Dissertations / Theses on the topic 'Multilayer Graphene'

The goal of the present investigation is to examine the processing-structure-property relationships of multilayer graphene nanowall materials. Various plasma enhanced chemical vapor deposition (PECVD) processing parameters were altered to control the structure and morphology of the material. Growth parameters and substrate material were the major structural features studied, and these were characterized by spectroscopic techniques. The direct synthesis of graphene without catalysis on dielectric substrates, compatible with the complementary metal oxide semiconductor technology, is a stimulating but complex task. The goal of the thesis has different tasks consisting of: a) The design and construction of a new inductively coupled plasma remote chemical vapor deposition reactor in the PECVD-FEMAN laboratory of the Universitat de Barcelona. b) Fabrication of vertical graphene nanostructures at low temperature on different conductive and nonconductive substrates. c) Characterization of the vertical graphene obtained through different synthesis parameters in order to optimize their physical and surface properties; such as structural and morphological studies by Raman spectroscopy, SEM and TEM. d) Functionalization of MLGNWs by MnO nanoparticles for hybrid supercapacitor systems. The thesis consists of the following main parts: In the first part of the thesis provides a brief introduction of carbon materials, graphene, graphene nanowalls and their history, discovery, outstanding properties and all the technologies that prompted their development during these years until the first application. Moreover, in this section the methods for the synthesis of carbon nanostructures and brief explanation of the fundamentals of each technique explains. In the second part the concepts and technologies of plasma, plasma enhanced chemical vapor deposition (PECVD) and PECVD related techniques are exposed. In addition, in this section a deposition reactor designed by us are described, where all experiments carried out during this thesis took place. Also, the basics and work principles of different characterization techniques are briefly described. Furthermore, this part discusses the growth mechanism of MLGNWs synthesized by PECVD. In the third part, which is the main results part, discusses the study of growing material along the entire length of the tube and the importance of sample location inside a tubular quartz reactor. The influence of the substrate material, growth time and growth temperature on the MLGNWs growth process have been examined by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and high resolution TEM (HRTEM) techniques. The chemical characteristics of as grown structures were studied by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectrometer. The hydrogen and carbon contents in grown samples were determined by elemental analysis (EA). To study the photoluminescent properties of the carbon structure grown in the whole length of the tubular reactor, the room temperature PL spectra were conducted. In addition, the chemical reactions inside the tube under plasma deposition were controlled by optical emission spectroscopy (OES). Also, in the part of results synthesis and characterization of MLGNWs/CNTs hybrid structure are discussed. The goal of MLGNWs/CNTs hybrid structure is increased chemical activity of CNTs. The morphological and structural characterization was carried out using SEM, High resolution TEM and Raman scattering analysis. Electrochemical properties of transferred MLGNWs/CNTs were studied by CV and charge/discharge measurements. The last section of the part of the results are exposed information about application of MLGNWs in supercapacitors, in particular, supercapacitive performance of manganese dioxide/ graphene nanowalls electrodes deposited on stainless steel current collectors and annealing temperature effect are discussed. Composite electrodes MLGNWs/MnO2 were characterized by FESEM and Raman shift spectroscopy. The electrochemical properties of the MLGNWs/MnO2 for supercapacitor applications were investigated by cyclic voltammetry (CV), charge/discharge and electrochemical impedance spectroscopy (EIS). The influence of annealing temperature on the electrochemical performance has been studied as well.
El grafeno, como material basado en el carbono, es un logro del desarrollo y los avances de la Nanotecnología. La síntesis directa de grafeno sin catálisis sobre sustratos dieléctricos, compatible con la tecnología de los semiconductores complementarios de óxido metálico, es una tarea estimulante pero compleja. La técnica PECVD, permite la síntesis directa de nanoestructuras de carbono a temperaturas más bajas y es el método principal utilizado en esta tesis. El objetivo de esta tesis es la síntesis y optimización de nanoparedes verticales de grafeno y su posible extensión a aplicaciones en sistemas que requieran superficies macroscópicas. Para ello, se han realizado diferentes tareas: a) Se ha diseñado y construido un reactor prototipo con plasma remoto en el laboratorio PECVD-FEMAN de la Facultad de Física (Universidad de Barcelona) con el fin último de crecer grafeno en forma de paredes/tabiques verticales nanométricos mediante la técnica PECVD. b) Se ha desarrollado un proceso PECVD modificado con el fin de mejorar los resultados actuales en términos de: 1) el tiempo de crecimiento, 2) la temperatura, 3) la naturaleza del substrato, 4) la presión, y 5) la cantidad de gas precursor para crecer grafeno vertical. Las muestras obtenidas fueron caracterizadas mediante microscopía TEM, SEM, XPS, XRD y mayormente mediante espectroscopia Raman, con el objetivo de optimizar el proceso y las propiedades físico-químicas y del grafeno vertical. c) Se ha desarrollado una estructura híbrida con nanoparedes y nanotubos de carbono. Para ello, se utilizaron tres equipos: el reactor “PEDRO” para la preparación del substrato, el reactor “CNTs” para el crecimiento de nanotubos de carbono y el reactor ICP-CVD para el crecimiento de nanoparedes de grafeno. En esta tesis se investigaron las caracterizaciones morfológicas y electroquímicas, pero aún se necesitan más estudios para confirmar posibles futuras aplicaciones. d) Para mejorar las propiedades de los supercapacitores basados en los electrodos desarrolladas con nanoparedes de grafeno y acero inoxidable, se ha realizado el crecimiento de capas delgadas de MnO2 mediante el método de electrodeposición. El efecto de la temperatura de recocido (annealing) en las propiedades electroquímicas de las muestras se ha estudiado en el rango de 70° C a 650° C.

Graphene is a new two-dimensional (2D) material with unique electrical transport, optical and mechanical properties. However, monolayer graphene (MLG) is a gapless semiconductor, which limits its relevance for transistor applications where a large on/off ratio of the current is required. In this work the investigation of transport properties of few-layer graphene (FLG) is presented. These 2D electronic systems offer a novel solution to the problem concerned the absence of an energy gap in single layer graphene, since they exhibit an electric field and stacking-dependent band gap in the energy dispersion. Thus far, a clear observation of a band-gap in multilayer graphene (e.g. Bernal-stacked bilayers) in transport measurements was hindered by the presence of disorder. Here we develop a reliable and effective method of fabrication of high-quality suspended double-gated graphene devices, which are of crucial importance for probing the low energy dispersion of few-layer graphene. The current annealing technique, described in details, improves transport characteristics like carrier mobility, which is typically higher than ∼ 104 cm2/Vs for our multilayer devices. Electrical transport experiments on suspended dual-gated ABC-stacked trilayer are performed. We report the direct evidence of the opening of a tunable band-gap with an external perpendicular electric field, ranging from 0 meV up to 5.2 meV for an electric field of 117 mV/nm. Thermally activated transport is observed in these samples over the temperature range 0.5 - 80 K. The values of energy gap extracted from both temperature dependence of minimum conductivity measurements and non-linear I –V characteristics correlate well. Our experimental results are in a good agreement with theoretical approximation, based on self-consistent tight-binding calculations. The high quality of our ABC trilayer samples is also demonstrated by a particularly high on/off ratio of the current (250 at applied electrical displacement as low as 80 mV/nm), which makes these devices promising for future semiconductor electronics. FLG samples with reduced disorder allow us to observe quantum Hall effect (QHE) at magnetic field as low as 500 mT. We present the first study of electric field- induced new QH states in ABC trilayer graphene (TLG). The transitions between spin-polarized and valley polarized phases of the sample at the charge neutrality point are investigated. Resolved novel broken symmetry states along with observed Lifshitz transition in rhombohedral TLG display exciting phenomena attributed to rich physics in these interactive electronic systems.

It has been shown recently that high-quality epitaxial graphene (EPG) can be grown on the SiC substrate that exhibits interesting physical properties and has great advantages for varies device applications. In particular, the multilayer graphene films grown on the C-face show rotational disorder. It is expected that the twisted layers exhibit unique new physics that is distinct from that of either single layer graphene or graphite. In this work, by combining density functional and tight-binding model calculations, we investigate the electric field and doping effects on twisted bilayer graphene (TBG), multiple layer effects on twisted triple-layer graphene, and wave packet propagation properties of TBG. Though these studies, we obtain a comprehensive description of the interesting interlayer interaction in this twisted multilayer graphene system.

Graphene has attracted significant research attention for next generation of semiconductor devices due to its high electron mobility and compatibility with planar semiconductor processing. In this dissertation, the influences of Ohmic metals and high dielectric (high-k) constant aluminum oxide (Al2O3) deposition on the structural and electrical properties of multi-layer epitaxial graphene (MLG) grown by graphitization of silicon carbide (SiC) substrates have been investigated. Uniform MLG was successfully grown by sublimation of silicon from epitaxy-ready, Si and C terminated, 6H-SiC wafers in high-vacuum and argon atmosphere. The graphene formation was accompanied by a significant enhancement of Ohmic behavior, and, was found to be sensitive to the temperature ramp-up rate and annealing time. High-resolution transmission electron microscopy (HRTEM) showed that the interface between the metal and SiC remained sharp and free of macroscopic defects even after 30 min, 1430 °C anneals. The impact of high dielectric constant Al2O3 and its deposition by radio frequency (RF) magnetron sputtering on the structural and electrical properties of MLG is discussed. HRTEM analysis confirms that the Al2O3/MLG interface is relatively sharp and that thickness approximation of the MLG using angle resolved X-ray photoelectron spectroscopy (ARXPS) as well as variable-angle spectroscopic ellipsometry (VASE) is accurate. The totality of results indicate that ARXPS can be used as a nondestructive tool to measure the thickness of MLG, and that RF sputtered Al2O3 can be used as a (high-k) constant gate oxide in multilayer grapheme based transistor applications.

This thesis reports the study of electronic band structure of single and few layered graphene grown by thermal decomposition of SiC at the surface and by C-sublimation on Ru single crystals. Growth conditions were optimized in order to obtain big few micrometer sized graphene domains. For the first system twisted multilayer graphene domains were found and chosen for study. On ruthenium only single layer graphene domains and also the domains with incorporated bilayer patches were obtained and their electronic properties were investigated after oxidation-reduction reactions at graphene/Ru interface. The electronic band structure was analyzed using high resolution angle resolved photoelectron spectroscopy. In order to obtain spectra from individual domains novel spectromicroscopy end station was used for focusing synchrotron radiation beam to sub-micrometer spot on the sample surface. Experimental results on twisted graphene confirmed interlayer coupling and resulting van Hove singularities, graphene Dirac fermions velocity renormalization and other exotic phenomena predicted by theoretical calculations and partially observed by scanning tunneling spectroscopy technique. Particular attention has been paid to poorly studied interlayer coupling in trilayer systems where middle layer has two different couplings being sandwiched between differently twisted layers. These multilayer graphene domains were also investigated in detail upon alkali metal intercalation and unexpected splitting of upper part of Dirac cone, related to graphene sublattice symmetry breaking in the middle graphene layer was found. In graphene on Ru it was first confirmed that oxidation of Ru under graphene decouples its strongly hybridized π orbitals making graphene p-doped. Our observations indicate that bilayer patches incorporated into single layer background remain n-doped and decorated by intercalated oxygen, thereby forming lateral p-n junctions in the same graphene layer. It was found that hydrogen atmosphere helps to reduce RuOx without the formation of carbon vacancy defects. However, structural wrinkle patterns appeared due to loss of original graphene/Ru epitaxial order remain, and in big graphene domains they can trap H2+RuOx reaction products, making graphene fully decoupled and undoped.

Orientador: Cesar Renato Foschini
Resumo: Os avanços na área da tecnologia e ciência dos materiais têm possibilitado grande evolução e contribuição para o desenvolvimento de materiais cerâmicos. Pesquisas associadas a esse tipo de material estão cada vez mais difundidas por apresentar características como: alta dureza, biocompatibilidade, estabilidade térmica, inércia química e resistência à corrosão. Entretanto, seu uso acaba sendo limitado por sua fragilidade. A utilização dos alótropos de carbono (grafeno, nanotubos e fulerenos) como material de reforço nas cerâmicas, têm sido muito estudados, no entanto, o desempenho desses alótropos está restrito a condições específicas de mistura e sinterização. Esta pesquisa trata da fabricação e caracterização mecânica do compósito Al2O3-MLG. Partindo de um pó cerâmico de Al2O3 e adicionando diferentes concentrações, 0,5%; 0,75%; 1,0% e 1,25%, em peso de MLG em sua matriz, corpos de prova foram fabricados através da mistura em meio úmido dos pós, secagem da mistura, compactação dos pós secos e posterior sinterização com atmosfera redutora. As amostras obtidas pelo método proposto foram submetidas a caracterização física, microestrutural e mecânica, sendo comparadas diretamente a amostra contendo apenas Al2O3. Foi verificado que a sinterização com atmosfera redutora, preservou a estrutura do MLG em matriz de Al2O3. A análise dos resultados indicou que a utilização de concentrações menores que 1,0% em peso de MLG, proporcionou a fabricação de compósitos com maior dispersão e me... (Resumo completo, clicar acesso eletrônico abaixo)
Abstract: Advances in the area of technology and materials science have enabled major evolution and contribution to the development of ceramic materials. Research associated with this type of material is becoming more widespread due to characteristics such as: high hardness, biocompatibility, thermal stability, chemical inertia and corrosion resistance. However, its use ends up being limited by its fragility. The use of carbon allotropes (graphene, nanotubes and fullerenes) as reinforcement material in ceramics has been widely studied, however, the performance of these allotropes is restricted to specific mixing and sintering conditions. This research is about the fabrication and mechanical characterization of the Al2O3-MLG composite. Starting from an Al2O3 ceramic powder and adding different concentrations, 0,50%; 0,75%; 1,0% and 1,25% by weight of MLG in their matrix, samples were fabricated by wet powder mixing, drying of the mixture, dry powder compaction and subsequent sintering with reducing atmosphere. The samples obtained by the proposed method were subjected to physical, microstructural and mechanical characterization, being directly compared to the sample containing only Al2O3. It was verified that the sintering with reducing atmosphere preserved the structure of the MLG in Al2O3 matrix. The analysis of the results indicated that the use of concentrations lower than 1.0% by weight of MLG, allowed the fabrication of composites with greater dispersion and less formation of aggl... (Complete abstract click electronic access below)
Mestre

Thesis (Ph. D.)--University of California, Riverside, 2009.
Includes abstract. Available via ProQuest Digital Dissertations. Title from first page of PDF file (viewed March 12, 2010). Includes bibliographical references (p. 96-107). Also issued in print.

A furação é um processo caracterizado por apresentar dificuldades em relação à formação e remoção do cavaco da região de corte, e à geração de altas temperaturas devido ao cisalhamento e encruamento do material no fundo do furo, principalmente, na usinagem do aço inoxidável austenítico. Com isso a presença do fluido de corte com boas propriedades de refrigeração e lubrificação faz-se necessária para preservar a vida da broca e garantir um acabamento satisfatório do furo. Contudo, com a tendência mundial de diminuição da quantidade de fluido de corte dispendido, tem-se buscado técnicas diferentes de aplicação que utilize o lubrirrefrigerante em quantidades reduzidas. Associado a essas novas técnicas, vem sendo empregado cada vez mais o uso de partículas sólidas adicionadas ao fluido com o intuito de aumentar sua eficiência. Um tipo de partícula, que vem sendo aplicada em diversos setores produtivos devido a suas ótimas propriedades mecânicas, químicas e elétricas, é o grafeno. Sendo assim, pretende-se analisar o comportamento da furação do aço inoxidável austenítico AISI 304 utilizando flocos de multicamadas de grafeno dispersos em fluido de corte empregando o projeto de experimentos Box-Behnken A aplicação do fluido é feita externamente utilizando a técnica de quantidade reduzida com três vazões: 1,5 l/h; 2,0 l/h e 2,5 l/h. O comportamento do processo é avaliado pelos esforços de corte (força de avanço e momento torsor), pela rugosidade (média e total), e pelos desvios (dimensional e de circularidade) na entrada e na saída do furo. O desempenho do grafeno é avaliado pela comparação dos resultados da sua utilização com os de outras duas condições de aplicação do fluido de corte sem adições: em quantidade reduzida e em abundância. Teve-se como resultado que todas as variáveis respostas analisadas sofreram alguma influência da velocidade de corte e/ou da vazão do fluido aplicado em quantidades reduzidas. Os benefícios da lubrificação e refrigeração do grafeno são sentidos em alguns resultados, como nos menores valores médios de rugosidade e de desvio dimensional. Mas, na análise do processo como um todo, não se afirma que o fluido de corte com flocos de multicamadas de grafeno aplicado externamente em quantidade reduzida traz mais benefícios que o sem adições.
Drilling is a machining process characterized by difficulties in the formation and removal of the chip from the cutting region and the generation of high temperatures due to shearing and hardening of the material at the bottom of the hole, especially in the machining of austenitic stainless steel. Thus the presence of cutting fluid with good cooling and lubrication properties is necessary to preserve the drill life and ensure a satisfactory hole finish. However, with the worldwide trend of decreasing the amount of cutting fluid expended, different application systems have been sought which use the coolant in reduced amounts. Associated with these new techniques, the presence of solid particles added to the fluid has been increasingly used in order to enhance its efficiency. One of these particles, which have been applied in several productive sectors due to its excellent mechanical, chemical and electrical properties, is graphene. Therefore, it is intended to analyze the drilling behavior of AISI 304 austenitic stainless steel using multilayer graphene flakes dispersed in the cutting fluid using the Box- Behnken Design The application of the fluid is done externally by means of reduced quantity lubricant with three flows: 1.5 l/h, 2.0 l/h and 2.5 l/h. The behavior of the process is evaluated via thrust force and torque, average and total roughness, and dimensional and circularity deviations at the input and output holes. The performance of graphene is evaluated by comparing the results of its use with two other cutting fluid conditions: quantity reduced and abundance. It was found that all the analyzed responses variables had some influence of the cutting speed and/or the flow of the applied fluid in reduced quantities. The benefits of graphene lubrication and cooling are felt in some results, such as the lower values of average roughness and dimensional deviation. However, in the analysis of the process as a whole, it is not possible to state that the multilayer graphene flakes cutting fluid externally applied brings more benefits than without additions.

The work function and electronic structure of epitaxial graphene as well as of quasi-freestanding graphene multilayer samples were studied by Kelvin probe and angle resolved photoelectron spectroscopy. The work function converges towards the value of graphite as the number of layers is increased. Thereby, n-type doped epitaxial graphene layers have a work function lower than graphite and p-type doped quasi-freestanding graphene layers exhibit a work function higher than graphite. We explain the behaviour by the flling of the pi-bands due to substrate interactions.

The work discussed in this thesis deals with the generation, control and modulation of optical interactions in two-dimensional materials, specifically in unpatterned, subwavelength graphene multilayers, using the process of Coherent Perfect Absorption (CPA). It aims to address the problem of inefficient light-matter coupling at the nanoscale by studying new geometries for enabling total absorption in 50% absorbing graphene films. Total optical absorption is demonstrated and a 80% modulation of the absorption and scattering is achieved by controlling the relative phases of the interacting optical beams. Degenerate four-wave mixing (DFWM) in graphene multilayers leads to the generation of optical phase conjugation and negative refraction. These nonlinear responses are generated with a conversion efficiency of 5 x 10-5, and using the CPA arrangement their amplitudes are modulated with a modulation contrast of 100%. It is shown that the two-dimensionality of graphene gives rise to a ‘phase-dependent’ nonlinearity, which differs significantly from that in bulk materials. The optical nonlinearity in graphene is seen to be controlled by the relative phases of the interacting optical fields in a manner such that the nonlinear polarisation itself can be switched on or off. The phase-dependent nonlinearity of the two-dimensional medium is then explored in three alternative geometries. The first one uses only two input beams, and a light-with-light modulation of the nonlinear signals is observed with a contrast of 90%. The second geometry involves a single beam interacting with the sample, wherein, nonlinear signals are generated in a self-pumping mode, due to reflection from a mirror placed very close to the graphene sample. The last configuration also uses a mirror in order to require only a single light beam and leads to the observation of a ‘negative reflection’ signal. Finally, a nonlinear imaging technique ‘phase-contrast imaging’ is performed using a traditional DFWM configuration with three input optical fields. A phase-object applied on one of the pump beams is transformed into an intensity object in the resulting negative refraction. A few basic phase objects are imaged on the negatively refracted beam and are reported in this work, offering a possible application for the advantages offered by two-dimensional optical nonlinearities.

COSTA, Diego Rabelo da. Transportes e confinamento em monocamada e bicamada de nanoestruturas de grafeno com diferentes bordas, interfaces e potenciais. 2014. 201 f. Tese (Doutorado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2014.
Submitted by Edvander Pires (edvanderpires@gmail.com) on 2015-06-01T22:18:12Z No. of bitstreams: 1 2014_tese_drcosta.pdf: 54910487 bytes, checksum: 82b386bac8259edaa10f6d5ff314bd42 (MD5)
Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2015-06-01T22:19:16Z (GMT) No. of bitstreams: 1 2014_tese_drcosta.pdf: 54910487 bytes, checksum: 82b386bac8259edaa10f6d5ff314bd42 (MD5)
Made available in DSpace on 2015-06-01T22:19:16Z (GMT). No. of bitstreams: 1 2014_tese_drcosta.pdf: 54910487 bytes, checksum: 82b386bac8259edaa10f6d5ff314bd42 (MD5) Previous issue date: 2014
Graphene, a two-dimensional lattice of carbon atoms, has been widely studied during the past few years. The interest in this material is not only due to its possible future technological applications, but also because it provides the possibility to probe interesting phenomena predicted by quantum field theories, ranging from Klein tunneling and other quasi-relativistic effects to the existence of new types of electron degrees of freedom, namely, the pseudo-spin, and the existence of two inequivalent electronic valleys in the vicinity of the gapless points of its energy spectrum. Several of the exotic properties observed in graphene originate from the fact that within the low energy approximation for the tight-binding Hamiltonian of graphene, electrons behave as massless Dirac fermions, with a linear energy dispersion. Just like in single layer graphene, the low-energy eletronic spectrum in bilayer graphene is gapless, but in this case it is dominated by the parabolic dispersion. Nevertheless, one interesting feature is shared by both monolayer and bilayer graphene: the valley degree of freedom. In this thesis, we theoretically investigate: (i) the dynamic properties in mono and bilayer graphene, performing a systematic study of wave packet scattering in different interface shapes, edges and potentials; and furthermore (ii) the energy levels of confined systems in graphene in the presence or absence of external magnetic and electric fields. In the first part of the work, we use the tight-binding approach to study the scattering of a Gaussian wave packet on monolayer graphene edges (armchair and zigzag) in the presence of real and pseudo (strain induced) magnetic fields and also calculate the transmission probabilities of a Gaussian wave packet through a quantum point contact defined by electrostatic gates in bilayer graphene. These numerical calculations are based on the solution of the time-dependent Schrödinger equation for the tight-binding model Hamiltonian, using the Split-operator technique. Our theory allows us to investigate scattering in reciprocal space, and depending on the type of graphene edge we observe scattering within the same valley, or between different valleys. In the presence of an external magnetic field, the well known skipping orbits are observed. However, our results demonstrate that in the case of a pseudo-magnetic field, induced by non-uniform strain, the scattering by an armchair edge results in a non-propagating edge state. We propose also a very efficient valley filtering through a quantum point contact system defined by electrostatic gates in bilayer graphene. For the suggested bilayer system, we investigate how to improve the efficiency of the system as a valley filter by varying parameters, such as length, width and amplitude of the applied potential. In the second part of the thesis, we present a systematic study of the energy spectra of graphene quantum rings having different geometries and edge types, in the presence of a perpendicular magnetic field. We discuss which features obtained through a simplified Dirac model can be recovered when the eigenstates of graphene quantum rings are compared with the tight-binding results. Furthermore, we also investigate the confined states in two different hybrid monolayer - bilayer systems, identifying dot-localized states and edge states for the suggested bilayer confinement structures, as well as we will study the behavior of the energy levels as a function of dot size and under an applied external magnetic field. Finally, using the four-band continuum Dirac model, we also derive a general expression for the infinite-mass boundary condition in bilayer graphene in order to apply this boundary condition to calculate analytically the confined states and the corresponding wave functions in a bilayer graphene quantum dot in the absence and presence of a perpendicular magnetic field. Our analytic results exhibit good agreement when compared with the tight-binding ones.
Grafeno, uma rede bidimensional de átomos de carbono, tem sido amplamente estudado durante os últimos anos. O interesse por este material não é apenas devido às suas possíveis aplicações tecnológicas futuras, mas também porque oferece a possibilidade de investigar fenômenos interessantes previstos pelas teorias quânticas de campo, que vão desde o tunelamento de Klein e outros efeitos quasi-relativísticos à existência de novos tipos de graus de liberdade do elétron, ou seja, o pseudo-spin, e a existência de dois vales eletrônicos não-equivalentes na vizinhança dos pontos sem gap do seu espectro de energia. Várias das propriedades exóticas observadas no grafeno originam-se do facto de que dentro da aproximação de baixas energias para o Hamiltoniano tight-binding do grafeno, elétrons se comportam como férmions de Dirac sem massa, com uma dispersão de energia linear. Assim como no caso de uma monocamada de grafeno, o espectro eletrônico de baixas energias para uma bicamada de grafeno é sem gap, mas, neste caso, é dominado pela dispersão parabólica. No entanto, uma característica interessante é compartilhada por ambas monocamada e bicamada de grafeno: o grau de liberdade de vale. Nesta tese, nós investigamos teoricamente: (i) as propriedades dinâmicas em mono e bicamadas de grafeno, realizando um estudo sistemático do espalhamento de pacotes de onda em diferentes formas de interfaces, bordas e potenciais; e, além disso, (ii) os níveis de energia de sistemas confinados no grafeno na presença ou ausência de campos magnéticos e elétricos externos. Na primeira parte do trabalho, nós utilizamos a abordagem tight-binding para estudar o espalhamento de um pacote de onda Gaussiano nas bordas de uma monocamada de grafeno (armchair e zigzag) na presença de campos magnéticos reais e pseudo-magnéticos (induzidos por tensão) e também calculamos as probabilidades de transmissão de um pacote de onda Gaussiano através de um contato de ponto quântico definido por potenciais eletrostáticos em bicamadas de grafeno. Estes cálculos numéricos são baseados na solução da equação de Schrödinger dependente do tempo para o Hamiltoniano do modelo tight-binding, usando a técnica Split-operator. Nossa teoria permite investigar espalhamento no espaço recíproco, e dependendo do tipo de borda do grafeno, nós observamos espalhamento dentro do mesmo vale, ou entre diferentes vales. Na presença de um campo magnético externo, as bem conhecidas órbitas skipping orbits são observadas. No entanto, nossos resultados demonstram que, no caso de um campo pseudo-magnético induzido por uma tensão não-uniforme, o espalhamento por uma borba armchair resulta em um estado de borda não-propagante. Nós também propomos um sistema de filtragem de vales muito eficiente através de um sistema de contato de ponto quântico definido por portas eletrostáticas em uma bicamada de grafeno. Para o sistema de bicamadas sugerido, nós investigamos a forma de melhorar a eficiência do sistema como um filtro de vales por diferentes parâmetros, como comprimento, largura e amplitude do potencial aplicado. Na segunda parte da tese, nós apresentamos um estudo sistemático dos espectros de energia de anéis quânticos de grafeno com diferentes geometrias e tipos de borda, na presença de um campo magnético perpendicular. Nós discutimos quais características obtidas por meio de um modelo simplificado de Dirac podem ser recuperadas quando os auto-estados de anéis quânticos de grafeno são comparados com os resultados do modelo tight-binding. Além disso, nós também investigamos os estados confinados em dois sistemas híbridos diferentes de monocamada - bicamada, identificando estados localizados dentro do ponto e estados de borda para as estruturas de confinamento em bicamadas sugeridas, assim como vamos estudar o comportamento dos níveis de energia em função do tamanho do ponto e sob um campo magnético externo aplicado. Finalmente, usando o modelo contínuo de Dirac de quatro bandas, nós também derivamos uma expressão geral para a condição de contorno de massa infinita em bicamada de grafeno, a fim de aplicar essa condição de contorno para calcular analiticamente os estados confinados e as correspondentes funções de onda em um ponto quântico em uma bicamada de grafeno na ausência e na presença de um campo magnético perpendicular. Nossos resultados analíticos apresentam boa concordância quando comparados com os resultados tight-binding.

Low friction, nanoscale multilayer carbon/chromium (C/Cr) coatings were successfully deposited by the combined steered cathodic arc/unbalanced magnetron sputtering technique (also known as Arc Bond Sputtering or ABS) using a Hauzer HTC 1000-4 PVD coater. The work described in this thesis has been directed towards understanding the effect of ion irradiation on the composition, microstructure, and functional properties of C/Cr coatings. This has been achieved by varying the bias voltage, U[B], over a wide range between -65 V and -550 V. C/Cr coatings were deposited in three major steps: (i) Cr+ ion etching using a steered cathodic arc discharge at a substrate bias voltage of -1200 V, (ii) deposition of a 0.25 mum thick CrN base layer by reactive unbalanced magnetron sputtering to enhance the adhesion, and (iii) deposition of C/Cr coatings by unbalanced magnetron sputtering from three graphite targets and one chromium target at 260°C. The coatings were deposited at different bias voltages (U[B]) from -65 V to -550 V in a non-reactive Ar atmosphere. C/Cr coatings exhibit excellent adhesion (critical load, L[C] > 70 N), with hardness ranging from 6.8 to 25.1 GPa depending on the bias voltage. The friction coefficient of C/Cr coatings was found to reduce from 0.22 to 0.16 when the bias voltage was increased from U[B] = -65 to -95 V. The relevance of C/Cr coatings for actual practical applications was demonstrated using dry high-speed milling trials on automotive aluminium alloy (Al-Si8Cu3Fe). The results showed that C/Cr coated cemented carbide ball-nose end mills prepared at U[B] = -95 V (70 at.% C, 30 at.% Cr) enhance the tool performance and the tool life compared to the uncoated tools by a factor of two, suggesting the potential for use in dry high-speed machining of "sticky" alloys such as aluminum. Different film morphologies were observed in the investigated bias voltage range between U[B] = -65 and -550 V using XTEM. With increasing bias voltage from U[B] = -65 to -95 V, the structure changed from columnar, with carbon accumulated at the column boundaries, to a dense structure which comprised randomly distributed onionlike carbon clusters. A novel nanostructure was observed within this bias voltage range, in which the basic nano-lamellae obtained as a result of substrate rotation in front of the C and Cr targets were modified by an ion-irradiation induced nanocolumnar structure. Further increases in the bias voltage to U[B] = -350 V and U[B] = -450 V led to segregation and self-organisation of the carbon atoms induced by the high energy ion bombardment and, finally, to the formation of a new type of self-organised multilayer structure. A coating growth model accounting for the influence of ion bombardment on the growing C/Cr film was introduced to explain the phase separation and formation of the selforganised layered nanostructure. A novel experimental set-up for the investigation of tribocorrosion was built based on a modification of the conventional Scanning Reference Electrode Technique (SRET). The device comprises a ball on rotating cylinder contact configuration combined with a SRET electrochemical device. This combination may contribute significantly to the understanding of wear-corrosion synergism.

Дисертацію присвячено комплексному дослідженню магніторезистивних і оптичних властивостей та газової чутливості приладових систем на основі масивів магнітних наночастинок (НЧ) NiFe2O4, СоFe2O4, Fe3O4 у провідній немагнітній матриці Ag або мультишарового графену. У роботі проаналізовано зв'язок між структурним станом НЧ і магніторезистивними, магнітооптичними та оптичними властивостями наноприладових систем. Встановлено механізми формування масивів спін-вентильних переходів при збільшенні товщини провідної матриці Ag від 5 до 20 нм і умов її температурної обробки (Тв = 600 К) та їх внесок у величину магнітоопору. Експериментально вивчено характер зміни електричного опору наноструктурованих шарів від умов температурної обробки (Тв = 1100 К) та досліджено вплив декорування поверхні мультишарового графену масивами НЧ NiFe2O4 для покращення чутливості до шкідливого газу NO2 на 40 %. Результати досліджень можуть бути використані як практичні рекомендації при побудові наноструктурованих чутливих елементів датчиків різного функціонального призначення.
Диссертация посвящена комплексному исследованию магниторезистивных и оптических свойств, газовой чувствительности приборных систем на основе массивов магнитных наночастиц (НЧ) NiFe2O4, СоFe2O4, Fe3O4 в проводящей немагнитной матрице Ag или мультислойного графена (МСГ). В работе изучены особенности и условия формирования двухмерных наноструктур из наночастиц и мультислойного графена на подложках SiO2 (500 нм) / Si (001) с использованием методик спин-коатинга и Ленгмюра – Блоджетт. Проанализирована связь между структурным состоянием НЧ и магниторезистивными, магнитооптическими и оптическими свойствами наноприборных систем. Установлены зависимости оптических параметров от характера распределения массивов наночастиц или фрагментов МСГ на подложке. Установлены механизмы формирования массивов спин-вентильных переходов при увеличении толщины проводящей матрицы Ag от 5 до 20 нм и условий ее температурной обработки (То = 600 К) и их вклад в величину магнитосопротивления. Разработана теоретическая модель, позволяющая оценить величину вклада рассеивания электронов на ферромагнитных частицах в электрическое сопротивление массивов магнитных наночастиц в проводящей матрице Ag до и после еѐ термообработки (То = 600 К). Экспериментально изучен характер изменения электрического сопротивления наноструктурированных слоев от условий температурной обработки (То = 1100 К) и исследовано влияние декорирования поверхности мультислойного графена массивами наночастиц NiFe2O4 для улучшения чувствительности к вредному газу NO2 на 40 %, а также предложен механизм взаимодействия чувствительного материала приборных структур и газа NO2. Исследовано влияние температуры (То = 1100 К) на величину газовой чувствительности МСГ и установлены оптимальные условия термообработки для получения наиболее эффективных чувствительных элементов с минимальным уровнем шума. Изучено влияние размеров фрагментов МСГ на газовую чувствительность датчиков на их основе. Результаты исследований могут быть использованы как практические рекомендации при построении наноструктурированных чувствительных элементов датчиков разного функционального назначения.
The thesis is devoted to the complex investigation of magnetoresistive and optical properties and sensitivity to gases of instrumentation systems based on arrays of nanoparticles (NP) NiFe2O4, СоFe2O4, Fe3O4 in Ag conductive matrix or multilayered graphene. In this work an interconnection between the structural features of the NP and magnetoresistive, magnetooptical and optical properties of instrumentation systems was analyzed. The mechanisms of formation of arrays of spin-valve junctions while increasing the thickness of Ag conductive matrix from 5 to 20 nm, conditions of it temperature treatment and their influence on the value of magnetoresistance were established. Electrical resistance changes of nanostructured layers depends on conditions of thermal treatment (Tt = 1100 K) were experimentally studied and an effect of decoration of multilayered graphene by arrays of NiFe2O4 NPs for increasing of sensitivity to NO2 gas by 40% was investigated. The results of research can be utilized as practical recommendation while develop the nanostructured sensitive elements of the sensors with different functional purposes.

Dans cette thèse, nous nous intéressons à un problème de fiabilité dans les réseaux multicouches IP-sur-WDM. Etant donné un ensemble de demandes pour lesquelles on connaît une topologie fiable dans la couche IP, le problème consiste à sécuriser la couche optique WDM en y cherchant une topologie fiable. Nous montrons que le problème est NP-complet même dans le cas d'une seule demande. Ensuite, nous proposons quatre formulations en termes de programmes linéaires en nombres entiers pour le problème. La première est basée sur les contraintes de coupes. Nous considérons le polyèdre associé. Nous identifions de nouvelles familles de contraintes valides et étudions leur aspect facial. Nous proposons également des algorithmes de séparation pour ces contraintes. En utilisant ces résultats, nous développons un algorithme de coupes et branchements pour le problème et présentons une étude expérimentale. La deuxième formulation utilise comme variables des chemins entre des terminaux dans le graphe sous-jacent. Un algorithme de branchements et génération de colonnes est proposé pour cette formulation. Par la suite, nous discutons d'une formulation dite naturelle utilisant uniquement les variables de design. Enfin, nous présentons une formulation étendue compacte qui, en plus des variables naturelles, utilise des variables de routage. Nous montrons que cette formulation fournit une meilleure borne inférieure
This thesis deals with a problem related to survivability issues in multilayer IP-over-WDM networks. Given a set of traffic demands for which we know a survivable logical routing in the IP layer, the aim is determine the corresponding survivable topology in the WDM layer. We show that the problem is NP-hard even for a single demand. Moreover, we propose four integer linear programming formulations for the problem. The first one is based on the so-called cut inequalities. We consider the polyhedron associated with the formulation. We identify several families of valid inequalities and discuss their facial aspect. We also develop separation routines. Using this, we devise a Branch-and-Cut algorithm and present experimental results. The second formulation uses paths between terminals of the underlying graph as variables. We devise a Branch-and-Price algorithm based on that formulation. In addition, we investigate a natural formulation for the problem which uses only the design variables. Finally, we propose an extended compact formulation which, in addition to the design variables, uses routing variables. We show that this formulation provides a tighter bound for the problem

L'analyse de données de plus en plus complexes, volumineuses et issues de différentes sources (e.g. internet, médias sociaux, etc.) est une tâche difficile. Elle reste cependant cruciale dans de très nombreux domaines d'application. Elle implique, pour pouvoir en extraire des connaissances, de mieux comprendre la nature des données, leur évolution ou les nombreuses relations complexes qu'elles peuvent contenir. La visualisation d'informations s'intéresse aux méthodes de représentations visuelles et interactives permettant d'aider un utilisateur à extraire des connaissances. C'est dans ce contexte que se situe le travail présenté dans ce mémoire. Dans un premier temps, nous nous intéressons à la visualisation de longues séries temporelles hiérarchiques. Après avoir analysé les différentes approches existantes, nous présentons le système MultiStream permettant de visualiser, explorer et comparer l'évolution de séries organisées dans une structure hiérarchique. Nous illustrons son utilisation par deux exemples d'utilisation : émotions exprimées dans des médias sociaux et évolution des genres musicaux. Dans un second temps nous abordons la problématique de données complexes modélisées sous la forme de graphes multicouches (différentes types d'arêtes peuvent relier les n÷uds). Plus particulièrement nous nous intéressons au requêtage visuel de graphes volumineux en présentant VERTIGo un système qui permet de construire des requêtes, d'interroger un moteur spécifique, de visualiser/explorer les résultats à différentes niveaux de détail et de suggérer de nouvelles extensions de requêtes. Nous illustrons son utilisation à l'aide d'un graphe d'auteurs provenant de différentes communautés
The analysis of data that is increasingly complex, large and from different sources (e.g. internet, social medias, etc.) is a dificult task. However, it remains crucial for many fields of application. It implies, in order to extract knowledge, to better understand the nature of the data, its evolution or the many complex relationships it may contain. Information visualization is about visual and interactive representation methods to help a user to extract knowledge. The work presented in this document takes place in this context. At first, we are interested in the visualization of large hierarchical time series. After analyzing the different existing approaches, we present the MultiStream system for visualizing, exploring and comparing the evolution of the series organized into a hierarchical structure. We illustrate its use by two examples: emotions expressed in social media and the evolution of musical genres. In a second time, we tackle the problem of complex data modeled in the form of multilayer graphs (different types of edges can connect the nodes). More specifically, we are interested in the visual querying of large graphs and we present VERTIGo, a system which makes it possible to build queries, to launch them on a specific engine, to visualize/explore the results at different levels of details and to suggest new query extensions. We illustrate its use with a graph of co-authors from different communities

碩士
中原大學
物理研究所
98
Graphene has emerged as an exciting material because of the novel properties associated with its special two-dimensional structure. Due to its special band structure, graphene has observed an abnormal half-integer quantum Hall effect. We would like to fabricated the graphite sheets (multilayer or monolayer graphene) and than to made devices for studying quantum transport. 　　We have made graphene by two methods, micromechanical cleavage and longitudinal unzipping of carbon nanotubes. The first method is the cleavage of the graphite with the tape to produce the graphene. This 3M scotch-tape is better in a variety of tape, and it is relatively apt to make graphene under the environment of low-temperature and low-humidity. The second method is by using KMnO4/H2SO4 to cause the chemical reaction for longitudinal unzipping of carbon nanotubes to make graphene nanoriboons, and than transfer graphene nanoribbons on SiO2/Si substrate. By checking on the optical microscope, we can find the graphite or few layer graphene. The thickness is distinguished of the graphite sheets from the color of image. And also analyzed the Raman spectrum and Atomic force microscope (AFM) of graphene, and determine the number of graphene layers. 　　In the future work, first, the weak localization effect will be studied for quantum interference in graphenes. Second, the graphenes will be fabricated for studying spin electronics.

碩士
國立中央大學
物理研究所
100
In this thesis, high quality monolayer graphene were grown on Cu foil by chemical vapor deposition, and we manufacture multilayer graphene by stacking monolayer graphene. The propose of stacking multilayer graphene is to obtain optimum condition both in optical and electrical properties. According to the results of Raman spectrum measurement, the full width at half maximum of G band and 2D band peaks are both smaller than 40 cm-1 and the ratio of 2D/G is larger than 1.5, which means the transferred graphene is monolayer. The average sheet resistivity of transferred graphene, which is measured by Hall measurement system, are about 2000 ohm/sq. For the optical and electrical properties of stacked multilayer graphene, the transmittance, Raman spectrum, and Hall measurement were taken. The Raman spectrum results shows that the peak positions of G and 2D band didn’t shift and the 2D/G ratio were almost the same between one and stacked multilayer graphene. In addition, no shoulders were observed in the 2D peak of Raman spectrum that means there should be no interactions between layers. In the optical transmittance results, the absorption of multilayer graphene is about 3.1%/layer at 555 nm wavelength. The transmittance of three layers graphene is about 90%. In electrical properties, the sheet resistivity of graphene were decreased to 50% and 75% with stacked layer number was two and three layers, respectively. When the layer number was increased more than five layers, the sheet resistivity of multilayer graphene was close to HOPG. In the thesis, we successfully manufacture certain layers graphene by stacking monolayer graphene. The sheet resistivity of stacked graphene was decreased and the optical transmittance of stacked graphene was above 90%.

Dissertação de mestrado integrado em Engenharia de Materiais
Biomedical applications and more precisely, implantable devices or scaffolds for regenerative medicine, typically require the use of biodegradable and/or biocompatible materials with adequate mechanical properties and, depending on the specific application, specific electric properties. In this work, such properties were attempted by combining natural polymers, to provide biodegradability and biocompatibility, and graphene, to provide adequate mechanical and electric properties. Four different types of graphene were produced through different chemical methods. Both raw multiwall carbon nanotubes (MWNTs) and raw exfoliated graphite (EG) were chemically functionalized by two methods, oxidation using a modified Hummers’ method, and functionalization through a 1,3-dipolar cycloaddition reaction. Both functionalization procedures were followed by unzipping/exfoliation in solution, through sonication. The result was functionalized graphene nanoribbons (f-GNRs) and graphene flakes (f-GFs) and oxidized graphene nanoribbons (o-GNRs) and graphene flakes (o-GFs), by subjecting functionalized MWNTs and EG and oxidized MWNTs and EG, respectively, to sonication. The graphene solutions produced were characterized by Ultraviolet-Visible Spectroscopy, Zeta Potential (ZP), and Scanning Transmission Electron Microscopy (STEM). Graphenes were characterized by Infrared Spectroscopy and Thermogravimetric Analysis (TGA). Additionally, f-GNRs were characterized by Scanning Tunneling Microscopy (STM), and to ease characterization they were previously characterized by Atomic Force Microscopy (AFM) and Optical Microscopy (OM). Thereafter, multilayer films combining those different types of produced graphene and two biodegradable and biocompatible natural polymers, namely chitosan (CHT) and alginate (ALG), were prepared through the layer-by-layer (LbL) technique, and real time monitored by a quartz crystal microbalance (QCM) for accurate mass sensing. The thickness, viscosity and shear modulus of the produced films were estimated by applying a Voigt based model.
Aplicações biomédicas, mais precisamente, dispositivos implantáveis e scaffolds para a medicina regenerativa, tipicamente, exigem materiais biodegradáveis e/ou biocompatíveis, com adequadas propriedades mecânicas e, dependendo da aplicação, propriedades eléctricas. Neste trabalho, polímeros naturais, pela sua biodegradibilidade e biocompatibilidade, foram combinados com grafeno, pelas suas adequadas propriedades mecânicas e elétricas, por forma a responder a tais requisitos. Quatro tipos de grafeno foram produzidos por diferentes métodos químicos. Nanotubos de paredes múltiplas (MWNTs) e grafite exfoliada (EG) foram funcionalizados quimicamente por dois métodos, oxidação segundo um método de Hummers modificado e funcionalização através da reacção de cicloadição dipolar 1,3. Deste processo resultaram nanofitas de grafeno oxidadas (o-GNRs) e flocos de grafeno oxidados (o-GFs) e, nanofitas de grafeno funcionalizadas (f-GNRs) e flocos de grafeno funcionalizados (f-GFs), pela exposição de MWNTs e EG oxidados e MWNTs e EG funcionalizados, respetivamente, a ultrassons, proporcionando a sua exfoliação. As soluções de grafeno produzidas foram caraterizadas por espectroscopia ultravioleta-visível, potencial zeta (ZP) e microscópia eletrónica de varredura por transmissão (STEM). Os grafenos foram caraterizados por espectroscopia de infravermelho e análises termogravimétricas (TGA). Adicionalmente, as f-GNRs foram caraterizadas por Microscopia Ótica (OM) e microscopia de força atómica (AFM) por forma a facilitar a observação das f-GNRs por microscopia de efeito de túnel (STM). Posteriormente, filmes multicamada combinando os diferentes tipos de grafeno produzidos com dois polímeros naturais biodegradáveis e biocompatíveis, nomeadamente quitosano (CHT) e alginato (ALG), foram produzidos através da técnica de deposição camada a camada (LbL), e monitorizados em tempo real através de uma microbalança de cristal de quartzo (QCM). A espessura, viscosidade e módulo de cisalhamento dos filmes multicamada produzidos foram estimados pela aplicação de um modelo de Voigt. Os filmes multicamada desenvolvidos no presente trabalho podem ser aplicados em diversas aplicações biomédicas, como biosensores, revestimentos de implantes e tecidos electroativos.

碩士
國立中央大學
物理學系
105
As the first identified 2-dimentional material, the unique properties of graphene, such as the ultrahigh electron mobility, had attracted lots of researchers in the related fields. However, the lack of large area and high quality graphene limited the application of graphene-based devices. In the past 10 years, a lot of graphene fabrication methods had been developed. Among these methods, CVD was seen as one of the most promising ways for graphene fabrication. CVD graphene is based on the need of large area continuous graphene films. However, the quality of CVD graphene films would be limited by the grain boundaries. The grain boundaries would reduce not only the stiffness of graphene films but also the electron mobility. Hence, how to reduce the effect of grain boundaries in CVD graphene became an important issue in related applications. The growth of graphene single crystals had been considered as a key to improve the quality of CVD graphene. In this study, we would demonstrate the growth of millimeter sized single crystalline graphene by a furnace CVD system. By visiting the effect of hydrogen to methane ratio and the effect of growth temperature, we found a suitable growth condition for large graphene single crystals. Besides, under the mm-sized graphene single crystals, we also found the growth of smaller few-layered graphene. Hence, we revisited the effect of growth temperature and found the sublimation of copper played an important role in the system. The reason the smaller grains could only appeared under large grains could be attributed to the copper sublimation would remove the smaller grains. The coverage of the large grains could limit the copper sublimation and created an environment for the smaller grains. We hoped this study could provide some idea about the growth mechanism of large area single crystals and multilayered graphene.

碩士
中原大學
機械工程研究所
106
Due to the outstanding mechanical stiffness, electrical conductivity, and flexibility, graphene is now widely used in transistor, detector, touch sensor, and supercapacitor. The continuous growth of wearable devices market such as smart phone and tablet, the development of touch sensor has attracted many interests in order to minimize size and reduced fabrication cost. In the present study, graphene ink was coated on glass substrate using spin coating technique to formed homogeneous thickness of multilayer graphene with approximately 20 μm. The process with an ultrafast laser system was used via a picosecond laser (Advanced Optowave Corp., USA). The laser system with scanner control software was used to deliver the pulse laser with 532 nm wavelength at a maximum power of 5 W. Varying laser fluences were investigated to analyze the effectiveness of using picosecond (PS) laser ablation in patterning graphene based devices. The ablation threshold fluence for machining multilayer graphene on the glass substrate was 0.84 mJ/cm2 at a constant scanning speed of 250 mm/s with a repetition rate of 300 kHz. The capacitance in the touched and untouched state as a function of electrode width and length were both investigated of the micro-capacitor. The experiments showed that the sensor capacitance was relatively sensitive to the length of the interdigitated electrode. The sensor capacitance increased approximated linearly as electrode width increased from 200 μm to 600 μm under touched and untouched state. As the electrode width continuously increase W? 600 μm, the sensor capacitance increased sharply under different electrode lengths, especially under touched state.

Polyelectrolyte multilayer capsules fabricated by layer-by-layer (LbL) self-assembly technique consistsing of core-shell structure have emerged as potential drug delivery systems along with their applications in micro-reactors, cosmetics, vaccines and antimicrobial coatings. Various ligands and stimuli responsive entities can be incorporated into the core and shell of the capsules for targeted delivery and/or controlled release applications. Though multilayer capsules have been studied extensively as delivery systems, their utility for encapsulation of hydrophobic drugs and multiple drugs have not been explored in detail so far. Application of traditional polyelectrolyte capsules has several limitations, which renders them inapplicable for encapsulation of multiple drugs, hydrophobic drugs and also for releasing drugs on demand without addition of the external photothermal agents such as metal nanoparticles into the shells of the capsules. Thus, in this thesis, an attempt has been made to develop novel multifunctional multilayered capsules to overcome the above mentioned limitations. We have formulated two novel methods to functionalize the core with cyclodextrin molecules and the shell of the capsules with two-dimensional material, graphene oxide (GO). The properties such as high surface area along with π bonds, broad NIR-absorption, superior photothermal conversion and antimicrobial activity of graphene oxide has been explored and it has been demonstrated that 2-D graphene oxide is unique compared to the regular polyelectrolytes. By functionalizing the shell of capsules with GO as one of the layer material, a simple and efficient way for encapsulating multiple drugs into core and shell of the capsules is achieved by utilizing the large surface area and amphiphilic nature of GO. Based on the unique optical absorption and photothermal conversion properties of GO, we have demonstrated a facile route for near-infrared (NIR)-laser triggered release with low laser power. In the second part, functionalization of the hollow core of the capsules has been functionalized using cylodextrin (CD)-incorporated CaCO3 porous sacrificial templates, where both CD-CaCO3 and CD-modified capsules are used as high efficient carriers for hydrophobic drugs. In the third part, synergistic antimicrobial therapy was achieved using composite graphene oxide/polymer LbL films by combining the intrinsic antimicrobial activity and photothermal conversion ability of graphene oxide and the results depicted superior antimicrobial activity towards E. coli. These composite films also can be used as efficient antimicrobial coatings on biomedical devices or implants. The thesis has been divided into five chapters based on the individual works. In Chapter 1, a brief review on the history of LbL self-assembly, mechanism of self-assembly along with factors affecting the process have been discussed. Followed by a brief discussion about the fabrication of multilayered hollow capsules (core-shell structure), their applications in drug delivery and fabrication of multifunctional multilayered capsules through core and shell have been discussed. Finally, recent developments in LbL self-assembly and multilayered hollow capsules using carbon based materials (fullerenes, carbon nanotubes and graphene oxide) and their biomedical applications have been presented. Chapter 2 deals with the study on fabricating multifunctional multilayered capsules for facile encapsulation of multiple drugs into the capsules, which is achieved by functionalizing the capsules with graphene oxide (GO) as one of the layer materials. The GO composite capsules exhibited unique permeability properties compared to traditional multilayered capsules made of two polyelectrolytes. Multiple drugs could be simultaneously encapsulated in the capsules in a simple and effective manner. These capsules were found to exhibit a “core-shell” loading property for encapsulation of dual drugs into the core and shell of the capsules respectively. In addition, the graphene oxide composite capsules showed excellent biocompatibility towards MCF-7 cells. This study is the first one that demonstrates the potential of hybrid polyelectrolyte capsules without the use of micelles or polymer-drug conjugates for multi-drug encapsulation. Chapter 3 deals with the development of a facile route for near-infrared (NIR)-light triggered release of encapsulated drugs from the multilayered capsules via incorporation of graphene oxide (GO) into layer-by-layer (LbL) assembled capsules without addition of any external additives such as metal nanoparticles (NPs) or carbon nanotubes (CNTs) into the shells of the capsules. Till now, there is no report on light-responsive drug delivery system by utilizing the NIR-optical absorption properties of GO. Here, graphene oxide (GO) plays a dual role, serving as a structural component of LbL capsules as well as strong NIR-light absorbing agent, which efficiently converts absorbed light into heat. Upon NIR-laser irradiation, the microcapsules were opened in “point-wise fashion” due to local heating caused by laser irradiation. The rupturing mechanism of the capsules has been clearly demonstrated using confocal fluorescence microscopy and high resolution transmission electron microscopy. The light-triggering ability of these capsules has been applied successfully to release the encapsulated anticancer drug, doxorubicin. Chapter 4 deals with simple and versatile simple routes for encapsulation of model hydrophobic drug. Encapsulation of hydrophobic drugs in pharmaceutical industries is always a big challenge due to limited number of available drug carrier systems and poor aqueous solubility of hydrophobic drugs. Here, by combining the special properties of cyclodextrins (CDs) with biodegradable inorganic calcium carbonate microparticles, the hybrid CD-CaCO3 mesoporous microparticles have been prepared for the first time. These CD-CaCO3 microparticles were utilized as sacrificial templates to prepare CDs-modified LbL capsules. We have demonstrated that both the hybrid CD-CaCO3 microparticles and CDs-modified capsules are potential carriers for encapsulation of model hydrophobic drugs (self-fluorescent coumarine and nile red dyes) with high loading efficiency using supramolecular host-guest interaction between entrapped CDs and hydrophobic dye molecules. Compared with other inorganic drug carrier systems (mesoporous silica), CaCO3 porous particles have better biocompatibility, biodegradability and cost-effective and without use of any organic solvents. Both these hybrid CD-CaCO3 microparticles and CDs-modified capsules can be good candidates for encapsulation of hydrophobic drugs without involving extreme chemical conditions for fabrication. Chapter 5 deals with development of facile synergistic method for killing pathogenic bacteria by combining the intrinsic antimicrobial activity of graphene oxide (GO) and unique photothermal conversion property of GO into a single material. We fabricated composite LbL films of graphene oxide (GO) and poly(allylamine hydrochloride) (PAH) films. Antimicrobial activity of these GO composite films has been studied using Escherichia coli (E. coli) cells by varying number of deposited layers on glass slides (20 to 80 layers) and results suggest that by increasing the number of deposited layers, antimicrobial activity is also increased gradually. Based on the unique optical properties of GO, photothermal therapy have been carried out for killing of E. coli using GO composite films by varying number of deposited layers (20 to 80 layers) by irradiation of NIR-pulse laser at 1064 nm wavelength (Nd:YAG, 10 ns pulse, 10 Hz). The photothermal results revealed the enhanced antimicrobial activity compared to GO composite films alone without NIR-laser irradiation. The synergistic photothermal killing ability along with intrinsic antimicrobial activity of GO films results in much faster killing compared to films alone.

Polyelectrolyte multilayer capsules fabricated by layer-by-layer (LbL) self-assembly technique consistsing of core-shell structure have emerged as potential drug delivery systems along with their applications in micro-reactors, cosmetics, vaccines and antimicrobial coatings. Various ligands and stimuli responsive entities can be incorporated into the core and shell of the capsules for targeted delivery and/or controlled release applications. Though multilayer capsules have been studied extensively as delivery systems, their utility for encapsulation of hydrophobic drugs and multiple drugs have not been explored in detail so far. Application of traditional polyelectrolyte capsules has several limitations, which renders them inapplicable for encapsulation of multiple drugs, hydrophobic drugs and also for releasing drugs on demand without addition of the external photothermal agents such as metal nanoparticles into the shells of the capsules. Thus, in this thesis, an attempt has been made to develop novel multifunctional multilayered capsules to overcome the above mentioned limitations. We have formulated two novel methods to functionalize the core with cyclodextrin molecules and the shell of the capsules with two-dimensional material, graphene oxide (GO). The properties such as high surface area along with π bonds, broad NIR-absorption, superior photothermal conversion and antimicrobial activity of graphene oxide has been explored and it has been demonstrated that 2-D graphene oxide is unique compared to the regular polyelectrolytes. By functionalizing the shell of capsules with GO as one of the layer material, a simple and efficient way for encapsulating multiple drugs into core and shell of the capsules is achieved by utilizing the large surface area and amphiphilic nature of GO. Based on the unique optical absorption and photothermal conversion properties of GO, we have demonstrated a facile route for near-infrared (NIR)-laser triggered release with low laser power. In the second part, functionalization of the hollow core of the capsules has been functionalized using cylodextrin (CD)-incorporated CaCO3 porous sacrificial templates, where both CD-CaCO3 and CD-modified capsules are used as high efficient carriers for hydrophobic drugs. In the third part, synergistic antimicrobial therapy was achieved using composite graphene oxide/polymer LbL films by combining the intrinsic antimicrobial activity and photothermal conversion ability of graphene oxide and the results depicted superior antimicrobial activity towards E. coli. These composite films also can be used as efficient antimicrobial coatings on biomedical devices or implants. The thesis has been divided into five chapters based on the individual works. In Chapter 1, a brief review on the history of LbL self-assembly, mechanism of self-assembly along with factors affecting the process have been discussed. Followed by a brief discussion about the fabrication of multilayered hollow capsules (core-shell structure), their applications in drug delivery and fabrication of multifunctional multilayered capsules through core and shell have been discussed. Finally, recent developments in LbL self-assembly and multilayered hollow capsules using carbon based materials (fullerenes, carbon nanotubes and graphene oxide) and their biomedical applications have been presented. Chapter 2 deals with the study on fabricating multifunctional multilayered capsules for facile encapsulation of multiple drugs into the capsules, which is achieved by functionalizing the capsules with graphene oxide (GO) as one of the layer materials. The GO composite capsules exhibited unique permeability properties compared to traditional multilayered capsules made of two polyelectrolytes. Multiple drugs could be simultaneously encapsulated in the capsules in a simple and effective manner. These capsules were found to exhibit a “core-shell” loading property for encapsulation of dual drugs into the core and shell of the capsules respectively. In addition, the graphene oxide composite capsules showed excellent biocompatibility towards MCF-7 cells. This study is the first one that demonstrates the potential of hybrid polyelectrolyte capsules without the use of micelles or polymer-drug conjugates for multi-drug encapsulation. Chapter 3 deals with the development of a facile route for near-infrared (NIR)-light triggered release of encapsulated drugs from the multilayered capsules via incorporation of graphene oxide (GO) into layer-by-layer (LbL) assembled capsules without addition of any external additives such as metal nanoparticles (NPs) or carbon nanotubes (CNTs) into the shells of the capsules. Till now, there is no report on light-responsive drug delivery system by utilizing the NIR-optical absorption properties of GO. Here, graphene oxide (GO) plays a dual role, serving as a structural component of LbL capsules as well as strong NIR-light absorbing agent, which efficiently converts absorbed light into heat. Upon NIR-laser irradiation, the microcapsules were opened in “point-wise fashion” due to local heating caused by laser irradiation. The rupturing mechanism of the capsules has been clearly demonstrated using confocal fluorescence microscopy and high resolution transmission electron microscopy. The light-triggering ability of these capsules has been applied successfully to release the encapsulated anticancer drug, doxorubicin. Chapter 4 deals with simple and versatile simple routes for encapsulation of model hydrophobic drug. Encapsulation of hydrophobic drugs in pharmaceutical industries is always a big challenge due to limited number of available drug carrier systems and poor aqueous solubility of hydrophobic drugs. Here, by combining the special properties of cyclodextrins (CDs) with biodegradable inorganic calcium carbonate microparticles, the hybrid CD-CaCO3 mesoporous microparticles have been prepared for the first time. These CD-CaCO3 microparticles were utilized as sacrificial templates to prepare CDs-modified LbL capsules. We have demonstrated that both the hybrid CD-CaCO3 microparticles and CDs-modified capsules are potential carriers for encapsulation of model hydrophobic drugs (self-fluorescent coumarine and nile red dyes) with high loading efficiency using supramolecular host-guest interaction between entrapped CDs and hydrophobic dye molecules. Compared with other inorganic drug carrier systems (mesoporous silica), CaCO3 porous particles have better biocompatibility, biodegradability and cost-effective and without use of any organic solvents. Both these hybrid CD-CaCO3 microparticles and CDs-modified capsules can be good candidates for encapsulation of hydrophobic drugs without involving extreme chemical conditions for fabrication. Chapter 5 deals with development of facile synergistic method for killing pathogenic bacteria by combining the intrinsic antimicrobial activity of graphene oxide (GO) and unique photothermal conversion property of GO into a single material. We fabricated composite LbL films of graphene oxide (GO) and poly(allylamine hydrochloride) (PAH) films. Antimicrobial activity of these GO composite films has been studied using Escherichia coli (E. coli) cells by varying number of deposited layers on glass slides (20 to 80 layers) and results suggest that by increasing the number of deposited layers, antimicrobial activity is also increased gradually. Based on the unique optical properties of GO, photothermal therapy have been carried out for killing of E. coli using GO composite films by varying number of deposited layers (20 to 80 layers) by irradiation of NIR-pulse laser at 1064 nm wavelength (Nd:YAG, 10 ns pulse, 10 Hz). The photothermal results revealed the enhanced antimicrobial activity compared to GO composite films alone without NIR-laser irradiation. The synergistic photothermal killing ability along with intrinsic antimicrobial activity of GO films results in much faster killing compared to films alone.

碩士
國立交通大學
電子物理系所
100
In this thesis, we study ultrafast dynamics of chemical vapor deposition multilayer graphene by the femtosecond spectroscopy. Graphene film which growth by chemical vapor deposition were transferred to the sapphire substrate. By repeating above steps we can obtain the multilayer graphene samples. Graphene could be applied to electro-optical devices potentially due to its extraordinary optical and electrical properties. Understanding the carriers cooling, carrier-carrier scattering and carrier dynamics of graphene are important topics. Using ultrafast optical pump-probe spectroscopy, we have measured the transient absorption. The change of absorption is mainly dominated by the electron-electron scattering and electron-phonon scattering. Electronics-phonon coupling constant can be extacted by two-temperature model. By analysis of ultrafast carrier dynamics we found the dependence of carrier relaxation dynamics to temperature and numbers of layers.

碩士
國立交通大學
電子物理系所
100
We use Terahertz-time domain spectroscopy (THz-TDS) to characterize electromagnetic property of graphene thin film. In particular, the extraordinary properties have made monolayer graphene a promising material for high speed electronic device and high sensitive electro-optical device. Graphene synthesized be CVD has been widely study because of not only the large scale area but also high uniformity of graphene. We then perform the THz-TDS to study the electromagnetic property of multilayer graphene via different transfer material in CVD technique. The Drude free carrier model and graphene optical conductivity model were performed in fitting the THz-TDS conductivity result to extracted values of carrier concentration and carrier mobility in the consideration of intraband transition. FTIR and Raman spectroscopy were both used to determine the properties of graphene.

碩士
國立中央大學
物理學系
103
The epitaxial graphene is grown for the first time on 15R-SiC(0001) substrate by employing a critically evaluated empirical potential, namely, the Tersoff-type Erhart- Albe potential [41] in the simulated annealing method. The factors that affect the growth process were studied. Three layers of graphene were successfully grown and they were examined by the calculated binding energy per atom, average bond-length, inter-layer and graphene-substrate separation distances, roughness parameter and graphene area coverage. We find that the threshold temperature at which one-layer graphene emerges is 1200 K which is the same as using 6H-SiC substrate. For the emergence of two-layer graphene, the 15R-SiC substrate yields 1000 K, which is lower than that from 6H-SiC substrate. The reasons for the disparity in threshod temperature grown on 6H- and 15R-SiC substrates are investigated and interpreted in terms of their geometrical differences. For the growth of three-layer graphene, we compared two annealing processes and discussed the difficulties in applying the same simulated method. A thorough analysis leads us to the present means of grow three-layer graphene. Also, we compared with related experiments for the various distance of separation parameters between the overlaid layers of graphene and substrate surface.

Thin films with the ability to impart oxygen and other types of gas barrier are crucial to commercial packaging applications. Commodity polymers, such as polyethylene (PE), polycarbonate (PC) and polyethylene terephthalate (PET), have insufficient barrier for goods requiring long shelf life. Current gas barrier technologies like plasma-enhanced vapor deposition (PECVD) often create high barrier metal oxide films, which are prone to cracking when flexed. Bulk composites composed of polymer and impermeable nanoparticles show improved barrier, but particle aggregation limits their practical utility for applications requiring high barrier and transparency. Layer-by-layer (LbL) assemblies allow polymers and nanoparticles to be mixed with high particle loadings, creating super gas barrier thin films on substrates normally exhibiting high gas permeability. Branched polyethylenimine (PEI) and poly (acrylic acid) (PAA) were deposited using LbL to create gas barrier films with varying pH combinations. Film thickness and mass fraction of each component was controlled by their combined charge. With lower charge density (PEI at pH 10 and PAA at pH 4), PEI/PAA assemblies exhibit the best oxygen barrier relative to other pH combinations. An 8 BL PEI/PAA film, with a thickness of 451 nm, has an oxygen permeability lower than 4.8 x 10^-21 cm^3 * cm/cm^2 * s * Pa, which is comparable to a 100 nm SiOx nanocoating. Crosslinking these films with glutaraldehyde (GA), 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide methiodide (EDC) or heating forms covalent bonds between PEI and/or PAA. Oxygen transmission rates (OTR) of 8 BL films crosslinked with 0.1M GA or 0.01M EDC show the best oxygen barrier at 100% RH. Graphene oxide (GO) sheets and PEI were deposited via LbL with varying GO concentration. The resulting thin films have an average bilayer thickness from 4.3 to 5.0 nm and a GO mass fraction from 88 to 91wt%. Transmission electron microscopy and atomic force microscopy images reveal a highly-oriented nanobrick wall structure. A 10 BL PEI/GO film that is 91 nm thick, made with a 0.2 wt% GO suspension, exhibits an oxygen permeability of 2.5 x 10^-20 cm^3 * cm/cm^2 * s * Pa. Finally, the influence of deposition time on thin film assembly was examined by depositing montmorillonite (MMT) or laponite (LAP) clays paired with PEI. Film growth and microstructure suggests that smaller aspect ratio LAP clay is more dip-time dependent than MMT and larger aspect ratio MMT has better oxygen barrier. A 30 BL PEI/MMT film made with 10 second dips in PEI has the same undetectable OTR as a film with 5 minute dips (with dips in MMT held at 5 minutes in both cases), indicating LbL gas barrier can be made more quickly than initially thought. These high barrier recipes, with simple and efficient processing conditions, are good candidates for a variety of packaging applications.

碩士
國立交通大學
機械工程系所
106
The study aims to characterize the mechanical and thermal properties of multilayer graphene nanocomposites using molecular dynamics (MD) simulation. The effects of surface functionalized and wavy graphene on nanocomposites were also investigated. The results indicated that the increase in the number of graphene layers will reduce the Young's modulus of the nanocomposite, which is mainly derived from the low load transfer efficiency of the graphene inner layer. The pullout simulation also shows that the interfacial shear stress of the multilayer graphene is lower than that of the monolayer graphene. Graphene surface functionalization can effectively improve the load transfer efficiency between graphene and epoxy matrix, thereby improving the Young's modulus of the nanocomposites, wherein the enhancement of the outermost layer is more significant than the inner layer. The wavy effect will significantly reduce the Young's modulus of the graphene nanocomposite, and minimize the difference between multilayer and monolayer graphene In terms of thermal properties, due to the specific area of graphene, low interfacial thermal conductivity and the thermal conductivity of surrounding epoxy matrix, the increase of the number of layers will reduce the in-plane thermal conductivity of nanocomposites, but the out-of-plane thermal conductivity is improved. In the in-plane direction, the surface functionalization can enhance the thermal conductivity of the surrounding epoxy matrix more significantly, and thus increase the thermal conductivity of nanocomposites. In the out-of-plane direction, the surface functionalization will improve the thermal conductivity of the graphene/epoxy interface, but at the same time, the thermal conductivity of the multilayer graphene itself will decrease, and the out-of-plane thermal conductivity of the nanocomposite will still improve. From the vibrational density of state (VDOS), it can be found that the VDOS mismatch between graphene outermost layer and epoxy can be reduced by the functional groups, but the VDOS mismatch between graphene outermost layer and inner layer increase, accordingly. Compare of Effective medium approximations (EMA) and Modified Mori-Tanaka model (MMT) with experimental data. The results show that when graphene with small lateral size or thickness, the EMA model will underestimate the thermal conductivity of nanocomposite. This is because of the EMA model ignore the interaction between graphene inclusions.

Magister Scientiae - MSc
In this study multilayer graphene nanosheets was synthesize by oxidizing graphite to graphene oxide using H2SO4 and KMnO4 followed by reduction of graphene oxide to graphene using NaBH4. The graphene nanosheets were characterized by Fourier Transform Infrared (FTIR) and Raman spectroscopy, high resolution transmission electron microscopy (HRTEM), Scanning electron microscopy (SEM) and X-ray diffraction (XRD). HRTEM images showed that the multilayer graphene were obtained. The graphene was immobilized directly onto a glassy carbon electrode using the drop coating technique followed by the in situ deposition of mercury, bismuth or antimony thin films to afford graphene modified glassy carbon metal film electrodes (Gr-GC-MEs). The experimental parameters (deposition potential, deposition time, rotation speed, frequency and amplitude) were optimized, and the applicability of the modified electrode was investigated towards the individual and simultaneous determination of Zn2+, Cd2+ and Pb2+ at the low concentration levels (μg L-1) in 0.1 M acetate buffer (pH 4.6) using square wave anodic stripping voltammetry (SWASV). The detection limits values for the Gr-GC-HgE was 0.08, 0.05 and 0.14 μg L-1 for Zn2+, Cd2+ and Pb2+, respectively. The Gr-GC-BiE the detection limits for was 0.12, 0.22 and 0.28 μg L-1 for Zn2+, Cd2+ and Pb2+ while the detection limits for the Gr-GC-SbE was 0.1, 0.3 and 0.3 μg L-1 for Zn2+, Cd2+ and Pb2+, respectively. A Gr-GCE prepared without any binding agents or metal film had detection limits for Zn2+, Cd2+ and Pb2+ of 3.9, 0.8 and 0.2 μg L-1 for Zn2+, Cd2+ and Pb2+. Real sample analysis of which was laboratory tap water was performed using the Gr-GCMEs. Only Gr-GC-HgE was sensitive enough to detect metal ions in the tap water samples at the 3ppb level whereas, the GC-BiE and GC-SbE detected the metal ions at the 10 μg L-1 to 30 μg L-1 level.

碩士
國立中正大學
光機電整合工程研究所
104
In this study, the surface hydrophobic property of the chemical vapor deposited (CVD) graphene was modified to hydrophilic property by using UV ozonation process. In order to enhance the visible reaction of graphene, antimony sulfide (Sb2S3) thin film was deposited onto graphene by chemical bath deposition (CBD). The quality of the single layer and multilayer graphene were compared in terms of defect, sheet resistance, carrier drift, and carrier concentration by using Raman spectra analysis and Hall measurement. Sb2S3 / single layer graphene and Sb2S3 / multilayer graphene photodetector devices were fabricated. The response time of multilayer graphene and single layer graphene was compared and we found that multilayer graphene owns shorter response time than single layer graphene. The optoelectronic characteristics of Sb2S3 / multilayer graphene photodetector devices which were irradiated under different incident light with different powers and applied different voltages were studied and discussed. As a result, we found that the device showed highest responsivity, which is 94.36 A/W, when it was irradiated by monochromator with the wavelength of 475 nm. Keywords : graphene, Sb2S3, chemical bath deposition method

The thesis presents experimental studies of device characteristics and vibrational properties of atomic layer thin graphene and molybdenum disulphide (MoS2). We carried out Raman spectroscopic studies on field effect transistors (FET) fabricated from these materials to investigate the phonons renormalized by carrier doping thus giving quantitative information on electron-phonon coupling. Below, we furnish a synoptic presentation of our work on these systems. Chapter1: Introduction Chapter1, presents a detailed introduction of the systems studied in this the¬sis, namely single layer graphene (SLG), bilayer graphene (BLG) and single layer molybdenum disulphide (MoS2). We have mainly discussed their electronic and vibrational properties in the light of Raman spectroscopy. A review of the Raman studies on graphene layers is presented. Chapter2: Methodology and Experimental Techniques Chapter 2 starts with a description of Raman instrumentation. The steps for isolating graphene and MoS 2 flakes and the subsequent device fabrication procedures involving lithography are discussed in detail. A brief account of the top gated field effect transistor (FET) using solid polymer electrolyte is presented. Chapter3: Band gap opening in bilayer graphene and formation of p-n junction in top gated graphene transistors: Transport and Raman studies In Chapter3 the bilayer graphene (BLG) field effect transistor is fabricated in a dual gate configuration which enables us to control the energy band gap and the Fermi level independently. The gap in bilayer energy spectrum is observed through different values of the resistance maximum in the back gate sweep curves, each taken at a fixed top gate voltage. The gate capacitance of the polymer electrolyte is estimated from the experimental data to be 1.5μF/cm2 . The energy gap opened between the valence and conduction bands using this dual-gated geometry is es¬timated invoking a simple model which takes into account the screening of gate induced charges between the two layers. The presence of the controlled gap in the energy band structure along with the p-n junction creates a new possibility for the bilayer to be used as possible source of terahertz source. The formation of p-n junction along a bilayer graphene (BLG) channel is achieved in a electrolytically top gated BLG FET, where the drain-source voltage VDS across the channel is continuously varied at a fixed top gate voltage VT(VT>0). Three cases may arise as VDS is varied keeping VT fixed: (i) for VT-VDS0, the entire channel is doped with electron, (ii) for VT-VDS= 0, the drain end becomes depleted of carriers and kink in the IDS vs VDS curve appears, (iii) for VT-VDS « 0, carrier reversal takes place at the drain end, accumulation of holes starts taking place at the drain end while the source side is still doped with electrton. The verification of the spatial variation of carrier concentration in a similar top gated single layer graphene (SLG) FET device is done using spatially resolved Ra¬man spectroscopy. The signature 2D Raman band in a single layer graphene shows opposite trend when doped: 2D peak position decreases for electron doping while it increases for hole doping. On the other hand, the G mode response being symmetric in doping can act as a read-out for the carrier concentration. We monitor the peak position of the G and the 2D bands at different locations along the SLG FET channel. For a fixed top gate voltage V T , both G and the 2D band frequencies vary along the channel. For a positive VTsuch that VT-VDS= 0, the peak frequencies ωGand ω2DωG/2D occur at the undoped frequency (ωG/2D)n=0 near the drain end while the source end corresponds to non-zero concentration. When VT-VDS<0, Raman spectra from hole doped regions (drain end) in the channels show an blue-shift in ω2Dwhile from the electron doped regions (near source) ω2Dis softened. Chapter4: Mixing Of Mode Symmetries In Top Gated Bilayer And Multilayer Graphene Field Effect Devices In Chapter4, the effect of gating on a bilayer graphene is captured by using Raman spectroscopy which shows a mixing of different optical modes belonging to differ¬ent symmetries. The zone-center G phonon mode splits into a low frequency (Glow) and a high frequency (Ghigh) mode and the two modes show different dependence on doping. The two G bands show different trends with doping, implying different electron-phonon coupling. The frequency separation between the two sub-bands in¬creases with increased doping. The mode with higher frequency, termed as Ghigh, shows stiffening as we increase the doping whereas the other mode, Glow, shows softening for low electron doping and then hardening at higher doping. The mode splitting is explained in terms of mixing of zone-center in-plane optical phonons rep¬resenting in-phase and out-of-phase inter-layer atomic motions. The experimental results are combined with the theoretical predictions made using density functional theory by Gava et al.[PRB 80, 155422 (2009)]. Similar G band splitting is observed in the Raman spectra from multilayer graphene showing influence of stacking on the symmetry properties. Chapter5: Anomalous dispersion of D and 2D modes in graphene and doping dependence of 2D ′and 2D+G bands Chapter 5 consists of two parts: Part A titled “Doping dependent anomalous dispersion of D and 2D modes in graphene” describes the tunability of electron-phonon coupling (EPC) associated with the highest optical phonon branch (K-A) around the zone corner K using a top gated single layer graphene field effect transistor. Raman D and 2D modes originate from this branch and are dispersive with laser excitation energy. Since the EPC is proportional to the slope of the phonon branch, doping dependence of the D and 2D modes is carried out for different laser energies. The dispersion of the D mode decreases for both the electron and the hole doping, in agreement with the recent theory of Attaccalite et. al [Nano Letters, 10, 1172 (2010)]. In order to observe D-band in the SLG samples, low energy argon ion bombardment was carried out. The D peak positions for variable carrier concentration using top-gated FET geometry are determined for three laser energies, 1.96 eV, 2.41 eV and 2.54 eV. However, the dispersion of the 2D band as a function of doping shows an opposite trend. This most curious result is quantitatively explained us¬ing a fifth order process rather than the usual fourth order double resonant process usually considered for both the D and 2D modes. Part B titled “Raman spectral features of second order 2D’ and 2D+G modes in doped graphene transistor” deals with doping dependence of 2D’ and 2D+G bands in single layer graphene transistor. The phonon frequency blue shifts for the hole doping and whereas it red shifts for electron doping, similar to the behaviour of the 2D band. The linewidth of the 2D+G combination mode too follows the 2D trend increasing with doping while that of 2D’ mode remains invariant. Chapter6: New Raman modes in graphene layers using 2eV light Unique resonant Raman modes are identified at 1530 cm−1 and 1445 cm−1 in single, bi, tri and few layers graphene samples using 1.96 eV (633 nm) laser excitation energy (EL). These modes are absent in Raman spectra using 2.41 eV excitation energy. In addition, the defect-induced D band which is observed only from the edges of a pristine graphene sample, is observed from the entire sample region using E L = 1.96 eV. Raman images with peak frequencies centered at 1530 cm−1, 1445 cm−1 and D band are recorded to show their correlations. With 1.96 eV, we also observe a very clear splitting of the D mode with a separation of ∼32 cm−1, recently predicted in the context of armchair graphene nanoribbons due to trigonal warping effect for phonon dispersion. All these findings suggest a resonance condition at ∼2eVdue to homo-lumo gap of a defect in graphene energy band structure. Chapter7: Single and few layer MoS2: Resonant Raman and Phonon Renormalization Chapter 7 is divided into two parts. In Part A “Layer dependent Resonant Raman scattering of a few layer MoS2”, we discuss resonant Raman scattering from single, bi, four and seven layers MoS2. As bulk crystal of MoS2is thinned down to a few atomic layers, the indirect gap widens turning into a direct gap semiconductor with a band gap of 1.96 eV in its monolayer form. We perform Raman study from MoS 2 layers employing 1.96 eV laser excitation in order to achieve resonance condition. The prominent Raman modes for MoS 2 include first order E12g mode at ∼383 cm−1 and the A1gmode at ∼408 cm−1 which are observed under both non resonant and resonant conditions. A1gphonon involves the sulphur atomic vibration in opposite direction along the c axis (perpendicular to the basal plane) whereas for E12g mode, displacement of Mo and sulphur atoms are in the basal plane. With decreasing layer thickness, these two modes shifts in opposite direction, the E12g mode shows a blue shift of ∼2cm−1 while the A1gis red shifted by ∼4cm−1 . Under resonant condi¬tion, apart from E12g and A1gmodes, several new Raman spectral features, which are completely absent in bulk, are observed in single, bi and few layer spectra pointing out the importance of Raman characterization. New Raman mode attributed to the longitudinal acoustic mode belonging to the phonon branch at M along the Γ-M direction of the Brillouin zone is seen at ∼230 cm−1 for bi, four and seven layers. The most intense region of the spectrum around 460 cm−1 is characterized by layer dependent frequencies and spectral intensities with the band near 460 cm−1 becoming asymmetric as the sample thickness is increased. In the high frequency region between 510-630 cm−1, new bands are seen for bi, four and seven layers. In Part B titled “Symmetry-dependent phonon renormalization in monolayer MoS2transistor”, we show that in monolayer MoS2the two Raman-active phonons, A1g and E21 g, behave very differently as a function of doping induced by the top gate voltage in FET geometry. The FET achieves an on-off ratio of ∼ 105 for electron doping. We show that while E12g phonon is essentially unaffected, the A1gphonon is strongly influenced by the level of doping. We quantitatively understand our experimental results through the use of first-principles calculations to determine frequencies and electron-phonon coupling for both the phonons as a function of carrier concentration. We present symmetry arguments to explain why only A1g mode is renormalized significantly by doping. Our results bring out a quantitative under¬standing of electron-phonon interaction in single layer MoS2.

The thesis presents experimental studies of device characteristics and vibrational properties of atomic layer thin graphene and molybdenum disulphide (MoS2). We carried out Raman spectroscopic studies on field effect transistors (FET) fabricated from these materials to investigate the phonons renormalized by carrier doping thus giving quantitative information on electron-phonon coupling. Below, we furnish a synoptic presentation of our work on these systems. Chapter1: Introduction Chapter1, presents a detailed introduction of the systems studied in this the¬sis, namely single layer graphene (SLG), bilayer graphene (BLG) and single layer molybdenum disulphide (MoS2). We have mainly discussed their electronic and vibrational properties in the light of Raman spectroscopy. A review of the Raman studies on graphene layers is presented. Chapter2: Methodology and Experimental Techniques Chapter 2 starts with a description of Raman instrumentation. The steps for isolating graphene and MoS 2 flakes and the subsequent device fabrication procedures involving lithography are discussed in detail. A brief account of the top gated field effect transistor (FET) using solid polymer electrolyte is presented. Chapter3: Band gap opening in bilayer graphene and formation of p-n junction in top gated graphene transistors: Transport and Raman studies In Chapter3 the bilayer graphene (BLG) field effect transistor is fabricated in a dual gate configuration which enables us to control the energy band gap and the Fermi level independently. The gap in bilayer energy spectrum is observed through different values of the resistance maximum in the back gate sweep curves, each taken at a fixed top gate voltage. The gate capacitance of the polymer electrolyte is estimated from the experimental data to be 1.5μF/cm2 . The energy gap opened between the valence and conduction bands using this dual-gated geometry is es¬timated invoking a simple model which takes into account the screening of gate induced charges between the two layers. The presence of the controlled gap in the energy band structure along with the p-n junction creates a new possibility for the bilayer to be used as possible source of terahertz source. The formation of p-n junction along a bilayer graphene (BLG) channel is achieved in a electrolytically top gated BLG FET, where the drain-source voltage VDS across the channel is continuously varied at a fixed top gate voltage VT(VT>0). Three cases may arise as VDS is varied keeping VT fixed: (i) for VT-VDS0, the entire channel is doped with electron, (ii) for VT-VDS= 0, the drain end becomes depleted of carriers and kink in the IDS vs VDS curve appears, (iii) for VT-VDS « 0, carrier reversal takes place at the drain end, accumulation of holes starts taking place at the drain end while the source side is still doped with electrton. The verification of the spatial variation of carrier concentration in a similar top gated single layer graphene (SLG) FET device is done using spatially resolved Ra¬man spectroscopy. The signature 2D Raman band in a single layer graphene shows opposite trend when doped: 2D peak position decreases for electron doping while it increases for hole doping. On the other hand, the G mode response being symmetric in doping can act as a read-out for the carrier concentration. We monitor the peak position of the G and the 2D bands at different locations along the SLG FET channel. For a fixed top gate voltage V T , both G and the 2D band frequencies vary along the channel. For a positive VTsuch that VT-VDS= 0, the peak frequencies ωGand ω2DωG/2D occur at the undoped frequency (ωG/2D)n=0 near the drain end while the source end corresponds to non-zero concentration. When VT-VDS<0, Raman spectra from hole doped regions (drain end) in the channels show an blue-shift in ω2Dwhile from the electron doped regions (near source) ω2Dis softened. Chapter4: Mixing Of Mode Symmetries In Top Gated Bilayer And Multilayer Graphene Field Effect Devices In Chapter4, the effect of gating on a bilayer graphene is captured by using Raman spectroscopy which shows a mixing of different optical modes belonging to differ¬ent symmetries. The zone-center G phonon mode splits into a low frequency (Glow) and a high frequency (Ghigh) mode and the two modes show different dependence on doping. The two G bands show different trends with doping, implying different electron-phonon coupling. The frequency separation between the two sub-bands in¬creases with increased doping. The mode with higher frequency, termed as Ghigh, shows stiffening as we increase the doping whereas the other mode, Glow, shows softening for low electron doping and then hardening at higher doping. The mode splitting is explained in terms of mixing of zone-center in-plane optical phonons rep¬resenting in-phase and out-of-phase inter-layer atomic motions. The experimental results are combined with the theoretical predictions made using density functional theory by Gava et al.[PRB 80, 155422 (2009)]. Similar G band splitting is observed in the Raman spectra from multilayer graphene showing influence of stacking on the symmetry properties. Chapter5: Anomalous dispersion of D and 2D modes in graphene and doping dependence of 2D ′and 2D+G bands Chapter 5 consists of two parts: Part A titled “Doping dependent anomalous dispersion of D and 2D modes in graphene” describes the tunability of electron-phonon coupling (EPC) associated with the highest optical phonon branch (K-A) around the zone corner K using a top gated single layer graphene field effect transistor. Raman D and 2D modes originate from this branch and are dispersive with laser excitation energy. Since the EPC is proportional to the slope of the phonon branch, doping dependence of the D and 2D modes is carried out for different laser energies. The dispersion of the D mode decreases for both the electron and the hole doping, in agreement with the recent theory of Attaccalite et. al [Nano Letters, 10, 1172 (2010)]. In order to observe D-band in the SLG samples, low energy argon ion bombardment was carried out. The D peak positions for variable carrier concentration using top-gated FET geometry are determined for three laser energies, 1.96 eV, 2.41 eV and 2.54 eV. However, the dispersion of the 2D band as a function of doping shows an opposite trend. This most curious result is quantitatively explained us¬ing a fifth order process rather than the usual fourth order double resonant process usually considered for both the D and 2D modes. Part B titled “Raman spectral features of second order 2D’ and 2D+G modes in doped graphene transistor” deals with doping dependence of 2D’ and 2D+G bands in single layer graphene transistor. The phonon frequency blue shifts for the hole doping and whereas it red shifts for electron doping, similar to the behaviour of the 2D band. The linewidth of the 2D+G combination mode too follows the 2D trend increasing with doping while that of 2D’ mode remains invariant. Chapter6: New Raman modes in graphene layers using 2eV light Unique resonant Raman modes are identified at 1530 cm−1 and 1445 cm−1 in single, bi, tri and few layers graphene samples using 1.96 eV (633 nm) laser excitation energy (EL). These modes are absent in Raman spectra using 2.41 eV excitation energy. In addition, the defect-induced D band which is observed only from the edges of a pristine graphene sample, is observed from the entire sample region using E L = 1.96 eV. Raman images with peak frequencies centered at 1530 cm−1, 1445 cm−1 and D band are recorded to show their correlations. With 1.96 eV, we also observe a very clear splitting of the D mode with a separation of ∼32 cm−1, recently predicted in the context of armchair graphene nanoribbons due to trigonal warping effect for phonon dispersion. All these findings suggest a resonance condition at ∼2eVdue to homo-lumo gap of a defect in graphene energy band structure. Chapter7: Single and few layer MoS2: Resonant Raman and Phonon Renormalization Chapter 7 is divided into two parts. In Part A “Layer dependent Resonant Raman scattering of a few layer MoS2”, we discuss resonant Raman scattering from single, bi, four and seven layers MoS2. As bulk crystal of MoS2is thinned down to a few atomic layers, the indirect gap widens turning into a direct gap semiconductor with a band gap of 1.96 eV in its monolayer form. We perform Raman study from MoS 2 layers employing 1.96 eV laser excitation in order to achieve resonance condition. The prominent Raman modes for MoS 2 include first order E12g mode at ∼383 cm−1 and the A1gmode at ∼408 cm−1 which are observed under both non resonant and resonant conditions. A1gphonon involves the sulphur atomic vibration in opposite direction along the c axis (perpendicular to the basal plane) whereas for E12g mode, displacement of Mo and sulphur atoms are in the basal plane. With decreasing layer thickness, these two modes shifts in opposite direction, the E12g mode shows a blue shift of ∼2cm−1 while the A1gis red shifted by ∼4cm−1 . Under resonant condi¬tion, apart from E12g and A1gmodes, several new Raman spectral features, which are completely absent in bulk, are observed in single, bi and few layer spectra pointing out the importance of Raman characterization. New Raman mode attributed to the longitudinal acoustic mode belonging to the phonon branch at M along the Γ-M direction of the Brillouin zone is seen at ∼230 cm−1 for bi, four and seven layers. The most intense region of the spectrum around 460 cm−1 is characterized by layer dependent frequencies and spectral intensities with the band near 460 cm−1 becoming asymmetric as the sample thickness is increased. In the high frequency region between 510-630 cm−1, new bands are seen for bi, four and seven layers. In Part B titled “Symmetry-dependent phonon renormalization in monolayer MoS2transistor”, we show that in monolayer MoS2the two Raman-active phonons, A1g and E21 g, behave very differently as a function of doping induced by the top gate voltage in FET geometry. The FET achieves an on-off ratio of ∼ 105 for electron doping. We show that while E12g phonon is essentially unaffected, the A1gphonon is strongly influenced by the level of doping. We quantitatively understand our experimental results through the use of first-principles calculations to determine frequencies and electron-phonon coupling for both the phonons as a function of carrier concentration. We present symmetry arguments to explain why only A1g mode is renormalized significantly by doping. Our results bring out a quantitative under¬standing of electron-phonon interaction in single layer MoS2.

碩士
國立中正大學
光機電整合工程研究所
104
Graphene is a suitable material in the application of photodetectors for its high carrier mobility and broad wavelength absorption ability. However, its optical transmittance and the carrier recombination speed is extremely high after illumination, resulting in poor photoresponse. In this study, the visible-light photoresponse of graphene is enhanced by growing cadmium sulfide (CdS) through Successive Ionic Layer Adsorption and Reaction (SILAR) method on graphene. Multilayer stacking CdS/graphene and non-stacking CdS/graphene devices were fabricated and their electrical properties were studied. Based on the experimental results, multilayer stacking CdS/graphene composite photodetector device shows better photoresponse. The optoelectronic characteristic of the multilayer stacking CdS/graphene was determined by irradiating laser sources with various incident light powers and different applied voltages. The full spectral response of 3-layer stacking CdS/graphene was measured by monochromntor, we found that the maximum responsivities were more than 100 A/W when the excitation wavelength changed from 300 nm to 500 nm. Therefore, by depositing CdS on graphene is proven to increase the photoresponse of graphene. Keywords: photodetector, graphene, SILAR, cadmium sulfide

碩士
國立中正大學
光機電整合工程研究所
107
Graphene is a suitable material in the application of photodetectors for its high carrier mobility and broad wavelength absorption ability. However, its optical transmittance and the carrier recombination speed is extremely high after illumination, resulting in poor photoresponse. In this study, the visible-light photoresponse of graphene is enhanced by growing Antimony sulfide through chemical bath deposition method on graphene. Previous experiments have found that strontium sulfide/graphene elements deposited for 6 hours have the best optical responsiveness, but have limited absorption in the red band, that is, when irradiating a 660 nm laser. Therefore, in this experiment, a stack of barium sulfide/graphene elements was fabricated, and two layers of barium sulfide/graphene were used to increase the absorption in the red band. It is hoped that the first layer of barium sulfide/graphene material is formed by a second layer of barium sulfide/graphene material. Failure to effectively absorb light again. Finally, the experimental results show that the stacked strontium sulfide/graphene can effectively increase the absorption of 660 nm laser, and has a response value of 66.6 A/W at 650 nm.

Graphene, a single layer of highly ordered carbon atoms, has captured tremendous interest within academia and industry recently. This is due to its unique, two-dimensional structure which yields a combination of outstanding properties ideal for enhancing the performance of existing applications and enabling future disruptive technologies. Nevertheless, the production of graphene particles devoid of structural defects that can be easily integrated and used in products and applications on a commercial scale has proved exceptionally challenging. The first part of this problem can be addressed using the ultrasonic exfoliation of graphene in aqueous surfactant solutions, which offers an established method of producing defect-free graphene suspensions suited to large scale production processes. As the graphene exists in the liquid phase, a variety of techniques are possible for the transfer and integration of the particles into products and applications. Many of these techniques rely on adsorption, which involves the net accumulation of molecules from a liquid at an interface through attractive intermolecular interactions. The intermolecular interactions inherent to surfactant-assisted exfoliated graphene have been used to investigate the prospect of employing such particles in a series of applications governed by adsorption mechanisms. The outcomes of these studies are described here in this thesis. Suspensions of graphene particles were prepared using the ultrasonic exfoliation of graphite, with continuous surfactant addition in the presence of the non-ionic surfactant, Pluronic® F108. The resultant suspensions and particles were characterised using UV-Visible spectroscopy, Raman spectroscopy, and transmission electron microscopy. The prepared graphene suspensions exhibited single and bilayer defect-free particles ranging up to 1 μm in size. Zeta potential measurements indicated the graphene particles exhibited with a low residual negative electrostatic charge originating from edge defects rather than the adsorbed surfactant. Four studies reflecting proof-of-concept applications for surfactant stabilised graphene were then investigated. Each study focused on a different type of intermolecular interaction or interface involved in the underlying adsorption process. In one study, multilayer thin films were sequentially constructed through the electrostatic layer-by-layer (LbL) deposition of Pluronic F108 exfoliated graphene and the cationic polyelectrolyte, polyethyleneimine on silica surfaces. Multilayer assembly was monitored using a quartz crystal microbalance and was shown to be strongly influenced by conditions such as graphene concentration, pH, ionic strength and ionic species. Consequently, it was shown that the thickness of the films could be specified by altering the number of layers deposited, while the viscoelastic properties of the films could be tailored by altering film deposition conditions. Hydrogen-bonded multilayer films consisting of Pluronic F108 exfoliated graphene and the weak polyelectrolyte, polyacrylic acid were also constructed using the LbL technique at low pH. In this study, quartz crystal microbalance measurements and Raman spectra suggested a superlinear film growth regime, whilst atomic force microscopy qualitative nanomechanical mapping measurements indicated that the mechanical properties of the films differed with the number of layers adsorbed. The films also underwent partial deterioration when exposed to aqueous solutions at neutral and basic pH. Thus, the hydrogen-bonded thin films demonstrated a series of features appropriate to functional coatings, such as pH responsiveness, surface roughness and internal film structures, which could be altered depending on deposition conditions and number of layers adsorbed. The third study described foam stabilisation through adsorption of Pluronic F108 exfoliated graphene at the liquid-air interface. Particle surface activity was confirmed through surface tension measurements and was shown using the evolution of bubble size distributions to cause a reduction in destabilisation mechanisms. The addition of alkali metal chlorides also enhanced foam stability by altering the wettability of graphene particles through changes in the hydration of polyethylene oxide groups present on the stabilising surfactant. Thus, the high-aspect ratio particles and adaptable surface interactions possible with Pluronic F108 exfoliated graphene are able to stabilise and enhance bubble surfaces in foams. In the final study, ionic and non-ionic surfactant exfoliated graphene were used to adsorb ionic organic dyes from solution. Dye adsorption was maximised when attractive electrostatic interactions were possible with the graphene particles. In particular, the adsorption of methylene blue, a cationic dye, by anionic SDS exfoliated graphene particles was shown to be exceedingly rapid. This process was shown to be consistent with the pseudo-second order kinetics model and Freundlich adsorption isotherm. Consequently, the surface interactions caused by the presence of surfactant at the graphene particle surface are sufficient to drive the adsorption of dye from solution. These results collectively demonstrate the versatility of surfactant stabilised graphene adsorption at interfaces, which is made possible by the nature of the stabilising surfactant and properties inherent to the 2D graphene lattice. They illustrate the potential for surfactant exfoliated graphene to be used in a variety of applications such as functional and responsive thin films, as well as their ability to act as effective foam stabilisers and carbon-based adsorbents.

碩士
國立臺灣大學
物理研究所
102
Graphene consists of the carbon atoms in the honeycomb structure. There are many exceptional properties of the graphene has been found. One of the unique properties is called Klein paradox, which states that the massless Dirac electrons could not be confined in the one-dimensional region by the electrostatic potential. Interestingly, recently experimental and theoretical studies have shown that the low-energy Dirac electron would be confined in the graphene bilayer with a small twist angle. Therefore, it is worth while to study the problem. In our thesis, we will focus on the electronic properties of twisted graphene multilayers. At first, we would discuss the energy splitting of the Van Hove singularities and the real space distribution of the localized states in twisted bilayer graphene based on the tight-binding model. A clear mathematical formula is able to explain the occurrence of the Van Hove singularities. Our results also show that the real space distribution of the first and the second localized states would follow circular and $C_3$ symmetry respectively. Second, we construct an approximate model to explain why there are existing multiple Van Hove singularities in twisted bilayer graphene with external uniform strains. At last, our bilayer system extends to the trilayer system, and multiple singularities appear due to the multilayer interaction, and we proposed several ways to predict the energy splitting of these singularity groups. Besides, it is possible to realize the origin of each singularity from the peak values of the local density of states. Therefore, in the thesis, we study several electronic properties of twisted graphene multilayers and give meaningful explanations for each problem. Hopefully, our research is helpful to solve the Morie pattern puzzles in the twisted graphene multilayers.

Dissertação de Mestrado Integrado em Engenharia do Ambiente apresentada à Faculdade de Ciências e Tecnologia
O principal objetivo deste trabalho é avaliar a viabilidade da incorporação de grafite recuperada de elétrodos em fim de vida provenientes da industria metalomecânica numa matriz de polipropileno, face à adição de grafeno multicamadas comercial. O polipropileno foi escolhido como material base pelo facto de ser um dos polímeros mais comercializados, cujo o seu processamento é fácil e apresenta uma reciclabilidade muito alta. Os nanocompósitos foram produzidos por um processo de fusão e a sua caracterização foi realizada a partir da injeção de provetes. O foco deste trabalho foi a caracterização das propriedades mecânicas com a realização de ensaio de tração e flexão em três pontos. Foram analisados reforços com cargas de 0.5%wt e 1%wt de grafite recuperada e grafeno multicamadas.O principal objetivo deste trabalho é avaliar a viabilidade da incorporação de grafite recuperada de elétrodos em fim de vida provenientes da industria metalomecânica numa matriz de polipropileno, face à adição de grafeno multicamadas comercial. O polipropileno foi escolhido como material base pelo facto de ser um dos polímeros mais comercializados, cujo o seu processamento é fácil e apresenta uma reciclabilidade muito alta. Os nanocompósitos foram produzidos por um processo de fusão e a sua caracterização foi realizada a partir da injeção de provetes. O foco deste trabalho foi a caracterização das propriedades mecânicas com a realização de ensaio de tração e flexão em três pontos. Foram analisados reforços com cargas de 0.5%wt e 1%wt de grafite recuperada e grafeno multicamadas.
The main objective of this work is to evaluate the feasibility of the incorporation of graphite recovered from end - of - life electrodes from the metal - mechanical industry into a polypropylene matrix, in addition to the addition of commercial multilayer graphene.Polypropylene has been chosen as the base material in that it is one of the most commercially available polymers whose processing is easy and has a very high recyclability.The nanocomposites were produced by a fusion process and their characterization was performed from the injection of test pieces. The focus of this work was the characterization of the mechanical properties with the accomplishment of test of traction and flexion in three points. Reinforcements with loads of 0.5% wt and 1% wt of recovered graphite and multilayer graphene were analyzed. The main objective of this work is to evaluate the feasibility of the incorporation of graphite recovered from end - of - life electrodes from the metal - mechanical industry into a polypropylene matrix, in addition to the addition of commercial multilayer graphene.Polypropylene has been chosen as the base material in that it is one of the most commercially available polymers whose processing is easy and has a very high recyclability.The nanocomposites were produced by a fusion process and their characterization was performed from the injection of test pieces. The focus of this work was the characterization of the mechanical properties with the accomplishment of test of traction and flexion in three points. Reinforcements with loads of 0.5% wt and 1% wt of recovered graphite and multilayer graphene were analyzed.

In a strong magnetic field, the psuedospin chirality of bilayer graphene’s low energy bands results in degenerate, zero energy n=0 and n=1 Landau or- bital states. In this thesis, we find that in addition to endowing states with energetic broadness disorder strongly mixes the zero energy orbitals of the lowest Landau level. We study the dependence of mixing and conductivity on inter-layer bias. Quantum Hall ferromagnetic states emerge when electronic interactions are included at the mean-field level. We study the character of ground states and quasiparticle excitations in the context of orbital degen- eracy and in the presence of interactions with the filled Dirac sea of states. Lastly, we study the effect of interactions in ABA-stacked trilayer graphene, and discover a metal-insulator transition and a separate propensity to break mirror symmetry in certain areas of parameter space.
text

碩士
國立成功大學
物理學系碩博士班
100
I use the tight-binding method to study the effect of AB-stacked graphite in the electric field. We found that the energy band will produce the energy gap in few-layers, and overlap in multi-layers. The overlap will become wider with the increased number of layers. The density of states of multi-layer graphene is very different compared to few-layers, the energy band will spread up and the density of states will become larger than few-layers. We also analyzed the the density of states on the Fermi energy with the changes of the electric field and the number of layers. Finally, we discuss the absorption spectra.

碩士
國立中央大學
物理研究所
99
The molecular dynamics simulation and simulated annealing method were applied to study the growth process of graphene and the thermal stability of layered graphene nanoribbons on 6H-SiC(0001) substrate. With an intention to understand the mechanisms that govern these panoramas, we tested two empirical potentials, i.e. the widely used Tersoff potential [Phys. Rev. B 39, 5566 (1989)] and its more refined version published years later by Erhart and Albe [Phys. Rev. B 71, 035211-1 (2005)]. We found that the modified Tersoff potential communicated by Erhart and Albe is generally more banausic for growing layered graphene on 6H-SiC substrate for the annealing temperature at which the graphene structure comes into view is very close to that observed in epitaxially grown graphene experiments. We evaluate our grown layered graphene by checking the reasonableness of the average carbon-carbon bond- length, pair correlation function, binding energy and also comparing with the experimentally grown epitaxial graphene the distances among the overlaid layers of graphene and substrate surface. The annealing temperature we obtained at 1325 K at which the graphitic structure just comes into view is reasonably close to the experimentally observed pit formation. On the thermal stability of layered graphene, the characteristics of the surface morphology of an infinite graphene sheet that we positioned near SiC substrate are consistent with other simulation works. Most importantly we obtained a threshold annealing temperature at around 2000 K below which the structural behavior of the carbon buffer layer is thermally stable and above which one sees the graphitic structures show tendency to slant up from the substrate.

Dissertação de mestrado integrado em Engenharia Biomédica (área de especialização em Biomateriais, Reabilitação e Biomecânica)
In several biomedical applications, one of the major disadvantages of natural polymers is their low mechanical performances. Such drawback has led scientists to search for new materials capable to improve their mechanical properties. In the last few years, graphene and graphene oxide (GO) nanocomposite materials have been proposed to be used in different applications due their outstanding mechanical and electrical properties. We hypothesized that the incorporation of such materials could be useful for biomedical applications. To achieve this goal, we transpose the layerby- layer technology for the production of nanostructured free-standing (FS) polymeric membranes that have such nanofillers in their composition. To this end, chitosan (CHI, polycation) and alginate (ALG, polyanion) were used as a biopolymeric matrix and GO (polyanion) as a reinforcement nanofiller. Prior to FS membranes production, different GO were synthetized, using a modified Hummers' method, from two diferente materials: exfoliated graphite and multi-walled carbon nanotubes, resulting in oxidized graphene flakes (o-GF) and graphene nanoribbons (o-GNR), respectively. Such oxidation process provided oxygen functional groups that among other features improve the bonding with biopolymers. Three membranes were developed, (CHI/ALG/CHI/ALG)100 that acted as controls, while (CHI/ALG/CHI/o-GF)100 and (CHI/ALG/CHI/o-GNR)100 were built up as proof of concept. The morphological analysis was performed by scanning electron microscopy, atomic force microcopy and Raman mapping. The physical properties were assessed by thermogravimetric analysis, water contact angle measurements, water uptake and weight loss. Tensile tests and dynamic mechanical analysis were employed to test the mechanical behavior of the FS membranes. Moreover, biological assays using L929 mouse fibroblasts line were executed to investigate their cytocompablity. Our results showed that the addition of both o-GF and o-GNR forms improved the mechanical properties however with no significant changes on the thermal properties. At the same time, the FS membranes presented a rough surface and an hydrophilic behavior. Concerning the cellular assays, the FS membranes with o-GF revealed a better promotion of cell adhesion and proliferation than both controls and o-GNR FS membranes. The outcomes of this thesis suggests that o-GF membranes may have potential for wound healing, cardiac and bone applications.
Em diferentes aplicações biomédicas, uma das maiores desvantagens apresentadas pelos polímeros naturais são as baixas propriedades mecânicas. Tais desvantagens têm levado a uma maior procura de soluções, nomeadamente através do desenvolvimento de nanocompósitos de grafeno e de grafeno oxidado. Neste trabalho, foi estudado os benefícios que o uso destes materiais podem trazer para aplicações biomédicas. Para tal, foi usada a técnica de camada-a-camada para a produção de membranas poliméricas free-standing, onde, quitosano (CHI, policatião) e alginato (ALG, polianião) foram usados como matriz biopolimérica e o grafeno oxidado (GO) como um nanomaterial de reforço. Antes da deposição das diversas camadas foi necessário proceder à síntese do GO através do método de Hummer modificado, usando para isso dois materiais: a grafite e os nanotubos de carbono de paredes múltiplas (MWNTs), resultando em flocos (o-GF) e nanofitas (o-GNR) de grafeno oxidados, respetivamente. De modo a alcançar os objetivos propostos foram desenvolvidos 3 tipos de membranas, (CHI/ALG/CHI/ALG)100 como controlos, (CHI/ALG/CHI/o-GF)100 e (CHI/ALG/CHI/o-GNR)100 como prova do conceito. A sua caracterização morfológica foi realizada através do recurso a técnicas como: microscopia eletrónica de varrimento, microscopia de força atómica e mapeamento de Raman. Já as propriedades físicas foram avaliadas através termogravimetria, medição dos ângulos de contacto, pela capacidade de absorção de água e pela perda de massa. Testes de tração e análise mecânica dinâmica foram usados para analisar as propriedades mecânicas. Para além disso, a aplicabilidade das membranas foi ainda testada através de ensaios celulares, usando-se para tal linhas celulares L929. Os resultados demonstraram que a adição de o-GF e o-GNR resultam num melhoramento mecânico das membranas sem que exista alteração das propriedades térmicas. Ao mesmo tempo foi também registada uma maior rugosidade e hidrofilicidade das membranas. No que diz respeito aos testes celulares, as membranas onde o-GF estava presente obtiveram uma maior adesão e proliferação celular. Sendo assim, as membranas com o-GF poderão encontrar várias aplicações biomédicas, nomeadamente em aplicações para a cicatrização de tecidos ou ainda para aplicações cardíacas ou ósseas.

碩士
國立成功大學
電機工程學系
104
In our research, the multilayers of silicon rich oxide/silicon dioxide and silicon/graphene were synthesized by reactive magnetron sputtering. The multilayers act as phonon block for the silicon substrate. Our objective is the reduction of thermal conductivity with preservation of electrical conductivity and seebeck coefficient. The structural and physical properties of all multilayers were investigated by TEM, XRD, XPS, TDTR and Harman method. The thermal conductivity successfully drops to accessible range because of phonon scattering in each interface. The variation of proportion and working pressure shows that the existence of silicon nanocrystal might be a key point to preserve power factor. Relation between the formation of multilayers and figure of merit would be discussed. The figure of merit could further increase via replacing the single layer graphene with silicon dioxide. Although the electrical conductivity was still low, it could be further improved after annealing. The result shows the capacity of eco-friendly and low cost Si material with optimized thermoelectric nanostructure.