Synthesis and Electron Emission Properties of Aligned Carbon Nanotube Arrays

165 Pages·2016·12.95 MB·English

Florida International University
FIU Digital Commons
FIU Electronic Theses and Dissertations University Graduate School
2-4-2014
Synthesis and Electron Emission Properties of
Aligned Carbon Nanotube Arrays
Suman Neupane
[email protected]
DOI:10.25148/etd.FI14040825
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Recommended Citation
Neupane, Suman, "Synthesis and Electron Emission Properties of Aligned Carbon Nanotube Arrays" (2014).FIU Electronic Theses and
Dissertations. 1168.
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FLORIDA INTERNATIONAL UNIVERSITY
Miami, Florida

SYNTHESIS AND ELECTRON EMISSION PROPERTIES OF ALIGNED CARBON
NANOTUBE ARRAYS

A dissertation submitted in partial fulfillment of
the requirements for the degree of
DOCTOR OF PHILOSOPHY
in
PHYSICS
by
Suman Neupane

2014
To: Dean Kenneth G. Furton
College of Arts and Sciences

This dissertation, written by Suman Neupane, and entitled Synthesis and Electron
Emission Properties of Aligned Carbon Nanotube Arrays, having been approved in
respect to style and intellectual content, is referred to your judgment.

We have read this dissertation and recommend that it be approved.

_______________________________________
Yesim Darici

_______________________________________
Chulei (Peggy) Wang

_______________________________________
Xuewen Wang

_______________________________________
Wenzhi Li, Major Professor

Date of Defense: February 4, 2014

The dissertation of Suman Neupane is approved.

_______________________________________
Dean Kenneth G. Furton
College of Arts and Sciences

_______________________________________
Dean Lakshmi N. Reddi
University Graduate School

Florida International University, 2014
ii
DEDICATION
I dedicate this dissertation to my family members– without their tremendous support and
understanding, it would have been impossible to complete this research.
iii
ACKNOWLEDGMENTS
I would like to sincerely thank my advisor, Dr. Wenzhi Li, for his unrelenting
support during the course of this dissertation. Working alongside him, I have developed
several skills which I cannot explain in this small paragraph. Dr. Li introduced the topic
of this dissertation and carefully guided me through it. I am grateful for his guidance and
tireless help at each and every step of my research. His constructive approach has guided
me through these years and will be a guiding factor towards my future research
endeavors.
I would also like to give my special gratitude to Dr. Xuewen Wang for his expert
guidance and analytical support. He was always available to answer my questions and
provide useful insights to the experimental results. I would also like to thank Dr. Yesim
Darici and Dr. Chunlei Wang for their timely support and encouragement at various
stages of my PhD research. I would also like to thank Dr. Jin He for his crucial support
and the feedback he provided me during the group meetings.
My special gratitude goes to all the group members in NanoPhysics lab at Florida
International University. Post-doctoral associates, Dr. Latha Kumari, Dr. Guohai Chen,
Dr. Yanli Zhao and, Dr. Mengjin Yang have been there besides me whenever I needed
them. I owe special appreciation to Dr. Latha Kumari who taught me several basic
laboratory skills during my initial days. Dr. Chen helped me understand the process of
electron field emission in carbon nanotubes and I am truly indebted to him. I would also
like to express gratitude to my lab mates Yuehai Yang, Kamal Kadel, and Rui Guo for
their role during this research. I thank undergraduate students Mauricio Lastres, Melissa
Chiarella, Ariel Cabrera, Daniel Martinez, Matthew Fey and High School student
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Matthew Llopis for their support and cooperation during this study. I am also thankful to
all the faculties and support staff members in the Physics department at Florida
International University.
I am grateful to my extended family members including my parents, uncles, aunts,
brothers, sisters, cousins, grandparents and in-laws. They have inspired me to join the
PhD program and finish it in a timely manner. Their support and understanding has been
phenomenal in the last six years. Finally, I would like to thank my wife Prakriti, and
daughter Suriti for their unswerving support, without which this work would not be
possible.

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ABSTRACT OF THE DISSERTATION
SYNTHESIS AND ELECTRON EMISSION PROPERTIES OF ALIGNED CARBON
NANOTUBE ARRAYS
by
Suman Neupane
Florida International University, 2014
Miami, Florida
Professor Wenzhi Li, Major Professor
Carbon nanotubes (CNTs) have become one of the most interesting allotropes of
carbon due to their intriguing mechanical, electrical, thermal and optical properties. The
synthesis and electron emission properties of CNT arrays have been investigated in this
work. Vertically aligned CNTs of different densities were synthesized on copper
substrate with catalyst dots patterned by nanosphere lithography. The CNTs synthesized
with catalyst dots patterned by spheres of 500 nm diameter exhibited the best electron
emission properties with the lowest turn-on/threshold electric fields and the highest field
enhancement factor. Furthermore, CNTs were treated with NH plasma for various
3
durations and the optimum enhancement was obtained for a plasma treatment of 1.0 min.
CNT point emitters were also synthesized on a flat-tip or a sharp-tip to understand the
effect of emitter geometry on the electron emission. The experimental results show that
electron emission can be enhanced by decreasing the screening effect of the electric field
by neighboring CNTs.
In another part of the dissertation, vertically aligned CNTs were synthesized on
stainless steel (SS) substrates with and without chemical etching or catalyst deposition.
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The density and length of CNTs were determined by synthesis time. For a prolonged
growth time, the catalyst activity terminated and the plasma started etching CNTs
destructively. CNTs with uniform diameter and length were synthesized on SS substrates
subjected to chemical etching for a period of 40 minutes before the growth. The direct
contact of CNTs with stainless steel allowed for the better field emission performance of
CNTs synthesized on pristine SS as compared to the CNTs synthesized on Ni/Cr coated
SS.
Finally, fabrication of large arrays of free-standing vertically aligned CNT/SnO
2
core-shell structures was explored by using a simple wet-chemical route. The structure of
the SnO nanoparticles was studied by X-ray diffraction and electron microscopy.
2
Transmission electron microscopy reveals that a uniform layer of SnO is conformally
2
coated on every tapered CNT. The strong adhesion of CNTs with SS guaranteed the
formation of the core-shell structures of CNTs with SnO or other metal oxides, which
2
are expected to have applications in chemical sensors and lithium ion batteries.

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TABLE OF CONTENTS

CHAPTER PAGE
1.0 Introduction 1
1.1 Overview 1
1.2 Objectives and scope of the research 5
1.3 Outline of the dissertation 6
1.4 References 8
2.0 Progress and trends in carbon nanotube research 9
2.1 Structure of carbon nanotubes 9
2.2 Synthesis methods 11
2.2.1 Arc-discharge method 11
2.2.2 Laser ablation method 12
2.2.3 Electrochemical synthesis 14
2.2.4 Diffusion flame synthesis 14
2.2.5 Chemical vapor deposition 15
2.2.6 Plasma enhanced chemical vapor deposition 16
2.3 Mechanical properties of CNTs 18
2.4 Thermal properties of CNTs 19
2.5 Electrical properties 22
2.5.1 Electrical conductivity of CNTs 22
2.5.2 Field emission 24
2.6 Applications of CNTs 28
2.6.1 Hydrogen storage 28
2.6.2 CNTs as sensors 33
2.6.3 CNTs for battery and supercapacitor applications 34
2.6.4 CNTs for photovoltaic device 37
2.6.5 CNTs for field emission displays and transistors 38
2.7 Other applications 39
2.8 References 40
3.0 Experimental procedure and characterization technique 46
3.1 Catalyst preparation by nanosphere lithography 46
3.2 Plasma enhanced chemical vapor deposition 49
3.3 Synthesis of CNT/SnO core-shell structure 51
2
3.4 X-ray diffraction analysis 51
3.5 Scanning electron microscopy analysis 52
3.6 Atomic force microscopy 52
3.7 Transmission electron microscopy analysis 53
3.8 Raman spectroscopic analysis 54
3.9 Field emission measurements 55
3.10 References 58
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4.0 Study of electron field emission of carbon nanotubes synthesized on copper 59
4.1 Introduction 59
4.2 Experimental 64
4.3 Results and discussions 66
4.3.1 Effects of densities of CNT emitters on their emission current 66
4.3.2 Effects of plasma treatment on the emission current of CNT
emitters 81
4.3.3 Effects of CNT emitter geometry on their emission currents 96
4.4 Conclusions 106
4.5 References 109
5.0 Field emission properties of vertically aligned carbon nanotubes on stainless steel
114
5.1 Introduction 114
5.2 Experimental methods 117
5.3 Results and discussions 118
5.3.1 Effect of synthesis time on the formation of CNTs 118
5.3.2 Effect of chemical etching on the formation of CNTs 120
5.3.3 TEM analysis 124
5.3.4 Field emission properties of CNTs 126
5.4 Conclusions 129
5.5 References 131
6.0 Fabrication of CNT/SnO core-shell structure 133
2
6.1 Introduction 133
6.2 Experimental methods 135
6.3 Results and discussions 136
6.4 Conclusions 141
6.5 References 142
7.0 Conclusions 143
7.1 Summary 143
7.2 Future work 145
Vita 147

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