Apresentamos neste trabalho uma técnica alternativa para a obtenção de filmes finos de silício amorfo hidrogenado (&#945-Si:H). Nós depositamos &#945-Si:H em um sistema de deposição que utiliza descarga luminescente a baixas freqüências (60Hz). Para tanto, nós projetamos todo o reator para que este objetivo pudesse ser atingido. Os filmes obtidos por nós mostram propriedades ópticas e eletrônicas bastante próximas aquelas dos filmes produzidos pela técnica convencional de descarga luminescente a radiofreqüência (13,56 MHz). A temperatura do substrato ótima para a técnica de descarga luminescente a baixas freqüências está na faixa 150-170&#176C, em torno de 100&#176C menor do que aquela usada para radiofreqüência. Neste trabalho nós apresentamos medidas das propriedades dos filmes, incluindo condutividade no escuro, fotocondutividade, comprimento de difusão ambipolar, absorção no infra-vermelho, gap óptico, e densidade de defeitos de níveis profundos. Para realizar parte destas medidas, nós construímos sistemas experimentais de caracterização exclusivos para o &#945-Si:H. / In this work we present an alternative technique for producing hydrogenated amourphous silicon thin films (&#945-Si:H). We deposited &#945-Si:H in a low-frequency (60 Hz) glow-discharge deposition system. For this purpose, we designed completely the reactor. The films we produced show electronic and optical properties nearly equivalent to those of films prepared by the conventional radio-frequency (13,56 MHz) glow-discharge technique. The optimal substrate temperature for the low-frequency glow-discharge technique is 150-170&#176C, about 100&#176C lower than that radio-frequency. In this work, we report measurements of film properties, including dark conductivity, photoconductivity, ambipolar diffusion lenght, infrared absorption, optical band gap, and deep defect density. To do these measurements, we assembled experimental systems used to characterize &#945-Si:H.

Submitted by Silvana Teresinha Dornelles Studzinski (sstudzinski) on 2015-10-19T12:41:55Z No. of bitstreams: 1 VIVIAN SEBBEN ADAMI_.pdf: 2714217 bytes, checksum: 0d285db93644d5612c6864a6baeacd87 (MD5) / Made available in DSpace on 2015-10-19T12:41:55Z (GMT). No. of bitstreams: 1 VIVIAN SEBBEN ADAMI_.pdf: 2714217 bytes, checksum: 0d285db93644d5612c6864a6baeacd87 (MD5) Previous issue date: 2015-05-29 / Nenhuma / Uma das prioridades da agenda sustentável mundial é a promoção do uso de fontes energéticas renováveis, como a energia eólica. O crescimento do uso dessa fonte desenvolveu uma indústria na Europa e nos Estados Unidos para atender à demanda desses mercados precursores. Na última década, o Brasil, juntamente com a China e a Índia, vem investindo na geração eólica, e destacando-se no cenário mundial. A implantação de parques eólicos acontece por meio de projetos interorganizacionais (PIOs), envolvendo atividades compartilhadas entre empresas com interesses e capacidades diferentes em um ambiente de riscos e incertezas. Esta pesquisa propõe-se a contribuir com a temática ainda nova dos PIOs da indústria eólica, tendo sustentação nos progressos recentes do conhecimento sobre redes e propriedades estruturais. O objetivo principal é compreender as implicações das propriedades estruturais das redes estabelecidas no contexto dos PIOs de geração de energia eólica para seu gerenciamento. Estudos recentes têm adotado a perspectiva de redes para a compreensão e o aprimoramento das relações nos PIOs. Nesses estudos, os PIOs são entendidos como uma rede de atores interdependentes que pode ser detalhada a partir de suas propriedades estruturais em vários níveis. O mapeamento da rede, sua análise e caracterização a partir de múltiplos níveis iluminam questões até então complexas para o desenvolvimento dos parques eólicos. A introdução do nível triádico permitiu novas visões aos aspectos estruturais e posicionais dos atores. As redes são um fenômeno relativamente recente, e o campo de estudos ainda se ressente com a falta de pesquisas que abordem as estruturas relacionais entre os atores. A utilização conjunta de abordagens quantitativas, com a adoção da Análise de Redes Sociais, e qualitativas, com a realização de entrevistas semiestruturadas, possibilitou incorporar diferentes (e complementares) perspectivas de análises ao fenômeno. Em termos acadêmicos, uma das contribuições da pesquisa foi o desenvolvimento de um conceitual teórico para as propriedades estruturais dos PIOs em vários níveis, reforçando o poder explicativo da estrutura frente ao dos atributos individuais dos atores. Outra contribuição foi a identificação de relações e papéis-chave para a estruturação de modelos de gestão mais efetivos. Relações informais, invisíveis nas redes de fornecimento e contratos, acontecem dentro de uma lógica cooperativa, e são importantes na coordenação do projeto. Em termos gerenciais, as contribuições centraram-se na compreensão da estrutura relacional dos PIOs, de seu funcionamento, e em aspectos relacionados às implicações das propriedades estruturais para seu gerenciamento. A composição e organização dos PIOs foi revelada assim como a importância da cooperação ou de uma estrutura relacional que estimule a cooperação entre os participantes do projeto. / One of the priorities of sustainable world agenda is promoting the use of renewable energy sources, like wind power. The growth of this source developed an industry in Europe and in the United States to supply the demand of these precursors markets. Recently (in the last decade), Brazil along with China and India has been investing in wind generation and emerging on the world scenario. The deployment of wind farms happens through inter-organizational projects (IOPs), involving shared activities among companies with different interests and capabilities in an environment of risks and uncertainties. This research aims to contribute to the still new topic of the wind industry IOPs, having support in recent progress of knowledge on networks and structural properties. Its main goal is to understand how the characteristics of networks established in the context of wind power generation IOPs affect the development of Brazilian initiatives. Recent studies have adopted social networking perspective for understanding and improving relations in IOPs. In these studies, IOPs are understood as a network of interdependent actors that can be detailed from their structural properties at various levels. Network mapping, analysis and characterization from multiple levels shed light to issues hitherto complex for the development of wind farms. The introduction of the triadic level, still little explored in studies of interorganizational networks, enables new visions to actors structural and positional aspects. The networks are a relatively new phenomenon, and the field of study still resents the lack of research that address the relational structures between the actors. The joint use of quantitative approaches, with the adoption of social network analysis, and qualitative, based on semi-structured interviews, made it possible to incorporate different (and complementary) analysis perspectives to the phenomenon. In academic terms, one of the research contributions was the development of a theoretical framework for the structural properties of IOPs on many levels, reinforcing the explanatory power of the structure over the individual attributes of the actors. Another result was the identification of relations and key roles for structuring more effective management models. Informal relations, invisible in supply and procurement networks, happen within a cooperative logic and are important in coordinating the project. In managerial terms, the contributions focused on understanding the relational structure of the IOPs, their operation, and issues related to the implications of the structural properties to its management. The composition and organization of IOPs was revealed as well as the importance of cooperation or of a relational structure that encourages cooperation between project participants.

Porous ceramic-metal composites, or cermets, such as nickel zirconia (Ni-YSZ), are widely used as the anode material in solid oxide fuel cells (SOFC). These materials need to enable electrochemical reactions and provide the mechanical support for the layered cell structure. Thus, for the anode supported planar cells, the thermomechanical behavior of the porous cermet directly affects the reliability of the cell. Porous cermets can be viewed as three-phase composites with a random heterogeneous microstructure. While random in nature, the effective properties and overall behavior of such composites can still be linked to specific stochastic functions that describe the microstructure. The main objective of this research was to develop the relationship between the thermomechanical behavior of porous cermets and their random microstructure. The research consists of three components. First, a stochastic reconstruction scheme was developed for the three-phase composite. From this multiple realizations with identical statistical descriptors were constructed for analysis. Secondly, a finite element model was implemented to obtain the effective properties of interest including thermal expansion coefficient, thermal conductivity, and elastic modulus. Lastly, nonlinear material behaviors were investigated, such as damage, plasticity, and creep behavior. It was shown that the computational model linked the statistical features of the microstructure to its overall properties and behavior. Such a predictive computational tool will enable the design of SOFCs with higher reliability and lower costs.

Τα άμορφα υλικά είναι μια ευρεία κατηγορία υλικών με σημαντικές ιδιότητες πολλές από τις οποίες δεν απαντώνται όταν αυτά βρίσκονται στην αντίστοιχη κρυσταλλική τους φάση. Στην παρούσα εργασία μελετώνται επιλεγμένα υμένια (πάχους ~ 1μm) από μια ειδική κατηγορία άμορφων υλικών, τις χαλκογονούχες ενώσεις (chalcogenides). Ως “χαλκογενή” (chlacogens) αναφέρονται τα στοιχεία της ομάδας VIA του περιοδικού πίνακα, δηλαδή το Θείο (S), το Σελήνιο (Se) και το Τελλούριο (Te) και συνεπώς οι ενώσεις που περιέχουν ένα ή περισσότερα από αυτά τα στοιχεία μαζί με στοιχεία όπως τα As, Ge, P, Bi, Si, Sb, Ga, Ag, κλπ. σχηματίζουν τις χαλκογονούχες ενώσεις. Το γεγονός ότι το ενεργειακό χάσμα των ενώσεων αυτό εμπίπτει στην φασματική περιοχή του ορατού φωτός και του κοντινού υπερύθρου έχει ως αποτέλεσμα την εμφάνιση πλήθους φωτο-επαγόμενων (μη-θερμικών) φαινομένων όταν τα υλικά αυτά ακτινοβοληθούν με φως κατάλληλου μήκους κύματος και πυκνότητας ισχύος. Τα φωτο-επαγόμενα φαινόμενα περιλαμβάνουν αλλαγές σε δομικές, μηχανικές, χημικές, οπτικές, ηλεκτρικές κ.α. ιδιότητες. Πιο συγκεκριμένα, μέσω της μελέτης των φωτο-επαγόμενων φαινομένων παρέχεται η δυνατότητα για ελεγχόμενη μεταβολή δομικών (μικροσκοπικών) αλλά και μακροσκοπικών ιδιοτήτων του υλικού. Επομένως τα υλικά αυτά έχουν έντονο τεχνολογικό ενδιαφέρον, σε εφαρμογές όπως στην οπτική, στην μικροηλεκτρονική και στην ανάπτυξη στοιχείων αποθήκευσης πληροφορίας (οπτικές μνήμες). Στόχος της παρούσας εργασίας είναι η μελέτη και η κατανόηση σε βασικό επίπεδο των μικρο-δομικών χαρακτηριστικών, υμενίων επιλεγμένων άμορφων χαλκογονούχων ενώσεων υπό την επίδραση διαφόρων εξωτερικών ερεθισμάτων καθώς και η επίτευξη συσχετισμού μεταξύ μικροσκοπικών χαρακτηριστικών και χρήσιμων για εφαρμογές μακροσκοπικών ιδιοτήτων. Πιο συγκεκριμένα, μελετήθηκε συστηματικά η επιφανειακή ηλεκτρονιακή δομή υμενίων του συστήματος AsxSe100-x, παρασκευασμένων με θερμική εναπόθεση (thermal evaporation, TE) και εναπόθεση με παλμικό laser (pulsed laser deposition, PLD) με επιφανειακά ευαίσθητες τεχνικές όπως Φασματοσκοπία Φωτοηλεκτρονίων από Ακτίνες-x (XPS) και από Υπεριώδες (UPS). H Φασματοσκοπία Φωτοηλεκτρονίων από Ακτίνες-x (XPS) χρησιμοποιείται για τον καθορισμό της χημικής σύστασης της επιφάνειας του στερεού. Η πολλαπλότητα των χημικών καταστάσεων για ένα συγκεκριμένο είδος ατόμου υποδηλώνει την ύπαρξη μιας ποικιλίας τοπικών ατομικών διατάξεων στην επιφάνεια του υμενίου. Επομένως οι αλλαγές των ηλεκτρονιακών ιδιοτήτων στην επιφάνεια μπορούν να συσχετιστούν άμεσα με αλλαγές που αφορούν στην επιφανειακή δομή, οι οποίες προκαλούνται είτε μεταβάλλοντας διάφορες παραμέτρους όπως η σύσταση του υλικού είτε με την επιβολή κάποιου εξωτερικού ερεθίσματος όπως η θέρμανση και η ακτινοβόληση, είτε με τη φωτο-διάλυση ατόμων μετάλλου (Ag) στο εσωτερικό τους. Μεταβάλλοντας την σύσταση σε PLD υμένια AsxSe100-x και υποβάλλοντας τα σε θέρμανση, σε θερμοκρασία 150ºC (δηλαδή λίγο πιο κάτω από το Τg) οι πιο έντονες αλλαγές παρατηρήθηκαν στο ηλεκτρονικό περιβάλλον των ατόμων αρσενικού στα υμένια με ενδιάμεσες συστάσεις (As50Se50, As60Se40). Στην συνέχεια, η συμμετρική σύσταση As50Se50 μελετήθηκε διεξοδικότερα λόγω της μεγάλης ποικιλομορφίας και ετερογένειας σε νανο-κλίμακα. Τα αποτελέσματα έδειξαν ότι η ακτινοβόληση και η θέρμανση οδηγούν την δομή σε δύο διαφορετικές άμορφες καταστάσεις με διαφορετικό ποσοστό δομικών μονάδων. Το φαινόμενο είναι αντιστρεπτό και επαναλήψιμο σε διαδοχικούς κύκλους θέρμανσης και ακτινοβόλησης για τα PLD υμένια ενώ δεν ισχύει το ίδιο για τα ΤΕ υμένια. Ο προσδιορισμός του δείκτη διάθλασης με την χρήση φασματοσκοπικής ελλειψομετρίας σε PLD και ΤΕ As50Se50 υμένια, σε διαδοχικές διεγέρσεις ακτινοβόλησης και θέρμανσης, αποκάλυψε την συσχέτιση των αλλαγών στη μικροδομή των υμενίων με τις μεταβολές σε αυτή την μακροσκοπική ιδιότητα του υμενίου. Επιπλέον, εκπονήθηκε μελέτη του φωτο-επαγόμενου φαινομένου της διάχυσης και διάλυσης ατόμων μετάλλου όπως ο Ag στην δομή των υμενίων PLD και ΤΕ As50Se50 με ακτινοβόληση ακτίνων- x και ορατού φωτός (laser ενέργειας συγκρίσιμης με το ενεργειακό χάσμα του ημιαγωγού). Σκοπός ήταν η μελέτη της εξέλιξης των σχηματιζόμενων χημικών ειδών κατά τα διάφορα στάδια του φαινομένου σε αντίθεση με την έως τώρα υπάρχουσα πρακτική που εστιάζει κυρίως στον μηχανισμό της κινητικής του φαινομένου. Μετρήσεις ανάλυσης σε βάθος με XPS και SIMS έλαβαν χώρα με σκοπό την διερεύνηση του προφίλ της συγκέντρωσης του μετάλλου στο εσωτερικό του υμενίου, πριν και μετά την επαγωγή του φαινομένου. / Amorphous, are a wide category of materials with significant properties that do not occur in their respective crystalline phase. In this work, a special category of selected amorphous chalcogenide compounds (chalcogenides) in the form of thin (1μm) films, is studied experimentally. Chalcogens are the elements from Group VIA, namely Sulfur (S), selenium (Se) and tellurium (Te) and therefore compounds containing one or more of these elements together with elements such as As, Ge, P, Bi, Si, Sb, Ga, Ag, etc. form chalcogenide compounds. The fact that their energy gap is within the range of visible light and near infrared has given rise to numerous of photo-induced (non-thermal) phenomena when these materials are irradiated with light of appropriate wavelength and power density. The photo-induced effects include changes in structural, mechanical, chemical, optical, electrical, etc. properties. More specifically, through the study of photo-induced effects it is possible to control micro-structural changes and macroscopic properties of the material. Therefore these materials have a strong technological interest for applications in optics, in microelectronics and as elements in circuits for optical data storage (optical memories). The aim of this work is to study and understand at a basic level the micro-structural characteristics of chalcogenide films of selected compounds under the influence of various external stimuli as well as to achieve a correlation between microscopic characteristics and useful for applications macroscopic properties. In the present work the electronic surface structure of AsxSe100-x films prepared by thermal evaporation (TE) and by pulsed laser deposition (PLD) was studied systematically with surface sensitive techniques such as X-ray and Ultraviolet Photoelectron Spectroscopies ( XPS, UPS). X-ray photoelectron spectroscopy is used to determine the chemical composition of the surface of a solid. The multiplicity of chemical states for a specific type of atom suggests the existence of a variety of local individual arrangements on the surface of the film. Therefore, the changes of electronic properties on the surface can be directly correlated with changes on the surface structure, which are caused either by altering various parameters such as the composition of the material or by imposing an external stimulus such as annealing and irradiation, or by photo-dissolution of silver atoms (Ag) in their structure. Changing the composition of PLD AsxSe100-x films and submitting them to annealing below the Tg, the most pronounced changes occurred in the electronic environment of atoms in films with intermediate compositions (As50Se50, As60Se40). The symmetrical composition As50Se50 was chosen and studied thoroughly because of the great diversity and heterogeneity of its micro-structural units in nano-scale. The results showed that irradiation and annealing lead the film to two different amorphous states, with different percentage of structural units. The phenomenon is reversible and repeatable in successive cycles of annealing and irradiation for the PLD films while this is not true for the TE films. The determination of the refractive index using spectroscopic ellipsometry in PLD and TE As50Se50 films, in successive irradiation and annealing stimuli, revealed the correlation of the changes in the microstructure of films with the changes in this macroscopic property. Furthermore, the photo-induced diffusion and dissolution of silver (Αg) atoms in the structure of PLD and TE As50Se50 films induced by x-rays and visible light (laser energy comparable to the energy gap of semiconductor) was studied. The purpose of these experiments was to follow the chemical species formed during the various stages of the diffusion procedure with XPS in contrast to most studies so far focusing mainly on the mechanism of kinetics of the diffusion reaction. Depth profile analysis by XPS and SIMS took place in order to investigate the concentration profile of the metal atoms in depth of the films, before and after the induction of the effect.

Le colmatage membranaire demeure l’un des verrous majeurs des bioréacteurs à membrane (BAM). Ces travaux visent à apporter une contribution à la compréhension de la dynamique de structuration de dépôts complexes formés lors de la filtration frontale de (bio)fluides et son impact sur les performances de filtration. Pour cela, différents outils permettant la mesure in-situ des propriétés structurelles locales des dépôts ont été utilisés de manière à pouvoir relier ces propriétés aux performances globales de filtration. Les propriétés électrocinétiques des dépôts ont également été étudiées. Deux méthodes, l’une optique et l’autre acoustique, ont été utilisées pour caractériser les propriétés structurelles locales des dépôts (épaisseur, cinétique de croissance). Dans un premier temps, les épaisseurs obtenues par les méthodes optiques et acoustiques ont été comparées. Il a été montré que, dans le cas de dépôts peu poreux, les deux méthodes conduisent aux mêmes épaisseurs de dépôts. Dans le cas de dépôts plus poreux, ces deux méthodes permettent l’obtention de données complémentaires à deux niveaux de profondeur différents au sein du dépôt. L’influence des conditions opératoires sur la structuration de dépôts de particules modèles a ensuite été étudiée. Grâce à la méthode optique il a été montré que, quelles que soient les conditions opératoires, il existe une variation temporelle des propriétés du dépôt. De plus, une répartition non homogène du dépôt à la surface de la membrane peut également exister, favorisée par de faibles répulsions entre particules. Enfin, l’applicabilité des différentes méthodes au cas des biofluides issus de BAM a été évaluée. La structure de dépôts complexes constitués lors de la filtration de ces biofluides a ensuite été étudiée. La compressibilité des dépôts, mise en évidence à l’échelle globale par suivi des performances de filtration, a également été observée à l’échelle locale : l’épaisseur du dépôt décroît avec une augmentation de la PTM. De plus, à l’échelle locale, une variabilité spatiale des propriétés du dépôt a été mise en évidence. Afin de contrôler la compressibilité des dépôts l’impact de l’ajout de particules au sein du biofluide a été évalué / Membrane fouling is one of the major drawbacks of membrane bioreactors. This study is thus a contribution to the understanding of the dynamic structuring of complex deposits build-up during dead-end filtration of complex biofluids. Several tools were used to perform in-situ characterization of cake layer structural properties at local scale and to link them to global filtration performances. The electrokinetics properties of the deposit were also investigated. An optical and an acoustic method were used to measure local cake thickness and growth kinetics. The thicknesses given by the two methods were first compared. It was shown that for compact deposits the two different methods lead to the same thickness of the deposit. For more porous deposits, these two methods allow to obtain complementary data at two different depths of the deposits. The influence of operating conditions on cake layer structuring during filtration of modelled particles was evaluated. Using the optical method it was shown that, whatever the operating conditions, the cake structure is not constant in time. Furthermore, a spatial variability of the cake layer thickness might exist especially in the case of weak particle-particle repulsion. Finally, it has been investigated whether the different methods could be applied or not to the case of biofluid filtration. The structural properties of the complex deposits built up during diluted activated sludge filtration were investigated. Cake compressibility was observed at global scale by a monitoring of process performances and was also observed at local scale: cake thickness decreases as TMP increases. Furthermore, at local scale, a spatial variability of deposit structure was found. Particle addition into the biofluid was assessed in order to mitigate compressibility effects

Apresentamos neste trabalho uma técnica alternativa para a obtenção de filmes finos de silício amorfo hidrogenado (&#945-Si:H). Nós depositamos &#945-Si:H em um sistema de deposição que utiliza descarga luminescente a baixas freqüências (60Hz). Para tanto, nós projetamos todo o reator para que este objetivo pudesse ser atingido. Os filmes obtidos por nós mostram propriedades ópticas e eletrônicas bastante próximas aquelas dos filmes produzidos pela técnica convencional de descarga luminescente a radiofreqüência (13,56 MHz). A temperatura do substrato ótima para a técnica de descarga luminescente a baixas freqüências está na faixa 150-170&#176C, em torno de 100&#176C menor do que aquela usada para radiofreqüência. Neste trabalho nós apresentamos medidas das propriedades dos filmes, incluindo condutividade no escuro, fotocondutividade, comprimento de difusão ambipolar, absorção no infra-vermelho, gap óptico, e densidade de defeitos de níveis profundos. Para realizar parte destas medidas, nós construímos sistemas experimentais de caracterização exclusivos para o &#945-Si:H. / In this work we present an alternative technique for producing hydrogenated amourphous silicon thin films (&#945-Si:H). We deposited &#945-Si:H in a low-frequency (60 Hz) glow-discharge deposition system. For this purpose, we designed completely the reactor. The films we produced show electronic and optical properties nearly equivalent to those of films prepared by the conventional radio-frequency (13,56 MHz) glow-discharge technique. The optimal substrate temperature for the low-frequency glow-discharge technique is 150-170&#176C, about 100&#176C lower than that radio-frequency. In this work, we report measurements of film properties, including dark conductivity, photoconductivity, ambipolar diffusion lenght, infrared absorption, optical band gap, and deep defect density. To do these measurements, we assembled experimental systems used to characterize &#945-Si:H.

Made available in DSpace on 2016-08-18T18:19:27Z (GMT). No. of bitstreams: 1 Edson Nunes Costa Paura.pdf: 4371864 bytes, checksum: eb38ecee742cbbf15a438597d2ab23de (MD5) Previous issue date: 2010-03-10 / In this work the structural and energetic properties of C60@(17,0) carbon peapods bundle were studied under hydrostatic pressure. To study such properties we use ab initio quantum calculations based on the density functional theory in the approach of the generalized gradient approximation. All simulations were performed using the SIESTA code. The results indicate that the bundles in the study present two phase transitions, the first transitions occurring in range 2.0 GPa - 3.0 GPa and the second occurring around 15.0 GPa, in good agreement with the literature. The analysis of electronics properties through energy bands indicates that the ( it Peapods) from P < 3.0 GPa are semiconductor with visible reduction in the energy gap, while above 3.0 GPa the systems become metallic. Also the energy levels of the band are changed according to the hydrostatic pressure is being applied, such as breaking the degeneracy of the levels of valence bands and conduction bands. To each interval of pressure applied we also calculated the relative volume percentual and cohesive energy as a function of pressure. / Neste trabalho foram estudadas as propriedades estruturais, energéticas e eletrônicas de bundles de peapods de carbono C60@(17,0) submetidos à variação de pressão hidrostática. Para estudar tais propriedades, utilizamos cálculos quânticos de primeiros princípios baseados na teoria do funcional da densidade com a aproximação do gradiente generalizado. Todas as simulações foram realizadas com a utilização do código SIESTA. Os resultados encontrados indicam que os bundles em estudo sofrem duas transiçõesde fase, a primeira ocorrendo no intervalo de pressão 2,0 - 3,0 GPa e a segunda ocorrendo entre 15 - 20.0 GPa, em perfeito acordo com a literatura.A análise das propriedades eletrônicas via estrutura de bandas de energia, indica que os peapods para P < 3,0 GPa são semicondutor com visível diminuição do gap de energia, enquanto que acima de 3,0 GPa os sistemas passam a ser metálico. Também os níveis de energia da banda são alterados conforme a pressão hidrostática está sendo aplicada, como por exemplo, a quebra na degenerecencia dos níveis das bandas de valência e condução. Para cada intervalo de pressão aplicada calculamos o percentual do volume relativo e a energia coesiva do sistema.

Made available in DSpace on 2016-08-18T18:19:29Z (GMT). No. of bitstreams: 1 Samir Silva.pdf: 2648726 bytes, checksum: c089ee5506a5e19afed5dafba9cbc09f (MD5) Previous issue date: 2007-08-21 / In this work were studied the electronic, vibrations and structural properties of boron nitride nanotube bundles (16,0), (12,0) and (8,0), when submitted to the hydrostatic pressure changes. To study such properties, we used the ab initio method employing density functional theory in the approach of the generalized gradient approximation. All the simulations were performed using of the siesta code. The obtained results showed that the circular cross section of each one of the studied bundles undergoes a deformation (elliptic) when applied pressure reaches the value of P > 1,0 GPa for the bundle (16,0), P > 2,0 GPa for the bundle (12,0) and for values greater then 6,5 GPa for the bundle (8,0). For each pressure interval it was calculated the percent relative volume, cohesive energy, modes of vibrations and the band structures. The analysis of the electronic properties, through the band structures, indicates that the characteristics semiconducting of boron nitride nanotubes is preserved during the pressure increasing. / Neste trabalho foram estudadas as propriedades eletrõnicas, vibracionais e estruturais de bundles de nanotubos de nitreto de boro (16,0), (12,0) e (8,0) submetidos à variação de pressão hidrostástica. Para estudar tais propriedades, utilizamos o método ab initio com uso da teoria do funcional da densidade na aproximação do gradiente generalizado. Todas simulações foram realizadas com a utilização do código siesta. Com os resultados encontrados foi observado que a seção transversal circular de cada um dos bundles estudados sofre uma deformação (elíptica) quando a pressão aplicada atinge o valor de P > 1,0 GPa para o bundles (16,0), P > 2,0 GPa para o bundles (12,0) e P > 6,5 GPa para os bundles (8,0). Para cada intervalo de pressão aplicada calculamos o percentual do volume relativo, energia coesiva, modos vibracionais e a estrutura de bandas. A análise das propriedades eletrônicas, através da estrutura de bandas, indica que as características semicondutoras dos nanotubos de nitreto de boro são preservadas durante o aumento da pressão.

La stratégie la plus utilisée aujourd'hui pour la conception de nouveaux matériaux luminescents est basée sur des méthodes d'essais-erreurs. Cependant, ces méthodes peuvent souvent entraîner une consommation d'argent et de temps. En ce sens, un modèle théorique agissant comme un outil prédictif peut servir comme une stratégie alternative. De tels modèles sont également étudiés et utilisés par des scientifiques du monde entier, mais ils sont pour la plupart difficiles à utiliser. Dans ce travail, deux modèles semi-empiriques conviviaux et faciles à utiliser ont été proposés pour la conception de luminophores intégrés dans le développement de technologies importantes, en particulier dans les éclairages à base de LED et les cellules solaires. Ces modèles sont: 1) le modèle de facteur environnemental (EF) basé sur la théorie diélectrique de la liaison chimique proposée par Philips et 2) le modèle de transfert de charge métal-métal (MMCT) utilisées pour trouver des relations entre les propriétés structurelles d'un matériau et sa luminescence. Le modèle EF a été appliqué à la famille des composés AIBIIPO4 (AI = cation monovalent, BII = cation divalent) dopés à Eu2+. Il était capable d'estimer l'énergie du bord d'excitation et d'identifier les sites de dopage à ± 1000 cm-1. Il peut donc être utilisé pour la conception de nouveaux luminophores appartenant à cette famille. Dans la deuxième partie, les deux modèles ont été utilisés pour identifier la nature de la luminescence dans les oxydes dopés à Bi3+. A cet effet, une méthode combinant les modèles motionnés avec les valeurs de Stokes shift a été trouvé fiable. En outre, la a été jugée utile pour expliquer la modification des propriétés luminescentes de YVO4:Bi3+ sous haute pression. Dans la troisième partie, les deux modèles ont été utilisés pour explorer la luminescence des oxydes dopés à Sb3+ par analogie avec Bi3+. Cependant, les résultats n'étaient pas assez bons pour identifier la nature de la luminescence dans ces matériaux. La raison pourrait être liée à la position décentrée du dopant (Sb3+). / The most used strategy for the design of new luminescent materials today is based on trial-error methods. However, these methods may often result in consummation of money and time. In this sense, a theoretical model acting as a predictive tool can serve as an alternative strategy. Such models are also studied and used by scientists around the world but they are mostly difficult to use. In this work, two friendly and easy to use semi-empirical theoretical models were proposed as a criterion for designing phosphors integrated in the development of important technologies especially in LED-based lightings and solar cells. These models are: 1) the environmental factor model (EF) based on the dielectric theory of chemical bonding proposed by Philips and 2) the metal to metal charge transfer (MMCT) model by Boutinaud that were both used for the purpose of finding relationships between the structural properties of a material and its luminescence. The EF model was applied on the family of AIBIIPO4 (AI= monovalent cation, BII= divalent cation) compounds doped with Eu2+. It was able to estimate the excitation edge energy and to identify the doping sites within uncertainty of ± 1000 cm-1. It can therefore, be used for the design of new phosphors belonging to this family. In a second part, both models were used to identify the nature of luminescence in Bi3+-doped oxides. For this purpose, a method combining the mentioned models along with the values of the Stokes shift was found reliable. In addition, the method was found useful to explain the change in luminescent properties of YVO4:Bi3+ under high pressure. In the third part, the two models were used to explore the luminescence of Sb3+-doped oxides by analogy with Bi3+. However, the results were not good enough to identify the nature of luminescence in these materials. The reason could be related to the off-centered position of the dopant (Sb3+).

Graphene is a single layer of carbon atoms arranged in honeycomb lattice. Over a long period of time it was treated as a hypothetical material to understand the properties of other allotropes of carbon, such as graphite, carbon nanotube etc. Half decade back, a single layer of graphene was finally isolated and since then the field has observed a flurry of activities. Low energy excitations in graphene are massless Dirac Fermions and quantum electrodynamic effects can be observed at room temperature in graphene, which makes it very popular among the condensed matter community. In addition graphene also shows many interesting mesoscopic effects, which is the focus of the present work. We study the electronic, magnetic and structural properties of the graphene nanostructures. The entire thesis based on the results and findings obtained from the present investigation is organized as follows. Chapter 1: provides a general introduction to the properties of graphene and graphene based nanostructures. Chapter2:describes the theoretical tools used in this thesis to investigate the properties of graphene nanoribbons. The first two chapters are meant to give the reader an overview about the field of graphene and a few of the computational techniques commonly used to investigate the properties of graphene. The following chapters are the new findings reported in this thesis. Chapter3:shows how zigzag graphene nanoribbons respond in a non-linear fashion when edges are subjected to some external potential such as magnetic field. Such response originates from the edge states present in zigzag ribbons and thus not observed in armchair nanoribbons. In the limit of ribbon width W→∞, an edge magnetic field produces a moment of ~ 1/3 per edge atom even for an infinitesimally small field, which is clearly a signature of non-linear response. Response of a finite width nanoribbon is size dependent and also depends on ln(V), the applied field. This is akin to Weber-Fechner law of audio visual perceptions. It is interesting to note that nature does provide a “quantum realization” of this in the form of biological sensing organs like the ear and eye. The magnetic response is found to scale inversely with the ribbon width. Chapter4:deals with the magnetic properties of the zigzag graphene nanoribbon. This is also a special property of the geometry of the zigzag edges and not observed in armchair nanoribbons. Our investigation reveals that the electron-electron repulsion (Hubbard U) energy creates a delta function like edge magnetic field in zigzag graphene nanoribbons. Starting from this, magnetic properties of zigzag graphene nanoribbons can be qualitatively and quantitatively explained from the non-linear response of zigzag nanoribbons. Zigzag graphene nanoribbons can exist in two possible ‘magnetic states’: antiferro (AF) where the two opposite edges have antiparallel magnetic moment and ferro (FM) where moment is parallel in the two opposite edges. First we describe the properties of undoped zigzag nanoribbons. They have AF ground state. Continuum theory can explain the size dependent bandgap and magnetic moment of the ground state. We present the first explicit derivation of the gap. Then we show that hole doping can change the ground state to FM, which is metallic. Thus the system has the property of magnetoresistance, which can be exploited by doping zigzag graphene nanoribbons externally with some gate voltage or internally by some electron acceptor element, such as boron. The critical doping for transition depends inversely with the ribbon width. We have found that the ‘phase transition’ on hole doping is a common phenomena for zigzag terminated nanostructures, such as hexagonal nanodots. Chapter5:discusses the effects of random edge shapes and random potential (Anderson disorder) on the magnetic properties of zigzag graphene nanostructures. Defects and disorders in the form of edge shape randomness and random potentials arising from substrate are very common in graphene. Our study reveals that edge state magnetism is very robust to shape randomness of the terminating edges of nanostructures; as long as there are three to four repeat units of a zigzag edge, the edge state magnetism is preserved. We also discover some “high energy” edges (ones where the edge atoms have only one nearest neighbor) can have very large moments compared to even the zigzag edges. Edge magnetism is also found to be robust to relatively small Anderson disorders, because a slowly varying small potential does not scatter the edge states. Chapter6:reveals how edge functionalization by O atom and OHgroup changes the properties of the zigzag graphene nanoribbons. Functionalization by various different molecules is a very popular method of tuning the properties of graphene. We have shown that it is possible to tune the properties of zigzag graphene nanoribbons by edge functionalization. Further, we have found that structures with clustered functionalization leads to “spatially” varying electronic structure, which can lead to interesting possibilities for electronic devices. Chapter7:describes structural stability, electronic and magnetic properties of graphene nanoribbons in presence of topological defects such as Stone-Wales defects. Our study reveals that the sign of stress induced by a SW defect in a graphene nanoribbon depends on the orientation of the SW defect with respect to the ribbon edge and the relaxation of the structure to relieve this stress determines its stability. Local warping or wrinkles arise in graphene nanoribbon when the stress is compressive, while the structure remains planar otherwise. The specific consequences to armchair and zigzag graphene nanoribbon can be understood from the anisotropy of the stress induced by a SW defect embedded in bulk graphene. We also have found localized electronic states near the SW defect sites in a nanoribbon. However, warping results in delocalization of electrons in the defect states. We have observed that, in zigzag graphene nanoribbons magnetic ordering weakens due to the presence of SW defects at the edges and the ground state is driven towards that of a nonmagnetic metal.