Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / This work presents a theoretical study about the interaction between semiconducting carbon nanotubes (8,0) single-wall (SWNT), with selenium nanostructures by first principles calculations using the density functional theory. To simulate the adsorption of selenium atoms on SWNTs are done with three initial configurations with the selenium atoms in positions (i) BC (C-C bond); (ii) HC (hexagonal center site) and (iii) TOP (above the carbon atom). For these configurations we analyze the adsorption of atomic, dimmer and wire selenium on SWNTs and for some structures it was observed a physical adsorption (weak interaction) while for others a chemical adsorption (strong interaction) is observed depending on how these structures were functionalized. In the configuration on which a physical interaction is achieved, no considerable changes on the electronics properties are seen, as a consequence of the absence of hybridization between the SWNTs and selenium orbitals. Never the less, in the arrangements with chemical interactions significant alterations are verified on the electronic properties, in other words, a relevant hybridization of the selenium levels and the tube band near to the Fermi energy is noticed. However, analyzing the structural properties, we also observed changes on the final distances of selenium atoms, dimmer and wires when compared with the initials distances. In that way, experimental literature works had demonstrated the importance of a theoretical understanding of structures and electronics properties of selenium interacting with SWNTs, making relevant the connection between the theoretical results and the experiments. / Nesta dissertação apresentamos o estudo teórico da interação de nanotubos de carbono semicondutores (8,0) de paredes simples (SWNT) com nanoestruturas de selênio utilizando cálculos de primeiros princípios fazendo uso da teoria do funcional da densidade. Para a adsorção dos átomos de selênio nos SWNTs escolhemos três possíveis configurações iniciais com os átomos de selênio nas posições (i) BC (centro da ligação); (ii) HC (centro do hexágono) e (iii) TOP (em cima do átomo de carbono). Para essas configurações analisamos a adsorção de selênio atômico, dímeros e fios nos nanotubos, sendo que em algumas estruturas tivemos uma adsorção física (interação fraca), enquanto que em outras tivemos uma adsorção química (interação forte), dependendo de como essas estruturas foram funcionalizadas. Nas configurações que tiveram interações físicas, não observamos mudanças significativas nas propriedades eletrônicas não ocorrendo hibridização entre os orbitais do nanotubo e dos átomos de selênio. Entretanto, nas configurações que tiveram interações químicas observamos alterações significativas nas propriedades eletrônicas, ou seja, nas bandas de energia percebemos uma hibridização relevante dos níveis de selênio e do tubo próximos ao nível de Fermi. Entretanto, na análise das propriedades estruturais, também observamos mudanças nas distâncias finais dos selênios atômicos, dímeros e fios quando comparadas com as distâncias iniciais considerando o selênio mais próximo ao carbono do SWNT. Dessa forma, trabalhos experimentais da literatura têm demonstrado a importância de um entendimento teórico das propriedades estruturais e eletrônicas de Se interagindo com SWNTs, o que nos leva a querer conectar resultados teóricos com experimentais.

Le niobate de lithium (LN) présente des propriétés piézo-électrique, électro-optique, optique non linéaire et photo-réfractive, qui sont riches d'applications matures (filtres acoustiques de surface, modulateurs EO, convertisseurs de fréquence) ou potentielles (mémoires holographiques, guide d'onde photo-inscrits). En utilisant la théorie de la fonctionnelle de la densité (DFT) couplée à la méthode des ondes planes augmentées linéarisées à potentiel total (FP-LAPW) implémentée dans le code WIEN2k_12, nous avons pu investiguer - avec plus de détails - les propriétés électroniques du LN stoechiométrique (SLN) tel que la structure de bande, les densités d'états, la densité de charges électroniques, les masses effectives, les intégrales du saut Nb-Nb et O-O, ainsi que les différentes propriétés optiques déduites de la fonction diélectrique à savoir l'indice de réfraction, le coefficient d'extinction, le coefficient d'absorption... Au cours de notre étude, nous avons essayé aussi d'analyser et d'interpréter, les désaccords qui apparaissent dans les résultats de calculs ab initio d'un auteur à un autre. Ces désaccords imputables aux méthodes de calcul employées sont parfois déjà très sensibles pour le matériau SLN exempt de défauts. La valeur expérimentale de l'énergie de gap (3.78 eV) correspondante à LN quasi stoechiométrique (NSLN) - fréquemment cité dans la littérature - fut suggérée plus grandes pour le SLN d'après des études ab initio récentes. Pour cette raison, nous avons tenté de la corriger en utilisant une nouvelle approche (en se référant à la valeur d'indice de réfraction expérimental du SLN). Ainsi, nous nous proposons une nouvelle valeur théorique Eg pour le LN pur. A cet effet, une correction de certains pics d'énergie des états excités des spectres optiques a été mise en évidence / Lithium niobate (LN) has piezoelectric, electro-optics, nonlinear optical and photorefractive properties, which are rich in mature (surface acoustic filters, EO modulators, frequency converters) or potential (holographic memories, photo-induced waveguide) applications. Using the density functional theory (DFT) combined with the full potential linearized augmented plane waves (FP- LAPW) method implemented in the WIEN2k_12 code, we investigated - with more details - the electronic properties of stoichiometric LN (SLN) such as the band structure, the density of states, the charge density, the effective masses, the hopping integral for Nb-Nb and O-O, as well as different optical properties derived from the dielectric function like the refractive index, the extinction index, the absorption coefficient ...etc. In our study, we also tried to analyze and interpret some disagreements that appear in the results of ab initio calculations of an author to another. These disagreements due to the methods of calculation employed may be very sensitive for the SLN material without any defects. The experimental value of the energy band gap (3.78 eV) corresponding to LN nearly stoichiometric (NSLN) - frequently cited in the literature - was suggested to be largest for SLN according to recent ab inito studies. For this reason, we have tried to correct it by using a new approach (referring to the value of the experimental refractive index of SLN). Thus, we propose a new theoretical value Eg for pure LN and we bring correction of some energy's peaks of excited states of the optical spectra