Gotowa bibliografia na temat „Semiconducting Nanostructures”

In this thesis we describe the development of a real-space implementation of the Bethe-Salpeter equation (BSE) and use it in conjunction with a semi-empirical tight-binding model to investigate the optoelectronic properties of colloidal quantum- confined nanostructures. This novel implementation exploits the limited radial extent and small size of the atomic orbital basis to treat finite systems containing up to ∼4000 atoms in a fully many-body framework. In the first part of this thesis our tight-binding model is initially benchmarked on zincblende CdSe nanocrystals, before subsequently being used to investigate the electronic states of zincblende CdSe nanoplatelets as a function of thickness. The band-edge electronic states are found to show minimal variation for a range of thicknesses and the results of our tight-binding model show good agreement with those predicted using a 14-band k·p model for a nanoplatelet of 4 monolayers (ML) in thickness. Optical absorption spectra were also computed in the independent-particle approximation. While the results of the tight-binding model show good agreement with those of the 14-band k·p model in the low-energy region of the spectrum, agreement with experiment was poor. This reflects the need for a many-body treatment of optical absorption in nanoplatelet systems. In the second part of this thesis we apply our tight-binding plus BSE model to study the excitonic properties of CdSe nanocrystals and nanoplatelets. Simulations performed on CdSe nanocrystals examined an approximation of the BSE equivalent to configuration interaction singles (CIS), and found that both the optical gap and the low-energy spectral features were unaffected by the approximation. A comparison of exciton binding energies with those predicted by CIS demonstrates the sensitivity of results to the exact treatment of dielectric screening and the decision of whether or not to screen exchange. Our model predicts optical gaps that are in strong agreement with average experimental data for all but the smallest diameters, but was not able to reproduce low-energy spectral features that were fully consistent with experiment. This was attributed to the absence of the spin-orbit interaction in the model. Simulations performed on CdSe nanoplatelets investigate the optical gaps and exciton binding energies as a function of thickness. Exciton binding energies were found to reach ∼200 meV for the thinnest system, however, optical gaps were slightly overestimated in comparison to experiment. This is attributed to the reduced lateral dimensions used in our simulations and our bulk treatment of dielectric screening. A two-dimensional treatment of dielectric screening is expected to further increase binding energies. Calculations of the excitonic absorption spectrum reproduce the characteristic spectral features observed in experiment, and show strong agreement with the spectra of nanoplatelets, with thicknesses ranging from 3 ML to 5 ML.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2011.<br>This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.<br>Cataloged from PDF version of thesis. Vita.<br>Includes bibliographical references (p. 163-174).<br>Semiconductor nanostructures exhibit distinct properties by virtue of nano-scale dimensionality, allowing for investigations of fundamental physics and the improvement of optoelectronic devices. Nanoscale morphological variations can drastically affect overall nanostructure properties because the investigation of nanostructure assemblies convolves nanoscale fluctuations to produce an averaged result. The investigation of individual nanostructures is thus paramount to a comprehensive analysis of nanomaterials. This thesis focuses on the study of individual GaAs, AlGaAs, and ZnO nanostructures to understand the influence of morphology on properties at the nanoscale. First, the diameter-dependent exciton-phonon coupling strengths of individual GaAs and AlGaAs nanowires were investigated by resonant micro-Raman spectroscopy near their direct bandgaps. The one-dimensional nanowire architecture was found to affect exciton lifetimes through an increase in surface state population relative to volume, resulting in Fröhlich coupling strengths stronger than any previously observed. Next, ZnO nanowire growth kinetics and mechanisms were found to evolve by altering precursor concentrations. The cathodoluminescence of nanowires grown by reaction-limited kinetics were quenched at the nanowire tips, likely due to point defects associated with the high Zn supersaturation required for reaction-limited growth. Further, cathodoluminescence was quenched in the vicinity of Au nanoparticles, which were found on nanowire sidewalls due to the transition in growth mechanism, caused by excited electron transfer from the ZnO conduction band to the Au Fermi level. Finally, ZnO nanowalls were grown by significantly increasing precursor flux and diffusion lengths over that of the ZnO nanowire growth. Nanowall growth began with the Au-assisted nucleation of nanowires, whose growth kinetics was a combination of Gibbs- Thomson-limited and diffusion-limited, followed by the domination of non-assisted film growth to form nanowalls. Nanoscale morphological variations, such as thickness variations and the presence of dislocations and Au nanoparticles, were directly correlated with nanoscale variations in optical properties. These investigations prove unequivocally that nanoscale morphological variations have profound consequences on optical properties on the nanoscale. Studies of individual nano-objects are therefore prerequisite to fully understanding, and eventually employing, these promising nanostructures.<br>by Megan Marie Brewster.<br>Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2015.<br>This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.<br>Cataloged from student-submitted PDF version of thesis.<br>Includes bibliographical references (pages 142-149).<br>III-V semiconducting nanostructures present a promising platform for the realization of advanced optoelectronic devices due to their superior intrinsic materials properties including direct band gap energies that span the visible light spectrum and high carrier mobilities. Additionally, the inherently high surface-to-volume ratio of nanostructures allows for the efficient relaxation of stress enabling the realization of defect free heterostructures between highly mismatched materials. As a result, nanostructures are being investigated as a route towards the direct integration of III-V materials on silicon substrates and as platforms for the fabrication of novel heterostructures not achievable in a thin film geometry. Due to their small size, however, many of the methods used to calculate stress and strain in 2D bulk systems are no longer valid as free surface effects allow for relaxation creating more complicated stress and strain fields. These inhomogeneous strain fields could have significant impacts on both device fabrication and operation. Therefore, it will be vital to develop techniques that can accurately predict and measure the stress and strain in individual nanostructures. In this thesis, we demonstrate how the combination of advanced transmission electron microscopy (TEM) and continuum modeling techniques can provide a quantitative understanding of the complex strain fields in nanostructures with high spatial resolutions. Using techniques such as convergent beam electron diffraction, nanobeam electron diffraction, and geometric phase analysis we quantify and map the strain fields in top-down fabricated InAlN/GaN high electron mobility transistor structures and GaAs/GaAsP core-shell nanowires grown by a particle-mediated vapor-liquid-solid mechanism. By comparing our experimental results to strain fields calculated by finite element analysis, we show that these techniques can provide quantitative strain information with spatial resolutions on the order of 1 nm. Our results highlight the importance of nanoscale characterization of strain in nanostructures and point to future opportunities for strain engineering to precisely tune the behavior and operation of these highly relevant structures.<br>by Eric James Jones.<br>Ph. D.

This project is a step forward in developing advanced two dimensional carbon-based materials for future nanoelectronics applications. It explores a new pathway towards nanoscale graphene fabrication compatible with the current semiconductor industry. Ribbons of graphene have been fabricated on silicon carbide wafers by nanoscale patterning as a first step towards developing graphene circuitry. A technology to decrease the interaction between the substrate and graphene has been developed to improve graphene flatness. The attenuation of electrons from the graphene layer have been also investigated, leading to a new insight in understanding electrons attenuation length.

This diploma thesis discusses the gas sensor preparation via anodic oxidation. It names sensor types, deals with the sensing principle of electrochemical sensors in detail and submits sensor parameters. It describes preparation technology and characterization technology methods. In the experimental part, it focuses on both the measurement methodology and the electrochemical oxygen sensor covered with titanium dioxide nanocolumns fabrication. Not the least it discusses acquired research results.

La présente thèse étudie les nanoparticules de ZnO incorporées dans une matrice d'acide polyacrylique (PAA) mésosphérique synthétisée via un protocole d'hydrolyse. La structure hybride mésosphérique de ZnO / PAA a précédemment démontré son efficacité pour émettre de la lumière visible dans une large gamme, qui résulte des défauts intrinsèques de niveaux profonds dans les nanocristaux de ZnO. Pour modifier davantage le spectre de photoluminescence (PL) et améliorer le rendement quantique de PL (PL QY) du matériau, le ZnO dopé au métal et le ZnO / PAA revêtu de silice sont fabriqués indépendamment. Au niveau du ZnO dopé avec des éléments métalliques, la nature, la concentration, la taille et la valence du dopant affectent la formation des mésosphères et par conséquent la PL et le PL QY. Les ions plus grands que Zn2+ avec une valence plus élevée ont tendance à induire des mésosphères plus grandes et des nanoparticules de ZnO non incorporées. Le dopage conduit généralement à l'extinction de la PL, mais le spectre PL peut toujours être ajusté dans une large plage (entre 2,46 eV et 2,17 eV) sans dégrader le PL QY en dopant avec de petits ions à une faible concentration de dopage (0,1 %). Concernant le ZnO / PAA revêtu de silice, un revêtement optimal est obtenu, qui dépend corrélativement de la quantité de TEOS et d'ammoniac dans le processus de revêtement. La quantité de TEOS n'affecte pas la structure cristalline de ZnO ou le spectre PL du matériau, mais une concentration élevée d'ammoniac peut dégrader les mésosphères de PAA et épaissir la couche de silice. Une fine couche de silice qui n'absorbe pas trop de lumière d'excitation mais recouvre complètement les mésosphères s'avère être la plus efficace, avec une amélioration drastique du PL QY d’un facteur six. En ce qui concerne l'application, les matériaux souffrent d’une dégradation thermique à des températures élevées jusqu'à 100 °C, auxquelles les diodes électroluminescentes blanches (WLEDs) fonctionnent généralement. Cependant, le ZnO / PAA revêtu de silice induit une intensité d'émission plus élevée à température ambiante pour compenser la dégradation thermique<br>The present thesis studies ZnO nanoparticles embedded in a mesospheric polyacrylic acid (PAA) matrix synthesized via a hydrolysis protocol. The mesospheric ZnO/PAA hybrid structure was previously proved efficient in emitting visible light in a broad range, which results from the deep-level intrinsic defects in ZnO nanocrystals. To further tune the photoluminescence (PL) spectrum and improve the PL quantum yield (PL QY) of the material, metal-doped ZnO and silica-coated ZnO/PAA are fabricated independently. For ZnO doped with metallic elements, the nature, concentration, size and valence of the dopant are found to affect the formation of the mesospheres and consequently the PL and PL QY. Ions larger than Zn2+ with a higher valence tend to induce larger mesospheres and unembedded ZnO nanoparticles. Doping generally leads to the quenching of PL, but the PL spectrum can still be tuned in a wide range (between 2.46 eV and 2.17 eV) without degrading the PL QY by doping small ions at a low doping concentration (0.1 %). For silica-coated ZnO/PAA, an optimal coating correlatively depends on the amount of TEOS and ammonia in the coating process. The amount of TEOS does not affect the crystal structure of ZnO or the PL spectrum of the material, but high concentration of ammonia can degrade the PAA mesospheres and thicken the silica shell. A thin layer of silica that does not absorb too much excitation light but completely covers the mesospheres proves to be the most efficient, with a drastic PL QY improvement of six times. Regarding the application, the materials suffer from thermal quenching at temperatures high up to 100°C, at which white light emitting diodes (WLEDs) generally operates. However, silica-coated ZnO/PAA induces higher emission intensity at room temperature to make up for the thermal quenching

Made available in DSpace on 2014-06-11T19:31:04Z (GMT). No. of bitstreams: 0 Previous issue date: 2009-11-16Bitstream added on 2014-06-13T19:01:19Z : No. of bitstreams: 1 savu_r_dr_bauru.pdf: 10688901 bytes, checksum: 4c1846c73d88b2e598b43e7a14ea1b7c (MD5)<br>Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)<br>A presente pesquisa teve como principal objetivo a obtenção de estruturas nanométricas de óxido de índio, óxido de estanho e óxido de zinco por evaporação térmica e síntese hidrotérmica e a construção e teste de sensores de gases e de fotodetectores de ultravioleta baseados nessas nanoestruturas. Foram realizados estudos da influência dos parâmetros experimentais das duas rotas de síntese usadas sobre as morfologias e as propriedades das estruturas. Para a obtenção das camadas nanoestruturadas por evaporação térmica foi especialmente construído um forno tubular que permitiu o controle da temperatura de deposição independente da temperatura de evaporação e da distância entre a fonte de evaporação e o substrato. Esses parâmetros, pouco explorados nas pesquisas reportadas na literatura, exerceram uma grande influência sobre a morfologia e as propriedades dos nanofios obtidos. O equipamento permitiu ainda um controle preciso da composição da atmosfera e da pressão de síntese. Na síntese química em solução, a construção de um reator hidrotérmico permitiu o estudo da influência da taxa de resfriamento sobre as dimensões, cristalinidade, morfologia e propriedades das nanoestruturas. Esse estudo, o primeiro do gênero na literatura, ressaltou a importância no controle deste parâmetro para sintetizar estruturas com propriedades melhoradas. As demais variáveis estudadas foram: a concentração das soluções, as camadas catalisadoras, a temperatura e o tempo de síntese. Foram testadas duas estratégias para a obtenção dos filmes nanoestruturados: spin-coating de suspensões de nanoestruturas sobre substratos de silício oxidado ou o crescimento das mesmas, durante a síntese, sobre substratos com camadas catalisadoras de zinco. Os nanofios e as camadas funcionais foram caracterizados por Difração de Raios-X (DRX), Microscopia Eletrônica de Varredura...<br>The subject of this thesis covers the synthesis and growth of indium, tin and zinc oxide nanostructures by thermal evaporation and hydrothermal synthesis and the fabrication and testing of gas sensors and ultraviolet photodetectors based on these nanosized structures. For both chemical and physical routes, the influence of processing conditions over the morphology, dimensions and electrical properties of the nanowires was investigated. In order to obtain nanostructured layers by thermal evaporation a tubular furnace was specifically builti, allowed the control of the source-substrate distance and the deposition temperature independently of the evaporation one. These parameters, slightly explored in the literature, granted a big influence over the nanowires morphology and properties. Moreover, the equipment permitted the control of deposition atmosphere and pressure. The design and assembly of a hydrothermal reactor allowed studying the influence of the cooling rate over the dimension, morphology, cristallinity and, consequently, the properties of the nanostructures. This study highlighted the importance of controlling this particular parameter in the hydrothermal process, yielding nanostructured materials with enhanced properties. Variables such as solution concentration, synthesis temperature and time, surfanctants and precursors were also explored in the hydrothermal process. In order to obtain nanostructured thin films using the chemical bath deposition, two processing techniques were employed: spin-coating of powder suspensions over oxidized silicon substrates and nanostructured anisotropic growth directly from solution using zinc coated substrates. The nanowires and the functional nanostructured layers were characterized by X-Ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FE - SEM), Transmission Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS)... (Complete abstract click electronic access below)

Resumo: A presente pesquisa teve como principal objetivo a obtenção de estruturas nanométricas de óxido de índio, óxido de estanho e óxido de zinco por evaporação térmica e síntese hidrotérmica e a construção e teste de sensores de gases e de fotodetectores de ultravioleta baseados nessas nanoestruturas. Foram realizados estudos da influência dos parâmetros experimentais das duas rotas de síntese usadas sobre as morfologias e as propriedades das estruturas. Para a obtenção das camadas nanoestruturadas por evaporação térmica foi especialmente construído um forno tubular que permitiu o controle da temperatura de deposição independente da temperatura de evaporação e da distância entre a fonte de evaporação e o substrato. Esses parâmetros, pouco explorados nas pesquisas reportadas na literatura, exerceram uma grande influência sobre a morfologia e as propriedades dos nanofios obtidos. O equipamento permitiu ainda um controle preciso da composição da atmosfera e da pressão de síntese. Na síntese química em solução, a construção de um reator hidrotérmico permitiu o estudo da influência da taxa de resfriamento sobre as dimensões, cristalinidade, morfologia e propriedades das nanoestruturas. Esse estudo, o primeiro do gênero na literatura, ressaltou a importância no controle deste parâmetro para sintetizar estruturas com propriedades melhoradas. As demais variáveis estudadas foram: a concentração das soluções, as camadas catalisadoras, a temperatura e o tempo de síntese. Foram testadas duas estratégias para a obtenção dos filmes nanoestruturados: spin-coating de suspensões de nanoestruturas sobre substratos de silício oxidado ou o crescimento das mesmas, durante a síntese, sobre substratos com camadas catalisadoras de zinco. Os nanofios e as camadas funcionais foram caracterizados por Difração de Raios-X (DRX), Microscopia Eletrônica de Varredura... (Resumo completo, clicar acesso eletrônico abaixo)<br>Abstract: The subject of this thesis covers the synthesis and growth of indium, tin and zinc oxide nanostructures by thermal evaporation and hydrothermal synthesis and the fabrication and testing of gas sensors and ultraviolet photodetectors based on these nanosized structures. For both chemical and physical routes, the influence of processing conditions over the morphology, dimensions and electrical properties of the nanowires was investigated. In order to obtain nanostructured layers by thermal evaporation a tubular furnace was specifically builti, allowed the control of the source-substrate distance and the deposition temperature independently of the evaporation one. These parameters, slightly explored in the literature, granted a big influence over the nanowires morphology and properties. Moreover, the equipment permitted the control of deposition atmosphere and pressure. The design and assembly of a hydrothermal reactor allowed studying the influence of the cooling rate over the dimension, morphology, cristallinity and, consequently, the properties of the nanostructures. This study highlighted the importance of controlling this particular parameter in the hydrothermal process, yielding nanostructured materials with enhanced properties. Variables such as solution concentration, synthesis temperature and time, surfanctants and precursors were also explored in the hydrothermal process. In order to obtain nanostructured thin films using the chemical bath deposition, two processing techniques were employed: spin-coating of powder suspensions over oxidized silicon substrates and nanostructured anisotropic growth directly from solution using zinc coated substrates. The nanowires and the functional nanostructured layers were characterized by X-Ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FE - SEM), Transmission Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS)... (Complete abstract click electronic access below)<br>Orientador: Maria Aparecida Zaghete Bertochi<br>Coorientador: Elson Longo<br>Banca: Antonio Ricardo Zanatta<br>Banca: Mônica Alonso Cotta<br>Banca: Talita Mazon Anselmo<br>Banca: Sidney José Lima Ribeiro<br>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<br>Doutor