Le nanotube de carbone double parois représente le cas idéal pour étudier la nature de l'interaction entre parois des tubes multiparois. En partant d'échantillons dispersés de DWNTs synthétisés par CVD, nous avons pu, grâce à la microscopie électronique en transmission haute résolution (METHR), établir une procédure robuste de détermination structurale des configurations. Il apparaît alors que certaines configurations structurales sont privilégiées alors que d'autres sont interdites, mettant en évidence les effets du couplage interparoi. À partir de simulations Monte Carlo réalisées sur des DWNTs de configurations interdites, nous avons montré que le tube interne modifie sa structure pour atteindre une stabilité énergétique, ce que nous avons pu rapprocher d'observations expérimentales. Pour étudier les propriétés électroniques des DWNTs observés expérimentalement, nous avons corrélé les techniques de METHR et d'absorption optique pour analyser des populations différenciées de tubes en nombre de parois, diamètre et nature électronique, grâce à la technique DGU (Ultracentrifugation de Gradient de Densité). À l'issue de trois tris successifs, nous avons pu isoler une population de tubes double parois pure à 95% et dont les tubes extérieurs sont de nature semi-conducteur à 90\%. / Double-walled carbon nanotube represents the ideal case to investigate the nature of the interaction between walls of multiwall tubes. Starting with scattered samples of DWNTs synthesized by CVD, we have established a robust procedure for structure determination of configurations based on high resolution electron microscopy transmission (HRTEM). After achieving a statistical study, it appears that some structural configurations have been favored while others are completely forbidden, highlighting the effects of inter-wall coupling. To go beyond, we have performed Monte Carlo simulations at atomic scale on DWNTs with forbidden configurations. As a result, we have shown that the inner tube changes its structure to achieve energy stability, in good agreement with experimental observations. To study the electronic properties of DWNTs observed experimentally, we correlated HRTEM and optical absorption techniques for analyzing differentiated tubes populations by number of walls, diameter and electronic nature, thanks to the technical DGU (Density Gradient Ultracentrifugation ). After three successive sorting, a pure population of double-walled tubes to 95% and of which 90% of the outer tubes are semiconductor has been isolated.

Les matériaux hybrides organiques inorganiques ont attirés l'attention vue qu'ils présentent des propriétés optiques et optoélectroniques fascinantes comme la forte photoluminescence même à température ambiante. Cet axe de recherche relativement nouveau, sur cette famille de matériaux, offre une variété d’opportunités technologiques. Dans ce contexte, nous nous sommes intéressés par l'étude des propriétés optiques des deux matériaux hybrides organiques inorganiques (C6H11NH3)2[PbI4] et (C6H11NH3)2[PbBr4], et principalement leurs propriétés de luminescence. Les résultats montre que sous excitation dans l'ultraviolet, (C6H11NH3)2[PbBr4] émet de la lumière blanche, même à température ambiante, ce qui présente un grand intérêt de l'utilisation de ces matériaux comme source d'émission de la lumière blanche. L'origine de cette émission a été étudié par différentes techniques comme la photoluminescence résolution en temps. / Inorganic organic hybrid materials have attracted a great attention do to their special structure and important optical such as the high luminescence, even at room temperature. This relatively new research on this family of materials, offers a variety of technological opportunities. In this context, we are interested in the study of optical properties of both inorganic and organic hybrid materials (C6H11NH3)2[PbI4] and (C6H11NH3)2[PbBr4], and mainly their luminescence properties. The results shows that under ultraviolet excitation, (C6H11NH3)2[PbBr4] show a strong white light emission, even at room temperature, which open a great interest in the use of these materials as a source of the white light emission. The origin of this large emission has been studied by different techniques such as the time resolved photoluminescence measurements .

Due to the study basic structural features of ultrafine ceramics, using the scanning electron microscopy method, were identified. Dependency of particle sizes on thermal treatment modes was determined, and average size of the sample grains was determined due to statistical processing of the SEM images, using hardware and software image analysis package SIAMS 700. It has been established that photoluminescent properties of single-crystalline samples under the study correspond to the publicly available literature data. Photoluminescence spectra were measured after sample finishing and dependence of optical absorption on exposure dose was identified. The ultrafine ceramic FL was examined and comparison study of the resulting parameters with a single-crystalline sample was carried out. Excitation and emission spectra under various modes of ceramic synthesis were also produced. This study makes it possible to consider ultrafine ceramics as a non-conventional material for high-dose measurements. / В результате исследования были выявлены основные структурные особенности ультрадисперсной керамики методом растровой электронной микроскопии. Определена зависимость размеров частиц от режимов термической обработки, а так же найден средний размер зерен образцов за счет проведения статистической обработки изображений РЭМ на программно-аппаратном комплексе анализа изображений SIAMS 700. Установлено, что фотолюминесцентные свойства исследуемых монокристаллических образцов соответствуют общеизвестным литературными данными. Измерены спектры фотолюминесценции после полировки образцов и найдена зависимость оптического поглощения от дозы облучения. Рассмотрена ФЛ ультрадисперсной керамики и проведен сравнительный анализ полученных параметров с монокристаллическим образцом. Так же получены спектры возбуждения и излучения при различных режимах синтеза керамики. Данные исследования позволяют рассматривать ультрадисперсную керамику как перспективный материал для высокодозных измерений.

Объект исследования – анодированный диоксид циркония. Цель работы – электрохимический синтез нанотубулярных структур диоксида циркония и исследование их люминесцентных и абсорбционных свойств. Методы исследования – растровая электронная микроскопия, рентгенофазовый анализ, абсорбционная и фотолюминесцентная спектроскопия. Новизна работы – исследована фотолюминесценция анодированного диоксида циркония в диапазоне температур 6.2-700 К. Показано, что интенсивность свечения в области 375 – 600 нм возрастает с уменьшением температуры до 30 К. Спектры свечения описываются двумя пиками гауссовой формы с максимумами Emax = 2.28 и 2.75 эВ и полуширинами ω = 0.77 и 0.67 эВ, соответственно. Измерены спектры диффузного отражения образцов до и после отжига. Посредством построения Тауца рассчитана энергия края оптического поглощения нанотубулярного диоксида циркония Eg = 5.4 ± 0.1 эВ. Возможная область применения анодированного диоксида циркония – матрицы солнечных батарей и фотокатализаторы. / The object of investigation is anodized zirconia. The aim of the work is the electrochemical synthesis of nanotubular zirconia structures and the study of their luminescence and absorption properties. The methods of investigation are scanning electron microscopy, X-ray phase analysis, absorption and photoluminescence spectroscopy. The novelty of the work is the photoluminescence of anodized zirconia in the temperature range of 6.2-700 K. It is shown that the luminescence intensity increases with decreasing temperature to 30 K. The emission spectra are described by two peaks of a Gaussian shape with maxima Emax = 2.28 and 2.75 eV and halfwidths ω = 0.77 and 0.67 eV, respectively. A possible field of application of anodized zirconia is the matrix of solar cells and photocatalysts. Spectra of diffuse reflection of samples before and after annealing were measured. By means of the Tauc plot, the energy of the edge of the optical absorption of nanotubular zirconium dioxide Eg = 5.4 ± 0.1 eV is calculated. A possible field of application of anodized zirconia is the matrix of solar cells and photocatalysts.

В работе проведено исследование структурных, оптических и электронных свойств графитоподобного нитрида углерода (g-C3N4), синтезированных методом пиролиза мочевины при температурах 450–600°С. Метод функционала плотности использован для подтверждения выводов, основанных на экспериментальных данных. Был выполнен анализ литературных данных по g-C3N4, были рассмотрены структурные особенности получаемого материала и их оптическое поглощение. Также приведено описание используемых экспериментальных методов и математическое описание используемого метода моделирования – теории функционала плотности. Структура синтезированных образцов g-C3N4 определена по спектрам рентгеновской дифракции и ИК поглощения. Оптическое поглощение образцов исследованы по спектрам диффузного отражения, преобразованным в соответствии с теорией Кубелки-Мунка, оценка ширины энергетической щели для прямых и непрямых оптических переходов проводилась по методу Тауца. Была установлена линейная зависимость параметров кристаллической решетки и ширины щели от температуры синтеза. Для подтверждения экспериментальных результатов было проведено моделирование возможных структур g-C3N4 методом функционала плотности в пакете Quantum ESPRESSO. Была проведена оптимизация положений атомов для различных модельных структур, рассчитаны ИК спектры и зонные структуры электронных состояний. / In this work we investigated structural, optical and electrical properties of graphitic carbon nitride (g-C3N4) synthesized by thermal treatment of urea at 450– 600°C. Density functional theory calculations were performed to support suggestions based on experimental data. Available research data for g-C3N4 was studied, with interest in structural and absorption properties of the material. Additionally, the experimental methods and theory behind DFT-calculations were described. We determined the structure of g-C3N4 samples based on XRD and IR spectra. Optical absorption was investigated using diffuse reflectance spectra transformed with Kubelka-Munk function. Energy band gap was evaluated with Tauc plot for direct and indirect optical transitions. We show that crystal lattice parameters and band gap linearly depend on the synthesis temperature. To further prove our assumptions based on experimental data we also performed DFT-calculations for g-C3N4 in Quantum ESPRESSO software package. We calculated IR spectra and electronic band structures for models of each sample.

[pt] A investigação da geometria quântica em semicondutores e isoladores tornou-se significativa devido às suas implicações nas características dos materiais. A noção de geometria quântica surge considerando a métrica quântica do estado de Bloch da banda de valência, que é definido a partir da sobreposição dos estados de Bloch em momentos ligeiramente diferentes. Ao integrar a métrica quântica em toda a zona de Brillouin, introduzimos uma quantidade que chamamos de número de fidelidade, que significa a distância média entre estados de Bloch adjacentes. Além disso, apresentamos um formalismo para expressar o número de fidelidade como um marcador de fidelidade local no espaço real que pode ser definido em qualquer sítio da rede. O marcador pode ser calculado diretamente diagonalizando o hamiltoniano da rede que descreve o comportamento das partículas na rede. Posteriormente, o conceito de número e marcador de fidelidade é estendido para temperatura finita utilizando a teoria de resposta linear, conectando-os a medições experimentais que envolvem analisar o poder de absorção óptica global e local quando o material é exposto à luz linearmente polarizada. Particularmente para materiais bidimensionais, a opacidade do material permite a determinação direta do número de fidelidade espectral, permitindo a detecção experimental do número de fidelidade. Finalmente, um marcador de fidelidade não local é introduzido considerando a divergência da métrica quântica. Este marcador é postulado como um indicador universal de transições de fase quântica, assumindo que o momento cristalino permanece um número quântico válido. Este marcador não local pode ser interpretado como uma função de correlação dos estados de Wannier, que são funções de onda localizadas que descrevem estados eletrônicos em um cristal. A generalidade e aplicabilidade destes conceitos são demonstradas através da investigação de vários isoladores topológicos e transições de fase topológicas em diferentes dimensões. Essas descobertas elaboram o significado dessas quantidades e sua conexão com vários fenômenos fundamentais na física da matéria condensada. / [en] The investigation of quantum geometry in semiconductors and insulators has become significant due to its implications for material characteristics. The notion of quantum geometry arises by considering the quantum metric of the valence-band Bloch state, which is defined from the overlap of the Bloch states at slightly different momenta. By integrating the quantum metric through-out the Brillouin zone, we introduce a quantity that we call fidelity number, which signifies the average distance between adjacent Bloch states. Furthermore, we present a formalism to express the fidelity number as a local fidelity marker in real space that can be defined on every lattice site. The marker can be calculated directly by diagonalizing the lattice Hamiltonian that describes particle behavior on the lattice. Subsequently, the concept of the fidelity number and marker is extended to finite temperature using linear-response theory, connecting them to experimental measurements which involves analyze the global and local optical absorption power when the material is exposed to linearly polarized light. Particularly for two-dimensional materials, the material s opacity enables straightforward determination of the fidelity number spectral, allowing for experimental detection of the fidelity number. Finally, a nonlocal fidelity marker is introduced by considering the divergence of the quantum metric. This marker is postulated as a universal indicator of quantum phase transitions, assuming the crystalline momentum remains a valid quantum number. This nonlocal marker can be interpreted as a correlation function of Wannier states, which are localized wave functions describing electronic states in a crystal. The generality and applicability of these concepts are demonstrated through the investigation of various topological insulators and topological phase transitions across different dimensions. These findings elaborate the significance of these quantities and their connection to various fundamental phenomena in condensed matter physics.

No description available.

Vidros teluritos são considerados fortes candidatos para aplicações em lasers e óptica não linear devido a importantes características, como o elevado índice de refração, baixa energia de fônons, baixa temperatura de transição vítrea e sua fácil preparação. Neste trabalho foram produzidas amostras dos sistemas vítreos TeO2- ZnO-Na2O, TeO2-Nb2O5-Na2O e Te2O-Na2O incluindo ou não dopagens de AgNO3 para estudos estruturais e ópticos. As propriedades estruturais e os mecanismos de recombinação destes vidros foram estudados por meio das técnicas de ressonância paramagnética eletrônica, termoluminescência e correntes de despolarização termicamente estimuladas. Já as propriedades ópticas como o coeficiente de absorção linear, índice de refração linear e não linear foram estudadas por meio das técnicas de absorção óptica, acoplamento de prismas e varredura Z. Foram ainda desenvolvidos estudos da precipitação de nanopartículas de prata para o sistema TeO2-ZnO-Na2O com dopagens de AgNO3 com diferentes temperaturas de tratamento térmico. A caracterização das nanopartículas metálicas foi realizada por meio da técnica de absorção óptica auxiliada por microscopia eletrônica de transmissão e espectrometria de energia dispersiva. Foram observadas por ressonância paramagnética eletrônica quatro respostas paramagnéticas dos vidros irradiados com raio , sendo uma de natureza desconhecida e as outras três identificadas como g¹, g² e g³ atribuídas respectivamente ao centro de buraco do telúrio e do oxigênio, centro de buraco do oxigênio terminal e centro de elétron do telúrio. Foi proposto ainda um modelo para explicar os mecanismos de formação destes centros. A partir dos resultados de termoluminescência observou-se que os processos de recombinação destes centros de defeitos são não radioativos. Neste projeto foram estudados pela primeira vez na literatura os fenômenos de polarização e despolarização em vidros teluritos, com e sem irradiação gama, por meio da técnica de correntes de despolarização termicamente estimuladas. Quanto às propriedades ópticas, foi investigada a influência da adição de prata dos sistemas vítreos TeO2-ZnO-Na2O e TeO2-Nb2O5-Na2O em relação à energia do gap, a cauda de Urbach, índice de refração linear e não linear.Nos resultados de varredura Z do sistema vítreo TeO2-Nb2O5-Na2O foi observado o aumento da assimetria na curva com aumento da adição de AgNO3 Este fenômeno foi explicado por meio de uma adaptação do modelo de Sumi para processos não radiativos, responsáveis pelo aumento do caráter térmico do índice de refração não linear. Por fim um modelo foi criado para explicar a precipitação de nanopartículas de prata na matriz vítrea TeO2-ZnO-Na2O mediante tratamento térmico. / Tellurite glasses are considered powerful candidates for applications in lasers and nonlinear optics due to their important properties such as high refractive index, low phonon energy, relatively low glass transition temperature and easy glass production at room atmosphere. In this work glass samples of the systems ZnO-Na2O, TeO2-Nb2O5-Na2O and Te2O-Na2O were produced, including or not the AgNO3 doping for the structural and optical study purposes. The structural properties and the recombination mechanisms of these glasses were studied by means of the techniques of electron paramagnetic resonance, thermoluminescence and thermally stimulated depolarization currents. The optical properties such as the optical linear absorption coefficient, linear and nonlinear refractive index were studied by means of the optical absorption techniques, prism coupling and Z-scan. Further studies were developed on the precipitation of silver nanoparticles embedded in the glass system TeO2-ZnO-Na2O doped with AgNO3, by means of thermal treatments at different temperatures. The characterization of the metallic nanoparticles was carried out by optical absorption aided by transmission electron microscopy and dispersive energy spectroscopy. Four paramagnetic responses were observed by electron paramagnetic resonance of the -irradiated glasses, one not yet identified and the other three identified as g0, g1 and g3 attributed respectively to the tellurium-oxygen hole center, non-bridging oxygen hole center and tellurium electron center. A model to explain the formation mechanisms of these centers was proposed. The thermoluminescence results indicated that the recombination processes of these centers are non-radiative. This is the first study that deals with polarization and depolarization phenomena in tellurite glasses with and without gamma irradiation, by the technique of thermally stimulated depolarization currents. In what regards the optical properties of the glass systems TeO2-ZnONa2O and TeO2-Nb2O5-Na2O, the effect of the silver doping on the gap energy, Urbach tail, linear and nonlinear refractive indices were investigated. Through the Z-scan technique results of the glass system TeO2-Nb2O5-Na2O, an increasing asymmetry of the curve, with increasing AgNO3 content was observed. This phenomenon was explained through an adaptation of Sumi´s model for non-radiative processes, applied to a silver doped insulating glass, to explain the thermal character of the nonlinear refractive index. Finally, a model was developed to explain the silver nanoparticles precipitation in the TeO2-ZnO-Na2O glass matrix submitted to thermal treatment.

Designs for deep ultraviolet (DUV) edge emitting laser diodes (LDs) based on the wurtzite III-nitride (III-N) material system are presented. A combination of proprietary and commercial advanced semiconductor LD simulation software is used to study the operation of III-N based DUV LDs theoretically. Critical factors limiting device performance are identified based on an extensive literature survey. A comprehensive design parameter space is investigated thoroughly with the help of advanced scripting capabilities. Several design strategies are proposed to eliminate the critical problems completely or partially. A DUV LD design is proposed based exclusively on AlInN active layers grown epitaxially on bulk AlN substrates because AlInN offers a promising alternative to AlGaN for the realization of LDs and LEDs operating in the DUV regime. The proposed AlInN-based design also features a tapered electron blocking layer (EBL) instead of a homogeneous one. Tapered EBLs redistribute the interfacial polarization charge volumetrically throughout the entire EBL thickness via compositional grading, and eliminate the parasitic inversion layer charge. AlGaN based DUV LD designs are explored also because at present, it may be difficult to grow AlInN epitaxially with superior crystalline quality. Polarization charge matching is proposed to improve electron and hole wavefunction overlap within the active region. Although the strategy of polarization charge matching has already been proposed in the literature to enhance performance of visible wavelength LEDs and LDs, the proposed design presents the first demonstration that polarization charge matching is also feasible for DUV LDs operating at sub-300 nm wavelengths. A lateral current injection (LCI) LD design is proposed featuring polarization-charge-matched barriers and regrown Ohmic contacts to avoid a group of issues related to the highly inefficient p-type doping of wide bandgap III-N materials in vertical injection designs. The proposed design partially decouples the problem of electrical injection from that of optical confinement. Although the idea of an LCI LD design has been proposed in the literature in the 90s to be used as longer wavelength active sources in optoelectronic integrated circuits using GaInAsP/InP and related material systems, the proposed design is the first theoretical demonstration that this concept can be applied to DUV LDs based on III-N material system. To solve the problem of hole transport in vertical injection designs, a DUV LD design based exclusively on AlGaN material system is presented, featuring an inverse-tapered p-waveguide layer instead of an EBL. Several EBL designs are investigated, and compared with conventionally-tapered EBL design. Through judicious volumetric redistribution of fixed negative polarization charge, inverse tapering may be exploited to achieve nearly flat valence band profiles free from barriers to hole injection into the active region, in contrast to conventional designs. Numerical simulations demonstrate that the inverse tapered strategy is a viable solution for efficient hole injection in vertical injection DUV LDs operating at shorter wavelengths (< 290 nm).

Fundação de Amparo a Pesquisa do Estado de Minas Gerais / The SNABP [40SiO2.30Na2CO3.1Al2O3.25B2O3.PbO2 (mol%)] glass system, nanostructured with PbS Quantum Dots (QDs) and/or doped with Er3+ ions, was successfully synthesized by the Fusion Method, when it was subjected to appropriate thermal annealing. The glass transition temperatures (Tg) were obtained by Differential Thermal Analyze (DTA), in which it was possible to define a suitable temperature to be used in the thermal annealing of the synthesized samples. As results of these thermal annealing, the formation and growth of PbS QDs have occurred in the glass environment. The optical properties of samples were investigated by Optical Absorption (OA), Photoluminescence (PL), and Photoluminescence with Temporal Resolution (PLTR). Moreover, the Atomic Force Microscopy (AFM) and X-Ray Diffractometry were employed in study of morphological and structural properties of samples, respectively. The size dispersions of PbS QDs were determined from OA spectra. Once using both the Method and OA data, it was also possible to estimate the average diameters of these nanoparticles, which grow with the increase in annealing time. The characteristic transitions of Er3+ ions were clearly identified in OA spectra. In addition, it was observed that the increase in annealing time of samples had provoked amplification in the overlapping between PL bands of both the PbS QDs and Er3+ ions, as well as with the absorption 4I15/2 -> 4I13/2 of these ions. Thus, the PLTR measurements have confirmed the decrease in lifetime of the 4I13/2 level (of Er3+ ions), with the amplification in the overlapping of PL emissions. Finally, as a main result of this work, it was proved that the SNABP glass system, nanostructured with PbS QDs and doped with Er3+ ions, displays to be quite favorable to the radiative energy transfer process (from PbS QDs to Er3+ ions), as well as the occurrence of stimulated emission of 4I13/2 level. / O sistema vítreo SNABP [40SiO2.30Na2CO3.1Al2O3.25B2O3.PbO2 (mol%)] nanoestruturado com pontos quânticos (PQs) de PbS e/ou dopados com íons Er3+ foi sintetizado com sucesso pelo Método de Fusão, quando submetido a tratamentos térmicos apropriados. As temperaturas de transição vítrea (Tg) foram obtidas por Análise Térmica Diferencial (DTA), em que foi possível definir uma temperatura adequada para ser utilizada nos tratamentos térmicos das amostras sintetizadas. Como resultados desses tratamentos térmicos, a formação e crescimento dos PQs de PbS ocorreram no ambiente vítreo. As propriedades ópticas das amostras foram investigadas por Absorção Óptica (AO), Fotoluminescência (PL) e Fotoluminescência com Resolução Temporal (PLRT). Além disso, a Microscopia de Força Atômica (AFM) e Difratometria de Raios-X (DRX) foram empregadas no estudo das propriedades morfológicas e estruturais das amostras, respectivamente. As dispersões de tamanho dos PQs de PbS foram determinadas a partir dos espectros de AO. Com a utilização do Método e os dados de AO, foi também possível estimar os diâmetros médios dessas nanopartículas, que aumentaram com o aumento no tempo de tratamento térmico. As transições características dos íons Er3+ foram claramente identificadas nos espectros de AO. Em adição, foi observado que o aumento no tempo de tratamento das amostras provocou um aumento na sobreposição entre as bandas de PL dos PQs de PbS e dos íons Er3+, bem como com a absorção 4I15/2 -> 4I13/2 desses íons. Assim, as medidas de PLRT confirmaram o decréscimo no tempo de vida do nível 4I13/2 (dos íons Er3+) com o aumento dessa sobreposição das emissões de PL. Finalmente, como principal resultado deste trabalho, foi comprovado que o sistema vítreo SNABP nanoestruturado com PQs de PbS dopados com íons Er3+ mostrou-se bastante favorável ao processo de transferência de energia radiativa (dos PQs de PbS para os íons Er3+), bem como a ocorrência da emissão estimulada do nível 4I13/2. / Mestre em Física