Cette thèse présente une étude du graphène complète depuis sa synthèse par dépôt de vapeur chimique (CVD), la fonctionnalisation de sa surface, jusqu'aux mesures à basse températures des propriétés de supraconductivité. Je présente tout d'abord une nouvelle technique de croissance CVD basée sur une injection impulsionnelle du gaz précurseur en présence constante d'hydrogène. Cette technique permet de s'affranchir de la présence de multicouches de graphène aux points de nucléation, un type de défaut connu qui avait limité jusqu'à présent les applications électroniques et optiques de ce matériau. Les dispositifs flexibles et optiquement transparents obtenus à partir de ce matériau présentent des caractéristiques améliorées par rapport à l'état de l'art notamment une homogénéité améliorée et une mobilité électronique dépassant 8,000 cm^2/V/s. Un traitement chimique est ensuite introduit et nous montrons par diverses techniques microscopiques, spectroscopiques optiques et électroniques qu'il est possible de contrôler précisément la densité de défauts dans le graphène par voie chimique. Cette thèse s'achève sur une étude à très basse température (<1K) des propriétés de transport électronique d'un nouveau type de systèmes basé sur un réseau de plots d'étain déposé sur un transistor en graphène. Ce type de système hybrides s'avère être un système modèle pour l'étude des transitions de phase quantique supra/isolant et supra/métal. Les résultats présentés sur les réseaux réguliers et dilués sont en bon accord avec les modèles théoriques prédisant un état métallique subsistant jusqu'à température nulle. / This thesis presents a wide range of graphene activity: from chemical vapor deposition (CVD) of graphene growth on Cu surface, to two dimensional mesoscopic graphene devices for superconductivity studies. I first show that, in the CVD process of fabricating graphene, a cyclic injection of carbon feedstock under a constant hydrogen background allows to suppress the formation of multilayer graphene, which has plagued this fabrication method since its early days. With the CVD grown graphene,I demonstrate applications such as flexible transparent electrodes. Mobility of our CVD graphene at 4 K temperature reaches as high as 8,000 cm^2/V/s. Chemical treatment studies based on copper etchant show that control of the density of defects in CVD graphene can be precisely achieved by a chemical route. This thesis finishes at sub 1 K temperature transport measurements of a new type of superconducting array decorated graphene transistor. This type of hybride system is a prototypical model for quantum phase transition studies. I then show that the experimental observations are in good agreement with the predicted superconductor-to-metal quantum phase transition at the zero-temperature limit.

An experimental investigation of the electronic transport properties of bismuth telluride nanocomposite materials is presented. The primary transport measurements are electrical conductivity, Seebeck coefficient and Hall effect. An experimental apparatus for measuring Hall effect and electrical conductivity was designed, constructed and tested. Seebeck coefficient measurements were performed on a commercial instrument. The Hall effect and Seebeck coefficient measurements are two of the most important tools for characterizing thermoelectric materials and are widely used in the semiconductor industry for determining carrier types, carrier concentration and mobility. Further, these transport parameters are used to determine the thermal to electrical conversion efficiency of a thermoelectric material. The Boltzmann transport equation was used to analyze the Seebeck coefficient, carrier mobility and electrical conductivity as a function of carrier concentration for eleven samples. The relationship between the electronic transport and material/composite composition is discussed.

O principal objetivo dessa dissertação de mestrado é o estudo das propriedades estruturais e eletrônicas de umdosmateriaismais promissores para a nanoeletrônica, os nanotubos de carbono, na presença de defeitos do tipo vacâncias. Os defeitos estudados neste trabalho são as monovacâncias e multivacâncias em nanotubos de carbono armchair (5, 5). Para isso, realizamos cálculos quânticos de primeiros princípios baseados na teoria do funcional da densidade. Primeiramente, foi realizado um estudo sistemático das monovacâncias variando a sua concentração, para que possamos entender qual é a influência da distância entre esses defeitos no sistema. A partir de 14.784Å o efeito do potencial inserido pela vacância praticamente é desprezível. Seguindo essa sistemática, observamos também que a partir de 19.712Å a interação elástica entre defeitos é desprezível. Em todas as estruturas de bandas ocorre o surgimento de bandas de impureza, caracterizadas por uma localização de carga no defeito, principalmente no \"dangling bond\". Utilizando o procedimento NEB (nudge elastic band) obtivemos um valor de 0.55eV para a barreira de migração da vacância. Esse valor fornece um tempo aproximado de 0.156ms a 300K, para que a vacância salte de um sítio para outro na rede do nanotubo. Tendo isso, determinamos que a monovacância é um defeito que apresenta uma grande mobilidade no sistema, o que pode eventualmente ocorrer de uma encontrar-se com outras, formando sistemas maiores, como clusters de monovacâncias ou multivacâncias. Seguindo o nosso objetivo, realizamos um estudo sistemático das multivacâncias. Determinamos todas as reconstruções desses defeitos, nas quais observamos que as multivacâncias de índice par, apresentavam somente pentágonos e uma região central, e em contrapartida as de índice ímpar apresentavam pentágonos, uma região central e um dangling bond na sua estrutura final. Definidas as estruturas e as energias das multivacâncias, calculamos suas propriedades eletrônicas, onde é notório que a influência desses defeitos é muito mais drástica que as monovacâncias, devido a sua grande extensão no sistema. Realizamos cálculos de transporte eletrônico para todos os sistemas, utilizando o método de funções de Green fora do equilíbrio (NEGF-DFT), desenvolvido no nosso grupo. A partir desses cálculos, observamos que todos os defeitos modificam a estrutura eletrônica do sistema, uns mais outros menos, mas até para as multivacâncias maiores como a hexavacância, o sistema mantém o seu caráter metálico. Através das técnicas de funções de Green, mostramos que quando estamos utilizando condições periódicas de contorno há uma surgimento de minigap\'s espúrios. Estes não aparecem quando o vínculo de simetria translacional é quebrado via técnicas de funções de Green. / The main purpose of this dissertation is the study of the electronic and structural properties in the presence of different types of vacancies, in one of the most promising materials for nanoelectronics, carbon nanotubes. The defects studied in this work are monovacancy and multivacancies in (5, 5) armchair carbon nanotubes. For that purpose perform quantum ab initio calculations based on density functional theory (DFT). Firstly, was made a systematic study of monovacancies varying its concentration, so we can understand what is the influence of the distance between these defects in the system. Undergoes reconstruction pentagon nonagon from14.784Å on words the effect of the potential created by the vacancy is practically negligible. Following this systematic approach, we also found that from 19.712Å the structural interaction between the defects tends to zero. In all band structures one observes impurity bands, characterized by charge localization in the defect, mainly in the dangling bond. Using the NEB (nudge elastic band) procedure we obtained a value of 0.55eV for the migration barrier of the monovacancy. This value provides an approximate time of 0.156ms at 300K for the vacancy to jump from one site to another on the nanotube network. Hence, we determined that the monovacancy presents a great mobility in the system, which may possibly inccur in two ore more vacancies merging with others to form a larger defect, such as clusters of monovacancies or a hole multivacancy. Following our goal, we conducted a systematic study of multivacancies. We determined all reconstructions of these defects. We observed that multivacancies with and even number of carbon atoms removed had only pentagons and a central region. In contrast the CNT with and odd numbers of carbon atoms removed have pentagons, a central region and a dangling bond in its final structure. Once we have defined the structures and the energies of the multivacancies, we calculated their electronic properties. It is clear that the influence of these defects is much more drastic than the monovacancy, due to the size of the defect in the system. We calculated the electronic transport for all systems, using a non-equilibrium Green\'s functions method (NEGF-DFT), developed in our group. From these calculations, we observed that all the defects alter the electronic structure of the system, but even for the largest multivacancy, the system maintains its metallic character. Using the Green\'s functions techniques, we show that the use of periodic boundary conditions leads to the emergence of spurious mini-gap\'s. These do not appear when the translational symmetry is broken using Green\'s function approach and a true open system is considered.

One dimensional carbon nanotubes (CNTs) and two-dimensional layered materials like graphene, MoS2, hexagonal boron nitride (hBN), etc. with different electrical and mechanical properties are great candidates for many applications in the future. In this study the synthesis and growth of carbon nanotubes on both conducting graphene and graphite substrates as well as insulating hBN substrate with precise crystallographic orientation is achieved. We show that the nanotubes have a clear preference to align to specific crystal directions of the underlying graphene or hBN substrate. On thicker flakes of graphite, the edges of these 2D materials can control the orientation of these carbon nanotubes. This integrated aligned growth of materials with similar lattices provides a promising route to achieving intricate nanoscale electrical circuits. Furthermore, short channel nanoscale devices consisting of the heterostructure of 1D and 2D materials are fabricated. In these nanoscale devices the nanogap is created due to etching of few layer graphene flake through hydrogenation and the channel is either carbon nanotubes or 2D materials like graphene and MoS2. Finally the transport properties of these nanoscale devices is studied.

The aim of this thesis in the nanoelectronics domain is to present a contribution to the analysis of the quantum electronic transport phenomena in nanostructures. For this purpose, we specifically develop the phase field matching theory (PFMT). Within this algebraic approach the electronic properties of the system are described by the tight-binding formalism, whereas the analysis of the transport properties based on the phase matching of the electronic states of the leads to the states of the molecular nanojunctions. By comparing some of our results with those of the first principles methods, we have shown the correctness and fonctionality of our approach. Moreover, our method can be considered as a practical and general alternative to the Green's function-based techniques, and is applied in this work to model the electronic transport across mono and diatomic nanojunctions, consisting of mono and multivalent Na, Cu, Co, C, Si, Ga and As elements.

A variety of nanostructures of tin (IV) oxide (TO) are synthesized using two fabrication methods: a solution spin-coating method followed by post-annealing in an oxygen flow and a newly developed catalyst-assisted pulsed laser deposition (PLD) technique. The spin-coating method is used to fabricate granular TO films with monodisperse, stable, ultra-small nanocrystallites (4-5 nm in size), the size of which is found to increase exponentially with post-anneal above 500??C. These nanocrystalline films are conductive and highly transparent, and their bandgap shows broadening due to a high carrier concentration. Their resistivity behavior as a function of temperature in the 50-280 K range can be explained by a two-medium transport model, i.e. transport through the crystalline grains and across the grain boundaries, and through the charge-depletion layer, where a potential barrier is found for transport across the grain boundaries. Electronic transport in these films follows a 3D-variable range hopping model, which reveals an increase in the localization length of carriers with increasing the TAnneal above the onset of exponential growth at TAnneal= 500??C. By homogenously doping Eu3+ in these nanocrystalline films up to a high doping level of ~ 8%, optical luminescence and magnetic orderings can be introduced into these nanocrystalline TO films. Both characteristic Eu3+ emission and defect-related TO emissions are observed in the otherwise transparent TO films upon UV-excitation. In spite of the non-magnetic nature of Eu3+ ions, magnetic orderings appear in the highly doped TO films below 50 K upon the emergence of Eu2Sn2O7 phase. In the second part of this work, we employ a layer of gold nanoislands with controlled sizes (10-50 nm) as catalysts for pulsed laser deposition of TO nanostructures. Highly crystalline TO nanobricks, cuboid nanoparticles, nanowires and nanobelts are obtained for the first time through vapour-solid or vapour-liquid-solid (VLS) mechanisms. Of particular interest are the micron long one-dimensional (1D) nanowires and nanobelts, with the smallest square and rectangular cross-sections, respectively, ever reported. These single-crystalline nanostructures are obtained at relatively low temperatures of 600??C, for nanowires, and 500??C, for nanobelts, and their cross-sectional sizes can be easily controlled by the size of the gold nanoislands. The nanobelts are found to grow along the [100] and [101] axes, while the nanowires appear to grow along the [100] axis. The growth evolution of the nanobelts are also investigated in detail revealing their VLS growth mode and their single-crystalline structure throughout the growth, which opens the prospect of controlling their growth axis and consequently their side-surface planes by pinning the base to the substrate at the desired crystalline orientation. Together, the two fabrication methods developed in the present work offer facile approaches to growing two scientifically and technologically important classes of TO nanostructures, i.e., nanocrystalline film and 1D nanostructures. Thorough characterization of the resulted nanostructured materials using advanced microscopic, spectroscopic and other techniques, including Helium Ion Microscopy, has been provided. Modification of structure, morphology and physical properties of these functional nanostructured materials are also illustrated by controlling the growth parameters and by (Eu-)doping, which pave the way for introducing new properties for applications in chemical sensing, (opto)electronics and displays.

Orientador: Antonio Carlos Ferreira Seridonio / Resumo: O físico italiano Ettore Majorana propôs, no campo da Física de altas energias, a existência de férmions peculiares que têm como característica serem suas próprias antipartículas. No contexto de Física da matéria condensada, tais férmions emergem como quasipartículas de Majorana (MQPs). Da perspectiva da compu- tação quântica, duas MQPs podem compor um férmion regular e atuar como um qubit protegido, que está desacoplado do ambiente e livre do efeito de decoerência. Até onde sabemos, a verificação experimental de uma MQP ainda é questionável, apesar de alguns resultados experimentais, e desta forma, o objetivo desta tese é de propor formas experimentais a fim de ajudar na busca das assinaturas de tais excitações. Como o efeito Fano é um efeito de interferência na qual canais de tunelamento competem entre si pelo transporte eletrônico, ele torna-se uma forma de capturar tais assinaturas das MQPs em sistemas de matéria condensada. Baseado nisto, a ideia é investigar teoricamente três diferentes interferômetros a fim de obter uma assinatura definitiva das MQPs. O primeiro é um interferômetro do tipo Aharanov-Bohm composto por dois quantum dots, sendo um deles acoplado a uma MQP, que se localiza na borda de um fio de Kitaev semi-infinito na fase topológica. Ajustando o nível de Fermi dos terminais e o detuning simétrico dos níveis dos dots, mostrou-se que regimes Fano opostos resultam em uma transmitância caracterizada por distintas regiões condutoras e isolantes, que são marcas ... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The Italian physicist Ettore Majorana proposed in the field of high-energy Physics the existence of peculiar fermions that constitute their own antiparticles. In the context of condensed matter Physics, these fermions are Majorana quasiparticles (MQPs). From the quantum computing perspective, two MQPs can compose a regular fermion acting as a protected qubit, which is indeed decoupled from the host environment and free of the decoherence effect. To the best of our knowledge, the experimental capture of a MQP up to now is still questionable despite some experimental results, then, the goal of this thesis is to propose helpful experiment manners in revealing signatures from such excitations. As the Fano effect is an interference phenomenon where tunneling paths compete for the electronic transport, it becomes a probe to catch fingerprints of MQPs lying on condensed matter systems. Based on this, the idea is to investigate theoretically three different interferometers in order to obtain a MQP smoking- gun signature. The first one was an Aharonov-Bohm-like interferometer composed by two quantum dots, being one of them coupled to a MQP, which is attached to one of the edges of a semi-infinite Kitaev wire within the topological phase. By changing the Fermi energy of the leads and the symmetric detuning of the levels for the dots, we show that opposing Fano regimes result in a transmittance characterized by distinct conducting and insulating regions, which are fingerprints of an iso... (Complete abstract click electronic access below) / Doutor

Junções túnel com eletrodos ferromagnéticos (Py/AlOx/Co) foram produzidas usando a técnica de desbastamento iônico e depositadas sobre condições de oxidação que garantem baixa altura da barreira de potencial, baixa assimetria da barreira, forte dependência da magnetorresistência túnel com a tensão aplicada e o tunelamento quântico como mecanismo de transporte eletrônico. As amostras foram produzidas com o objetivo de corroborar um modelo recentemente publicado e que prevê inversão da magnetorresistência túnel com a tensão aplicada devido à baixa altura do potencial da barreira. As medidas de magneto-transporte eletrônico (resistência de tunelamento em função do campo magnético aplicado) mostram uma inversão da magnetorresistência túnel com a tensão aplicada para temperatura constante de 77 K. O sistema (Py/AlOx/Co) é bem conhecido por apresentar magnetorresistência positiva onde a altura da barreira de potencial é geralmente igual ou maior a 2,0 eV (Moodera et al. 1995 e Boeve et al. 2000). Esta inversão não foi anteriormente reportada e se deve preferencialmente à baixa altura do potencial da barreira e à forte dependência com a tensão aplicada. A explicação física para a inversão é baseada no fator de coerência quântica, D(Ex , V), como previsto por Li et al. (2004a,b) e Ren et al. (2005) para a região de tensão intermediária. Ajustes às curvas I-V, medidas a temperatura ambiente, com os modelos de Simmons (1963b,c), Simmons (1964) e Chow (1965) mostram valores menores que os reportados anteriormente para a altura do potencial da barreira (≈ 1,0 eV) e barreiras com baixa assimetria (≈ 0,2 eV). Também, as curvas I-V para temperatura ambiente e baixa temperatura, as curvas I-T para tensão constante e o crescimento exponencial da resistência de tunelamento em função da espessura efetiva da barreira mostram que o tunelamento quântico é um mecanismo de transporte eletrônico. Este resultado sugere a possibilidade de constatar o aparecimento de áreas efetivas de tunelamento e indicando a presença de uma distribuição não uniforme da corrente de tunelamento. O efeito combinado da baixa altura da barreira de potencial, da baixa assimetria da barreira, da forte dependência da magnetorresistência túnel com a tensão aplicada e do tunelamento quântico como mecanismo de transporte eletrônico possibilitaram não somente a inversão da magnetorresistência túnel com a tensão aplicada, mas também o crescimento exponencial da resistência de tunelamento em função da espessura efetiva da barreira. / Tunneling junctions with ferromagnetic electrodes (Py/AlOx/Co) were produced by magnetron sputtering technique and deposited under oxidation conditions that lead to low potential barrier height, low asymmetrical barrier, strong tunneling magnetoresistance dependence with applied bias and quantum tunneling as the charge transport mechanism. The samples were deposited to verify a recently published model which predicts tunneling magnetoresistance inversion with applied bias due to low enough potential barrier height. Electronic transport measurements (tunneling resistance as a function of the applied magnetic field) show inverse (negative) tunneling magnetoresistance with applied bias at 77 K. Tunneling junctions of (Py/AlOx/Co) are well known positive magnetoresistance system where the potential barrier height is usually equal or higher than 2.0 eV (Moodera et al., 1995 e Boeve et al., 2000). This inverted tunneling magnetoresistance behavior has not been reported before and is due mainly to the low potential barrier height and the strong bias dependence The physical explanation for the inversion is based on the quantum coherence factor, D(Ex , V), following the Li et al. (2004ab) and Ren et al. (2005) model for intermediate voltage range. Room temperature I-V curves fitted with both Simmons’ (1963b,c), Simmons’ (1964) and Chow’s (1965) models showed potential barrier height values (≈ 1.0 eV), lower than those previously reported, and low asymmetry of the barrier (≈ 0.2 eV). Also, I-V curves for room and low temperature, I-T curves for constant applied bias and the exponential growth of the tunneling resistance as a function of the effective barrier thickness showed quantum tunneling as the charge transport mechanism. This result suggests the presence of effective tunneling areas or hot spots, leading to a non-uniform current distribution. The combined effect of low potential barrier height, low barrier asymmetry, strong tunneling magnetoresistance dependence with applied bias and quantum tunneling as the charge transport mechanism allowed not only the tunneling magnetoresistance inversion with applied bias but also, the exponential growth of the tunneling resistance as a function of the effective barrier thickness.

xix, 190 p. : ill. A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / The electrical properties of conducting polymer-based devices are investigated in order to better understand charge transport through conducting polymers and charge transfer at conducting polymer interfaces with metals and inorganic semiconductors. Experiments on two specific systems are reported: (1) an anionically functionalized conducting polymer between metal electrodes and (2) nanostructured doped conducting polymer-semiconductor interfaces. Temperature dependent impedance measurements are reported on an anionically functionalized polyacetylene sandwiched between two gold electrodes (Au|P A |Au). These measurements provide key quantities regarding the ionic carriers in this system, such as the characteristic frequency for electrode polarization, ionic DC conductivity, activation energy, effective ion concentration, and hopping frequency. Impedance measurements are also reported on samples where excess electronic carriers had been introduced with a DC bias and at temperatures sufficiently low so as to freeze out the ionic carriers. In addition to providing information about the dielectric relaxation of electronic carriers such as the characteristic frequency for electrode polarization and activation energy, these low-temperature impedance measurements also support the ionic dielectric relaxation assignments. Temperature-dependent potential step experiments, in combination with the dielectric measurements probing ionic carriers, demonstrate the direct connection between the redistribution of ions and an enhancement in carrier injection in the Au|P A |Au system. Further potential step experiments followed by relaxation through either a short- or open-circuit configuration demonstrate that the electric field distribution is closely related to the amount of injected electronic carriers. The electric field distribution changes from being mostly determined by ionic carriers to being jointly determined by both ionic and injected electronic carriers when the density of injected electronic carriers is higher than that of the effective ionic carriers. To investigate charge depletion and transport at length scales less than the depletion width of a semiconductor interface, nanoscale metal-InP contacts with low barrier height were embedded within conducting polymer-InP contacts with high barrier height. Electrical measurements on these hybrid interfaces indicate that charge transport across the nanoscale metal contacts is affected by the neighboring high barrier region when the size of the metal contacts is less than the depletion width of the conducting polymer-InP background. / Adviser: Mark Lonergan

abstract: Scaling of the Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) towards shorter channel lengths, has lead to an increasing importance of quantum effects on the device performance. Until now, a semi-classical model based on Monte Carlo method for instance, has been sufficient to address these issues in silicon, and arrive at a reasonably good fit to experimental mobility data. But as the semiconductor world moves towards 10nm technology, many of the basic assumptions in this method, namely the very fundamental Fermi’s golden rule come into question. The derivation of the Fermi’s golden rule assumes that the scattering is infrequent (therefore the long time limit) and the collision duration time is zero. This thesis overcomes some of the limitations of the above approach by successfully developing a quantum mechanical simulator that can model the low-field inversion layer mobility in silicon MOS capacitors and other inversion layers as well. It solves for the scattering induced collisional broadening of the states by accounting for the various scattering mechanisms present in silicon through the non-equilibrium based near-equilibrium Green’s Functions approach, which shall be referred to as near-equilibrium Green’s Function (nEGF) in this work. It adopts a two-loop approach, where the outer loop solves for the self-consistency between the potential and the subband sheet charge density by solving the Poisson and the Schrödinger equations self-consistently. The inner loop solves for the nEGF (renormalization of the spectrum and the broadening of the states), self-consistently using the self-consistent Born approximation, which is then used to compute the mobility using the Green-Kubo Formalism. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2017