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ELECTROCHEMICAL SYNTHESIS AND STUDY OF ZnO AND ZnCdO NANOSTRUCTURES

a

TRILOK SINGH

Department of Physics

Submitted

In fulfillment of the requirement of the degree of

Doctor of Philosophy

to the

INDIAN INSTITUTE OF TECHNOLOGY DELHI

NEW DELHI-110016

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CERTIFICATE

This is to certify that the thesis entitled, "Electrochemical Synthesis and Study of ZnO and ZnCdO Nanostructures" being submitted by Mr. Trilok Singh, to the Department of Physics, Indian Institute of Technology Delhi, India, for the award of the degree of Doctor of Philosophy in Physics is a record of bonafide research work carried out by him under our supervision and guidance. He has fulfilled the requirements for the submission of thesis, which to best of our knowledge has reached the requisite standard.

The results contained in this thesis have not been submitted in part or full to any other University or Institute for the award of any degree or diploma.

Dr. Rajendra Singh Assistant Professor Department of Physics

Indian Institute of Technology Delhi Delhi

New Delhi-110016 (India)

Prof. D.K. Pandya Professor

Department of Physics

Indian Institute of Technology

New Delhi-110016 (India)

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ACKNOWLEDGEMENTS

Looking back, I am surprised and at the same time very grateful for all I have received throughout these years. It has certainly shaped me as a person and has led me where I am now. All these years of Ph.D studies are full of such gifts.

1 would like to express my love and gratitude to Babtyi who envisioned to make a master from the family. Today you are not with us for feeling this success but your dream come true!

It is my pleasure to tfianka((the people who directly or indirectly influenced me and were helpful for making this thesis possible.

I am deeply grateful to my Ph.D supervisors Dr. Rajendra Singh and Prof.

D.K. Pandya for their invaluable guidance, patience, constant encouragement, objective criticism and moral support, sound advice, good teaching, good company, and lots of good ideas. I consider it as a great privilege to have worked under their proficient guidance. Dr. Singh has always been open to new ideas and has encouraged attempting tasks even if they seemed difficult. Apart from academics, Dr.

Singh has been fabulous advisor, sharp, cheery, perceptive, and mindful of the things that truly matter. Prof. Pandya has been a source of inspiration throughout this arduous journey. He introduced me to this field and always happy to discuss Physics.

In a nutshell he is an excellent teacher. Training and experience gained from their supervision has been a value addition. I express deep gratitude to both of them for supporting me at every stage of work.

I would like to express my gratitude to the Heads of the Department (during research period) Prof. D.K. Pandya and Prof B.P. Pal for providing necessary infrastructure and research facilities to carry out my research work at IIT Delhi. I express my sincere thanks to Dr. Sujeet Chaudhry, Dr. Pankaj Srivastva, Dr. Santanu Ghosh and Dr. J.P. Singh for their valuable suggestions, critical comments and discussions in my various academic presentations.

I sincerely acknowledge the help of Dr. C .Dhanavantri (CEERI Pilani), Dr.

Dinesh Kumar (Kurukshetra University), and Mr. Khanna for PL, UV-VIS and TEM measurements respectively; Mr(s) Raju, Ravi, Anil and Mrs. Amrita for XRD measurements, Dr. Chatar Singh and Mr. D.C. Sharma for SEM/EDAX measurements.

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I am thankful to my SRC members Prof. B.R. Mehta and Prof. M. Jagdesh Kumar for their valuable suggestions, comments and encouragement at all the stages of this work.

I am thankful to Mr. Jaspal Singh and Mr. Rajaram for their technical assistance in mechanical and other works. I extend my sincere thanks to Mr.

Nagendra Singh for his kind help in initial stages of my work.

I sincerely acknowledge the help and support ofDr. B. C. Joshi, Dr. Amanpal Singh, Dr. Vinod, Dr. Hardeep, Dr. Jaswinder, Dr. Gargi, Dr. Priyanka, Dr. Pradeep and Dr. Bhawana.

I have enjoyed working all these years in IIT Delhi where I found a healthy, comfortable and friendly ambience to work. Working together with research scholars from different streams, departments, centres and discussions both academic and non- academic will be always remembered.

I am thankful to have friends like Nandan, Darshan, Girja, Vjay, Tulsi, Nirmala, Hardeep, Sandeep, Naresh, Vikram and Puneet, for helping me get through the difficult times, and for all the emotional support, comraderie, entertainment, and caring they provided. These truly great friends are hard to find, difficult to leave, and impossible to forget. You guys made many things simpler in my life.

I am thankful to Wide Bandgap Semiconductor Laboratory labmates Ashish, Mandeep, Sudheer, Uday, Vipin and Ashutosh. I also want to express my heartfelt thanks to Ashish for his help in all odds and even of this work. He was always there to cheer up the environment in the lab and for endless discussion of all sorts of stuffs from research to politics. I would like to thank Mandeep for his help in maintaining computers up to date and giving quick response to any software or hardware related problems. You guys (Ashish and Mandeep) will be remembered for your kind help.

It is my great pleasure to have the company of my labmates, Daljit, Shikha, Raju, Anil, Ankit, Braj bhushan, Himanshu, Himani, Suneet, Nisha and Monika.

Frequent discussions with them over tea break have been both very refreshing and encouraging.

I would like to thank my seniors and juniors in TFL, Nanostech, CARE and other laboratories at IIT Delhi. I am thankful to Suneet, Pradeep, Atul, Ruchi, Hardik and Monika for their kind cooperation and help at different stages of my work.

Words are inadequate to thank my family for their selfless love, care, patience, support and positive attitude throughout my life. I would like to express my deep

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gratitude to my mother. Smt. Shobha Devi, Tauji and TaUi and I also thank my brothers (Umed, Manoher), Bhabhis (Kushum and Lalita), sisters (Hema, Yashoda, Mohini) and Jiju (Durga Singh, Narayan Singh and Manoher singh) and my In-laws for their tolerance and support during this arduous journey.

I sincerely acknowledge IIT DELHI for providing the research assistance through JRF and SRF. This fellowship gave me an opportunity to kick off my research career.

Finally, I would like to thank my wonderful wife, Hema. If I wrote down everything I ever wanted in a wife and best friend I would not have believed I could meet someone better! She is source of inspiration for hard work, punctuality and caring. It was her support and understanding that helped me for culminating this great job. Still today, learning to love her and to receive her love makes me a better person. I would be a very different person today without her. Special thanks to her for helping me with the figures, editing and making presentation.

I am remembered of saying: Dream is not what you see in sleep, dream is the thing which does not let you sleep!

I hope readers of this thesis find the work interesting and enjoy reading it.

Trilok Singh

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ABSTRACT

Semiconductor nanostructures represent a unique system for exploring phenomena at the nanoscale and are also expected to play a critical role in future electronic and optoelectronic devices. Nanostructured materials have attracted great interest due to their unique chemical, physical, optical and electrical properties which can be influenced not only by their shape and size but also by the preparation procedure. The morphology of the nanostructures plays a key role especially on the optoelectronic properties of the materials, which determine the performance of semiconductors to be used in ultraviolet lasers,solar cell, photo transistors and diodes. Among wide bandgap semiconducting oxide materials, ZnO, CdO and their alloys are one of the most attractive functional semiconductor materials.

The most important issues in this research is to grow nanostructures with controlled shape and size, uniform distribution of nanostructures, bandgap tuning and surface plasmon study for most of the optoelectronic applications. Although there exist many deposition techniques, but electrodeposition is generally preferred. The electrodeposition methods are not only inexpensive and relatively simple, but often produce materials or nanostructures that cannot be accessed by other deposition methods.

In the present work ZnO, CdO and Znl_XCddO (x = 0 to 1) thin films and nanostructures (nanorods) have been prepared by electrodeposition technique and their morphological, structural, and optical properties have been studied. Annealing effect on basic physical and optical properties of ZnO, CdO and ZnCdO nanostructures have also been investigated. In the present work enhancement of UV-emission and suppression of defects related peaks by sputtering of Au nanoparticles on the nanostructures have also been investigated.

Exercisingcontrol over deposition parameters such as deposition temperature, potential, electrolyte concentration, pH values and supporting electrolytes ZnO nanowires with tuned opto-electronic properties have been grown using ITO and Si substrates and porous

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templates. The growth depends on the nature of substrate; ITO being the better one and electrolyte concentration plays an important role in the growth, with higher concentration producing flakes and lower concentration producing vertically oriented nanorods with diameters from 50-150 nm. It has been found that the supporting electrolytes (such as KNO3) also affect the growth process due to diffusion of Zn2+ ions being high and electrodeposition rate of ZnO nanorods thus increases leading to the growth of dense ZnO nanorods. The bath temperature also plays an important role on the growth of ZnO nanorods by controlling the release and supply of Zn ions. PL spectra show a UV-emission peak centered at 373 nm and a broad defects related emission peaks centered on 520 and 590 nm.

The CdO thin films and nanostructures prepared from cadmium nitrate and cadmium chloride bath, respectively. The thin films and nanostructures were polycrystalline in nature, highly transparent in the visible region, also their optical bandgap varied in the range of 2.20 — 2.54 eV. The results demonstrated that CdO thin films and nanostructures have fcc structure of good crystallinity. XPS measurement did not record any traces of metallic Cd.

Variable bandgap ternary Znl_XCdO alloy semiconductor thin films and nanostructures have been grown for 0 <x< 1. For x? 0.25 mixed phase structures with bandgap varying from 3.30 to 2.36 eV was obtained. However for 0 <x< 0.16, a single wurtzite phase solid solution was formed with bandgap variation from 3.32 to 3.08 eV. The nanorods possessed a compressive stress along c-axis direction. ZnO nanostructures were found to be highly transparent with 80% transmittance in the visible range. After the incorporation of Cd into ZnO the average transmittance decreased to — 60%.

Both the ZnO and ZnCdO nanorods with the Au nanoparticles attached to their surfaces revealed a strong enhancement in the 373 nm band edge emission and almost extinction of oxygen defects related band in the green and yellow regions. A significant achievement is

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about 10 fold enhancement in band edge emission which is higher than the existing literature value.

The significant finding of this chapter is a correlation between growth related microstructures and defect structure. A clear correspondence has been established especially between oriented growth behavior obtained at low concentration and the related enhancement of oxygen interstitial defects (O;). When the growth conditions are changed to higher electrolyte concentration or lattice mismatched substrate, like Si, the relatively disordered growth promotes the formation of oxygen vacancies. Interestingly when the growth is modified by adding supporting electrolytes not only lateral dimension of nanorods increased but also a new defect of zinc interstitials (Vz„) is incorporated corresponding to 405 nm defect emission. The addition of strongly ionic KNO3 suppresses the dissociation of Zn(NO3)2 and hence the availability of Zn ions. This could explain the increase in nanorods diameter. But at the same time a slight decrease in pH of the electrolyte also occurs on addition of KNO3, thus lowering the OH- concentration. This may be responsible for the presence of Zn interstitials in nanorods.

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TABLE OF CONTENTS

Page No.

CERTIFICATE i

ACKNOWLEDGEMENT ii

ABSTRACT v

TABLE OF CONTENTS viii

LIST OF FIGURES xiii

LIST OF TABLES xx

Chapter 1 Introduction

1.1 Introduction 1

1.2 Properties of ZnO and CdO 3

1.2.1 Crystal Structure 4

1.2.2 Optical Properties of ZnO 6

1.2.3 Defects Related Optical Transition in ZnO 7

1.2.4 Optical Properties of CdO 9

1.3 Bandgap Engineering 11

1.4 Surface Plasmon driven enhancement of UV-emission 13

1.5 Motivation and objectives of present work 16

1.6 Thesis Organization 16

Chapter 2 Experimental Techniques

2.1 Introduction 21

2.2. Synthesis 22

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2.2.1 Electrodeposition 22

2.2.2 Process of Electrodeposition 22

2.2.3 Faradays Laws of Electrolysis 24

2.2.4 Advantages of Electrodeposition 25

2.2.5 Cyclic Voltammetry 26

2.2.6 Cyclic Voltammetry of ZnO 27

2.2.7 Cyclic Voltammetry of CdO 29

2.2.8 Cyclic Voltammetry of Znl_XCddO 29

2.2.9 Chronoamperometry 30

2.3 Vacuum Thermal Evaporation 31

2.4 Post deposition Annealing 32

2.5 Characterization Techniques 33

2.5.1 Structural and Morphological Characterization 33

2.5.1.1 X-Ray Diffraction 33

2.5.1.2 Scanning Electron Microscopy 38

2.5.1.3 Energy Dispersive X-ray Analysis (EDX) 39

2.5.1.4 Transmission Electron Microscopy 40

2.5.1.5 High Resolution Transmission Electron Microscopy 42

2.5.1.6 Atomic Force Microscopy 42

2.5.1.7 X-Ray Photoelectron Spectroscopy 44

2.5.2 Optical Characterization 45

2.5.2.1 Optical Absorption 45

2.5.2.2 Photoluminescence 46

ix

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Chapter 3 ZnO Thin Films and Nanostructures

3.1 Introduction 49

3.2 Synthesis 50

3.3 Results and Discussion about nanostructured ZnO thin films 50

3.3.1 Microstructural Study 50

3.3.2 Phase and Structural Analysis 51

3.3.3 Optical Study 52

3.3.4 Photoluminescence 53

3.4 Results and Discussion about ZnO Nanostructures grown on

ITO and Si substrates 55

3.4.1 Microstructural Study 55

3.4.2 Phase and Structural Analysis 56

3.4.3 Optical Study 56

3.4.4 Photoluminescence 57

3.4.5 Discussion of ZnO nanostructures 58

3.5 Results and Discussion about ZnO Nanorods grown using various electrolyte 63

3.6.1 Microstructural Study 63

3.6.2 Phase and Structural Analysis 64

3.6.3 Photoluminescence 65

3.7 Growth model for ZnO Nanostructures 67

3.6 Results and Discussion about ZnO NWs grown using templates 69

Chapter 4 CdO Thin Films and Nanostructures

4.1 Introduction 72

4.2 Synthesis 72

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4.3 Results and Discussion 73

4.3.1.1 Microstructural Study 73

4.3.1.2 Phase and Structural Analysis 75

4.3.1.3 Optical Study 78

4.3.2 Results and Discussion CdO Nanostructures 81

4.3.2.1 Microstructural Study 82

4.3.2.2 Phase and Structural Analysis 84

4.3.2.3 Chemical State Analysis 85

4.3.2.4 Optical Study 88

Chapter 5 Synthesis and Characterization of Zn1

..

CdgO Nanostructures

5.1 Introduction 90

5.2 Synthesis 91

5.3 Results and Discussion about Znl_XCddO with x =0 to 1 91

5.3.1.1 Microstructural Study 91

5.3.1.2 Phase and Structural Analysis 93

5.3.1.3 Optical Study 95

5.3.2 Results and Discussions about Znl_XCddO with x =0 to 0.16 96

5.3.2.1 Microstructural Study 97

5.3.2.2 Phase and Structural Analysis 99

5.3.2.3 Optical Study 101

Chapter 6 Surface plasmon enhanced UV-emission of ZnO and Znl_gCdgO nanostructures

6.1 Introduction 103

xi

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6.2 Synthesis 104 6.3 Results and Discussion about ZnO nanostructures 105

6.3.1 Microstructural Study 105

6.3.2 Phase and Structural Analysis 107

6.3.3 Optical Study 108

6.4 Results and Discussion about ZnCdO nanostructures 112

6.4.1 Microstructural Study 112

6.4.2 Phase and Structural Analysis 113

6.4.3 Optical Study 114

7. Conclusions and Future Direction of Work

7.1 Conclusions 117

7.1.1 ZnO nanorods/nanowires 117

7.1.2 CdO thin films and nanostructures 118

7.1.3 ZnCdO nanostructures 119

7.1.4 Enhancement of UV emission in ZnO/ZnCdO mediated by Au

nanoparticles 120

7.2 Future Direction of the Present Work 121

References

123

List of publications

138

Bio - data

140

References

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