This thesis describes studies on two-dimensional electron gases (2DEG) in graphene and related 2D systems. Magnetotransport investigations specifically in graphene and its bilayer system are demonstrated in detail, while the experimental techniques presented in this thesis are widely applicable to a large variety of other 2D materials. Chapter 1 gives an introduction and motivation for the principal topic presented in this thesis, with a general introduction to carbon nano-materials and an overview of the current state of graphene-related research and technological development (RTD). Chapter 2 establishes a basic theoretical framework which is essential for interpreting the results presented in this thesis, starting with the crystal and electronic band structures of graphene and its bilayer, followed by high magnetic fields effects on transport properties in these 2D systems. Chapter 3 details the experimental methods directly related to the presented work. The next three chapters report experimental results of three specific magnetotransport studies. Chapter 4 reports the disorder effects on epitaxial graphene in the vicinity of the Dirac point. Quadratic increases of carrier densities with temperature are found to be due to intrinsic thermal excitation combined with electron-hole puddles induced by charged impurities. It is also shown that the minimum conductivity increases with increasing disorder strength, in good agreement with quantum-mechanical numerical calculations. Chapter 5 reports measurements of the quantum Hall effect in epitaxial graphene showing the widest quantum Hall plateau observed to date extending over 50 T, attributed to a magnetic field dependent charge transfer process from charge reservoirs with exceptionally high densities of states in close proximity to the graphene. Using a realistic framework of broadened Landau levels this process is modelled in excellent agreement with experimental results. In Chapter 6, energy relaxation of hot carriers in graphene bilayer systems is investigated from measurements on Shubnikovde Haas oscillations and weak localisation. The hot-electron energy loss rate follows the predicted T⁴ power-law at carrier temperatures from 1.4 up to about 100 K, due to electron-acoustic phonon interactions. Comparisons are made between graphene monolayer and bilayer systems and a much stronger carrier density dependence of the energy loss rate is found in the bilayer system. This thesis concludes with a summary of the most important findings of the topics that have been discussed. The significance and limitations of the present research are listed. Some suggestions and outlook are given for possible improvements and interesting areas of future research and development.

Orientador: Aldo Eloizo Job / Banca: Marcos Augusto de Lima Nobre / Banca: Marcelo Ornaghi Orlandi / O Programa de Pós-Graduação em Ciência e Tecnologia de Materiais, PosMat, tem caráter institucional e integra as atividades de pesquisa em materiais de diversos campi da Unesp / Resumo: O objetivo deste trabalho foi estudar "efeitos de escala e tamanho" através de medidas de permissividade dielétrica de partículas nanométricas e/ou nanoestruturadas do óxido policátion niobato de potássio dopado com estrôncio (K'Sr IND. 2' N'b IND. 5' 'O IND. 15'), um óxido ferroelétrico da família tetragonal tungstênio bronze (TTB). A determinação da constante dielétrica de nanopartículas foi realizada através da técnica de mistura. Nesta técnica, nanopartículas de permissividade dielétrica desconhecidas são dispersas em um meio de permissividade dielétrica conhecida. A partir da resposta dielétrica da mistura a permissividade dielétrica das nanopartículas foram calculadas utilizando modelagem numérica via circuitos elétricos equivalentes. A fase K'Sr IND. 2' N'b IND. 5' 'O IND. 15, foi preparada por rota química (método poliol modificado), a temperatura de calcinação necessária à obtenção de nanopartículas foi otimizada e partículas nanoestruturadas de K'Sr IND. 2' N'b IND. 5' 'O IND. 15' monofásico foram avaliadas. A caracterização estrutural foi realizada utilizando as técnicas de difratometrica de raiox x (DRX), espectroscopia vibracional de absorção na região do infravermelho (FTIR) e ultravioleta visível (UV/vis) a partir das quais se avaliaram os parâmetros de rede, cristalinidade relativa, tamanho médio de cristalito, volume da cela unitária e energia de gap. Utilizando a técnica de espectroscopia de impedância entre 5 Hz e 3 MHz foram estudadas as propriedades dielétricas das amostras através de medidas de permissividade confirmando a existência de ferroeletricidade nas amostras e foi identificado o efeito de tamanho nas propriedades dielétricas das nanopartículas. / Abstract: This work aimed the study of "size effect" through permittivity measurements of nanometric particles and/or nanostructured of niobate oxide K'Sr IND. 2' N'b IND. 5' 'O IND. 15', a ferroelectric oxide belonging to the tetragonal tungsten bronze family (TTB). Determination of nanoparticles dielectric constant was done using the mixture technique. In this technique, nanoparticles of unknown dielectric permittivity are dispersed in a medium of know dielectric permittivity. From the dielectric response of the mixture, the dielectric permittivity of the nanoparticles is calculated using numerical modeling by means of equivalent circuits. The K'Sr IND. 2' N'b IND. 5' 'O IND. 15' phase was prepared using a chemical route (polyol modified method) and the optmization of the temperature calcination was performed aiming to obtain nanoparticles and the resulting nanostructured particles were evaluated. The structural characterization was carried out by X-ray diffraction (XRD), infrared absorption spectroscopy (FTIR) and UV-Vis spectroscopy allowing the evaluation of cell parameters, relative crystallinity and crystallite size, unitary cell volume and gap energy. The impedance spectroscopy technique in the range from 5 Hz to 3 MHz to study dielectric properties of samples was used. The presence of ferroelectric phase in samples was confirmed and the size effect was identified on nanoparticles. / Mestre

Orientador: Varlei Rodrigues / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-31T16:45:14Z (GMT). No. of bitstreams: 1 Ferreira_RafaelCintraHensel_M.pdf: 43759995 bytes, checksum: 8636f117e63053d6b540d65b95c34072 (MD5) Previous issue date: 2016 / Resumo: Os filmes orgânicos ultrafinos tem permitido funcionalizar superfícies a fim de introduzir propriedades específicas que diferem daquelas apresentadas em escala macroscópica. Um processo versátil para modificar superfícies e criar sistemas multifuncionais é a técnica de automontagem por adsorção física (LbL, do inglês Layer-by-Layer), na qual nanoestruturas são formadas devido a adsorção de moléculas com cargas opostas em arquiteturas moleculares multicamadas. O monitoramento elétrico do crescimento dos filmes é essencial em experimentos nos quais não são permitidas caracterizações ópticas. Além disto, é importante compreender as propriedades elétricas destes filmes devido a sua utilização em sensores capacitivos para análise química. Deste modo, neste trabalho desenvolvemos um equipamento capaz de acompanhar o crescimento de filmes LbL por meio de medidas de capacitância após a deposição de cada camada de polieletrólitos sobre eletrodos interdigitados (IDEs, do inglês Interdigitated Electrodes). Observamos um aumento linear da capacitância devido ao acúmulo de material dielétrico sobre os IDEs, com uma alternância na medida de capacitância de acordo com o polieletrólito depositado, isto é, há uma reversão de carga na camada mais externa quando a adsorção do policátion é compensada pela do poliânion, ou vice-versa. Utilizando o modelo para o potencial eletrostático de um IDE proposto por M. W. Den Otter, desenvolvemos uma nova metodologia para investigar a constante dielétrica da estrutura multicamadas formada. Obtivemos (16 ± 1) para a constante dielétrica do filme LbL (PDDA/CuTsPc), e (21 ± 3) para a arquitetura (PDDA/PSS), ambos em satisfatória concordância com a literatura. Ademais, desenvolvemos um modelo para interpretar o deslocamento na capacitância medida após a adsorção do policátion a fim de determinar sua densidade de carga superficial, que diminuiu de (0,23 ± 0,02) e/µm2 para (0,08 ± 0,01) e/µm2 quando o PDDA (policátion fortemente carregado) foi substituído pelo PAH (policátion fracamente carregado) para ambos os poliânions analisados. Portanto, desenvolvemos um equipamento para monitorar o crescimento de filmes LbL e uma metodologia que permite obter informações sobre o comportamento dielétrico dos filmes a cada camada depositada, o que pode auxiliar na escolha dos materiais e espessura dos filmes LbL utilizados como elementos transdutores em desenvolvimentos futuros / Abstract: Organic ultrathin films have enabled surface¿s functionalization in order to introduce specific properties which differ from those in macroscopic scale. A versatile process to modify surfaces and create multifunctional systems is the layer-by-layer (LbL) technique, in which the nanostructures are formed due to the adsorption of charged molecules in multilayered molecular architectures. The electrical monitoring of the film's growth is essential in experiments in which optical characterizations can not be used. Moreover, it is important to comprehend the electrical properties of these films owing to their application on capacitive sensors for chemical analysis. In this research we developed a home-made setup to keep track of the LbL film¿s growth by measuring the capacitance after each deposited layer onto interdigitated electrodes (IDEs). We have observed a linear increase in the capacitance due to accumulated dielectric material onto the IDEs, along with an alternation in the measured capacitance according to the deposited polyelectrolyte, i.e., a charge reversal of the outermost layer as the polycation adsorption is compensated by the polyanion adsorption, or vice-versa. Using the IDEs electrostatic potential model proposed by M. W. Den Otter, we have developed a new methodology to investigate the dielectric constant of the formed multilayered structure. A dielectric constant of (16 ± 1) was obtained for (PDDA/CuTsPc) films and (21 ± 3) for (PDDA/PSS) architecture, both in satisfactory agreement with the literature. Furthermore, we have developed a model to interpret the capacitance measurement shift after the polycation adsoption in order to investigate its surface charge density, which decreased from (0.23 ± 0.02) e/µm2 to (0.08 ± 0.01) e/µm2 when PDDA (strong polycation) was replaced by PAH (weak polycation) for both analyzed polyanions. Therefore, we have developed an equipment to monitor the LbL film's growth as well as a methodology that enables the obtaining of information about the film dielectric behavior at each deposited layer, which can assist in the choice of the materials and thickness of the LbL films that are used as transductor elements in future developments / Mestrado / Física / Mestre em Física / 147530/2014 / CNPQ

Thermal and flash photolysis studies of ligand-substitution reactions of cis-(pip)(L)M(CO)_4 by L' (pip = piperidine; L, L' = CO, phosphines, phosphites; M = Cr, Mo) implicate square-pyramidal [(L)M(CO)_4], in which L occupies a coordination site in the equatorial plane, as the reactive species. In chlorobenzene (= CB) solvent, the predominant species formed after flash photolysis and a steady-state intermediate for the thermal reaction is cis—[(CB)(L)M(CO)_4], for which rates of CB-dissociation increase with increasing steric demands of coordinated L. Rates of CB-dissociation from trans-[(CB)(L)M(CO)_4] intermediates, formed after photolysis but not thermally, exhibit no observable dependence on the steric properties of the coordinated L.

In this study, lumped parameter cardiovascular modeling has been used to understand the influence of muscle properties on mechanical dyssynchrony (MD) as well as general muscle dynamics. Incorporating viscous influence into the model allowed for an expanded view when analyzing muscle parameter response to MD. A unique method of ventricle segmentation was introduced that allowed fast analysis of regional and global ventricular properties. This segmentation process produced a ventricle with four identical sections each consisting of separately tunable muscle properties in the form of minimum and maximum elastance, elastance waveform delay, and myocardial viscous friction, yet these regional sections remained globally dependent. Elastance waveform delay proved to be the most influential property on MD as measured by internal flow fraction (IFF), followed by regional elastance magnitude, and finally regional viscosity influence. Due to the unique segmentation of this model, two metrics for IFF were derived: (1) the "true" IFF (IFF-4seg) and (2) the IFF as would be measured by an ideal conductance catheter (IFF-CC). The results of IFF-CC versus IFF-4seg show that conductance catheters are not capable of measuring IFF during a side-to-side volume transfer within the stacked cylinder under measurement. Finally, unique energetic situations were observed with this model that point to likely myocardium remodeling situations.

This master’s thesis is focused on the electrical properties of biological tissues and flesh in particular. Their electrical characteristics depend on the physical and chemical parameters that determine the concentration and mobility of ions in metabolic fluids. From the electrical point of view, flesh can be simply substituted by a field of elongated conductive cells which are separated by the insulating membrane from each other. In the next part, the Fricke model is introduced. The model describes the measurement of impedance of the tissue at low and high frequencies. The aim of the work is to assess how the impedance of the dielectric sample is dependent on the frequency of the electrical signal during the optimal aging or ripening of flesh.

L'objectif principal de cette thèse a été d'expérimenter de nouvelles voies d'exfoliation des feuillets de graphène dans des matrices polysaccharides telles que la cellulose et le chitosane dopé avec des nanoparticules d'or (Au). Notre stratégie a été d'explorer de nouvelles voies de greffage de molécules et de macromolécules sur des feuillets d'oxyde de graphène (GO). Dans un premier temps, nous avons donc oxydé un graphite commercial par la méthode de Hummers qui est apriori la méthode la plus simple à mettre en oeuvre pour produire une suspension stable de feuillets d'oxyde de graphène totalement exfoliés dans l'eau. L'intérêt de cette oxydation est l'obtention de fonctions acide carboxylique et époxyde susceptibles d'être fonctionnalisées en deux étapes par l'éthylénediamine puis par un polysaccharide tel que la cellulose. En effet, afin d'améliorer la compatibilité du graphite oxydé avec des matrices organiques telle que la cellulose, l'idée est de lui greffer des chaînes polysaccharides. Ces résultats ont permis de mettre en évidence l'exfoliation partielle des feuillets de graphène après fonctionnalisation et l'obtention d'un taux de greffage massique d'environ 35% pour la cellulose. La conductivité électrique des nano-composites correspondants a aussi été étudiée par des mesures diélectriques à des températures variées. L'augmentation de la conductivité électrique après la fonctionnalisation du graphite oxydé a mis en évidence une solvo-thermoréduction simultanément à la fonctionnalisation. Enfin, le dopage de ce matériau par des particules d'or a permis d'obtenir une conductivité électrique de 1.60 10-4 S m-1. En ce qui concerne le matériau composite à base de chitosane, la démarche scientifique a été la même que pour l'analogue cellulose, le taux de greffage massique a été d'environ 68% et nous avons en plus testé son activité catalytique vis-à-vis de la conversion du 4-Nitrophénol en 4-Aminophénol et les résultats obtenus ont été très satisfaisants / The main objective of this thesis has been to experiment a new ways of exfoliation of graphene sheets in polysaccharide matrices such as cellulose and chitosan doped with gold nanoparticles (Au). Our strategy was to explore new routes for the grafting of molecules and macromolecules onto graphene oxide (GO). First, we have oxidized commercial graphite by the method of Hummers which is a priori the simplest method to implement to produce a stable suspension of graphene oxide sheets totally exfoliated in water. The advantage of this oxidation is the formation of carboxylic acid and epoxy functional groups onto the graphite surface that can be functionalized in two stages by ethylenediamine and then by a polysaccharide such as cellulose. Indeed, in order to improve the compatibility of graphite oxide with organic matrix such as cellulose, the idea is to graft it onto polysaccharide chains. These results made it possible to demonstrate the partial exfoliation of the graphene sheets after functionalization and to obtain a percentage of grafting of about 35wt% for cellulose. The electrical conductivity of the corresponding nanocomposites has also been studied by dielectric measurements at various temperatures. The increase of the electrical conductivity after the functionalization of graphite oxide showed a solvo-thermo reduction simultaneously with the functionalization. Finally, the doping of this material by gold particles made it possible to obtain an electrical conductivity of 1.60 10-4 S m-1. Concerning chitosan-based composite materials, the scientific approach was the same as cellulose substrate and we obtained a percentage of grafting of 68wt%. In addition, its catalytic activity for the conversion of 4-Nitrophenol to 4-Aminophenol was of high efficiency

A resonant microwave cavity technique was employed to study the dielectric behavior of some polar and non-polar liquids near the phase transition temperatures at microwave frequencies of 7.2, 9.2 and 10.1 GHz. The Slater perturbation equations for a resonant microwave cavity are briefly discussed to show that the above technique can be used to determine both the real and imaginary parts of dielectric response. Abrupt changes in dielectric response were observed near the phase transition temperatures for the polar liquids studied in this investigation. The dielectric relaxation phenomenon in liquids has been treated as a chemical rate process and the abrupt change in the dielectric response of the liquids near phase transition temperatures is shown to be related to the dramatic changes in the free energy of activation of the molecules. Some values of the free energy of activation were deduced for the various compounds from data obtained in this investigation.

Carbon emissions associated with deep learning and high-performance computing have reached critical levels and must be addressed to mitigate the potential damage to the environment. Optical solutions have been widely accepted as a necessary part of any comprehensive intervention, primarily in the form of ultra-broadband wavelength-division multiplexing (WDM) optical interconnects to connect spatially distanced compute nodes and, in the further term, as dedicated photonic deep learning accelerators and photonic quantum computers. Silicon photonic interconnects provides the most promising platform for satisfying the required performance, device density, and total wafer throughput by leveraging the same mature complementary metal–oxide–semiconductor (CMOS) infrastructure used to fabricate modern electronic chips. However, implementing these links at scale requires unprecedented levels of integration density in the associated silicon photonic integrated circuit (PICs). The potential explosion in PIC density poses a significant design challenge towards guaranteeing that designers are capable of both an exhaustive design space exploration and rigorous design optimization within reasonable design cycles. Higher level design abstractions—that is, representations of designs that accurately capture system behavior while simultaneously reducing model complexity—are needed for moreefficient design and optimization of PICs. This work contributes two novel design abstractions for the rapid optimization of ultra-high-bandwidth silicon photonic interconnects. The first contribution is a novel process variation-aware compact model of strip waveguides that is suitable for circuit-level simulation of waveguide-based process design kit (PDK) elements. The model is shown to describe both loss and—using a novel expression for the thermo-optic effect in high index contrast materials—the thermo-optic behavior of strip waveguides. Experimental results prove the reported model can self-consistently describe waveguide phase, loss, and thermo-optic behavior across all measured devices over an unprecedented range of optical bandwidth, waveguide widths, and temperatures. The second contribution is a generalized abstraction for designing WDM links in the multi-freespectral range (FSR) regime, a technique for avoiding aliasing while using microresonators with FSRs smaller than the total optical bandwidth of the link. Extensive simulation and experimental results prove that the aforementioned abstractions described collectively provide a powerful toolset for rapid interconnect design and optimization. The advances in this thesis demonstrate the utility of higher-level design abstractions for fully realizing the potential silicon photonics holds for keeping pace with ever-growing bandwidth demands computing systems in the post-Moore’s Law era and beyond.

Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Previous work has shown that microwave heating of mineral ores induces micro cracks within the ore structure, which can be attributed to the difference in the adsorption of microwaves amongst the different mineral phases. This reduces the energy required during subsequent grinding and enhances the liberation of valuable minerals. In order to design microwave applicators for this purpose, knowledge of the effective dielectric properties of the crushed ore is required. Of particular interest is the effective complex permittivity of the bulk crushed ore. The measurement of the effective permittivity of a large volume of crushed ore is most readily accomplished using the waveguide measurement technique. In this method a representative sample of the material is placed in a defined and fixed volume in a standard size rectangular section metallic waveguide. The magnitude and phase angle of the transmitted and reflected low power microwaves through and from the sample are measured. The complex permittivity can be extracted from these so-called scattering, or Sij parameters. In this study the effective complex permittivities for two porphyry copper ores and a copper carbonatite ore were determined as a function of particle size distribution (-26.5+2mm) using two sizes of waveguide (WR284 and WR340). The sample holders incorporate dielectric windows for the location of the material under test. The extraction of dielectric properties from Sij parameter measurements is problematic using standard algorithms in such cases. Accordingly a new Database Extraction (DBE) Algorithm has been developed. In this method, a database of scattering parameters is established through electromagnetic modelling of the measurement system. A search algorithm is used to determine the effective complex permittivity of the modelled load whose scattering parameters provide the best fit to the experimental data. The goodness of the experimental fit of the simulated to the measured Sij parameters is determined by a root mean squared deviation minimisation metric. Results show that the method can be used successfully to determine an effective complex permittivity for a bulk volume of the crushed material. It is concluded that the dielectric property extraction over the full operational frequency interval (2.3-3 GHz) is preferred as it has a larger degree of extraction confidence and hence reliability. Results show that with increasing particle size, the experimental fit between the simulated and measured Sij parameters becomes increasingly poor, as wall effect become more prominent. The effect is most prominent for the smaller WR284 waveguide size. It is shown that for a waveguide size of similar size to the particle size, the Sij parameter fitting is poorer compared to when a larger waveguide size is used. The extracted complex permittivity reproducibility between repeated dielectric property measurements is improved for the WR340 waveguide size, as the extractions in the WR284 waveguide is dominated by the combined particle size and wall-effects of the sample holder. Ore mineralogy is identified as a key parameter that influences the dielectric properties of the crushed ore. For ores with a dominant microwave absorbent mineral phase, the dielectric constant and loss factor is found to be larger, compared with ores with a more dominant microwave transparent gangue mineral phase. / AFRIKAANSE OPSOMMING: Navorsing toon dat die verhitting van mineraal erts, met mikrogolwe, mikroskaal frakture in die mineraalstruktuur teweeg bring weens die verskil in die adsorpsie van mikrogolwe in die verskillende mineraalfases. Gevolglik verminder die energievereiste vir die vergruising van die erts en verbeter die vrystelling van waardevolle minerale wat vasgevang is in die mineraalmatriks. Vir die ontwerp van mikrogolfapplikators vir dié doel, word die effektiewe diëlektriese eienskappe van die vergruisde erts benodig. Van spesifieke belang is die effektiewe komplekse permittiwiteit van die erts. Die effektiewe permittiwiteit van `n vergruisde materiaal monster word met behulp van die golfgeleier tegniek gemeet. Vir dié tegniek word `n verteenwoordigende monster van die materiaal in `n rigiede volume in `n standaard grootte reghoekige golfgeleier geplaas. Die grootte en fasehoek komponente van die deurgelate en weerkaatste mikrogolwe deur en van die oppervlak van die materiaal word gemeet. Die komplekse permittiwiteit van die vergruisde materiaal kan geëkstrakteer word vanaf hierdie sogenaamde verspreide, of Sij parameters. In hierdie studie word die effektiewe permitiwiteit van twee porforie koper ertse en `n koper karbonatiet erts bepaal as funksie van partikel grootte (-26.5+2 mm) deur gebruik te maak van twee standaard grootte golfgeleiers. Die monster houers inkorporeer diëlektriese vensters om die vergruisde materiaal monster in posisie te hou. In so `n geval is die ekstraksie van die diëlektriese eienskappe vanuit die Sij parameter metings problematies. Gevolglik is ‘n nuwe Databasis Ekstraksie Algoritme ontwikkel wat `n databasis van verspreide parameters opstel deur die elektromagnetiese simulasie van die metingsisteem. `n Soek-algoritme word gebruik om die effektiewe komplekse permitiwiteit van die gesimuleerde monster te bepaal wat die beste ooreenstem met dié van die gemete eksperimentele Sij parameter data. Die mate van ooreenstemming tussen die parameters, word bepaal aan die hand van die minimaliserings prosedure. Resultate toon dat dié metode geskik is vir die bepaling van die effektiewe komplekse permitiwiteit van die vergruisde monster. Dit word vasgestel dat die betroubaarheid van die geëkstraeerde Sij parameters, en gevolglik die diëlektriese eienskappe van die erts, toeneem indien die algoritme oor `n groter frekwensie band uitgevoer word. Resultate toon verder dat met toenemende partikel grootte, die mate waartoe die absolute grootte en fasehoek komponente van die gesimuleerde en gemete Sij parameters ooreenstem, versleg. Dit word toegeskryf aan wand-effekte. Hierdie verskynsel is veral opmerklik vir die kleiner grootte golfgeleier. Dit word getoon dat vir metings waar die golfgeleier dieselfde orde grootte geometriese afmetings het as die vergruisde erts self, die passing tussen die gesimuleerde en gemete Sij parameters swakker is, wanneer dit vergelyk word met metings waar dit nie die geval is nie. Die reproduseerbaarheid van die geëkstraeerde diëlektriese eienskap waardes verbeter vir lesings wat uitgevoer word in `n groter grootte golfgeleier. Laasgenoemde word toegeskryf aan die meer dominante wand-effekte wat kenmerklik is vir `n kleiner golfgeleier. Erts mineralogie word geïdentifiseer as `n sleutel parameter wat die diëlektriese eienskappe van die vergruisde materiaal beïnvloed. Beide die diëlektriese konstante en verliesfaktor is groter vir ertse met `n oorheersende mikrogolf absorberende mineraalfase.