Study of field emission characteristics of ultrathin film coated carbon nanotubes core shell structures 1

5.2.5 FE Study of Tungsten Oxides Coated CNT Nanostructures 110 TETRAHEDRAL AMORPHOUS CARBON COATED CARBON NANOTUBES 6.3 Thickness Effect of Ta-C Films on FE Properties of Composite Emi

Second, I would like to express my heartfelt gratitude to Asst Prof Sow Chorng Haur and Loh Kian Ping for providing the field emission and microwave plasma chemical vapor deposition facilities during my research, without which I could not have finished my experiments

I shall extend my thanks to Mr Chen Gin Seng in the Department of Physics, who has helped me a lot in the setup and maintenance of the metal-organic chemical vapor deposition (MOCVD) facility

I am sincerely thankful for the kindness, discussion and constructive suggestions

of Dr Niu Lifang

I am also gratefully to my dear friends and group members: Tang Zhe, Foong Yuan Mei, Koh Ting Ting Angel, Wang Hongyu, Le Quang Tri, Lim Su Ru, and Hsieh Jovan, who gave me their help and time in listening to me and helping me work out

my problems during the difficult course of my Ph D study

I would like to acknowledge Dr Binni Varghese and Ms Lim Xiaodai Sharon

from Asst Prof Sow Chorng Haur’s group for demonstrating the field emission operation procedures in details to me

Additionally, I am greatly indebted to all the staff and postgraduates in the Department of Materials Science and Engineering, who have ever sincerely helped me

in various aspects

Last but not the least my special thanks would go to my beloved families for their loving considerations, care, support and great confidence in me all through these years

CHAPTER 3 EXPERIMENTAL TECHNIQUES 33

3.1.2 Plasma-Enhanced Chemical Vapor Deposition (PECVD) 34

4.3.1 Characterization of the As-Grown Carbon Nanotubes 56

5.2.5 FE Study of Tungsten Oxides Coated CNT Nanostructures 110

TETRAHEDRAL AMORPHOUS CARBON COATED CARBON NANOTUBES

6.3 Thickness Effect of Ta-C Films on FE Properties of Composite Emitters 125

6.3.1 Confirmation of Core-shell Nanostructures of the Emitters 125 6.3.2 Confirmation of High sp3 Content of the Coating Films 126

6.3.4 FE Performance of the Composite Emitters with Varied Thicknesses of

6.4 Hydrogenation Effect on FE Properties of the Composite Emitters 135

6.4.2 FE Properties of the Hydrogenated Composite Emitters 141

CHAPTER 7 CONCLUSIONS AND FUTURE WORKS 152

Abstract

Application of carbon nanotubes (CNTs) for field emission (FE) has attracted great interest across the world In order to conserve energy, tremendous effort has been made to further enhance the FE properties of pristine CNTs by various approaches such as coating the CNTs with appropriate ultrathin films However, a thorough and systematic study of the FE enhancement mechanism of the CNTs is considerably lacking

The aim of this dissertation was to explore alternative materials to modify CNTs

so as to further enhance their FE characteristics and to investigate the enhancement mechanisms of FE for the modified or coated CNTs To achieve these purposes, the tetrahedral amorphous carbon (ta-C) and metal oxides such as molybdenum oxide and tungsten oxide ultrathin films were coated onto high density vertically-aligned CNT substrates and their FE characteristics were examined The metal oxide films were deposited by custom-designed metal-organic chemical vapor deposition (MOCVD) technique at varying temperatures The metal oxides coated CNTs nanostructures obtained at 400 °C exhibited enhanced FE properties The underlying principles for the enhancement are probably due to the Schottky junction formed at the interface, which leads to lowered electron emission barrier height In addition, novel cactus-shaped nanostructures were obtained for the 600 °C tungsten oxides coated CNTs and their growth mechanism may be attributed to the dendritic growth The numerous branches perpendicularly aligned along the main stems may distort the

applied electric field and remarkably enlarge the local field of the emission sites, thus explaining the FE enhancement of the composite emitters

The ta-C films were coated with different thicknesses followed by hydrogen plasma treatments with diverse durations to investigate the influence on FE properties

It was found that there was an optimum film thickness demonstrating the best FE performance Systematic studies showed that with either thinner or thicker films, the effective emission potential barrier and the electron transport would be affected, and surface work function would be changed as well Further work on modifying the surface of CNTs with hydrogen plasma showed enhanced FE performance due to the positive C-H dipoles generated at the surface and the reduced surface barrier height resulted from the energy band bending caused by the charge transfer between the ta-C and the absorbed water layer on its surface However, longer duration of hydrogen plasma treatments (> 10 s) would degrade the enhancement by severely damaging the structure of the composite emitters thus making the electron transport within the emitters become difficult

In conclusion, new CNT-based core-shell composite emitters with enhanced FE properties have been successfully fabricated and their enhancement mechanisms have been intensively discussed The main factors influencing the FE properties of the composite emitters have been determined as well, such as the effective potential emission barrier, field enhancement factor and the electron transport ability By understanding these factors, better control can be achieved for improving FE characteristics of the electron emitters

List of Figures

Fig 1.1 Low magnification SEM image of high density vertically-aligned CNTs 3

Fig 1.3 Illustration of the ways to roll up a grapheme sheet to make a nanotube The

vector C h can be donated by the integers (n, m) T denotes the tube axis, and a1and a2 are the unit vectors of graphene in real space 5Fig 1.4 Schematic illustration of sp1, sp2 and sp3 hybrid carbon orbitals 6

Fig 1.5 Illustration of energy of the electrons with wavenumber k in graphene 9

Fig 2.1 Schematic potential energy diagram illustrating the effect of an external electric field on the energy barrier for electrons at a metal surface, with

consideration of an image potential E vac represents the vacuum level, E F refers to

the Fermi level, and Ø is the work function of the metal 21

Fig 2.2 The electron emission current density versus applied field (J-E)

characteristics of the specimen The corresponding Fowler–Nordheim (F-N) plot

Fig 2.3 Energy band bending near the surface of a semiconductor induced by the

external electrical field E c represents the conduction band minimum, E F refers to

the Fermi level, E v is the valence band maximum, V 0 donates the original

emission barrier height, and V is the barrier height with band bending 24

Fig 3.2 Schematic illustration of XPS photoemission process 45Fig 3.3 Schematic illustration of the sample stage for field emission testing 48

Fig 4.3 (a) Low magnification and (b) high magnification top view and (c) cross-section SEM images of CNT samples obtained with Fe catalyst 57Fig 4.4 TEM images of the as-grown CNTs obtained with Fe catalyst 58

List of Figures

Fig 4.5 Schematic illustration of formation process of bamboo-like compartment structures in the center hollow part of CNTs (a) CNT growth; (b) nucleation of a partial bamboo-knot carbon layer at the carbon wall-catalyst junction; (c) subsequent growth of bamboo-like compartment in the hollow center of the CNT

59

Fig 4.6 TEM image of the tip of the as-grown CNT with metal catalyst embedded in

Fig 4.7 Schematic illustration of catalytic tip growth mechanism of CNTs 61

Fig 4.8 (a) SEM image of the Fe catalyst particles resulted from thermal expansion;

63

Fig 4.9 Cross-sectional SEM images of CNTs grown for (a) 3 min, (b) 5 min, (c) 10 min, and (d) 20 min, showing the different CNT lengths 66Fig 4.10 The average CNT length as a function of the growth duration 67

Fig 4.11 Top view SEM images of CNTs grown for (a) 3 min, (b) 5 min, and (c) 10

Fig 5.1 Schematic setup of the custom-designed MOCVD system 75

Fig 5.3 SEM images of the pristine CNT substrates (a) top view and (b)

Fig 5.4 SEM images of the coated samples obtained by MOCVD treatments at [(a) and (b)] 200 °C, [(c) and (d)] 400 °C, and [(e) and (f)] 700 °C 80Fig 5.5 XPS wide scan spectrum for the coated CNT sample obtained at 700 °C 81

Fig 5.6 Mo 3d XPS core level spectra of the coated CNT samples obtained at (a)

Fig 5.9 The field emission J-E characteristics of the coated samples obtained at (a)

200 °C, (b) 400 °C, and (c) 700 °C Their corresponding F-N plots are shown in

Fig 5.10 Illustration of electric field concentration at sharp tips of the emitters 90

Fig 5.11 UPS spectra of the pristine CNT substrate and the coated CNT sample obtained at 400 °C The extrapolation of the coated CNTs curve (Kinetic energy from 3.97 eV to 4.98 eV) on to x-axis gives its work function value 92

Fig 5.12 (a) HRTEM image of the core-shell nanostructure of the MoO3 coated CNTs (b) a schematic band diagram of field emission from coated CNTs thin film to

vacuum Ø represents the electron emission barrier height of CNTs and χ is the

electron affinity of MoO3 Electron injected from the Fermi energy E F of CNTs to the conduction band of MoO3 After thermalization to the conduction band minimum (CBM), field emission happened at MoO3/vacuum interface A

potential drop ∆V was induced due to the applied electric field VL refers to

Fig 5.13 Low magnification SEM images of tungsten oxides coated CNT samples obtained by MOCVD treatments at [(a) and (b)] 200 °C, [(c) and (d)] 400 °C,

Fig 5.14 High magnification top view SEM image of the tungsten oxides coated CNT

Fig 5.20 UPS spectra of the pristine CNT substrate and the WOx coated CNT samples

112

List of Figures

Fig 5.21 Schematic electric concentration on the tips of WOx nanorods 113Fig 6.1 Schematic illustration of the custom-degisned PLD system 122Fig 6.2 The laser-route of the custom-designed PLD system 122

Fig 6.3 Illustration of the preparation procedures of the samples (a) CNT growth on the silicon substrate (b) DLC thin films coating on the CNTs (c) Hydrogen plasma post-treatment on the surface of DLC coated CNTs 125Fig 6.4 Core-shell structure of a DLC thin film coated CNT confirmed by TEM 126

Fig 6.5 Carbon 1s core level XPS spectrum confirms high sp3 content of the DLC

500 nm; (f) 1000 nm Their tilted images are shown in the insert respectively 130

Fig 6.8 The FE J-E characteristics of the pristine CNT substrate and the composite

emitters with varied ta-C film thicknesses The corresponding F-N plots are

Fig 6.9 Schematic illustration of effective potential area (shadowed parts) of electron tunneling varying with the change of the thickness of the coating ultrathin film

E vac represents the vacuum level, E F donates the Fermi energy, Ø is the work

function of CNT, CBM means the conduction band minimum, and VBM is the

Fig 6.10 Top and cross-sectional view SEM images of composite emitters (a) and (b)

50 nm ta-C coated CNTs; (c) and (d) 50 nm ta-C coated CNTs with a 10 s hydrogenation treatment; (e) and (f) 50 nm ta-C coated CNTs with a 20 s hydrogenation treatment; (g) and (h) 50 nm ta-C coated CNTs with a 30 s

Fig 6.11 Top and cross-sectional view SEM images of composite emitters (a) and (b)

100 nm ta-C coated CNTs; (c) and (d) 100 nm ta-C coated CNTs with a 10 s hydrogenation treatment; (e) and (f) 100 nm ta-C coated CNTs with a 20 s hydrogenation treatment; (g) and (h) 100 nm ta-C coated CNTs with a 30 s

hydrogenation treatment 139

Fig 6.12 High resolution TEM images of (a) 50 nm ta-C coated CNT sample, (b) 50

nm ta-C coated CNTs with a 10 s hydrogenation treatment, and (c) 50 nm ta-C

Fig 6.13 The FE J-E characteristics of the pristine CNTs substrate and the 50 nm ta-C

coated composite emitters with varied hydrogenation durations (10, 20 and 30 s)

Fig 6.14 Illustration of the band bending of the ta-C film in equilibrium with the

absorbed water layer in air E vac represents the vacuum level, E c refers to the

conduction band minimum and E v is the valence band maximum Driven by the

potential difference of the ta-C Fermi energy level (E F) and absorbed water electrochemical potential (µe), electrons at the ta-C surface would transfer to the

Fig 6.15 Carbon 1s core level XPS spectra of the (a) 50 nm ta-C coated CNTs and (b)

10 s hydrogenated 50 nm ta-C coated CNT samples, indicating an increased sp3

Fig 6.16 The FE J-E characteristics of the pristine CNT substrate and the 100 nm

ta-C coated composite emitters with varied hydrogenation durations (10, 20 and

30 s) The corresponding F-N plots are shown in the insert 147

HRTEM high-resolution transmission electron microscopy

MOCVD Metal-organic chemical vapor deposition

PECVD Plasma-enhanced chemical vapor deposition

sccm Standard cubic centimeters per minute

UPS Ultraviolet photoelectron spectroscopy

f (E) Fermi-Dirac function

P Tunneling probability

T Temperature in Kelvin; transmission coefficient

V, V 0 Potential barrier height; voltage

Ø Electron emission barrier height; work function

Ø eff Effective work function

Chapter 1 Introduction

Chapter 1 Introduction

1.1 Background

Field emission (FE) properties of carbon nanotubes (CNTs) have been widely

studied recently since the combination of the extraordinary properties of CNTs, namely, the nanometer-size diameter, structural integrity, high electrical and thermal conductivity as well as the chemical stability, makes CNTs excellent electron emitters [1] Although the emission of current through a single nanotube is constrained because

of its very small cross sectional area, the CNTs can be arranged into a vertically-aligned array allowing an unprecedented amount of current to pass through it This makes it possible to generate a high FE current at a low voltage, which exactly satisfies the need

of low electric consumption of FE devices Therefore, it would be of great significance

to study and enhance the FE characteristics of CNT-based materials

In the past couple of years, plenty of efforts have been made by researchers to improve the FE performance of CNTs and there have been considerate studies reporting the FE properties on CNTs or CNT-based materials A major milestone in this area was the successful synthesis of large arrays of CNTs with well alignment with respect to the substrates, because FE favors the vertical alignment geometry of emitters [2, 3] Further advancement was achieved by the thorough study and well control of the adjacent distance between CNTs, thus making it possible of reduction

or elimination of field-screening effect [4, 5] Application of lithographic technique in