensional interfaces. In addition, such studies are crucial for practically employing 2D materials in device applications. Keeping this as the motivation, a major part of the present thesis is focussed on investigating optical, electronic and interfacial properties of 2D materials/thin films and nanoparticles/thin films material systems.

The growth of MoS2 layers of desired dimensionsat predefined locations is essential for fabricating opto-electronic devices based solely on MoS2 or on heterostructures based on MoS2. A new route for patterned growth of 2D MoS2 layers by combining radio frequency (RF) magnetron sputtering, stencil mask lithography and vapour phase sulfurization is presented. The given method does not involve use of chemical etchants and organic photoresist, hence provides a simplified process of achieving 2D MoS2 arrays. The control over the number of layers (mono, few and bulk) of MoS2 is achieved by varying the thickness i.e. sputtering time of Mo films. The statistical variation in thickness i.e. number of MoS2 layers within the individual patterns is investigated from Raman mappings while Kelvin probe force microscopy (KPFM) is used to understand surface potential variation across an individual pattern. The present approach is further extended for achieving MoS2 (2D)/WS2

heterojunctions arrays at pre-defined locations. Surface potential analysis using KPFM across individual MoS2 patterns indicates Fermi level at 4.6 eV with n-type nature, whereas similar analysis on uncovered WS2 portion reveals that Fermi level is located just below the middle of the band gap i. e. p type nature of WS2 layer.

Based on these measurements, band alignments in the heterojunction are proposed. Conducting atomic force microscopy technique is employed to obtain

viii localised I-V curves at random locations in the patterned arrays of MoS2/WS2

hetero-junctions. Rectifying I-V behaviour with a good point-to-point uniformity is observed.

Next, interfaces between MoS2, a 2D layered material with ZnS, a three dimensional (3D) semiconductor are investigated by carrying out KPFM based surface potential measurements in surface and junction modes under white light exposure. The difference in the surface potential values in surface and junction modes has been assigned to the interface photovoltages at the heterojunction. Enhanced interface photovoltage is observed in junctions having mono and few layers MoS2 in comparison to bulk MoS2 layer. This suggests active participation of 2D MoS2 layer in photon absorption and charge separation processes taking place close to the junction. This study provides a new method for studying the interfaces of 2D materials based devices without surfaces and adsorbents effects.

Multilayered WS2/MoS2 heterojunctions are achieved by sequential RF magnetron sputtering of W and Mo metal and vapor phase sulphurization. Structural, optical and electrical properties of van der Waals heterojunction are investigated using Raman, spectroscopic ellipsometry (SE) and KPFM, respectively. Charge generation and separation across WS2/MoS2 heterojunction is investigated by carrying out KPFM measurements under white light illumination. Macroscopic I-V characterizations under dark and light revealed rectifying behaviour under dark while efficient carrier generation and separation across interface under white light exposure.

Heterojunctions of MoS2 layers with nanocrystalline ZnTe film (0D) with MoS2

thicknesses varying from few to multilayers have been fabricated by RF magnetron sputtering of ZnTe nanocrystalline film on partially masked MoS2 films which are prepared prior to ZnTe deposition by vapor phase sulfurization of Mo films.

Spectroscopy ellipsometry technique has been employed to investigate optical constants of heterostructures while KPFM measurements under illumination are used to explore generation and separation of carriers by junction field. Findings of SE and KPFM measurements are used to propose possible band alignments, band bending and built in field at the interface.

ix Charge transfer taking place at interface between nanoparticles and 3D semiconductor is also investigated in the present thesis. For this, CdSe NPs/ZnS thin film heterojunctions with varying diameters of CdSe NPs have been prepared using sequential RF magnetron sputtering technique. CdSe NPs/ZnS thin-films show an absorption edge at about 3.5 eV corresponding to ZnS and contribution due to size dependent absorption edge in the energy range 1.74-2.16 eV corresponding to CdSe NPs. Kelvin probe force microscopy studies show that surface potential values of CdSe NPs/ZnS thin-films lie in between that of ZnS (209 meV) and CdSe/ZnS (-4 meV) thin-films which confirms charge transfer between CdSe NPs and ZnS thin-films. The evolution of CdSe NPs/ZnS nanoscale heterojunction has been observed with shift in the surface potential values by varying size and coverage of CdSe NPs.

A part of this thesis is devoted to the investigation of oxygen induced modifications in structural, optical and electronic behaviour of ZnTe thin films. On incorporating oxygen, nanocrystalline character of ZnTe is increased with change in optical properties due to absorption through sub band states and increase in fundamental absorption edge. From density functional theory analysis, origin of these sub band gap states is attributed to oxygen incorporation induced electronic states and Te vacancies. In addition, photoelectrochemical performances of ZnTe with and without oxygen have been investigated where a change over from photocathodic response for ZnTe to enhanced photoanodic response for ZnTe:O thin films along with increased response for low energy photons is observed. These findings are explained in terms of oxygen induced modification in visible light absorption, enhanced surface area due to increased nanocrystalline character and modified electronic properties of ZnTe:O thin films. Modifications in optical properties and enhancement in PEC performance by oxygen incorporation shown in the present study may be useful for developing ZnTe based photovoltaic devices.

xi

औ , MoS2, दो- (2D TMD) ने अपने

लचीलेपन, , औ का

MoS2 n- ( n = 0, 1, 2, 3) कर , - उपयोग , - / औ / , औ

MoS2 MoS2 MoS2 हेटरोस्ट्रक्चर्ड - (RF) , औ सल्फ्युराइजेशन - MoS2 गया

औ , - MoS2 की यह सरल MoS2 ( , औ ) Mo गया - MoS2 गयी

(KPFM) - . MoS2 (2D)/WS2 KPFM - MoS2 MoS2 जो कक n-type कदखाता

है WS2 p-type का उपयोग MoS2/WS2 I-V गया I-V - -

xii आगे, MoS2, - औ ZnS, KPFM तकनीक को सफेद रोशनी में औ में अपयोग की गयी है औ औ MoS2 - MoS2 औ - औ

WS2/MoS2 , W औ Mo RF औ गया , औ , (SE) औ KPFM गयी

WS2/MoS2 औ KPFM तकनीक रोशनी गयी I-V रोशनी जनन औ

MoS2 औ ZnTe (0D) MoS2 , MoS2 ,, MoS2

Mo गयी

, जनन औ KPFM तकनीक रोशनी SE औ KPFM , औ - -

औ , CdSe NPs/ZnS CdSe NPs - RF CdSe NPs/ZnS 3.5 eV ZnS 1.74-2.16 eV CdSe NPs KPFM CdSe NPs/ZnS , ZnS (209 meV) औ CdSe/ZnS (-4 meV) CdSe NPs औ ZnS CdSe NPs औ CdSe NPs/ZnS

xiii उत्रूप्रेररत ZnTe , औ ZnTe ढ़ औ - औ , - उत्रूप्रेररत औ Te , ZnTe उत्रूप्रेररत औ , ZnTe ZnTe:O ढ़ , उत्रूप्रेररत , , औ ZnTe:O औ PEC , ZnTe

xv

T a b l e o f c o n t e n t s

Certificate i

Acknowledgments iii

Abstract vii

Abstract in Hindi xi

Table of Contents xv

List of Figures xxi

List of Tables xxix

Nomenclature xxxi

Chapter 1: Introduction………. 1

1.1 Materials on different dimensions…..……… 3

1.2 Graphene and beyond graphene 2D materials……….. 4

1.3 Two dimensional transition metal dichalcogenides……….. 5

1.4 Molybdenum disulphide………. 6

1.4.1 Crystal structure……….. 6

1.4.2 Properties.……… 7

1.4.3 Preparation methodology.……….. 8

1.4.4 Applications.………... 10

1.5 Heterostructures based on 2D MoS2 semiconductors.……… 10

1.5.1 MoS2/WS2 heterojunctions………. 12

1.5.2 MoS2/3D heterojunctions……….. 13

1.5.3 MoS2/0D heterojunctions……….. 14

1.6 Nanoparticles/thin film nanoscale heterojunctions……… 16

1.7 ZnTe:O alloys films.………. 16

1.8 Objective.………. 18

1.9 Thesis overview………. 18

1.10 References……… 20

xvi

Chapter 2: Experimental techniques……… 27

2.1 Sample preparation.……… 29

2.1.1 Substrate cleaning...……… 29

2.1.2 Sputter deposition...………. 29

2.1.3 Sulfurization...……… ………... 31

2.1.4 Thermal Evaporation……… 32

2.2 Stencil mask lithography...……… 32

2.3 Structural characterisations.……… 34

2.3.1 X-ray diffraction...………. 34

2.3.2 Electron microscopy...………. 34

2.3.2.1 Field emission scanning electron microscopy..……… 35

2.3.2.2 High resolution transmission electron microscopy...…………. 36

2.4 Stylus profilometry……….. 37

2.5 Spectroscopic characterisation……… 38

2.5.1 Variable angle spectroscopic ellipsometry……… 38

2.5.2 Raman Effect...……….. 40

2.5.2.1 Resonance Raman scattering………. 42

2.5.2.2 Raman mapping………. 42

2.5.3 Photoluminescence measurements………. 43

2.6 Surface topography, surface potential and microscopic electrical characterisations……….. 43

2.6.1 Atomic force microscopy………. 43

2.6.2 Kelvin probe force microscopy...……….. 45

2.6.3 Conducting atomic force microscopy.………. 48

2.7. References……….. 48

Chapter 3: Growth of large area 2D MoS2 and MoS2/WS2 arrays at pre- defined locations using stencil mask lithography………. 51

3.1 Introduction... 53

3.2 Sample preparation and experimental details ……….. 55

3.3 Results and discussion ………. 56

3.3.1 Dependence of MoS2 morphology on Mo thickness ………. 56

3.3.2 Patterned growth of MoS2...……….. 58

3.3.2.1 FESEM, topographical and Raman analysis ……… 58

3.3.2.2 Surface potential measurements……….. 63

3.3.3 Patterned MoS2/WS2 heterojunctions……… 65

xvii

3.3.4.1 Experimental details and sample preparation………. 66

3.3.4.2 Result and discussion……….. 67

3.3.4.2.1 MoS2 patterns………. 67

3.3.4.2.2 MoS2/WS2 patterns……… 70

3.3.4.2.3 Electrical characterization of MoS2/WS2 heterojunctions using KPFM and CAFM techniques……… 71

3.4 Conclusions……….……… 74

3.5 References……….………. 74

Chapter 4: Enhanced charge separation at 2D MoS2/ZnS heterojunction: KPFM measurements in surface and junction modes….. 79

4.1 Introduction……….……… 81

4.2 Sample preparation and experimental details ………...……… 82

4.3 Results and discussion ………... 83

4.3.1 Structural characterization of MoS2/ZnS heterojunction……… 83

4.3.2 Surface potential and interface photovoltage studies of MoS2/ZnS heterojunctions ………. 87

4.3.3 Energy band alignments at MoS2/ZnS heterointerface and charge separation under illumination……….. 90

4.4 Conclusions………. 92

4.5 References……… 93

Chapter 5: Optical absorption and electronic properties of multilayers WS2/MoS2 heterojunction …………... 95

5.1 Introduction……….………. 97

5.2 Sample preparation and experimental details……….. 98

5.3 Results and discussion.……… 100

5.3.1 Raman Studies………. 100

5.3.2 Dielectric constant and band gap findings………. 101

5.3.3 Surface potential and charge transfer at WS2/MoS2 heterojunctions ……….. 104

5.4 Conclusions ………. 109

5.5 References………...………. 109

xviii Chapter 6: Optical properties and band alignments in ZnTe

nanoparticles/MoS2 layer hetero-interface using SE and KPFM studies… 111

6.1 Introduction……….. 113

6.2 Sample preparation and experimental details ………...………. 114

6.3 Results and discussion ……… 116

6.3.1 HRTEM and Raman studies………. 116

6.3.2 Dielectric constants and band gaps……….. 117

6.3.3 Surface potential analysis for ZT/MS(M) ………... 120

6.4 Conclusions……….. 123

6.5 References………..………... 123

Chapter 7: Spectroscopic ellipsometry, photoluminescence and Kelvin probe force microscopy studies of CdSe nanoparticles dispersed on ZnS thin film...……….. 127

7.1 Introduction………..……….. 129

7.2 Sample preparation and experimental details ………. 130

7.3 Results and discussion.. ………. 131

7.3.1 HRTEM and X-ray diffraction studies ………. 131

7.3.2 Dielectric constant and optical absorption studies ……… 133

7.3.3 Photoluminescence studies………. 135

7.3.4 Work-function and Kelvin probe force microscopy studies …………. 137

7.4 Conclusions……….. 140

7.5 References………. 140

Chapter 8: Oxygen induced enhanced photoanodic response of ZnTe:O thin films: modifications in optical and electronic Properties………. 143

8.1 Introduction………..……….. 145

8.2 Sample preparation and experimental details ……… 146

8.3 Results and discussion……… 147

8.3.1 Modification in structural and optical properties of ZnTe thin film on oxygen incorporation………. 147

8.3.2 Density functional theory based theoretical calculations and correlation with experimental findings………... 154

8.3.3 Photoelectrochemical measurements ………. 158

8.4 Conclusions………..……….. 162

8.5 References………... 163

xix

Chapter 9: Summary and future perceptive ………. 167

9.1 Summary………..……… 169

9.2 Future perspective ………..………. 171

Publications in International Journals………...………. 173

International/National Conference Presentations ………...………. 174

Bio-data………..………...………. 177

xxi

L i s t o f f i g u r e s

Figure 1.1: Density of states relation with energy as a function of

dimension………. 3 Figure 1.2: Illustration of 2D monolayer materials and examples of flexible

smart systems ………... 5 Figure 1.3: (a) Top view of a monolayer of MoS2. The lattice vectors (a1 and

a2) that define the unit cell are indicated by vectors, and the outline of the unit cell is indicated by dashed lines. (b) The Brillouin zone, with the relevant high-symmetry k points indicated. (c) The structure of trilayer 2H- MoS2. In the 2H stacking the S atoms in each layer are located directly

above and below the Mo atom in the neighbouring layer………. 7 Figure 1.4: Band structure of bulk MoS2 (a), 4-layers MoS2 (b), 2-layers

MoS2 (c) and monolayer MoS2 (d). Bulk MoS2 shows an indirect band gap.

The direct excitonic transitions occur at high energies at K point. With reduced layer thickness, the indirect bandgap becomes larger, while the direct excitonic transition barely changes. For monolayer MoS2 (d) band gap changes to direct band gap, leading to photoluminescence in monolayer

MoS2………... 8 Figure 1.5: van der Waals heterostructures formed by integrating 2D

layered materials with 0D nanoparticles or quantum dots (a), 1D nanowires (b), 1.5 D nanoribbons (c), 3D bulk materials (d) and 2D nanosheets

(e)………. 11 Figure 2.1: Schematic of RF magnetron sputtering process. A mixer of Ar

and 2%O2 in Ar is used for reactive RF magnetron sputtering and only Ar for

RF magnetron sputtering.…………... 30 Figure 2.2: Schematic of sulfurization process in two zone tube furnace.

Prior to sulfurization, Mo/W thin films have been deposited from RF

magnetron sputtering process………... 32 Figure 2.3: Stencil mask fabrication process flow. Process starts with a

double sided polished silicon wafers (a), onto which a low stress silicon nitride layer is deposited on both sides (b). After that patterns are defined on the front side silicon nitride by using EBL and reactive ion etching (c). (d) Patterns are defined on the backside by photolithography for KOH etching

which releases the membrane (e)………. 33 Figure 2.4: Schematic of the operation of scanning electron microscopy……. 36 Figure 2.5: Transmission electron microscope operation in imaging and

diffraction modes……… 37 Figure 2.6: A schematic showing the operating principle of ellipsometry……. 38 Figure 2.7: Block diagram of spectroscopic ellipsometer……… 40 Figure 2.8: Possible scattering processes on the interaction of light with

matter………. 41

xxii Figure 2.9: Electronic energy levels of the sample and the tip (a) tip and

sample are not in electrically contact, (b) tip and sample are so close to make an electrical contact, and (c) an external bias (VDC) is applied between the tip and the sample to nullify the VCPD and, therefore, the tip-sample electrical force. Here, vacuum energy level, Fermi level of sample and Fermi

level of tip are represented by EV, Efs and Eft, respectively……….. 46 Figure 2.10: Schematic of KPFM measurement set up……… 48 Figure 3.1: Schematic of process steps for growth of MoS2 patterns by RF

sputtering of Mo metal through stencil mask followed by sulfurization………. 55 Figure 3.2: FESEM images for MoS2 obtained after sulfurization of Mo

deposited for (a) 2 min, (b) 30 sec and (c) 20 sec. Bulk and few layer continuous MoS2 films are shown in (a) and (c), respectively whereas isolated nano or micron size flakes of MoS2 of varying thickness (ranging monolayer to bulk) at random locations are shown in (b). Scale drawn is 10 µm for (a)

and (c) and 50 μm for (b) .……….. 57 Figure 3.3: (A) and (B) are FESEM images for Mo and MoS2 patterns,

respectively. (a), (b) and (c) are the images corresponding to S10, S2 and S1 samples, respectively. S10, S2 and S1 are deposited through stencil masks consisting of arrays of 10×10, 2×2 and 1×1 µm2 feature sizes, respectively.

Scale drawn is 40 µm ……….. 58 Figure 3.4: (a)-(c) are FESEM image for MoS2 single patterns of S10, S2 and

S1, the magnified images of central (A) and edge (B) points of S10 sample are shown in (d) and (e), respectively. S10 show morphology similar to S2 and S1 at the edges. This is due to screening of Mo atoms in sputtering by aperture walls, which is more in smaller size apertures. Scale drawn is

1μm………. 59 Figure 3.5: (a) and (b) are topographical (AFM) images for S10 Mo and S10

MoS2 samples, respectively. Scale drawn is 10µm. (c) and (d) are profiles of topography for a single horizontal line (red) of the topographical images of Mo and MoS2, respectively. (e) Magnified image of a single feature of S10 sample. Scale drawn is 3 µm. From Mo to MoS2, an increase in thickness from ~4 to 4.5 nm is observed which corresponds to 3-4 layers of

MoS2……... 60 Figure 3.6: Raman spectra of few layers MoS2 corresponding to S10, S2 and

S1 samples which are prepared by sulfurization of Mo deposited through stencil mask along with Raman spectra of bulk and monolayer MoS2

prepared through sulfurization of Mo deposited without stencil mask for ~2

min and ~30s, respectively………. 61 Figure 3.7: (A) (a) Raman intensity map for S10 sample. (b) Raman intensity

map of a single feature of S10 sample. Intensity decreases on going from centre of square features towards the edges. It can be clearly seen that boundaries are not sharply defined but there is a gradual decrease at the edges. (c) Peak positions difference map for intensity map of Figure (b). (B) Histograms for peak position difference for four complete squares marked as 1, 2, 3 and 4 of sample S10. Δ for the most of patterns lies in the range 22-

24 cm^-1 which corresponds to 2-4 layers.………….………. 63

xxiii Figure 3.8: (a) and (b) are surface potential images for S10 Mo and S10

MoS2, respectively. Scale drawn is 10 µm. (c) and (d) are profiles of surface potentials for a single horizontal line of the Mo and MoS2 arrays. (e) Magnified image of a single feature of MoS2, scale drawn is 3 µm. Surface potential values in the range -350 to -370 meV correspond to 2-4 layers and Fermi level position at 0.83-0.85 eV below the conduction band indicates n

type nature of grown MoS2 patterns……….. 64 Figure 3.9: (a) and (b) FESEM images for Mo and MoS2 patterns deposited

through stencil mask consisting of arrays of 5×5 µm² feature size, respectively. Scale bars in (a) and (b) are 40 µm each. (c) FESEM image for

single MoS2 pattern……… 67

Figure 3.10: (a) Raman intensity maps for the MoS2 patterns and (b) magnified intensity map of the marked single pattern. (c) and (d) are peak position difference map and corresponding histogram of this marked single feature, respectively. Peak position difference lies in the range 22-24 cm^-1

which confirms the presence of 2-4 MoS2 layers ……… 68 Figure 3.11: (a) Topographical (AFM) images for patterned Mo and (b) MoS2

samples, scales in (a) and (b) are 10µm, each. Line profiles corresponding to a single horizontal line (red) of the topographical images of Mo (c) and MoS2

(d). (e) Magnified image of a single feature of MoS2 with scale bar of 2 µm…… 69 Figure 3.12: Typical micro Raman spectra of MoS2 pattern/WS2, MoS2

pattern/SiO2 and WS2 layer/SiO2. Raman peaks corresponding to MoS2 as well as WS2 in MoS2/WS2 patterns confirm the formation of MoS2/WS2

patterned hetero-junctions. (b) Optical image of such patterned arrays of

MoS2/WS2 hetero-junction……….. 70 Figure 3.13: Surface potential image (a) arrays of MoS2 patterns, (b) a single

feature of MoS2 and (c) WS2 layer. Scale shown in (a), (b) and (c) correspond to 10, 2 and 1µm, respectively. (d) Surface potential profiles for the marked area of MoS2 pattern and WS2 layer. MoS2 shows an average surface potential value -65 meV which corresponds to 2-4 layers of MoS2. Fermi level is found to be located at 0.6 eV below the conduction band which indicates n type nature of grown MoS2 patterns. Average surface potential across WS2 layer is found to be -776 mV which gives Fermi level position at around 0.8 eV with respect to conduction band suggesting p type nature of WS2 layer. N- and P- type nature of MoS2 and WS2 confirms the formation of 2D materials based hetero-junction.(e) Band diagrams derived from KPFM measurements for MoS2/WS2 heterojunction (i) before and (ii) after contact

formation...……….. 72

Figure 3.14: (a) Schematic of CAFM set up used for characterising MoS2/WS2 heterojunction. (b) and (c) are current scans at -2 and +2V, respectively for MoS2 2D/WS2 heterojunctions Localized I-V measurements are performed by contacting the CAFM tip with MoS2 pattern at 20 random and (d) Typical I-V curves obtained by sweeping the voltage between -2 and +2 V at three random location of the sample. Inset of (d) shows I-V for WS2

layer only which are obtained by contacting CAFM tip with WS2 layer. Linear and rectifying I-V characteristics observed for WS2 layer and MoS2/ WS2

hetero-junction, respectively confirms that formation of 2D materials based

xxiv patterned heterojunctions. (e) and (f) show histogram for the current images

(b) and (c) and indicate uniformity of the rectification ratio in the MoS2/WS2

junction.………... 73 Figure 4.1: Schematics of KPFM set up used for carrying out measurements

in surface and junction modes.………. 83 Figure 4.2: (a), (b), (c) and (d) optical micrographs for ZnS, MS(M)/ZS,

MS(F)/ZS and MS(B)/ZS samples, respectively, scale drawn is 10 μm. (e) Typical micro Raman spectra for MoS2/ZnS samples where E12g and A1g modes of MoS2 are observed while no Raman mode corresponding to ZnS can be seen. (f) XRD diffractogram for ZnS and MoS2 2D layer/ZnS thin film samples, showing wurtzite phase of ZnS. (g) Magnified view of (002) peaks which shows shift towards high 2θ value indicating strain in the underling

ZnS layer especially in case of monolayer MoS2 ………. 85 Figure 4.3: PL spectra of ZnS, MS(M)/ZS, MS(F)/ZS and MS(B)/ZS sample.

Resonance Raman lines superimposed on PL spectra can be seen.………. 86 Figure 4.4: (a) and (b) are surface potential images in mode S and J under

white light illumination for MS(M)/ZS sample. (c)-(d) and (e)-(f) show similar

images for MS(F)/ZS and MS(B)/ZS samples.………. 87 Figure 4.5: Topographical images (a) MS(M)/ZS, (b) MS(F)/ZS and (c)

MS(B)/ZS samples ……….... 88

Figure 4.6: Surface potential profiles for MS(M)/ZS, MS(F)/ZS and MS(B)/ZS samples in mode S and J under white light illumination.

Horizontal arrows represent magnitudes of interface photo voltages.…………. 89 Figure 4.7: (A) Proposed energy band diagrams for (a) ZnS, (b) monolayer,

(c) few layers and (b) bulk MoS2, when ZnS and MoS2 are isolated. (B) Band diagrams for MS(F)/ZS heterojunctions (a) no illumination (dark) and (b) under white light illumination showing interface photovoltage of 0.292

eV.………... 92 Figure 5.1: Raman spectra for WS2, MoS2 layers and WS2/MoS2

heterojunction. Multilayers nature of both MoS2 as well as WS2 layer and

their heterojunction is clearly visible.………. 101 Figure 5.2: (a) and (b) are Psi (Ψ) and delta (∆) plots, respectively for MoS2

layer. Similar plots for WS2 layers and WS2/MoS2 heterojunction are shown in (c, d) and (e, f) respectively. Experimental data is shown in open squares (55^o), circles (65^o) and triangles (75^o), while the fitted data is shown in solid

lines.……… 102

Figure 5.3: (a) Imaginary parts of dielectric constants for individual MoS2, WS2 layers and WS2/MoS2 heterojunction. (b) Tauc’s plot for MoS2, WS2

layers and WS2/MoS2 hetero-junction………. 104 Figure 5.4: (a) and (b) are topography and surface potential images,

respectively under light conditions for WS2 layer. Similar images for MoS2

layer and WS2/MoS2 heterojunction are shown in (c)-(d) and (e)-(f),

respectively………... 105 Figure 5.5: Surface potential profiles for (a) WS2, (b) MoS2 and (c) WS2/MoS2

heterojunction..……….. 106

xxv Figure 5.6: (a) Proposed band alignments for MoS2 and WS2 layers in

isolated condition. (b) Band alignments on the formation of MoS2/WS2

heterojunction under dark condition.(c) Charge generation and separation

under illumination across the WS2/MoS2 heterojunction……… 107 Figure 5.7: I-V characteristics under dark and white light exposure for

WS2/MoS2 heterojunction……… 108 Figure 6.1: (a), (b) and (c) are TEM, HRTEM and SAED patterns for ZnTe

nanocrystalline film. (d) Raman spectra for ZnTe, MS(F), MS(M), ZT/MS(F)

and ZT/MS(M) samples.……….. 117

Figure 6.2: (a), (b) and (c) are Ψ plots for ZnTe nanocrystalline film, MoS2

multilayers and MoS2 few layers, respectively. (d) and (e) are similar plots for

ZT/MS(M) and ZT/MS(F) heterojunctions, respectively……… 118 Figure 6.3: (a) and (b) are imaginary parts of dielectric constants for ZnTe

and MoS2 layers in ZT/MS(M) and ZT/MS(F) heterojunction samples, respectively. (c) and (d) are absorption coefficients of ZnTe and MoS2 layers of above mentioned samples. (e) and (f) are Tauc’s plots for ZnTe and MoS2

layers.………. 119 Figure 6.4: (a) and (b) are topography and surface potential images under

white light exposure for MoS2 multilayers, respectively. (c)-(d) and (e)-(f) are similar images for ZnTe nanocrystalline film and ZnTe/MoS2 heterojunction,

respectively………….………..………... 120 Figure 6.5: Surface potential profiles for MoS2 layers, ZnTe and ZnTe/MoS2

heterojunction under dark and light conditions, respectively……… 121 Figure 6.6: (a) Band alignments for MoS2 and ZnTe, when both are isolated.

(b) and (c) are band alignments on contact formation under dark and under

white light illumination conditions, respectively..……….. 122 Figure 7.1: (a), (b) and (c) show, TEM images for CdSe nanoparticles of

diameter 5, 7 and 10 nm, respectively. (d), (e) and (f) show the HRTEM images for the same samples. (g) XRD diffractogram for ZnS, C5/Z, C7/Z and C10/Z samples. Samples are polycrystalline in nature and consist of

hexagonal CdSe NPs and cubic ZnS thin-film.………... 132 Figure 7.2: (a) and (b) show real (ɛ1) and imaginary (ɛ2) parts of dielectric

constants for CdSe layers of C5/Z, C7/Z and C10/Z sample while (d), (e) and (f) show absorption coefficients for C5/Z, C7/Z and C10/Z samples, respectively. (g) Tauc’s plot for the ZnS layers and EMA layers of C5/Z, C7/Z and C10/Z samples. (h) Schematic of three layers model used for the

analysis of ellipsometeric data……….. 134 Figure 7.3: (a) shows photoluminescence spectra for ZnS, C5/Z, C7/Z and

C10/Z samples, embedded figure show schematic of structure. (b) Magnified view of sub band gap luminescence. (c) PL spectra for three layers sample consisting of alternating layers of ZnS and CdSe NPs, embedded figures show schematic of structure. (d) Magnified view of PL emission, emission

features corresponding to CdSe NPs are observed………. 136 Figure 7.4: (a), (b)-(d) and (e) show surface potential and topography

(embedded) images for ZnS thin-film, C5/Z, C7/Z, C10/Z and CdSe/ZnS thin-films, respectively. (f) Schematics of charge transfer process at the

xxvi interface of CdSe NPs and ZnS thin-film.……….. 137 Figure 7.5: (a) shows surface potential profiles for ZnS film, CdSe/ZnS film

and C5/Z, C7/Z and C10/Z. (b) Band alignment of CdSe NPs and ZnS, before and after contact. On incorporation of CdSe NPs, the surface potential

of ZnS decreases due to accumulation of electrons at the interface.…………... 139 Figure 8.1: (a) HRXRD diffractogram for ZnTe, ZnTe:O (0.02%) and ZnTe:O

(0.2%) nanocrystalline films of thickness ~500 nm deposited at RT and 300ºC on glass substrates. (b) and (c) show (111) peaks of ZnTe and ZnTe:O (0.02%) samples which are deposited at RT and 300ºC, respectively, at magnified scale. The incorporation of oxygen results in broadening and shift of peaks toward higher 2θ values due to decrease in crystallinity and

formation of highly mismatched ZnTe1-xOx alloys……… 148 Figure 8.2: (a)-(c) show HRTEM images for ZnTe, ZnTe:O (0.02%) and ZnTe

(0.2%) nanocrystalline films deposited at room temperature, respectively while insets show corresponding SAED patterns. (d)-(f) show similar images for the ZnTe, ZnTe:O (0.02%) and ZnTe (0.2%) samples deposited at 300oC, respectively. Planes (311) and (220) of zinc blende crystal structure are clearly visible. More diffuse diffraction rings in ZnTe:O as compared to ZnTe nanocrystalline film indicate less crystallinity of ZnTe:O. At a particular substrate temperature, as oxygen concentration increases, more diffuse diffraction rings are observed. Increase in deposition temperature results in

the improvement of the crystallinity of both ZnTe and ZnTe:O.……… 149 Figure 8.3: (a) and (b) represent real (ɛ1) and imaginary (ɛ2) parts of

dielectric constant for ZnTe, ZnTe:O (0.02%) and ZnTe:O (0.2%) nanocrystalline films prepared at RT. Similar images for samples prepared at substrate temperature of 300oC are shown in (c) and (d). Sharp features in ɛ(E) spectra of samples prepared at 300oC show better crystallinity. Non- zero values of ɛ2 for ZnTe:O (0.02%) and ZnTe:O (0.2%) nanocrystalline films below the band edge of ZnTe show absorption through sub band gap

states.……… 151 Figure 8.4: (a) and (b) show absorption coefficients (α(E)) spectra for ZnTe,

ZnTe:O (0.02%) and ZnTe:O (0.2%) nanocrystalline films prepared at RT and 300°C, respectively. Non-zero values of α at energy below the band gap of ZnTe are observed in ZnTe:O. (c) and (d) show Tauc’s plots for ZnTe and ZnTe:O prepared at RT and 300°C. A long tail and a hump in Tauc’s plot of ZnTe:O (0.2%) nanocrystalline films prepared at RT and 300°C, respectively

can be seen along with an increase in the fundamental absorption edge…….. 153 Figure 8.5: (a) to (f) show PL spectra for ZnTe, ZnTe:O (0.02%) and ZnT:O

(0.2%) nanocrystalline films prepared at RT and 300^oC. ZnTe nanocrystalline films show emissions corresponding to band edge only while ZnTe:O show sub band PL emissions. Dominate emissions in range 1.60-2.00 eV are due to oxygen and number of small sharp peaks in range 1.20-1.50 eV are

attributed to Te vacancies as verified from DFT calculations………. 154 Figure 8.6: (a), (b) and (c) show calculated band structures for ZnTe, ZnTe:O

and ZnTe:VTe, respectively. The horizontal lines through reference zero represent Fermi energy positions. Arrows indicate the band gap values and

separation of IB from the bottom of CB.……….... 155

xxvii Figure 8.7: (a) DOS spectra for ZnTe, ZnTe:O and ZnTe:VTe systems near

the band gap region. Valence band maxima are represented by the energy position 0 eV. (b) Magnified view of DOS spectra for ZnTe and ZnTe:O near

the CB edge.……… 157 Figure 8.8: (a) Current density versus RHE curves under dark and light

conditions for ZnTe. (b) Similar plots for ZnTe:O (0.02%) and ZnTe:O (0.2%) samples. (c) and (d) are photocurrent density versus RHE plots for ZnTe and

ZnTe:O thin films, respectively.………. 159 Figure 8.9: (a), (b) and (c) are photocurrent density versus RHE plots for

ZnTe, ZnTe:O (0.02%) and ZnTe:O (0.2%) thin films, respectively under 2.4

and 1.8 eV energy.………. 160

Figure 8.10: MS plots for (a) ZnTe , (b) ZnTe:O (0.02%) and ZnTe:O (0.2%) thin films under dark condition with a scan rate of 20 mV/s as a function of

applied potential (V vs. RHE).……… 161 Figure 8.11: EIS Nyquist plots for ZnTe, ZnTe:O (0.02%) and ZnTe:O (0.2%)

photoelectrodes……….. 162

xxix

L i s t o f t a b l e s

Table 1.1: Electronic and magnetic properties of different layered TMDs…… 6 Table 1.2: Advantages and disadvantages of methods used for MoS2

growth……… 9 Table 1.3: Various interface characterization techniques which are used

for the investigation of 2D MoS2 layers based interfaces………. 15 Table 1.4: Properties and applications of the materials which are studied in

the present thesis………... 17 Table 8.1: Photocurrent ratios of 2.4 to 1.8 eV energy for ZnTe, ZnTe:O

(0.02%) and ZnTe:O (0.2%) samples.………. 161

xxxi

N o m e n c l a t u r e

Symbols

dMo-Mo Interlayer distance between MoS2 layers θB Incidence angle

d Interplaner spacing

β Broadening of the peak at the full width half maximum intensity k Shape factor

L Average size of crystallites

θ Angle of incidence in ellipsometry

Rp/ Rs Complex Fresnel reflection coefficients for the p and s directions

tan ψ Magnitude of the ratio of p and s direction complex reflection coefficients δ Phase difference between p and s reflection coefficients

ρ Complex ellipsometric parameter

P Azimuthal angle between polarizer axis and plane of incidence n Refractive index

k Extinction coefficient

ε1/ε2 Real and imaginary parts of dielectric constant E12g In plane vibration of Mo/W and S atoms

A1g Out of plane vibration of S atoms

I2LA/IA1g Intensity ratio of 2LA(M) and A1g modes of WS2

∆ (cm^-1) Raman peak frequencies of E12g and A1g Φtip Work function of tip

Φsample Work function of sample VCPD Contact potential difference

Z C 

 / Gradient of the capacitance between sample surface and the tip The direction normal to the sample surface

ω Frequency

FDC DC part of the force Fω Force at the frequency ω F2ω Force at the frequency 2ω ΦM Work function of metal tip

Χs Electron affinity of semiconductor Conduction band edge

Fermi level position

xxxii EC-EF Position of Fermi level with respect to conduction band

a Hexagonal lattice constant χ Electron affinity

ΔΦ The difference in the Fermi level positions Eg Band gap

Absorption coefficient h Planck’s constant

υ Frequency of incident radiation Isc Short circuit current

Open circuit voltage Jcv Joint density of state U[(0)]2 Excitonic effect

Ґ Centre of Brillion Zone Jph Photocurrent density

2

,



VZn

Os Interstitial sulphur and complexes of native defects



Vs Sulphur vacancy

Abbreviations

2D Two dimensional

TMDs Transition metal dichalcogenides SCCM Standard cubic centimetres

NPs Nanoparticles 0D Zero dimensional 1D One dimensional 3D Three dimensional h-BN Hexagonal boron nitride

DOS Density of states

KPFM Kelvin probe force microscopy

CAFM Conducting atomic force microscopy

HRTEM High resolution transmission electron microscopy HPOG Highly oriented graphite

TMCs Transition metal carbides CB Conduction band

VB Valence band

xxxiii BZ Brillouin Zone

CVD Chemical vapour deposition RF Radio frequency

ALD Atomic layer deposition PLD Pulsed layer deposition CBM Conduction band minima VBM Valence band minima

PL Photoluminescence

SE Spectroscopic ellipsometry AFM Atomic force microscopy LEDs Light emitting diodes

PEC Photoelectrochemical QDs Quantum dots

IBs Intermediate band

HMAs Highly mismatched alloys BAC Band anticrossing

PLM Pulsed laser melting

VRHE Standard hydrogen electrode potential VASE Variable angle spectroscopic ellipsometry

XRD X-ray diffraction TMP Turbo molecular pump EBL Electron beam lithography SEM Scanning electron microscopy TEM Transmission electron microscopy SAED Selected area electron diffraction

DP Diffraction pattern CCD Charged couple detector

2LA(M) Second order longitudinal acoustical RRS Resonance Raman scattering

STM Scanning tunnelling microscopy VCPD Contact potential difference

VDC DC voltage

PMMA Polymethylmethacrylate VAC AC voltage

1L Monolayer I-V Current voltage

LO Longitudinal optical mode

xxxiv SO Surface optical mode

SPs Surface potentials TO Transverse optical mode MBE Molecular beam epitaxy

HRXRD High resolution X-ray diffraction DFT Density functional theory

EIS Electrochemical impedance spectroscopy MS Mott-Schottky

CPs Critical points

HOVB Highest occupied valence band LUCB Lowest unoccupied conduction band

LCA Local density approximation

GGA Generalized gradient approximation RT Room temperature

J-V Current density-voltage

J-V Electrochemical impedance spectroscopy MS Mott-Schottky