This thesis presents the development of chemical vapour deposited (CVD) graphene and multi-walled carbon nanotubes (MWCNTs) as enabling technologies for flexible transparent conductors offering enhanced functionality. The technologies developed could be employed as thin film field emission sources, optical sensors and substrate-free wideband optical polarisers. Detailed studies were performed on CVD Fe and Ni catalysed carbon nanotubes and nanofibres on indium tin oxide, aluminium and alumina diffusion barriers. Activations energies of 0.5 and 1.5 eV were extracted supporting surface diffusion limited catalysis forCNTs and CNFs. For the first time an activation energy of 2.4 eV has been determined for Cu-catalysed growth of CVD graphene. Graphene was shown to deviate significantly from the more traditional rate-limited surface diffusion and suggests carbon-atom-lattice integration limited catalysis. An aligned dry-transferred MWCNT thin film fabrication technique was developed using MWCNTs of varied lengths to control the optical transparency and conductivity. A process based on the hot-press lamination of bilayer CVD graphene (HPLG) was also developed. Transport studies revealed that these thin films behave, in a macroscopic sense, similar to traditional c-axis conductive graphite and deviate toward tunnel dominated conduction with increasing degrees of network disorder. Various MWCNT-based thin film field emitters were considered. Solution processing was shown to augment the surface work function of the MWCNTs resulting in reduced turn-on electric fields. Integrated zinc oxide nanowires were investigated and were shown to ballast the emission, thereby preventing tip burn out, and offered lower than expected turn-on fields due to the excitation of a hot electron population. To obviate nearest neighbour electrostatic shielding effects an electrochemical catalyst activation procedure was developed to directly deposit highly aligned sparse carbon nanofibres on stainless steel mesh. Highly-aligned free-standing MWCNT membranes were fabricated through a solid-state peeling technique. Membranes were spanned across large distances thereby offering an ideal platform to investigate the unambiguous photoresponse of MWCNTs by removing all extraneous substrate interfaces, charge traps and nanotube-electrode Shottky barriers as well as using pure, chemically untreated material. Oxygen physisorbtion was repeatedly implicated through in-situ lasing and in-situ heated EDX measurements, FT-IR and low temperature transport and transfer measurements. A MWCNT membrane absorptive polariser was fabricated. Polarisers showed wideband operation from 400 nm to 1.1 μm and offered operation over greater spectral windows than commercially available polymer and glass-support dichroic films. Ab-initio simulations showed excellent agreement with the measured polarisation attributing the effect to long-axis selective absorption.

The aim of this research was to demonstrate a high current and stable field emission (FE) source based on carbon nanotubes (CNTs) and electron multiplier microchannel plate (MCP) and design efficient field emitters. In recent years various CNT based FE devices have been demonstrated including field emission displays, x-ray source and many more. However to use CNTs as source in high powered microwave (HPM) devices higher and stable current in the range of few milli-amperes to amperes is required. To achieve such high current we developed a novel technique of introducing a MCP between CNT cathode and anode. MCP is an array of electron multipliers; it operates by avalanche multiplication of secondary electrons, which are generated when electrons strike channel walls of MCP. FE current from CNTs is enhanced due to avalanche multiplication of secondary electrons and in addition MCP also protects CNTs from irreversible damage during vacuum arcing. Conventional MCP is not suitable for this purpose due to the lower secondary emission properties of their materials. To achieve higher and stable currents we have designed and fabricated a unique ceramic MCP consisting of high SEY materials. The MCP was fabricated utilizing optimum design parameters, which include channel dimensions and material properties obtained from charged particle optics (CPO) simulation. Child Langmuir law, which gives the optimum current density from an electron source, was taken into account during the system design and experiments. Each MCP channel consisted of MgO coated CNTs which was chosen from various material systems due to its very high SEY. With MCP inserted between CNT cathode and anode stable and higher emission current was achieved. It was ~25 times higher than without MCP. A brighter emission image was also evidenced due to enhanced emission current. The obtained results are a significant technological advance and this research holds promise for electron source in new generation lightweight, efficient and compact microwave devices for telecommunications in satellites or space applications. As part of this work novel emitters consisting of multistage geometry with improved FE properties were was also developed.

In the current age of fast-depleting conventional energy sources, top priority is given to exploring non-conventional energy sources, designing highly efficient energy storage systems and converting existing machines/instruments/devices into energy-efficient ones. ‘Energy efficiency’ is one of the important challenges for today’s scientific and research community, worldwide. In line with this demand, the current research was focused on developing two highly energy-efficient devices – field emitters and Li-ion batteries, using beneficial properties of carbon nanotubes (CNT). Interface-engineered, directly grown CNTs were used as cathode in field emitters, while similar structure was applied as anode in Li-ion batteries. Interface engineering was found to offer minimum resistance to electron flow and strong bonding with the substrate. Both field emitters and Li-ion battery anodes were benefitted from these advantages, demonstrating high energy efficiency. Field emitter, developed during this research, could be characterized by low turn-on field, high emission current, very high field enhancement factor and extremely good stability during long-run. Further, application of 3-dimensional design to these field emitters resulted in achieving one of the highest emission current densities reported so far. The 3-D field emitter registered 27 times increase in current density, as compared to their 2-D counterparts. These achievements were further followed by adding new functionalities, transparency and flexibility, to field emitters, keeping in view of current demand for flexible displays. A CNT-graphene hybrid structure showed appreciable emission, along with very good transparency and flexibility. Li-ion battery anodes, prepared using the interface-engineered CNTs, have offered 140% increment in capacity, as compared to conventional graphite anodes. Further, it has shown very good rate capability and an exceptional ‘zero capacity degradation’ during long cycle operation. Enhanced safety and charge transfer mechanism of this novel anode structure could be explained from structural characterization. In an attempt to progress further, CNTs were coated with ultrathin alumina by atomic layer deposition technique. These alumina-coated CNT anodes offered much higher capacity and an exceptional rate capability, with very low capacity degradation in higher current densities. These highly energy efficient CNT based anodes are expected to enhance capacities of future Li-ion batteries.

Téma diplomové práce je zaměřeno na metodiku charakterizace autoemisního zdroje pro elektronový mikroskop. Použitá metoda – Fowlerova-Nordheimova analýza je založená na přítomnosti kvantového tunelování, ke kterému dochází v průběhu autoemise elektronů. Práce je obsahově dělena na teoretický úvod do problematiky a praktickou část. Teoretický úvod popisuje emisi elektronů, kvantové tunelování a vlastnosti emisních zdrojů. Druhá část diplomové práce se věnuje experimentální analýze autoemisní katody. Součástí analýzy je výpočet základních charakteristik této katody. Vypočtené hodnoty stanovují celkovou kvalitu elektronové emise a zároveň poskytují informace o aktuálním stavu autoemisní katody. Výsledky F-N analýzy jsou poté srovnány s výsledky počítačové simulace autoemise elektronů. Simulace je provedena v programu COMSOL Multiphysics, který k výpočtu používá metodu konečných prvků a je nástrojem pro hodnocení významu a přesnosti Fowlerovy-Nordheimovy analýzy.

Consistent charge or defect control in oxide grown on silicon carbide (SiC) continues to be difficult to achieve and directly impacts the electrical performance of SiC-based metal oxide semiconductor (MOS) devices. This research applied non-contact Corona-Kelvin metrology to investigate the charge transport in oxides grown on n-type 4H-SiC epitaxial substrates. The cost and engineering science impact of this metrology are significant as device fabrication is avoided leading to quick determination of electrical characteristics from as-grown oxide films. Non-contact current-voltage (I-V) measurements of oxide on SiC were first demonstrated within this work and revealed that Fowler-Nordheim (F-N) current emission was the dominant conduction mechanism at high electric fields. Oxides on SiC were grown at atmospheric pressure (thermal oxides) or at a reduced pressure (afterglow oxides) ambient and examined using non-contact charge-voltage (Q-V), capacitance-voltage (C-V), equivalent oxide thickness (EOT), and I-V methods. The F-N conduction model was modified to address charge trapping and effective barrier effects obtained from experimental oxide films. Trap densities determined with this metrology were used to show that the F-N model including their density and position was adequate for thermal oxides on SiC but not for afterglow films. Data from the latter films required further modification of the theory to include a chemical effect of the oxide growth process on the effective conduction band offset or barrier. This work showed that afterglow chemistry was able to vary the effective conduction band offset from 2.9 eV, typical of thermal oxidation of SiC, up to 3.2 eV. Stress induced leakage current (SILC), an excess above the F-N base current resulting from prolonged current through the dielectric films, was also investigated. Multiple point SILC testing was used to identify statistical effects of process variations and defects in as-grown oxide films on SiC. These results open the possibility to improve oxide manufacture on SiC using methods common in the silicon IC industry. This work demonstrated the first non-contact F-N current determination in oxides on SiC and showed both charge trapping and chemical dependencies of as-grown films. Future studies may extend the findings of this work to further improve this important dielectric-semiconductor system.

The effects of Cs deposition on the field emission (FE) properties of single-walled carbon nanotube (SWNT) bundles were studied. In addition, a comparative study was made on the effects of O2, Ar and H2 gases on the field emission properties of SWNT bundles and multiwall carbon nanotubes (MWNTs). We observed that Cs deposition decreases the turn-on field for FE by a factor of 2.1 - 2.9 and increases the FE current by 6 orders of magnitude. After Cs deposition, the FE current versus voltage (I-V) curves showed non-Fowler-Nordheim behavior at large currents consistent with tunneling from adsorbate states. At lower currents, the ratio of the slope of the FE I-V curves before and after Cs deposition was approximately 2.1. Exposure to N2 does not decrease the FE current, while exposure to O2 decreases the FE current. Our results show that cesiated SWNT bundles have great potential as economical and reliable vacuum electron sources. We find that H2 and Ar gases do not significantly affect the FE properties of SWNTs or MWNTs. O2 temporarily reduces the FE current and increases the turn-on voltage of SWNTs. Full recovery of these properties occurred after operation in UHV. The higher operating voltages in an O2 environment caused a permanent decrease of FE current and increase in turn-on field of MWNTs. The ratios of the slopes before and after O2 exposure were approximately 1.04 and 0.82 for SWNTs and MWNTs, respectively. SWNTs compared to MWNTs would appear to make more economical and reliable vacuum electron sources.

Since the discovery of carbon nanotubes (CNT) in 1999, they have attracted much attention due to their unique mechanical and electrical properties and potential applications. Yet their nanosize makes the study of individual CNTs easier said than done. In our laboratory, carbon fibers with nanotube cores have been synthesized with conventional chemical vapor deposition (CVD) method. The single multiwall carbon nanotube (MWNT) sticks out as a tip of the carbon fiber. In order to pick up the individual CNT tips, focused ion beam (FIB) technique is applied to cut and adhere the samples. The carbon fiber with nanotube tip was first adhered on a micro-manipulator with the FIB welding function. Afterwards, by applying the FIB milling function, the fiber was cut from the base. This enables us to handle the individual CNT tips conveniently. By the same method, we can attach the nanotube tip on any geometry of solid samples such as conventional atomic force microscopy (AFM) silicon tips. The procedures developed for the FIB assisted individual CNT tip fabrication will be described in detail. Because of their excellent electrical and stable chemical properties, individual CNTs are potential candidates as electron guns for electron based microscopes to produce highly coherent electron beams. Due to the flexibility of the FIB fabrication, the individual CNT tips can be easily fabricated on a sharpened clean tungsten wire for field emission (FE) experimentation. Another promising application for individual CNT tips is as AFM probes. The high aspect ratio and mechanical resilience make individual CNTs ideal for scanning probe microscopy (SPM) tips. Atomic force microscopy with nanotube tips allows us to image relatively deep features of the sample surface at near nanometer resolution. Characterization of AFM with individual CNT tips and field emission properties of single CNT emitters will be studied and presented.

<p dir="ltr">This dissertation reports experiments and simulations of micro-/nanoscale electrical breakdown, connects them to the microscale breakdown theories, relates them to field emission and space-charge-limited conditions, and demonstrates the modification of the approach to microwave fields. It provides the first comprehensive experimental assessment of the transitions between electron emission and gas breakdown mechanisms at microscale and nanoscale and extension of semi-empirical laws for ionization process in DC and microwave. These findings will be valuable in developing theories to predict electron emission and gas breakdown mechanisms, which provides guidance for nanoscale device design.</p>

No description available.

Dispositivos de emissão de elétrons por efeito de campo elétrico (FE - Field Emission Devices) têm sido propostos para aplicações como fontes eficientes de elétrons em nanolitografia, microscopia eletrônica, sensores microeletrônicos de vácuo, entre outras. Atualmente os sistemas tradicionais utilizados para caracterização de dispositivos FE fornecem apenas o comportamento geral da emissão de elétrons, ou seja, não permitem efetuar uma investigação mais seletiva e precisa dos centros emissores que constituem o dispositivo, durante seu funcionamento como dispositivo. Frente a esta lacuna, este trabalho propõe o desenvolvimento de um sistema de caracterização de dispositivos emissores de elétrons através do processamento de imagens integrado ao tradicional levantamento da característica corrente-tensão (I-V) do dispositivo. Tais imagens são obtidas através do impacto dos elétrons em uma tela fosforescente. A plataforma LabVIEW foi aplicada para o desenvolvimento do algoritmo de processamento dos sinais, que incorpora desde a etapa de aquisição dos dados (ou seja, medições realizadas pelos instrumentos) até a apresentação dos resultados em diferentes formatos (imagem ou representações gráficas), à escolha do usuário. O sistema desenvolvido permitiu avaliar, quantificar e identificar a distribuição da corrente de emissão em distintas regiões de interesse de amostras compostas por microestruturas ou filmes emissores de elétrons, resultando em representações gráficas bi e tridimensionais. Complementarmente, o sistema também permitiu comparar o desempenho dos dispositivos de emissão de elétrons por efeito de campo elétrico e estudar os efeitos físicos relacionados ao impacto dos elétrons em diferentes tipos de telas fosforescentes. Esta proposta resultou em uma ferramenta inovadora para análise de emissão de elétrons, com a vantagem de manter o dispositivo dentro do seu ambiente de operação, em alto vácuo, representando um grande avanço na metodologia aplicada para se obter características operacionais de dispositivos FE e proporcionando uma caracterização com qualidade superior, visto que proporcionou uma avaliação localizada dos centros emissores numa dada matriz. / Field emission devices (FE devices) have been proposed as efficient sources of electrons for applications in nanolithography, electron microscopy, microelectronic vacuum sensors, among others. Nowadays, the traditional FE characterization systems provide only information related to the general behavior of the electron emission, that is, they do not have specific tools to execute more selective and precise investigation of the emitting centers during their operation as device. As an alternative, this work proposes the development of a dedicated system for characterization of FE by integrating image processing with the traditional current-voltage (I-V) characteristics of the devices. Such images are obtained by the impact of electrons on a phosphorescent screen. The LabVIEW platform was applied to develop the algorithm of signal processing. This algorithm processes information from the data acquisition stage (instruments measurements) to the presentation of the results in different ways (image or graphical representation), according to the researcher´s choices. The developed system allowed evaluating, quantifying and identifying the emission current distribution in different regions of interest of samples composed by microstructures or thin emitting electron films, whose results could be represented in two and threedimensional graphics. Additionally, the system also allowed comparing the performance of electron emission devices by electrical field effect and studying physical effects related to the impact of electrons in different types of phosphorescent screens. This proposal resulted in an innovative tool for electron emission analysis, with the advantage of keeping the device inside its high vacuum characterization environment, which represents a significant advance in the methodology applied to obtain operational characteristics of FE devices and to provide a characterization with superior quality, since it provided a localized evaluation of the emitting centers in a given matrix.