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

Acknowledgements Acknowledgements I would like to express my appreciations to many people, without whom this thesis would not have been completed My deepest gratitude goes first and foremost to my supervisor Asst Prof Daniel Chua, a very kind, patient and resourceful gentleman, who has provided me with valuable guidance, inspiration and encouragement in every stage of my studies and research during the past four years 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 i Acknowledgements 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 ii Table of Contents Table of Contents ACKNOWLEDGEMENTS TABLE OF CONTENTS ABSTRACT I III VII LIST OF TABLES IX LIST OF FIGURES X LIST OF ABBREVIATIONS LIST OF SYMBOLS XV XVII CHAPTER INTRODUCTION 1.1 Background 1.2 Carbon Nanotubes 1.2.1 Discovery of Carbon Nanotubes 1.2.2 Bonding Structures of Carbon Nanotubes 1.2.3 Properties of Carbon Nanotubes 1.2.4 Applications of Carbon Nanotubes 2 13 1.3 Motivation and Objectives 15 References 17 CHAPTER PHYSICS OF FIELD EMISSION 20 2.1 Field Emission and Fowler-Nordheim Theory 20 2.2 Field Emission from Semiconductors 23 2.3 Influencing Parameters of Field Emission 25 References 29 iii Table of Contents CHAPTER EXPERIMENTAL TECHNIQUES 33 3.1 Carbon Nanotubes Growth Techniques 3.1.1 Magnetron Sputtering 3.1.2 Plasma-Enhanced Chemical Vapor Deposition (PECVD) 33 33 34 3.2 Thin Film Deposition and Preparation Systems 3.2.1 Metal-Organic Chemical Vapor Deposition (MOCVD) 3.2.2 Pulsed Laser Deposition (PLD) 3.2.3 Microwave Plasma CVD System 36 36 38 39 3.3 Thin Film Characterization Techniques 3.3.1 Scanning Electron Microscopy (SEM) 3.3.2 Transmission Electron Microscopy (TEM) 3.3.3 X-ray Diffraction (XRD) 3.3.4 Photoemission Spectroscopy 40 40 41 42 43 3.4 Field Emission Apparatus 47 References 49 CHAPTER NANOTUBES SYNTHESIS OF VERTICALLY-ALIGNED CARBON 51 4.1 Introduction 51 4.2 Experimental Details 54 4.3 Results and Discussion 4.3.1 Characterization of the As-Grown Carbon Nanotubes 4.3.2 Catalytic Growth Mechanism of Carbon Nanotubes 4.3.3 Function of Metal Catalyst in CNT Growth 4.3.4 Growth Rate of Carbon Nanotubes with Fe Catalyst 56 56 60 62 66 4.4 Summary 68 References 70 CHAPTER FIELD EMISSION CHARACTERISTICS OF METAL OXIDE ULTRATHIN FILMS COATED CARBON NANOTUBES CORE-SHELL NANOSTRUCTURES 72 iv Table of Contents 5.1 FE Study of Molybdenum Oxide Thin Films Coated CNTs 73 5.1.1 Introduction 73 5.1.2 Fabrication of Molybdenum Oxides Coated CNT Nanostructures 74 5.1.3 Characterization of Molybdenum Oxides Coated CNT Nanostructures 79 5.1.4 FE Study of Molybdenum Oxides Coated CNT Nanostructures 86 5.1.5 Enhanced FE Mechanism of Molybdenum Oxides Coated CNT Nanostructures 91 5.1.6 Summary 95 5.2 FE Study of Tungsten Oxide Thin Films Coated CNTs 96 5.2.1 Introduction 96 5.2.2 Preparation of Tungsten Oxides Coated CNT Nanostructures 97 5.2.3 Characterization of Tungsten Oxides Coated CNT Nanostructures 98 5.2.4 Growth Mechanism of Cactus-Shaped Tungsten Oxides Coated CNT Nanostructures 106 5.2.5 FE Study of Tungsten Oxides Coated CNT Nanostructures 110 5.2.6 Summary 114 References 115 CHAPTER FIELD EMISSION STUDY OF HYDROGENATED TETRAHEDRAL AMORPHOUS CARBON COATED CARBON NANOTUBES CORE-SHELL NANOSTRUCTURES 119 6.1 Introduction 119 6.2 Preparation of Hydrogenated Ta-C Coated CNT Nanostructures 6.2.1 Setup of the PLD System Used 6.2.2 Preparation Procedures of the Samples 121 121 124 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 125 126 6.3.2 Confirmation of High sp Content of the Coating Films 6.3.3 Surface Morphology of the Composite Emitters 128 6.3.4 FE Performance of the Composite Emitters with Varied Thicknesses of Coating Films 131 6.4 Hydrogenation Effect on FE Properties of the Composite Emitters 6.4.1 Characterization by SEM and TEM 6.4.2 FE Properties of the Hydrogenated Composite Emitters 135 136 141 6.5 Summary 147 References 149 v Table of Contents CHAPTER CONCLUSIONS AND FUTURE WORKS 152 7.1 Conclusions 152 7.2 Recommendation for the Future Works 155 References 158 LIST OF PUBLICATIONS 159 vi Abstract 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 vii Abstract 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 viii List of Tables List of Tables Table 5.1 Parameters of the pristine CNTs and the WOx coated CNT samples obtained at 200, 400 and 600 °C 111 Table 6.1 Specifications of the PLD laser system used 123 ix List of Figures List of Figures Fig 1.1 Low magnification SEM image of high density vertically-aligned CNTs Fig 1.2 TEM image of a multiwalled CNT Fig 1.3 Illustration of the ways to roll up a grapheme sheet to make a nanotube The vector Ch can be donated by the integers (n, m) T denotes the tube axis, and a1 and a2 are the unit vectors of graphene in real space Fig 1.4 Schematic illustration of sp1, sp2 and sp3 hybrid carbon orbitals Fig 1.5 Illustration of energy of the electrons with wavenumber k in graphene 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 Evac represents the vacuum level, EF 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 is shown in the inset 23 Fig 2.3 Energy band bending near the surface of a semiconductor induced by the external electrical field Ec represents the conduction band minimum, EF refers to the Fermi level, Ev is the valence band maximum, V0 donates the original emission barrier height, and V is the barrier height with band bending 24 Fig 3.1 Schematic principles of Bragg's law 43 Fig 3.2 Schematic illustration of XPS photoemission process 45 Fig 3.3 Schematic illustration of the sample stage for field emission testing 48 Fig 4.1 Schematic setup of the PECVD system used 55 Fig 4.2 The macrograph of the as-grown CNT samples 56 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 57 Fig 4.4 TEM images of the as-grown CNTs obtained with Fe catalyst 58 x Chapter Introduction directions in the hexagonal crystal lattice of graphene Each pair of integers (n, m) represents a possible tube structure The nanotube can be either zigzag (m = 0), armchair (n = m), or chiral (all other types with independent n and m) [22] Fig 1.3 Schematic representation of a graphene sheet showing chiral vectors for formation of different types of single-walled CNTs The vector Ch can be donated by the integers (n, m) T denotes the tube axis, and a1 and a2 are the unit vectors of graphene in real space [21] 1.2.2 Bonding Structures of Carbon Nanotubes Carbon is the sixth element in the periodic table and the atomic carbon has a 1s22s22p2 electronic ground state configuration The 2s2 and 2p2 orbitals are close in energy thereby resulting in atomic orbital configuration consisting of 2s, 2px, 2py, 2pz orbitals These orbitals can readily mix with each other to optimize the bonding energy of carbon with its neighbors This intermixing of atomic orbitals gives rise to three new Chapter Introduction electronic configurations designated by sp3, sp2 and sp1 that are kno known as hybrid molecular orbitals [23, 24] Their schematic illustration is shown in Fig 1.4 24] Fig 1.4 Schematic illustration of sp1, sp2 and sp3 hybrid carbon orbitals Hybridization determines the type of bonding of the carbon atoms with their types n neighbors, and different types of bonding are responsible for the varied properties of , diverse carbon forms For instance, in diamond, the four valence electrons located in 2s and 2p orbitals occupy the sp3 hybrid orbitals and create four equivalent σ covalent r bonds, forming a tetrahedral structure that makes the diamond the hardest know material In contrast, for graphite or graphene, the carbon atom binds with three neighbors and three of its valence electrons occupy the sp2 hybrid orbi orbitals, forming Chapter Introduction three in-plane σ covalent bonds with an out of plane π bond More accurately, the π bond points perpendicularly to the graphene plane This sp2 hybridization gives graphite or graphene planar structures held together by van de Waals force with a spacing of 0.34 nm CNTs essentially consist of sp2 carbon bonding However, due to the curvature of the shells, some sp3 bonding is also incorporated on their caps On the other hand, CNTs possess both the σ and π bonds The σ bonds form the hexagonal network of the graphene walls while the π bonds are perpendicular to the tube’s surface and responsible for the van der Waals interaction between the tube layers This unique structure has granted CNTs with distinctive properties as listed in the following section 1.2.3 Properties of Carbon Nanotubes Owing to their unique bonding structures, CNTs possess remarkable physical properties in electronic, mechanical, optical and thermal areas, etc Here, only the mechanical and electronic properties of CNTs will be covered since these two properties are critical important for electron emitters Field emission properties of CNTs will be reviewed in this section as well Chapter Introduction Mechanical Properties In sp2 carbon orbital, the σ bond length is 0.14 nm long and the bond energy is 420 kcal mol-1 whereas in sp3 bond, it is 0.15 nm and 360 kcal mol-1 [23] As such, graphite or graphene is stronger in-plane than diamond However, defect-free CNTs are generally stronger than graphite due to the considerably increase of the axial component of σ bonding The unique structures thus result in excellent mechanical properties for CNTs For instance, CNTs are one of the stiffest known materials, and they can reach as high Young’s modulus as TPa [25, 26] This value is independent of tube chirality but is relative to the tube diameter Larger diameter makes CNT approach to graphite but smaller one makes CNT less mechanically stable In addition, multiwalled CNTs always exhibit higher Young’s modulus than single-walled CNTs due to the contribution of van der Waals force CNTs possess remarkable elastic properties as well They can sustain up to 15% tensile stain before fracture [27] For a individual nanotube with Young’s modulus of TPa, its tensile strength can reach as high as 150 GPa [23] Such a high tensile strength is attributed to the elastic buckling through which the high stress is released Electronic Properties It is already known that a single-walled CNT can be considered to be a rolled up graphene sheet In order to better understand the electronic properties of CNTs, the electronic properties of a graphene sheet can be investigated first Graphene is Chapter Introduction composed of both carbon σ and π bonds The σ bonds not contribute much to its electronic properties In contrast, it was found that the bonding and anti anti-bonding π bands cross at the K point of the Brillouin zone, in other words, the π valence band and π* conduction band cross the Fermi level Normal metals may have spherical * Fermi surfaces with many available quantum states whereas graphe possesses a graphene unique Fermi surface consisted of only six allowed momentum as shown in Fig 1.5 [28] Therefore, graphene is a semimetal material, which can be considered as either a ne material, very poor metal or a zero-bandgap semiconductor [29, 30] bandgap Fig 1.5 Illustration of e energy of the electrons with wavenumber k in graphene [28] graphene The electronic properties of single single-walled CNTs are pretty similar to th re those of graphene The electric conductivity of a single-walled CNT depends on the diameter Chapter Introduction and the tube direction upon which the graphene sheet is rolled up Based on the classification method of the tube direction by the chiral vector (n, m), all armchair single-walled CNTs are metals; those with (n – m) is a multiple of are semiconductors with a tiny bandgap; and all others are semiconductors with a bandgap that inversely depends on the nanotube diameter [22, 31] The case of multiwalled CNTs are more complicated Multiwalled CNTs have a very high degree of order since they can be modeled as aggregation of parallel, independent single-walled CNTs with similar spacing Each layer can be randomly alternate between semiconducting and metallic However, the electronic communication between these layers is minimal Thus, multiwalled CNTs possess a radial bandgap variation from layer to layer A typical multiwalled CNT might possess electronic bandgaps of 0.3 eV at the inner diameter and 0.03 eV at the outer diameter with a graphene layer diameter ranging from to 20 nm [29] Some researchers have found out that the current was essentially conducted by the outer shell of the CNTs, whatever type of electronic properties may be [32, 33] Field Emission Properties of Carbon Nanotubes It is known that a CNT is a very thin filament-like carbon molecule whose diameter is in the nanometer range but whose length can be quite long, i.e., 10 - 100 microns, depending on how it is grown or prepared The high aspect ratio (length/diameter) of the CNT makes it an excellent electron emitter Besides, CNTs 10 Chapter Introduction possess intrinsically suitable properties for acting as field emitters such as high mechanical stiffness, chemical inertness and high electrical conductivity [34] The FE properties of CNTs were experimentally revealed as early as 1995 when Rinzler et al demonstrated laser-irradiation-induced electron emission from an individual CNT [35] In this case, the nanotubes were not aligned and the density of nanotubes at the substrate surface was very low Right after that, de Heer et al used arrays of carefully aligned CNTs to produce electron emission sources [36], but the tubes were still not well distributed To be better utilized for FE application, the nanotubes should be highly oriented and well distributed Thus, Wang et al fabricated buckybundle CNTs with aligned array via an arc deposition technique, resulting in an onset field of 0.8 V µm-1, which was a much lower value than that reported previously [37] Subsequently, Saito et al manufactured cathode ray tubes equipped with field emitters composed of multiwalled CNTs [38] This manufacture is the first known practical attempt of CNTs on an industrial scale Despite this attempt, the commercial use of CNTs in FE devices is still far from actual implementation due to its many unsolved problems For instance, FE devices require emitters to be grown perpendicularly to the substrate as well as very high current densities (> 500 mA cm-2), indicating the need for high density growth of emitters To address the alignment and density problems, Ren et al fabricated large-scale well-aligned CNTs on nickel-coated glass below 666 °C via plasma-enhanced hot filament chemical vapor deposition technique [6] Chhowalla’s group also successfully 11 Chapter Introduction obtained high density vertically-aligned CNTs using a direct current plasma enhanced chemical vapor deposition system [39] This group performed a detailed parametric study of various factors influencing the growth of aligned CNTs and came up with specific CNT growth recipes These recipes were later widely regarded as a CNT growth manual to access various morphologies of CNTs From the known growth process of CNTs, the fabrication of CNT emitters with the required morphology was easily achieved However, it is obvious that CNTs with very high density are not ideal for FE application because the close packing of nanotubes would screen the local electric field, thus reducing the field enhancement advantages of the two-dimensional emitters [40, 41] This means that a desirable current density cannot be obtained by a limitless increase of CNT density Other approaches should also be considered based on appropriate adjacent CNTs distance Fortunately, the growth of CNTs with appropriate neighboring distance can be well controlled with the aid of photolithography [42] Regardless of CNTs adjacent distance, various methods have been reported in the past several years on the enhancement of the FE characteristics of CNTs, a majority of which employed the method of modifying the CNT surface Nagatsu et al coated the CNTs with amorphous carbon thin film of around 0.6 ~ µm thick and observed an improved FE performance with the reduction of ignition voltage for electron emission from 240 V to 110 V [43] Jin et al successfully lowered the work function of the CNTs from 4.5 eV to 1.9 eV by covering the surface of CNTs with a thin layer of 12 Chapter Introduction barium strontium oxide while preserving their geometry [44] Some other studies reported a lower turn-on field and higher field enhancement by modifying the CNTs with wide bandgap materials (WBGMs) such as SiO2, MgO and BN [15, 16] The WBGMs possess low or negative electron affinity (NEA) property, which is believed to be reason for the decreased effective barrier height of the electron emission and thereby inducing the electron emission at lower turn-on electric field 1.2.4 Applications of Carbon Nanotubes Due to the excellent properties, CNTs are promising in many application areas For instance, CNTs possess high sensitivity, fast response and good reversibility properties at room temperature due to their large surface area, and these properties enable CNTs to be potential in molecular sensor applications [22, 45] The unique geometry and conducting properties make CNTs useful as probe tips in applications such as scanning probe microscopy, atomic force microscopy and magnetic force microscopy Meanwhile, the remarkable mechanical properties of CNTs significantly increase the probe life and minimize the damage to the sample surface [46, 47] CNTs can also be incorporated into composite materials in order to improve the electrical conductivity and achieve enhanced mechanical properties as well as performing as reinforcing elements for the polymer and ceramic materials [48-51] In addition, CNTs are very good candidates for micro-electro-mechanical systems (MEMS) 13 Chapter Introduction applications as well, which has been demonstrated as nanotweezers by attaching two CNTs onto two independent metal electrodes deposited onto tapered glass micropipettes [52] In this study, attention will be paid on the FE applications of CNTs Individual CNT emitter constitute a small commercial market, but arrays of well-aligned CNTs can be used for general lighting, flat panel displays and high power electronics [29] The largest potential market for CNTs field emitters is for field emission display (FED), a technology that could replace most monitors used today in computers and televisions such as cathode ray tube (CRT), liquid crystal display (LCD) and plasma display panel FED is expected to have advantages over previous displays for that its image quality can be comparable to CRT and low power consumption can be comparable to LCD Furthermore, as electron source, the CNTs can be placed very close to the screen, leading to an ultrathin display In addition, CNTs are found particularly suitable for generating X-rays Before nanotubes, FE is not practically applied in X-ray tubes because no emitter structures could survive in high voltage conditions However, CNT-based sources not only can produce high flux X-rays, but also possess advantages such as low weight, portable, power-efficient, high resolution and long lifetime, making CNT-based X-rays sources potential for medical imaging or scientific equipment [23, 29] 14 Chapter Introduction 1.3 Motivation and Objectives Based on the above literature review, it is clear that CNTs are promising in the FE application and plenty of work has been done in this research area However, the commercial application of CNTs in FE devices is still in the initial stage It will be of great significance to extend the work to further enhance the FE characteristics of CNTs towards the advanced commercial application In addition, to my knowledge, few researchers have ever conducted a systematic and intensive study of the FE enhancement mechanism of the CNTs previously Therefore, it is worthwhile to carry out this study As many materials have been attempted to coat the surface of CNTs and promising results have been achieved, coupled with the advantages of simple procedures, low cost, high productivity and capability of conducting large area uniform coating, the method of surface modification or coating has been employed in this research The main aim of this study was to explore appropriate materials that could enhance the FE characteristics of pristine CNTs through coating ultrathin films onto the surface of CNTs More specifically, the objectives of this study were to: explore alternative materials to modify CNTs so as to further enhance their FE characteristics investigate the FE enhancement mechanism of the modified or coated CNTs determine the main influencing factors for the FE properties of the modified or coated CNTs 15 Chapter Introduction The results of this study should contribute to a better understanding of the enhancement mechanism of the FE for modified CNTs emitters In addition, a new field emitter with low cost, low turn-on voltage, low threshold field, high chemical stability and longer lifetime may be fabricated This emitter may provide an alternative choose for the 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Background 1. 2 Carbon Nanotubes 1. 2 .1 Discovery of Carbon Nanotubes 1. 2.2 Bonding Structures of Carbon Nanotubes 1. 2.3 Properties of Carbon Nanotubes 1. 2.4 Applications of Carbon Nanotubes 2 13 1. 3... Nanostructures 10 6 5.2.5 FE Study of Tungsten Oxides Coated CNT Nanostructures 11 0 5.2.6 Summary 11 4 References 11 5 CHAPTER FIELD EMISSION STUDY OF HYDROGENATED TETRAHEDRAL AMORPHOUS CARBON COATED CARBON NANOTUBES. .. CHAPTER FIELD EMISSION CHARACTERISTICS OF METAL OXIDE ULTRATHIN FILMS COATED CARBON NANOTUBES CORE- SHELL NANOSTRUCTURES 72 iv Table of Contents 5 .1 FE Study of Molybdenum Oxide Thin Films Coated