Rozprawy doktorskie na temat „CNT MODEL FOR FIELD EMISSION”

Since the discovery of X-rays over a century ago the techniques applied to the engineering of X-ray sources have remained relatively unchanged. From the inception of thermionic electron sources, which, due to simplicity of fabrication, remain central to almost all X-ray applications at this time, there have been few fundamental technological advances. The emergence of new materials and manufacturing techniques has created an opportunity to replace the traditional thermionic devices with those that incorporate Field Emission electron sources. One of the most important attributes of Field Emission X-ray sources is their controllability, and in particular the fast response time, which opens the door to applying techniques which have formerly been the preserve of optical systems. The work in this thesis attempts to bridge the gap between the fabrication and optimisation of the vacuum electronic devices and image processing aspects of a new approach to high speed radiographic imaging, particularly with a view to addressing practical real-world problems. Off the back of a specific targeted application, the project has involved the design of a viable field emission X-ray source, together with the development of an understanding of the failure modes in such devices, both by analysis and by simulation. This thesis reviews the capabilities and the requirements of X-ray sources, the methods by which nano-materials may be applied to the design of those devices and the improvements and attributes that can be foreseen. I study the image processing methods that can exploit these attributes, and investigate the performance of X-ray sources based upon electron emitters using carbon nanotubes. Modelling of the field emission and electron trajectories of the cathode assemblies has led me to the design of equipment to evaluate and optimise the parameters of an X-ray tube, which I have used to understand the performance that is achievable. Finally, I draw conclusions from this work and outline the next steps to provide the basis for a commercial solution.

The outstanding properties of carbon nanotubes (CNTs) keep attracting the attention of researchers from different fields. CNTs are promising candidates for applications e.g. in lightweight construction but also in electronics, medicine and many more. The basis for the realization of the manifold applications is a detailed knowledge of the material properties of the carbon nanotubes. In particular for applications in lightweight constructions or in composites, the knowledge of the mechanical behavior of the CNTs is of vital interest. Hence, a lot of effort is put into the experimental and theoretical determination of the mechanical material properties of CNTs. Due to their small size, special techniques have to be applied. In this research, a modified molecular structural mechanics model for the numerical determination of the mechanical behavior of carbon nanotubes is presented. It uses an advanced approach for the geometrical representation of the CNT structure while the covalent bonds in the CNTs are represented by beam elements. Furthermore, the model is specifically designed to overcome major drawbacks in existing molecular structural mechanics models. This includes energetic consistency with the underlying chemical force field. The model is developed further to enable the application of a more advanced chemical force field representation. The developed model is able to predict, inter alia, the lateral and radial stiffness properties of the CNTs. The results for the lateral stiffness are given and discussed in order to emphasize the progress made with the presented approach.

In this work we developed a 3-D theoretical model for plasma-enhanced thermo-field emission from nanostructured cathodes in the absence of significant erosion. Our first studies indicated that very dense arrays of vertically-aligned carbon nanotubes (CNT) acting as electron emitters in vacuum could sustain the temperatures resulting from very high surface-averaged current densities such as those found in the cathode spots of arc discharges on non-refractory cathodes. A comparative study of the electron emission models for cold surfaces subjected to strong electric field revealed the existence of a simple relation between the inaccurate Fowler-Nordheim (F-N) equation and the accepted result provided by the theory of Murphy and Good (M-G). We therefore proposed a parametric equation for the emitted current density which was as convenient as the F-N equation but more accurate. The use of M-G theory has also provided an explanation for the tip cooling effect described in a previous study on the destruction of field emitting CNT. We identified the source of the tip cooling effect as the Nottingham effect. For very short emitters, this particular effect heats the surface and accelerates the destruction process and for long emitters, it creates a small isothermal zone at the emitter's tip which is destroyed when it reaches a critical temperature (approximately 1850 K according to our calculations). When combined to existing data on the emitter's length, its diameter, the applied voltage and the measured current, our model can provide the emitter's work function, its room temperature resistivity and the value of the thermal contact resistance between the emitter and its substrate. Another version of this model includes a calculation of the surface electric field in the presence of a non-thermal plasma. To this end we modified the model developed by Mackeown and obtained a general result for 3-D surfaces. This general expression requires the calculation of the ion flux enhancement factor which can be obtained by solving Laplace equation above the surface of interest. This simple approach allows us to describe how the ions are redistributed within the sheath towards the tip of the CNT where the surface field increases. These theoretical predictions were tested by developing simultaneously a fabrication process for a composite electrode matching the optimized design we suggested. Anodic aluminum oxide templates were used as substrates to grow CNT arrays. In order to facilitate their large scale use we modified a standard CNT production process to allow the direct use of as anodized commercial aluminum. Our resulting electrodes were then used as cathodes in low pressure gas discharges. The operating parameters of these discharges are different from the typical voltages and current densities found in glow discharges using as electrodes bare aluminum surfaces. In fact, due to the very low work function of the sharp and relatively ordered emission sites and the simultaneous presence of a ceramic template around them, our electrodes produced very diffuse attachment points for the plasma in a similar fashion as thermionic cathodes do for high pressure arcs. They also required lower (38-140 V) sustaining voltages than what is necessary to sustain a conventional glow discharge. Our electrodes also showed the ability to sustain these low voltage discharges for as much as 500 hours if their bulk temperature was maintained below 60 Celsius and if water vapour was added to the feed gas. Our experiments in nitrogen-water mixtures demonstrated the feasibility of producing large amounts of UV photons at an operating voltage (anode, grounded cathode) of 90-100 V. These results are very promising for future applications in lighting.
Dans le cadre de ce projet nous avons développé un modèle en 3-D pour l'émission électronique par effet thermo-champ stimulée par un plasma. Nos premiers résultats ont indiqué que des réseaux denses de nanotubes de carbone (NTC) agissant en tant qu'émetteurs dans le vide pouvaient supporter les températures résultant de densité de courant moyennes très élevées de l'ordre de celles présentes dans les taches cathodiques d'arcs opérant sur des cathodes non-réfractaires. Une étude comparative des modèles pour l'émission électronique pour les surfaces froides soumises à de forts champs électriques a révélé l'existence d'une relation simple entre l'équation imprécise de Fowler-Nordheim (F-N) et le résultat accepté fourni par la théorie de Murphy et Good (M-G). Nous avons donc proposé une équation paramétrique précise et simple pour la densité de courant émise. L'usage de la théorie de M-G a aussi fourni une explication pour l'effet de refroidissement à la pointe décrit dans une étude précédente sur la destruction de NTC émettant par effet de champ: l'effet Nottingham. Pour des émetteurs très courts, cet effet particulier chauffe la surface et accélère le processus de destruction et pour de longs émetteurs, il crée une petite zone isotherme à la pointe de l'émetteur qui est détruite lorsqu'elle atteint une température critique (approximativement 1850 K). Lorsque combiné à des données sur la longueur de l'émetteur, son diamètre, la tension appliquée et le courant mesuré, notre modèle peut fournir le travail de sortie de l'émetteur, sa résistivité à la température ambiante et la valeur de la résistance de contact thermique entre l'émetteur et son substrat. Une autre version de ce modèle inclut un calcul du champ électrique de surface en présence d'un plasma froid. À cette fin, nous avons modifié le modèle développé par Mackeown et obtenu un résultat général pour des surfaces en 3-D. Cette expression générale requiert le calcul du facteur d'amplification du flux ionique, lequel peut être obtenu en résolvant l'équation de Laplace au dessus de la structure d'intérêt. Cette approche simple nous permet de décrire comment les ions sont redistribués à l'intérieur de la gaine vers la pointe des NTC où le champ électrique augmente. Ces prédictions ont été testées en développant simultanément un procédé de fabrication pour une électrode composite correspondant au schéma optimisé que nous suggérions. Des patrons d'oxyde anodique d'aluminium furent utilisés en tant que substrats pour faire croître nos réseaux de NTC mais afin de faciliter leur usage à grande échelle nous avons modifié le procédé de production des NTC pour permettre l'usage directe d'aluminium commercial anodisé. Nos électrodes furent ensuite utilisées comme cathodes dans des décharges à basse pression. Les tensions et densités de courant mesurées sont différentes des valeurs typiques rencontrées dans les décharges électroluminescentes utilisant comme électrodes, des surfaces d'aluminium. En fait, en raison du très faible travail de sortie des sites d'émission pointus et relativement ordonnés et de la présence simultanée d'un patron de céramique autour d'eux, nos électrodes ont produit des points d'attachement très diffus pour le plasma d'une façon similaire à ce qui est observé pour des cathodes réfractaires chaudes telles que le tungstène. Les tensions d'opération, dans la plage 38-140 V, sont de beaucoup inférieures aux tensions observées avec des électrode d'aluminium (>200 V). Nos électrodes ont aussi démontré leur capacité de maintenir ces décharges à basse tension pour au moins 500 heures si leur température moyenne était maintenue sous 60 Celsius et si de la vapeur d'eau était ajoutée au gaz injecté. Nos expériences dans des mélanges d'azote et d'eau ont démontré la faisabilité de produire de larges quantités de photons UV pour un potentiel anodique (cathode mise à la terre) de 90-100 V. Ces résultats sont très prometteurs pour de futures applications en éclairage.

Uncertainty in the emission estimates is one the main reasons for shortcomings in the Chemistry Transport Models (CTMs) which can reduce the confidence level of impact assessment of anthropogenic activities on air quality and climate. This dissertation focuses on understating the uncertainties within the CTMs and reducing these uncertainties by improving emission estimates The first part of this dissertation focuses on reducing the uncertainties around the emission estimates from oil and Natural Gas (NG) operations by using various observations and high-resolution CTMs. To achieve this goal, we used Weather Research and Forecasting with Chemistry (WRF-Chem) model in conjunction with extensive measurements from two major field campaigns in Colorado. Ethane was used as the indicator of oil and NG emissions to explore the sensitivity of ethane to different physical parametrizations and simulation set-ups in the WRF-Chem model using the U.S. EPA National Emission Inventory (NEI-2011). The sensitivity analysis shows up to 57.3% variability in the modeled ethane normalized mean bias (NMB) across the simulations, which highlights the important role of model configurations on the model performance. Comparison between airborne measurements and the sensitivity simulations shows a model-measurement bias of ethane up to -15ppb (NMB of -80%) in regions close to oil and NG activities. Under-prediction of ethane concentration in all sensitivity runs suggests an actual under-estimation of the oil and NG emissions in the NEI-2011 in Colorado. To reduce the error in the emission inventory, we developed a three-dimensional variational inversion technique. Through this method, optimal scaling factors up to 6 for ethane emission rates were calculated. Overall, the inversion method estimated between 11% to 15% higher ethane emission rates in the Denver-Julesburg basin compared to the NEI-201. This method can be extended to constrain oil and NG emissions in other regions in the US using the available measurement datasets. The second part of the dissertation discusses the University of Iowa high-resolution chemical weather forecast framework using WRF-Chem designed for the Lake Michigan Ozone Study (LMOS-2017). LMOS field campaign took place during summer 2017 to address high ozone episodes in coastal communities surrounding Lake Michigan. The model performance for clouds, on-shore flows, and surface and aircraft sampled ozone and NOx concentrations found that the model successfully captured much of the observed synoptic variability of onshore flows. Selection of High-Resolution Rapid Refresh (HRRR) model as initial and boundary condition, and the Noah land surface model, significantly improved comparison of meteorology variables to both ground-based and aircraft data. Model consistently underestimated the daily maximum concentration of ozone. Emission sensitivity analysis suggests that increase in Hydrocarbon (HC). Variational inversion method and measurements by GeoTAS and TROPOMI instruments and airborne and ground-based measurements can be used to constrain NOx emissions in the region.

Relations de dispersion des porteurs dans le plan des couches, avec attention particuliere aux sous-bandes de valence issues des extrema gamma ::(8) des materiaux hotes; etude des problemes coulombiens avec resolution du probleme de l'exciton et etude de la raie d'emission associee aux recombinaisons electron-trou piege dans les puits quantiques gaas/algaas. Effets d'un champ electrique longitudinal sur les niveaux d'energie a une et deux particules; interpretations de la stabilite de l'exciton et etude des niveaux d'energie d'impurete et des super reseaux "dents de scie". Etude de la capture des porteurs depuis le continuum vers les etats lies du puits quantique (emission de phonon ou niveau-relais quasidiscret). Niveaux d'energie dans les fils quantiques

ABSTRACT The present work examines the field emission from Conducting Hemispherical Carbon Nanotune (CNT) tip including the Effect of Image Force. An expression for electrostatic potential for a Hemispherical CNT tip at a distance from the centre of CNT has been derived. Using the time-independent Schrodinger equation corresponding expressions for transmission coefficient and field emission current density have been derived for the Hemispherical Conducting Carbon Nanotubes. The numerical calculations of potential, transmission coefficient and the current density function have been calculated for a typical set of carbon nanotube parameters. From the expression of potential energy we found that the potential energy for the hemispherical CNT tip first increases and then decreases with the radial distance. The transmission coefficient increases with the normalized radial energy. And the current density function also increases with the normalized Fermi energy. An important outcome of the present work is that both transmission coefficient and field emission current density function decreases as the hemispherical CNT tip radius increases.

碩士
國立中興大學
電機工程學系
92
In this research, the carbon nano tubes (CNTs) are grown by controlling methane flow, plasma power and nitrogen flow. The Raman spectrum is measured to analyze the relative concentration of diamond structure (D-band) and graphite structure (G-band). The growth rate of CNTs increases as the methane flow rate increase, however, the properties of CNT is not as good as that grown by lower flow rate. The tubes are twisty, some carbon black spots are observed in CNTs. The growth rate of CNTs becomes slow when nitrogen is mixed to dilute the reactant, but on the other side the quality of CNTs becomes better and the tubes are more straightly. The field emission data shoes that the current emitted has certain correlation with the D-band over G-band intensity ratio. The smaller the ratio means the higher the graphite structure concentration in the CNTs, and the better of the emission property. This observation is consistent with morphologic observations mention above. Key words: Carbon nano tubes, field emission, graphite structure, diamond structure

碩士
國立臺灣科技大學
電子工程系
100
Anatase titanium dioxide (A-TiO2) were grown on top of carbon nanotube (CNT) bundle arrays by metal organic chemical vapor deposition (MOCVD) using titanium-tetraisopropoxide (TTIP, Ti[OCH(CH3)2]4) as the source reagents. The N-doped A-TiO2/CNTs nanocomposite was then fabricated with nitrogen plasma treatment. The surface morphology, structural and spectroscopic properties of the A-TiO2/CNTs and N-doped A-TiO2/CNTs nanocomposites were characterized using Field-emission scanning electron microscopy (FESEM), Raman spectroscopy, Transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The FESEM image showed a dense coalescence of A-TiO2 with uniform size distribution on the nanotube walls. Raman spectra revealed that nanostructural A-TiO2 had been deposited on the CNT nanocrystals and a new vibration mode of D^'-band at higher wavenumber side was also found. The XPS spectra in the region of N 1s, Ti 2p and O 1s provided a conclusive evidence of the formation of O-Ti-N bond during nitrogen treatment process. The TEM image of A-TiO2 deposited CNT showed uniform distribution, and random directions of A-TiO2 had been grown on the surface of the CNT. The current density versus electric field measurements yielded turn-on field of 1.8 V/?慆 and 1.0 V/?慆 at a current density of 10 ?嫀/cm2, threshold field of 3.6 V/?慆 and 1.9 V/?慆 at a current density of 1 mA/cm2, and field enhancement factor of 2700 and 3000 for the A-TiO2/CNTs and N-doped A-TiO2/CNTs nanocomposites, respectively. Long term stability studies were also carried out. The results indicated that nitrogen doping decreased the turn-on field and threshold field of A-TiO2/CNTs, providing stable field emission applications. The probable mechanisms of field emission enhancement for N-doped A-TiO2/CNTs composite were proposed and discussed.

碩士
崑山科技大學
電子工程研究所
95
The iron, cobalt and nickel are three kinds of element most used as catalyst to grow CNT. However the research report pointed out that, the alloy catalyst which contains two different kinds of element has special effect on controlling CNT growth and caliber. The present research mainly uses RF Magnetron sputtering system to prepare the metal catalysts. The metal catalyst used in this study is nickel metal doped with different percent of iron metal and annealed to become a Ni-Fe alloy catalyst. By changing the composition of alloy catalyst, the growth of CNT and the character of field emission were studied. The alloy catalysts were etched by the microwave plasma enhanced chemical vapor deposition system in hydrogen atmosphere to produce nano scale catalysts. Then the mixture of methane and hydrogen were fed into the system to grow CNT at 500℃. The as-grown CNT samples were characterized by FE-SEM、TEM and Raman Spectroscopy. Its field emission properties were characterized by the I-V Measurement. The result shows that using Ni-Fe alloy catalyst will produce branched shape CNT. These CNT observed by the TEM showing a hollow bamboo structure with multi-walled CNT. Increasing the Fe amount in catalyst, the behavior of field emission is better. When Fe content in alloy catalyst is 39.4wt%, the lowest initial voltage of emission is 6.8V/μm and the highest current density is 314μA/cm2.

碩士
國立嘉義大學
光電暨固態電子研究所
94
The present paper mainly was used the microwave plasma enhanced-chemical vapor deposition(MP-CVD) growing carbon nanotube (CNTs) by (Ni) the array (10μm x 10μm) and different spaces (20μm、15μm、10μm、5μm) with four different metals (TiN，Ti，Ta) to be the barrier layer。 The advantage used MP-CVD in the catalyzed metal (Ni) the array to grow the high density and vertically aligned CNTs。Then changed diverse parameters to grow CNTs and compared field emission characteristic by penetration vacuum electric properties measurement system measured field emission properties，the scanning electron microscope (SEM) to visit the appearance of carbon nanotube。And the Raman spectrum to obtain D-band and G-band, two area compare by a type, if the degree more greatly graphitization is better。 By the experiment we can suppose that higher temperature or higher microwave power cause better degree of graphitization。Degree of graphitization:TiN ＞Ta＞Ti，Diameter of CNTs:TiN＞Ta＞Ti。Field emission properties: Ti ＞Ta＞TiN。

Carbon Nanotubes (CNTs) have great potentials for Field Electron Emission (FEE) and Flow Boiling Heat Transfer (FBHT) applications. However, their weak adhesion on metallic substrates limits the development of CNTs in both applications. Diamond has high thermal conductivity and develops strong bonding with CNTs. The development of a diamond interlayer between CNTs and substrates is a feasible approach to address the adhesion problems. The purpose of this research was to develop a new CNT-based materials with a diamond interlayer for FEE and FBHT applications by focusing on four objectives: (1) enhancement of diamond thin film adhesion on a Cu substrate, (2) improvement of the CNT FEE stability, (3) reduction of the CNT FEE turn-on field, and (4) investigation of the FBHT performance of CNT based structures. The CNTs and diamond thin films in this thesis were prepared by Microwave Plasma enhanced Chemical Vapor Deposition (MPCVD) and Hot Filament enhanced Chemical Vapor Deposition (HFCVD). The structure and chemical states of the diamond films and CNTs were characterized by Scanning Electron Microscopy (SEM), cross-sectional Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD), Raman spectroscopy, synchrotron based X-ray Absorption Spectroscopy (XAS). To deposit diamond thin films on a Cu substrate with sufficient adhesion strength, a sandblasting pretreatment and alloying with a tiny amount of Al were investigated. The adhesion of diamond thin films to substrates was evaluated by Vickers micro-hardness indentation. The FEE stability and turn-on field were measured by a Keithley 237 high voltage measuring unit. The FBHT property of the structures was tested repeatedly at different flow velocities to explore the dependence of heat transfer performance on certain parameters, including the flow patterns, Critical Heat Flux (CHF), and stability. The results show that sandblasting pretreatment increases the surface roughness and surface defect density, thereby increasing diamond nucleation density and adhesion to the Cu substrate. Al alloying appears to inhibit the formation of graphite at the interface between diamond and the Cu substrate, which improves the chemical bonding between diamond and the Cu substrate and increases the adhesion strength between them. The FEE testing results show that ultra-high FEE stability (more than 5000 minutes) was achieved for the CNTs with a diamond interlayer. This is attributed to the good contact at the diamond-CNT and diamond-substrate interfaces. The main factors that affect the CNT FEE turn-on field were also studied. By optimizing the structure, an FEE turn-on field of 5.1 V/μm was achieved and an emission barrier model for CNTs with a diamond interlayer on Cu substrate was used to explain the results. FBHT testing was done on CNTs with different structures and the results show that high heat transfer efficiency can be achieved on CNTs with a diamond interlayer at low mass fluxes.

碩士
逢甲大學
紡織工程所
95
This study discussed field emission characteristics， film surface resistance and film surface morphology of CNT film contained sodium metasilicate and TEOS inorganic binder. The experimental results showed that the CNT paste containing sodium metasilicate(50 wt%) and TEOS inorganic binder both had good dispersion with CNT. SEM images showed that the CNT film containing sodium metasilicate(50 wt%) and TEOS inorganic binder had very smooth film surface . The CNT film containing sodium metasilicate(50 wt%) had better field emission characteristic than that of containing TEOS， and the turn-on electric field was 6.4 V/μm. In order to decrease CNT film surface resistance， nano Ag powder， nano Ag solution and silver epoxy were filled in the CNT paste containing sodium metasilicate(50 wt%). The surface resistance of CNT fim were decreased when nano Ag powder， nano Ag solution and silver epoxy were added and as a result of descending the turn-on electric field. The CNT film containing nano Ag powder had best field emission characteristics， the turn-on electric field was 3.7 V/μm and the film surface resistance was 550 (Ω•cm). The better dispersion of the CNT film was achieved when the nano Ag powder and nano Ag solution were added comparing with that of silver epoxy. Consequently， the optimal condition for a low turn-on electric field and high surface film conductivity were nano Ag powder and nano Ag solution instead of silver epoxy.

We have advanced the state-of-the-art for nano-fabrication of carbon nanotube (CNT) based field emission devices, and have conducted experimental and theoretical investigations to better understand the reasons for the high reduced brightness achieved. We have demonstrated that once the CNT emitter failure modes are better understood and resolved, such CNT emitters can easily reach reduced brightness on the order of 10⁹ A m⁻² sr⁻¹ V⁻¹ and noise levels of about 1%. These results are about 10% better than the best brightness results from a nanotip emitter archived to date. Our CNT emitters have order of magnitude better reduced brightness than state-of-the-art commercial Schottky emitters. Our analytical models of field emission matched our experimental results well. The CNT emitter was utilized in a modified commercial scanning electron microscope (SEM) and briefly operated to image a sample. We also report a successful emission from a lateral CNT emitter element having a single suspended CNT, where the electron emission is from the CNT sidewall. The lateral CNT emitters have reduced brightness on the order of 10⁸ A m⁻² sr⁻¹ V⁻¹, about 10X less than the vertical CNT emitters we fabricated and analyzed. The characteristics of the lateral field emitter were analyzed for manually fabricated and directly grown CNT emitters. There was no significant difference in performance based on the way the CNT emitter was fabricated. We showed that the fabrication technique for making a single CNT emitter element can be scaled to an array of elements, with potential density of 10⁶-10⁷ CNT emitters per cm². We also report a new localized, site selective technique for editing carbon nanotubes using water vapor and a focused electron beam. We have demonstrated the use of this technique to cut CNTs to length with 10s of nanometers precision and to etch selected areas from CNTs with 10s of nanometers precision. The use of this technique was demonstrated by editing a lateral CNT emitter. We have conducted investigations to demonstrate the effects of higher local water pressure on the CNT etching efficiency. This was achieved by developing a new method of localized gas delivery with a nano-manipulator.
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碩士
東海大學
工業工程與經營資訊學系
94
Recently, technology is considered as one of the most important driving forces to gain competitive advantages for business under the influence of ever increasing competitive pressures and shortening product life cycles. According to a survey by PDMA (Product Development and Management Association), more than 50% of the sales in high technology companies are coming from new product. Hence, it is clear that the accelerating new product development (NPD) is crucial for companies to be survival and even success. Thus, the attention on management of NPD is essential. On the other hand, the poor selection and management of NPD would result in significant losses of financial and human resources. Therefore, it is very important for product managers to evaluate the viability of a new product at every stage of its development, especially the initial ides screening stage. However, the NPD process is complex and involved varieties and uncertainties of environment problems such like technological competitiveness, customer needs, and so on. Nevertheless, in comparisons with previous study in this area, most literature focuses on technological and financial aspects. As aimed at these issues, this study devises a feasible and systematical mechanism based on Analytic Hierarchical Process (AHP) and scoring techniques to deal with the technology evaluation and provides more complete evaluative criteria, especially bringing manufacturing aspect into evaluative consideration. Then, this study implements a promising technology, Carbon Nanotube Field Emission Back Light Unit (CNT-BLU), to proposed evaluative mechanism and analyzed the result. In conclusions, manufacturing dimension is considered as essential as others for managers to evaluate technology.

An mathematical expression has been derived and analysed for total potential energy V and field emission current density function ϕ of emitted electrons by taking into account the distribution of charge along the length of the carbon nanotubes (CNTs), matching to the representation for the tunnelling coefficient. This tunnelling coefficient is obtained from solving of the time independent Schrödinger’s wave equation. Mathematical calculations for the potential energy V, tunnelling coefficient T and current density function ϕ have been worked out for a distinctive set of the CNTs factors. It is concluded that the potential energy in ergs declines with the radial distance r but increases with z coordinate along the length of the CNT. Moreover, the current density function at the spherical CNT tip is substantially larger than along the length of cylindrical metallic CNT. In addition, the field emission current density function decreases with radius of CNTs in both the cases (i.e., cylindrical and spherical tip). Some of our theoretical results are in agreement with the current experimental observations.

博士
國立清華大學
工程與系統科學系
94
In this thesis, we will separate three parts: first: introduce the growth of single vertically-aligned carbon nanotube (CNT), secondly: field emission measurement and analysis with various shape of single CNT, and finally: the application of scanning probe microscopy and triode structure device with single CNT. First, the vertically aligned CNTs were fabricated by a combination of EBL and ICP-CVD deposition. The positive photoresist of polymethylmehtacrylate (PMMA) 35 nm thickness was coated on a <100> p-type silicon substrate, followed by EBL. The spacing between dots were 1~3 μm apart. The acceleration voltage was 20 KV, and the exposure time was varied between 5 ms and 10 ms to control the size of holes after development.The minimum and average sizes of the nickel dots after lift-off ranged were 30 nm and 60 nm, respectively. Nickel, as the catalyst, was then deposited either by E-gun evaporation or radio frequency sputter, which thickness was about 10 nm. An ICP-CVD system was used to grow vertically aligned CNTs under the following process conditions: ICP power of 1000 W, substrate RF bias of 300 W, feed gas mixture of C2H2/H2/Ar with 8/24/0.5 sccm flow rates, and total pressure of 20 mtorr. The substrate temperature was about 550o C and the growth time was 10 minutes. To separate the field emission characteristics with various shape of CNT, two different types (24 cm and 4cm) of graphite electrodes supporting silicon substrates were used. In additionally, the aspect ratio of CNT is directly proportional to the exposure time of electron beam after using EBL and ICP-CVD. Secondly, we have designed and constructed the three-axis nano-positioning device for carbon nano-tip assembly and FE measurement inside a SEM chamber in order to investigate the field emission characteristics with various shape and aspect ratio of single CNT (Free standing VACNTs of various aspect ratios (height/radius) were explained at first part). This device consists of two parts: inertial walker for x-y-axis and inchworm unit for the z-axis. The x and y stages are independent and actuated with piezoelectric stack respectively. The dimension of the x-y stage is 25 mm × 25 mm. The dimension of the inchworm is 48 mm × 20 mm. Based on the results of experiment, it clearly shows that the lengths and diameters of CNT are varied with the exposure times (the larger the Ni dot size, the large the CNT diameter and the faster the CNT growth rate). It was believed that the increasing growth rate for larger Ni dot size is attributed to the increasing carbon flux around Ni. The large height and small radius of CNT lead to high aspect ratios and thus better enhancement of the electric field at the tip. That means required turn-on electric filed is substantially reduced as the height increases or radius is decreases due to the increased geometrical field enhancement factor. Furthermore, the turn-on field was also showed a result of FE simulation at the apex of the CNT emitter with various aspect ratio by using a commercial code, SIMION 7.0.The SIMION code simulation conditions are: (1) diameter of anode probe of 3 μm,(2) spacing between anode and cathode 200 nm, and (3) anode voltage 24 V. The results were also showed that the maximum field enhancement factor and field strength was related to the aspect ratio. Finally, we have constructed the scanning probe microscopy and triode structure device with single vertically-aligned CNT emitter source by using electron beam lithography and inductively-couple plasma (ICP) chemical vapor deposition. In the scanning probe microscopy measurement part, it was apparent that the silicon pillar array with height 150 nm and length 1.25~1.5 μm have been completely scanning by the single CNT probe. In the triode measurement, the field emission was the synthesized effect of the gate structure; in other words, in addition to the geometric factor of CNTs, the field strength at the CNT apex in triode structure also depends on the gate hole size, position of CNT at gate hole, and the height of CNT tip relative to the gate height (gate-to-cathode spacing). In this study, the gate hole size, gate-to-cathode spacing, and the position of CNT were fixed (i.e. CNT is positioned at gate hole center). Thus the emphasis was on the effect of CNT length and the consequent difference of CNT tip to gate distance. It was evident that the optimized CNT height for highest field at the CNT apex is that equal to the gate-to-cathode spacing. Furthermore, the turn-on field was also showed a result of FE simulation at the apex of the CNT emitter with various CNT heights by using a commercial code, SIMION 7.0. The simulation conditions are: (1) diameter of aperture hole and anode probe of 1.5 μm and 4 μm respectively, (2) applied gate voltage -10 V and anode voltage 0 V, and (3) cathode voltage -27.7 V. The results were also showed that the maximum field strength was related to the CNT length. Based on the above results, it was evident that in the triode measurement, although the difference of field emission is “controlled” by varying the “CNT length”, the difference is not simply due to the aspect ratio, but also due to the difference in distance between CNT tip height and gate height.

碩士
國立雲林科技大學
環境與安全工程系碩士班
94
The greenhouse effect is an important mechanism of maintaining the temperature and ecological system of the Earth. However, many recent researches show that mankind over-using petrochemical fuels and excessively developing land surface have produced the aggravation of greenhouse effect and climate change. Paddy field is a kind of artificial wetlands--flooding rice fields would produce methane by methogentic bacteria under anaerobic conditions in soils. Methane emission from rice paddies is about 10-30 percent of atmospheric methane on the world. Since rice is the major food crop in world as well as Taiwan, methane emission from rice paddy is an important source. This study collected and analyzed past researches on emission factors and data, based and developed system dynamic model through the theory of bacterial energetics, and modeled the system by a software tool STELLA. On the other hand, this study also use the Fourier Transform Infrared method to monitor the emission of GHGs from paddy rice field in the second season of 2004 and in the first season of 2005. The local emission coefficient of paddy rice field was then calculated and compared with the model. The model integrated the influencing factors of methane emission to established a system dynamic model of paddy rice in order to evaluate and analysis emission amount and possible scenarios in the future. These simulation results could be adopted for management strategies and policy for GHG reduction on paddy fields. Based on of reduction trends for cultivation areas from 1990 to 2000, the system dynamic model estimates methane emission of 24.7 Gg (109g = 103 ton ) in 2010. If the average temperature raises 1 or 2℃, methane emission from rice paddy would increase 6.8 and14% due to flooding field. By extending the draining period, methane emission could reduce 20%. In addition, postponing cultivation for half month to one month could help emission reduction by 1.5%-2.3%. Considering the zone management of cease cultivation, the middle region should take priority due to its large temperature effect on methane emission. Because the second crop season emitted more methane than the first one, a 50% overall reduction in planting areas for the second crop season could yield 33% reduction in yearly emission according to our model simulation.

Field emission is the emission of electrons from a solid under an intense electric field, of the order of 109 V/m. Emission occurs by the quantum mechanical tunneling of electrons through a potential barrier to vacuum. Field emission sources offer several attractive features such as instantaneous response to field variation, resistance to temperature fluctuation and radiation, a high degree of focusing ability in electron optics, good on/off ratio, ballistic transport, and a nonlinear current-voltage relationship. Carbon nanotubes (CNTs) are potential candidates as field emitters since they possess high aspect ratio and are chemically inert to poisoning, and physically inert to sputtering during field emission. They can carry a very high current density and do not suffer field-induced tip sharpening like metallic tips. In addition, the CNT field emitters have the advantage of charge transport through 1D channels and electron emission at the sharp tips due to large enhancement. But the injection of electrons from the back contact remains a technical challenge which requires binding of CNT emitters to metallic substrate. Also, detachment of the CNT from the substrate tends to occur with time. The electrically conducting mixtures of CNTs and polymer can provide an alternative route to address these issues in the field emission of CNTs. The composites can be casted on any substrate in desired shape and the polymer matrix provides necessary support. The research work reported in this thesis includes the preparation of high quality multiwall carbon nanotubes (MWCNTs), MWCNT-polystyrene (PS) composites, and experimental investigation on field emission properties of MWCNT¬PS composites in two different configurations. Electrical conductivity and percolation threshold of the MWCNT-PS composites are also investigated to ensure their high quality prior to the field emission studies. The study has been further extended to reduced graphene oxide (rGO) coated on polymer substrate. The main results obtained in present work are briefly summarized below. This thesis contains eight chapters. Chapter 1 provides an overview of basics of field emission, and the potential of CNT and CNT-polymer composites as field emitters. Chapter 2 deals with the concise introduction of various structural characterization tools and experimental techniques employed in this study. Chapter 3 describes the synthesis of MWCNTs and characterization by using electron microscopy and Raman spectroscopy. MWCNTs are synthesized by chemical vapor deposition (CVD) of toluene [(C6H5) CH3] and ferrocene [(C5H5)2 Fe] mixture at 980 °C. Here toluene acts as carbon source material and ferrocene provides catalytic iron (Fe) particles. The MWCNT formation is based on the thermal decomposition of the precursor mixture. Scanning electron microscopy (SEM) characterization shows that the MWCNTs are closely packed and quite aligned in one direction. The average length of MWCNTs is about 200 μm and outer diameter lies in the range of 50-80 nm. The high quality of as-prepared MWCNT sample is confirmed by Raman spectroscopy. The as-grown MWCNTs are encapsulated with catalytic Fe nanoparticles, revealed by transmission electron microscopy. The Fe nanoparticles trapped within the MWCNT serve as fantastic system for studying the magnetic properties. Three types of MWCNT samples filled with Fe nanoparticles of different aspect ratio (~10, 5 and 2) are synthesized by varying the amount of ferrocene in the precursor material, and their magnetic properties are investigated. Enhanced values of coercivity (Hc) are observed for all samples, Hc being maximum (~2.6 kOe) at 10 K. The enhancement in Hc values is attributed to the strong shape anisotropy of Fe nanoparticles and significant dipolar interactions between Fe nanoparticles. Chapter 4 deals with the field emission studies of MWCNT-PS composites in the parallel configuration. By incorporating as-prepared MWCNTs in PS matrix in a specific ratio, composites with varying loading from 0.01-0.45 weight (wt.) fraction are prepared using solution mixing and casting. High degree of dispersion of MWCNTs in PS matrix without employing any surfactant is achieved by ultrasonication. Low percolation threshold (~0.0025 wt. fraction) in the MWCNT-PS composites ensures the good connectivity of filler in the fabricated samples. Field emission of MWCNT¬PS composites is studied in two different configurations: along the top surface of the film (parallel configuration) and along the cross section of the sample (perpendicular configuration). In this chapter field emission results of the MWCNT-PS composites in parallel configuration are presented. The effect of charge transport in limiting the field emission of MWCNT-PS composite is discussed. Field emission results of MWCNT-PS composites in parallel configuration indicate that the emission performance can be maximized at moderate wt. fraction of MWCNT (0.15). The obtained current densities are ~10 µA/cm2 in the parallel configuration. Chapter 5 presents the study of field emission characteristics of MWCNT¬PS composites of various wt. fractions in the perpendicular configuration. Till date most studies using nanotube composites tend to have the nanotubes lying in two dimensional plane, perpendicular to the applied electric field. In the perpendicular configuration, the nanotubes are nearly aligned parallel to the direction of the applied electric field which results in high field enhancement, and electron emission at lower applied fields. SEM micrographs in cross-sectional view reveal that MWCNTs are homogeneously distributed across the thickness and the density of protruding tubes can be scaled with wt. fraction of the composite film. Field emission from composites has been observed to vary considerably with density of MWCNTs in the polymer matrix. High emission current density of 100 mA/cm2 is achieved at a field of 2.2 V/µm for 0.15 wt. fraction. The field emission is observed to follow the Fowler– Nordheim tunneling mechanism, however, electrostatic screening plays a role in limiting the current density at higher wt. fractions. Chapter 6 highlights the field emission response of rGO coated on a flexible PS film. Field emission of rGO coated PS film along the cross section of the sample is studied in addition to the top film surface of the film. The effect of geometry on the improved field emission efficiency of rGO coated polymer film is demonstrated. The emission characteristics are analyzed by Fowler–Nordheim tunneling for field emission. Low turn-on field (~0.6 V/µm) and high emission current (~200 mA/cm2) in the perpendicular configuration ensure that rGO can be a potential field emitter. Furthermore, stability and repeatability of the field emission characteristics are also presented. Chapter 7 deals with the synthesis, characterization, and field emission of two different kinds of hybrid materials: (1) MWCNT coated with zinc oxide (ZnO) nanoparticles (2) ZnO/graphitic carbon (g-C) core-shell nanowires. The field emission from the bucky paper is improved by anchoring ZnO nanoparticles on the surface of MWCNT. A shift in turn on field from 3.5 V/µm (bucky paper) to 1.0 V/µm is observed by increasing the ZnO nanoparticle loading on the surface of MWCNT with an increase in enhancement factor from 1921 to 4894. Field emission properties of a new type of field emitter ZnO/g-C core-shell nanowires are also presented in this chapter. ZnO/g-C core/shell nanowires are synthesized by CVD of zinc acetate at 1300 °C. Overcoming the problems of ZnO nanowire field emitters, which in general possess high turn on fields and low current densities, the core-shell nanowires exhibit excellent field emission performance with low turn on field of 2.75 V/µm and high current density of 1 mA/cm2. Chapter 8 presents a brief summary of the important results and future perspectives of the work reported in the thesis.

Field emission is the emission of electrons from a solid under an intense electric field, of the order of 109 V/m. Emission occurs by the quantum mechanical tunneling of electrons through a potential barrier to vacuum. Field emission sources offer several attractive features such as instantaneous response to field variation, resistance to temperature fluctuation and radiation, a high degree of focusing ability in electron optics, good on/off ratio, ballistic transport, and a nonlinear current-voltage relationship. Carbon nanotubes (CNTs) are potential candidates as field emitters since they possess high aspect ratio and are chemically inert to poisoning, and physically inert to sputtering during field emission. They can carry a very high current density and do not suffer field-induced tip sharpening like metallic tips. In addition, the CNT field emitters have the advantage of charge transport through 1D channels and electron emission at the sharp tips due to large enhancement. But the injection of electrons from the back contact remains a technical challenge which requires binding of CNT emitters to metallic substrate. Also, detachment of the CNT from the substrate tends to occur with time. The electrically conducting mixtures of CNTs and polymer can provide an alternative route to address these issues in the field emission of CNTs. The composites can be casted on any substrate in desired shape and the polymer matrix provides necessary support. The research work reported in this thesis includes the preparation of high quality multiwall carbon nanotubes (MWCNTs), MWCNT-polystyrene (PS) composites, and experimental investigation on field emission properties of MWCNT¬PS composites in two different configurations. Electrical conductivity and percolation threshold of the MWCNT-PS composites are also investigated to ensure their high quality prior to the field emission studies. The study has been further extended to reduced graphene oxide (rGO) coated on polymer substrate. The main results obtained in present work are briefly summarized below. This thesis contains eight chapters. Chapter 1 provides an overview of basics of field emission, and the potential of CNT and CNT-polymer composites as field emitters. Chapter 2 deals with the concise introduction of various structural characterization tools and experimental techniques employed in this study. Chapter 3 describes the synthesis of MWCNTs and characterization by using electron microscopy and Raman spectroscopy. MWCNTs are synthesized by chemical vapor deposition (CVD) of toluene [(C6H5) CH3] and ferrocene [(C5H5)2 Fe] mixture at 980 °C. Here toluene acts as carbon source material and ferrocene provides catalytic iron (Fe) particles. The MWCNT formation is based on the thermal decomposition of the precursor mixture. Scanning electron microscopy (SEM) characterization shows that the MWCNTs are closely packed and quite aligned in one direction. The average length of MWCNTs is about 200 μm and outer diameter lies in the range of 50-80 nm. The high quality of as-prepared MWCNT sample is confirmed by Raman spectroscopy. The as-grown MWCNTs are encapsulated with catalytic Fe nanoparticles, revealed by transmission electron microscopy. The Fe nanoparticles trapped within the MWCNT serve as fantastic system for studying the magnetic properties. Three types of MWCNT samples filled with Fe nanoparticles of different aspect ratio (~10, 5 and 2) are synthesized by varying the amount of ferrocene in the precursor material, and their magnetic properties are investigated. Enhanced values of coercivity (Hc) are observed for all samples, Hc being maximum (~2.6 kOe) at 10 K. The enhancement in Hc values is attributed to the strong shape anisotropy of Fe nanoparticles and significant dipolar interactions between Fe nanoparticles. Chapter 4 deals with the field emission studies of MWCNT-PS composites in the parallel configuration. By incorporating as-prepared MWCNTs in PS matrix in a specific ratio, composites with varying loading from 0.01-0.45 weight (wt.) fraction are prepared using solution mixing and casting. High degree of dispersion of MWCNTs in PS matrix without employing any surfactant is achieved by ultrasonication. Low percolation threshold (~0.0025 wt. fraction) in the MWCNT-PS composites ensures the good connectivity of filler in the fabricated samples. Field emission of MWCNT¬PS composites is studied in two different configurations: along the top surface of the film (parallel configuration) and along the cross section of the sample (perpendicular configuration). In this chapter field emission results of the MWCNT-PS composites in parallel configuration are presented. The effect of charge transport in limiting the field emission of MWCNT-PS composite is discussed. Field emission results of MWCNT-PS composites in parallel configuration indicate that the emission performance can be maximized at moderate wt. fraction of MWCNT (0.15). The obtained current densities are ~10 µA/cm2 in the parallel configuration. Chapter 5 presents the study of field emission characteristics of MWCNT¬PS composites of various wt. fractions in the perpendicular configuration. Till date most studies using nanotube composites tend to have the nanotubes lying in two dimensional plane, perpendicular to the applied electric field. In the perpendicular configuration, the nanotubes are nearly aligned parallel to the direction of the applied electric field which results in high field enhancement, and electron emission at lower applied fields. SEM micrographs in cross-sectional view reveal that MWCNTs are homogeneously distributed across the thickness and the density of protruding tubes can be scaled with wt. fraction of the composite film. Field emission from composites has been observed to vary considerably with density of MWCNTs in the polymer matrix. High emission current density of 100 mA/cm2 is achieved at a field of 2.2 V/µm for 0.15 wt. fraction. The field emission is observed to follow the Fowler– Nordheim tunneling mechanism, however, electrostatic screening plays a role in limiting the current density at higher wt. fractions. Chapter 6 highlights the field emission response of rGO coated on a flexible PS film. Field emission of rGO coated PS film along the cross section of the sample is studied in addition to the top film surface of the film. The effect of geometry on the improved field emission efficiency of rGO coated polymer film is demonstrated. The emission characteristics are analyzed by Fowler–Nordheim tunneling for field emission. Low turn-on field (~0.6 V/µm) and high emission current (~200 mA/cm2) in the perpendicular configuration ensure that rGO can be a potential field emitter. Furthermore, stability and repeatability of the field emission characteristics are also presented. Chapter 7 deals with the synthesis, characterization, and field emission of two different kinds of hybrid materials: (1) MWCNT coated with zinc oxide (ZnO) nanoparticles (2) ZnO/graphitic carbon (g-C) core-shell nanowires. The field emission from the bucky paper is improved by anchoring ZnO nanoparticles on the surface of MWCNT. A shift in turn on field from 3.5 V/µm (bucky paper) to 1.0 V/µm is observed by increasing the ZnO nanoparticle loading on the surface of MWCNT with an increase in enhancement factor from 1921 to 4894. Field emission properties of a new type of field emitter ZnO/g-C core-shell nanowires are also presented in this chapter. ZnO/g-C core/shell nanowires are synthesized by CVD of zinc acetate at 1300 °C. Overcoming the problems of ZnO nanowire field emitters, which in general possess high turn on fields and low current densities, the core-shell nanowires exhibit excellent field emission performance with low turn on field of 2.75 V/µm and high current density of 1 mA/cm2. Chapter 8 presents a brief summary of the important results and future perspectives of the work reported in the thesis.

碩士
國立成功大學
光電科學與工程研究所
95
The Fowler-Nordheim (F-N) plots of the carbon nanotubes (CNTs) often exhibit a saturation-like phenomenon in the high-voltage region. This phenomenon is attributable to the resistance effect of the CNTs and/ or the interface effect between the substrate and CNTs. Jo et al. established a modified F-N equation to take the resistance effect into account. And their model can fit the experiment data well by adjusting the bulk resistance of the CNTs [52]. In this study, the carrier transport model is applied to investigate the resistance effect of the 1-D nanostructure grown on silicon substrate. The classical transport equation is used to describe the carrier transport in the material and solved together with the Poisson,s equation. The field emission at the emitter-vacuum interface is modeled by the F-N equation. My thesis simulation results exhibit that the F-N plots obtained from the simulation can also be fitted well by Jo,s modified F-N equation. And more importantly, the fitted resistance of the 1-D nanostructure is very close to the calculated resistance from the material mobility used in the simulation. Furthermore, the interface effect can also be considered as a large resistor which is in series with the bulk resistance of the 1-D nanostructure. The effect of carrier,s temperature on carrier,s mobility and resistance in the 1-D nanostructure is also examined.