Orientador: Prof. Dr. Gustavo Martini Dalpian / Tese (doutorado) - Universidade Federal do ABC, Programa de Pós-Graduação em Nanociências e Materiais Avançados, 2015. / A propriedade de chaveamento da resitência ou memoristiva é a habilidade de um material de alterar seu estado de resistência elétrica devido a um campo elétrico. O memoristor é um dispositivo de dois terminais com tal propriedade capaz de armazenar informação através de sua resistência, constituído de uma estrutura metal/isolante/metal. Este dispositivo pode revolucionar a indústria de memórias por apresentar tempos de chaveamento rápidos e de retenção longos, assim como altas densidades. Entretanto, seu princípio de funcionamento não é totalmente entendido a nível atômico, logo sua aplicação é impedida. Dois mecanismos são propostos: o mecanismo de difusão-deriva de íons afirma que campos elétricos e gradientes de temperatura formam e dissolvem canais condutores, alterando a resistividade. Por outro lado, modelos eletrônicos consideram o aprisionamento e liberação de cargas como causa da mudança da resistividade. Neste trabalho utilizamos uma abordagem heurística¿cálculos de teoria do funcional da densidade e soluções numéricas¿para entender os processos ocorrendo em escala atômica no interior de dispositivos baseados em TiO2. Os resultados mostram que a dificência em oxigênio neste caso leva à formação de fases TinO2n..1 que apresentam uma banda intermediária, a qual pode se tornar carregada quando propriamente interfaceada. A resolução numérica da equação de Poisson apresenta múltiplas soluções relacionadas a diferentes estados de resistência, estas soluções são usadas em um código de transmissão que fornece curvas teóricas i X V para o memoristor. / The resistive switching or memristive property is the ability of a material to change its electrical resistance due to the application of an electric field. The memristor is a two-terminal device with this property that is capable of storing information as its resistance state, being architectured in a metal/insulator/metal stacking. This device can revolutionize the memory industry by providing fast switching and large retention times as well as high-density capabilities. However, its working principle is not completely understood at an atomic level, thus its application as next-generation resistive memories is hindered. Two mechanisms are proposed: ion drift mechanisms claim that the electric field and temperature gradients inside the device can form and dissolve a conducting filament, changing the electrical resistivity. On the other hand, electronic models consider charge trapping and de-trapping inside the insulator layer as the cause of the resistivity change. In this work we use a heuristic computational approach¿density functional theory calculations and other numerical solutions¿to understand the processes developing at the atomic scale inside TiO2-based devices. Our results show that the oxygen deficiency in this material leads to the formation of a series of phases TinO2n..1 that present an intermediate band which can become charged when properly interfaced. The self-consistent-numerical solver of the Poisson equation shows multiple solutions that are related to the resistance states, and finally the potential is used in a transmission code that results in theoretical i X V curves for the memristor.

L’étude du phénomène d’adsorption peut être réalisée théoriquement dans le cadre de la physique statistique, à l’échelle microscopique, en mettant en jeu une interface entre un fluide et un solide. L’objectif de cette thèse est de proposer une modélisation moléculaire de fluides tels que le méthane, confinés dans des matériaux poreux de géométrie quelconque. Le cadre théorique est ainsi directement développé à l’échelle microscopique et ses résultats sont confrontés à ceux obtenus avec des simulations moléculaires. À l’échelle macroscopique, le formalisme théorique nous permet de retrouver des résultats expérimentaux tels que des isothermes et des chaleurs d’adsorption.Tout d’abord, nous présentons les principaux résultats de la théorie de la fonctionnelle de la densité classique (cDFT), qui permet de formuler les lois de la physique statistique à partir de la densité du fluide en chaque point de l’espace. Cette théorie permet de décrire des fluides inhomogènes, c’est-à-dire des fluides pour lesquels la densité n’est pas constante en tout point de l’espace. Nous devons également considérer une équation d’état moléculaire de référence. Nous choisissons pour cela la théorie statistique des fluides associatifs (SAFT), formulée à partir de l’énergie libre du système. Le potentiel d’interaction d’une molécule telle que le méthane est alors modélisé comme celui d’une sphère dure entourée d’une couronne attractive. Nous décrivons la sphère dure à l’aide de la théorie de la mesure fondamentale (FMT), qui utilise des densités pondérées, c’est-à-dire des fonctions exprimées en un point de l’espace, mais qui dépendent du voisinage immédiat de ce point. Les fonctions pondérées sont nécessaires pour modéliser les fluides inhomogènes confinés à l’échelle microscopique. L’étude menée à partir de la FMT nous a conduit à définir de nouvelles fonctions pondérées, permettant de décrire des fluides de sphères attractives.Dans ce nouveau cadre théorique, il est nécessaire d’utiliser des approximations dans l’écriture de la fonctionnelle d’énergie libre. Nous proposons quatre approches, avec lesquelles nous pouvons prédire la distribution de la densité du fluide dans l’espace. Ces profils étant décrits à l’échelle microscopique, nous avons réalisé des simulations moléculaires de type Monte Carlo pour en évaluer la qualité par comparaison, pour des systèmes définis à potentiel équivalent. Nous retenons alors une des nouvelles formulations décrivant le fluide inhomogène. Puis, nous nous intéressons à la modélisation du solide. De nombreuses approches utilisent des expressions analytiques des densités pondérées, ce qui ne permet d’étudier que des milieux poreux à géométrie simple et idéale. Dans le cadre de cette thèse, nous écartons ce type d’approche et nous proposons de calculer les densités pondérées à l’aide de transformées de Fourier rapides dans un espace à trois dimensions, pour une forme quelconque de pore. La conséquence numérique de cette approche est que l’on considère un espace de calcul discret. Cela demande alors d’utiliser des résultats mathématiques issus de la géométrie discrète, afin de décrire correctement les interactions entre le fluide et une surface solide discrète quelconque.Cette nouvelle combinaison entre la théorie de la fonctionnelle de la densité et la géométrie discrète permet notamment d’étudier l’adsorption de méthane dans des pores cylindriques de silice. Nous réalisons en même temps des mesures expérimentales avec ce système, en nous servant de nouveaux substrats de silice préalablement caractérisés. Nous comparons alors les isothermes et les chaleurs d’adsorption obtenues expérimentalement aux prédictions théoriques, ce qui valide l’ensemble du formalisme de l’échelle microscopique à l’échelle macroscopique, en nous servant de tous les nouveaux développements que nous présentons dans cette thèse, liés à la modélisation du fluide et à la modélisation du solide. / The study of adsorption, for systems presenting an interface between a fluid and a solid, can be undertaken theoretically with the statistical physics formalism, at the microscopic scale. The objective of this PhD thesis is to propose a molecular modelling of fluids like methane, confined into porous materials of various geometry. This way, the theoretical framework is directly developed at the microscopic scale and its results are compared with molecular simulations. At the macroscopic scale, the theoretical formalism leads us to obtain the same results than experimental measurements of isotherm and heat of adsorption.First of all, the main results of the classical density functional theory (cDFT) - which gives laws of statistical physics with the fluid density in every point of the space - are presented. Inhomogeneous fluids are thus described with this theory. A molecular equation-of-state has also to be considered as a reference. The statistical associating fluid theory (SAFT), formulated with the free energy of the system has been chosen. Then, the interaction potential of a molecule such as methane is described by a hard-sphere surrounded with an attractive range. The hard-sphere is described with the fundamental-measure theory (FMT), using weighted densities, corresponding to functions defined in a point of the space, but depending on the immediate neighbourhood of this point. Weighted functions are necessary for inhomogeneous fluids modelling confined at the microscopic scale. The study undertaken from the FMT led us to define new weighted functions, allowing us to describe fluids of attractive spheres.With this new theoretical framework, it is necessary to use approximations of the free energy functional. Four different approaches are proposed allowing to predict the spatial distribution of the fluid density. Because these profiles are described at the microscopic scale, Monte Carlo molecular simulations have been performed in order to evaluate their quality by comparison, for systems defined with an equivalent potential. Thereby, one of the new formulations describing the inhomogeneous fluid has been selected for its superiority among the others. Then, special attention has been given to the modelling of the solid. Indeed, most of the existing approaches use analytical expressions of weighted densities to that extent, which limits studies to porous media with simple and ideal geometries. In our work, we exclude this kind of approaches and we propose to compute weighted densities with fast Fourier transforms in a three-dimensional space, for any pore geometry. The consequence of this approach is that a numerical discrete space is considered. This implies the use of mathematical results from discrete geometry, in order to correctly compute interactions between the fluid and any discrete solid surface.This new combination of the density functional theory and discrete geometry has allowed us to study methane adsorption into cylindrical pores of silica. To do so, experimental measurements have been performed on new silica substrates specially synthetized and characterised for this thesis. Theoretical predictions were compared with experimental isotherms and heat of adsorption. It allowed to validate the whole formalism presented in this thesis and developed both for the fluid and the solid modelling from the microscopic to the macroscopic scale.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / This work presents a theoretical study about the interaction between pristine or functionalized B3 and C vitamins with semiconducting carbon nanotubes (8,0) through first principles simulations. We evaluate the structural and electronic properties of the B3 and C vitamins adsorbed on nanotubes, using the density functional theory with the SIESTA computational code. We observe that there is a strong dependence of the interaction between B3 and C vitamins with the carbon nanotubes subject to the modification of the original structure (functionalization) of these vitamins. For the configurations which the pristine vitamins are adsorbed on the nanotubes, the interaction occurs by physical adsorption. On the other hand, we verify that in the case of functionalized vitamins, the adsorption occurs by a chemical interaction with the nanotube. In the case of vitamins interacting by physical adsorption, no significant changes in the electronics properties are observed. However, for the chemical adsorption of the vitamins on the tube, we notice that the changes in the electronic properties are dependent of how the molecules are functionalized, being able to create half-filled levels in the Fermi energy region, or even though to restore the electronic structure of the semiconducting carbon nanotube. These results show that carbon nanotubes can be considered chemical agents for attachment or selectivity for organic molecules, as in the case of the B3 and C vitamins. / Este trabalho apresenta um estudo teórico da interação das vitaminas B3 e C puras ou funcionalizadas com nanotubos de carbono semicondutores (8,0) via simulações de primeiros princípios. Avaliamos as propriedades estruturais e eletrônicas das vitaminas B3 e C adsorvidas nos nanotubos, fazendo-se uso da teoria do funcional da densidade por meio do código computacional SIESTA. Observamos que existe uma forte dependência da interação das vitaminas B3 e C com os nanotubos de carbono condicionada a modificação da estrutura original (funcionalização) dessas vitaminas. Para as configurações em que as vitaminas puras são adsorvidas nos nanotubos, a interação ocorre via uma adsorção física. Por outro lado, observamos que no caso das vitaminas funcionalizadas, a adsorção se dá via uma interação química com o nanotubo. No caso das vitaminas interagindo via adsorção física, não observamos mudanças expressivas nas propriedades eletrônicas. Entretanto, para a adsorção química das vitaminas no tubo, notamos que as alterações nas propriedades eletrônicas são dependentes da forma como as moléculas são funcionalizadas, podendo gerar níveis semi-preenchidos na região da energia de Fermi, ou até mesmo recuperar a estrutura eletrônica do nanotubo semicondutor. Estes resultados mostram que os nanotubos de carbono podem ser considerados agentes químicos para ancoradouro ou seletividade de moléculas orgânicas, como no caso das vitaminas B3 e C.

L'intérêt de la recherche fondamentale pour les morceaux nanométriques de métaux nobles est principalement dû à la résonance localisée des plasmons de surface (LSPR) dans l'absorption optique. Différents aspects, liés à la compréhension théorique de la LSPR dans le cas de clusters de métaux nobles de taille dite intermédiaire, sont étudiés dans ce manuscrit. Afin d'avoir une vision plus large nous utilisons deux approches : l'approche électromagnétique classique et le formalisme ab initio en temps réel de la théorie de la fonctionnelle de la densité dépendant du temps (RT-TDDFT). Une comparaison systématique et détaillée de ces deux approches souligne et quantifie les limitations de l'approche électromagnétique lorsqu'elle est appliquée à des systèmes de taille quantique. Les différences entre les excitations plasmoniques collectives et celles impliquant les électrons d, ainsi que leurs interactions, sont étudiées grâce au comportement spatial des densités correspondantes. Ces densités sont obtenues en appliquant une transformée de Fourier dans l'espace à la densité obtenue par les simulations DFT utilisant une perturbation delta-kick. Dans ce manuscrit, des clusters de métaux nobles nus et protégés par des ligands sont étudiés. En particulier, motivé par de récents travaux sur les phénomènes d'émergence de plasmon, l'étude par TD-DFT de nano-alliages Au-Cu de taille tout juste inférieure à 2nm à fourni de subtiles connaissances sur les effets d'alliages sur la réponse optique de tels systèmes. / The fundamental research interest in nanometric pieces of noble metals is mainly due to the localized surface-plasmon resonance (LSPR) in the optical absorption. Different aspects related to the theoretical understanding of LSPRs in `intermediate-size' noble-metal clusters are studied in this thesis. To gain a broader perspective both the real-time \ai formalism of \td density-functional theory (RT-TDDFT) and the classical electromagnetics approach are employed. A systematic and detailed comparison of these two approaches highlights and quantifies the limitations of the electromagnetics approach when applied to quantum-sized systems. The differences between collective plasmonic excitations and the excitations involving $d$-electrons, as well as the interplay between them are explored in the spatial behaviour of the corresponding induced densities by performing the spatially resolved Fourier transform of the time-dependent induced density obtained from a RT-TDDFT simulation using a $\delta$-kick perturbation. In this thesis, both bare and ligand-protected noble-metal clusters were studied. In particular, motivated by recent experiments on plasmon emergence phenomena, the TDDFT study of Au-Cu nanoalloys in the size range just below 2~nm produced subtle insights into the general effects of alloying on the optical response of these systems.

abstract: Nanoscale semiconductors with their unique properties and potential applications have been a focus of extensive research in recent years. There are many ways in which semiconductors change the world with computers, cell phones, and solar panels, and nanoscale semiconductors having a promising potential to expand the efficiency, reduce the cost, and improve the flexibility and durability of their design. In this study, theoretical quantum mechanical simulations were performed on several different nanoscale semiconductor materials, including graphene/phosphorene nanoribbons and group III-V nanowires. First principles density functional theory (DFT) was used to study the electronic and structural properties of these nanomaterials in their fully relaxed and strained states. The electronic band gap, effective masses of charge carriers, electronic orbitals, and density of states were most commonly examined with strain, both from intrinsic and external sources. For example, armchair graphene nanoribbons (AGNR) were found to have unprecedented band gap-strain dependence. Phosphorene nanoribbons (PNRs) demonstrate a different behavior, including a chemical scissors effect, and studies revealed a strong relationship between passivation species and band gap tunability. Unlike the super mechanical flexibility of AGNRs and PNRs which can sustain incredible strain, modest yet large strain was applied to group III-V nanowires such as GaAs/InAs. The calculations showed that a direct and indirect band gap transition occurs at some critical strains and the origination of these gap transitions were explored in detail. In addition to the pure nanowires, GaAs/InAs core/shell heterostructure nanowires were also studied. Due to the lattice mismatch between GaAs and InAs, the intrinsic strain in the core/shell nanowires demonstrates an interesting behavior on tuning the electronic properties. This interesting behavior suggests a mechanical way to exert compressive strain on nanowires experimentally, and can create a finite quantum confinement effect on the core. / Dissertation/Thesis / Doctoral Dissertation Physics 2016

La Théorie de la Fonctionnelle de la Densité (DFT) combinée avec la méthode de la Symétrie Brisée (BS) est aujourd'hui très utilisée dans le domaine du magnétisme moléculaire pour le calcul des constantes d'échange. Cette méthode (DFT-BS) reste cependant semi-quantitative et elle souffre de défauts déjà discutés dans la littérature. Dans le but de mieux en comprendre l'origine, nous avons réexaminé les contributions physiques qui participent au mécanisme d'échange. Nous proposons alors plusieurs formules analytiques construites suivant deux approches complémentaires (orbitales moléculaires et liaison de valence). Au cours de notre analyse, nous avons soulevé un problème inédit relatif à l'état de symétrie brisée tel que livré par le calcul DFT. Nos modèles seront appliqués au cas des dimères de cuivre(II) et nous verrons comment quantifier les différents paramètres afin de reconstruire les constantes d'échange. Qui plus est, notre travail permet d'établir une correspondance quantitative originale entre les deux approches pré-citées. / Density Functional Theory (DFT) combined with the Broken Symmetry (BS) method is today widely used in the field of molecular magnetism for the computation of exchange coupling constants. But this method (DFT-BS) remains semi-quantitative as it suffers from a series of drawbacks already discussed in the literature. In order to better understand the origin of such problems, we reexamined the physical contributions acting in the exchange phenomenon. We then propose alternative analytical expressions built along two complementary approaches (molecular orbitals and valence bond). During our analysis, we found a new problem linked to the broken symmetry state as it comes out of a DFT calculation. Our models will be applied to copper(II) dimers and we will show how to quantify the different parameters involved in order to reconstruct the coupling constants. Moreover, our work allows for an original quantitative correspondence between the two above-mentioned approaches.

Le sujet de cette thèse porte sur l'étude des propriétés magnétiques de métaux de transition et leurs alliages sous haute pression au moyen de calculs ab initio. D'abord, les résultats de mesures de dichroïsme magnétique circulaire des rayons X (XMCD) au seuil K du nickel et du cobalt sont interprétés. Je montre que les données expérimentales doivent être comparées à celle de l'aimantation d'orbite projetée sur les états ``p''. Je mets en avant que la pression affecte différemment le spin et le moment orbitalaire. Dans le cas de l'alliage FeCo, la transition structurelle s'effectue sous une pression appliquée de l'ordre de 35 GPa. Je propose que l'émergence de l'antiferromagnétisme peut expliquer la disparition du signal XMCD au seuil K du fer et du cobalt. Ensuite, la transformation de phase dans FePd3, induite sous une pression de 12 GPa, est étudiée. Je démontre que le système est décrit par un modèle de Heisenberg étendu, contenant interactions d'échange biquadratiques forts. Selon nos résultats, FePd3 subit une transition de l'etat ferromagnétique à l'état triple-Q non-colinéaire, lorsqu'il est compressé. Enfin, une mise en oeuvre du tenseur des contraintes dans le code BigDFT est présentée. Il est montré qu'un traitement explicite des électrons de coeur permet de réduire considérablement les erreurs introduites par les pseudo-potentiels. Ainsi, les estimations des propriétés structurales peuvent être améliorées. / The subject of the present thesis is the investigation of magnetic properties of transition metals and their alloys under high pressure by means of first-principles calculations. First, the results of the K-edge x-ray magnetic circular dichroism (XMCD) experiments on Ni and Co are interpreted. It is shown that the experimental pressure evolution of the data should be compared with that of the p-projected orbital magnetization. I emphasize that the spin and orbital moments have different behavior upon compression. In the case of FeCo alloy the structural transition occurs under the pressure of 35 GPa. I propose that the emergence of antiferromagnetism can explain the disappearance of the XMCD signal at the Fe and Co K-edges. Then, the phase transformation in FePd3 , induced under pressure of 12 GPa, is investigated. I demonstrate that the system is described by an extended Heisenberg model, containing strong biquadratic exchange interactions. According to the results, FePd3 undergoes a transition from the ferromagnetic to the noncollinear triple-Q state when compressed. Finally, the implementation of the stress tensor in the BigDFT software package is presented. It is shown that an explicit treatment of core electrons can considerably reduce the errors introduced by the pseudopotentials. Thus the estimates of the structural properties can be improved.

Abstract The rare-earth elements (REEs) and bismuth, being classified as the ‘most critical raw materials’ (European Raw Materials Initiatives, 2017), have a high economic importance to the EU combined with a high relative supply risk. REEs are highly important for the evolving technologies such as clean-energy applications, high-technology components, rechargeable batteries, permanent magnets, electric and hybrid vehicles, and phosphors monitors. This scientific research work aims at building a fundamental knowledge base concerning the electronic/molecular structure and properties of rare-earth element (REE) and bismuth complexes with dithiocarbamate (DTC) and 1,10-phenanthroline (PHEN) by employing state-of-the-art experimental techniques such as nuclear magnetic resonance (NMR) spectroscopy and X-ray diffraction (XRD) techniques together with ab initio quantum mechanical computational methods. This combination of methods has played a vital role in analysing the direct and significant effect of the heavy metal ions on the structural and magnetic resonance properties of the complexes, thereby, providing a framework of structure elucidation. This is of special importance for REEs, which are known to exhibit similar chemical and physical properties. The objectives of the work involve i) a systematic investigation of series of REE(III) as well as bismuth(III) complexes to get a profound understanding of the structure-properties relationship and ii) to find an appropriate theoretical modelling and NMR calculation methods, especially, for heavy metal systems in molecular and/or solid-state. This information can later be used in surface interaction studies of REE/bismuth minerals with DTC as well as in design and development of novel ligands for extraction/separation of metal ions. The REE(III) and bismuth(III) complexes with DTC and PHEN ligands have all provided a unique NMR fingerprint of the metal centre both in liquid and solid phase. The solid-state ¹³C and ¹⁵N NMR spectra of the diamagnetic REE(III) and bismuth(III) complexes were in accord with their structural data obtained by single crystal XRD. The density functional theory (DFT) methods were used to get complementary and refined structural and NMR parameters information for all diamagnetic complexes in the solid-state. The relativistic contributions due to scalar and spin-orbit correlations for the calculated ¹H/¹³C/¹⁵N chemical shifts of REE complexes were analysed using two-component zeroth-order regular approximation (ZORA)/DFT while the ‘crystal-lattice’ effects on the NMR parameters were calculated by combining DFT calculations on molecular and periodic solid-state models. The paramagnetic REE complexes display huge differences in their ¹H and ¹³C NMR spectral patterns. The experimental paramagnetic NMR (pNMR) chemical shifts, as well as the sizable difference of the ¹H and ¹³C NMR shifts for these isoelectronic complexes, are well reproduced by the advanced calculations using ab initio/DFT approach. The accuracy of this approach is very promising for further applications to demanding pNMR problems involving paramagnetic f-block elements. The results presented in this thesis demonstrate that a multidisciplinary approach of combined experimental NMR and XRD techniques along with computational modelling and property calculations is highly efficient in studying molecular complexes and solids containing heavy metal systems, such as rare-earths and bismuth.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this work we perform a theoretical study about fluorine adsorption in BC2N nanostructures. The fluorine interaction with the BC2N nanostructures was studied using only atomic fluorine on two nanotubos with different chiralities (zigzag(5,0) and armchair(3,3)) and a monolayer. We used first principles calculations based on the density functional theory (DFT) taking account the effects of the spin polarization. For the exchange and correlation term, we use the generalized gradient approximation (GGA) and to describe the electron-ion interaction we use the pseudopotential approximation. The charge density is obtained solving the selfconsistent Kohn-Sham equations and to represent the Kohn-Sham wave functions a linear combination of atomic orbital is used. Our results show that when a single fluorine atom is adsorbed, the most stable configuration occurs when the fluorine atom is adsorbed on the boron atom (FB). The configurations with the fluorine adsorbed on carbon atom (FCI and FCII) are less stable than the FB configuration and the FN reaction is unstable. When the fluorine atom is adsorbed in BC2N nanostructure the local configuration is modified. It is observed that the atom bonded to the fluorine atom moves outward from the surface of the nanotubes and the monolayer. The electronic properties present similar characteristics in both nanotubes and also in the monolayer. The FCII configuration introduce acceptors properties in a BC2N nanostructures and the FCI configuration introduce donor properties. The fluorine adsorption in the most stable configuration gives rise to electronic levels in the band gap. When we investigated the BC2N monolayer with different fluorine coverage, the most stable configuration is obtained when the fluorine atoms are adsorbed on the boron atoms, forming a line. Defects levels are observed near to the top of the valence band and the system exhibit a p-type semiconductor character to low fluorine density and metallic character to the high fluorine density. We observe that the fluorine adsorption induces spin polarization effects in BC2N nanostructures leaving the system to present a spin magnetic moment. The absolute value of the spin magnetic moment depends on the density of fluorine adsorbed. / Nesse trabalho realizamos o estudo teórico da adsorção de flúor em nanoestruturas de BC2N. A interação de flúor com as nanoestruturas de BC2N foi estudada a partir da adsorção de flúor atômico em nanotubos de diferentes quiralidades, zigzag (5,0) e armchair (3,3), além da monocamada. Usamos cálculos de primeiros princípios dentro do formalismo da teoria do funcional da densidade (DFT), levando em consideração os efeitos de polarização de spin. Para o termo de troca e correlação, utilizamos a aproximação do gradiente generalizado (GGA) e para descrever a interação elétron-caro¸co utilizamos a aproximação do pseudopotencial. A densidade de carga é obtida resolvendo-se as equações de Kohn-Sham de maneira autoconsistente, com as funções de onda de Kohn-Sham expandidas em uma combinação linear de orbitais atômicos. Nossos resultados mostram que para a adsorção de um átomo de flúor, a configuração mais estável ocorre quando o átomo de flúor é adsorvido sobre o átomo de boro (FB). As configurações com adsorção de flúor em carbono (FCI e FCII) são menos favoráveis do que a adsorção ao FB, enquanto que a reação FN é instável. Quando o átomo de flúor é adsorvido nas nanoestruturas de BC2N a configuração local é modificada. Observa-se que o átomo ligado ao átomo de flúor desloca-se para fora da superfície dos nanotubos e em relação à monocamada temos um deslocamento deste átomo para fora da sua superfície. As propriedades eletrônicas apresentam características semelhantes em ambos os nanotubos e também para a monocamada. A configuração FCII faz com que as nanoestruturas de BC2N apresentem características aceitadoras e a configuração FCI faz com que essa estrutura apresente características doadoras. Para a adsorção de flóuor na configuração mais estável temos níveis rasos presentes na estrutura de bandas. Quando é feita a variação da densidade de flúor em uma monocamada de BC2N, a configuração mais estável é obtida quando os átomos de flúor são adsorvidos sobre átomos de boro formando uma linha, e na região do topo da banda de valência níveis de defeito podem ser observados. Em relação às propriedades eletrônicas é possível observar que em todas as estruturas ocorre uma perturbação no fundo da banda de condução. Na região do topo da banda de valência níveis de defeitos podem ser observados. Onde podemos obsevar que essas estruturas apresentam características de semicondutor do tipo-p para baixas densidades de flúor e metálicas para altas densidades. Temos que a adsorção de flúor provoca uma quebra de degenerescência de spin o que faz com que o sistema apresente um momento magnético de spin. O valor absoluto desse momento magn´etico de spin depende da densidade de flúor adsorvido.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this work we study simultaneously the structural, electronic and thermoelectric properties of PbSe and PbTe nanowires, analyzing the quantum confinement effects, the dependence with the planar stoichiometry and the spin-orbit interactions. We also study these nanowires in the presence of intrinsic defects (vacancies and antisites) and doped with group III (Al, Ga, In, and Tl) impurity. We use first principles calculations within the formalism of the density functional theory (DFT). We observed that the nanowires are more stable in the rock salt structure and aligned along the (001) direction. The electronic properties of nanowires are in uenced by three effects: the quantum confinement, spinorbit interactions and the planar stoichiometry. The quantum confinement increases the thermoelectric efficiency of the PbSe and PbTe nanowires when compared to the system in the bulk phase, reaching an increase up to two orders in the magnitude, leading the PbSe and PbTe nanowires with great potential to be used in thermoelectric devices. We studied the in uence of intrinsic defects and group III impurity doping in the main properties of PbSe and PbTe, we show that these defects give rise to different electronic properties in the nanowires as compared to the bulk one. Intrinsic defects and group III impurities, which modify the electronic density of states (DOS) near to the top of the valence band or near to the bottom of the conduction band increase the thermoelectric efficiency of the PbSe and PbTe nanowires. However, defects that introduce electronic levels in energy band gap are shown to cause and degradation in the thermoelectric efficiency. The increase (decrease) in thermoelectric efficiency is associated with a lower (higher) value of electronic part of the thermal conductivity. In summary, we show that PbSe and PbTe nanowires are very promising materials to be used in thermoelectric, electronic and optical devices. / Neste trabalho estudamos simultaneamente as propriedades estruturais, eletrônicas e termoelétricas de nanofios de PbSe e PbTe, analisando os efeitos do confinamento quântico, a dependência com a estequiometria planar e a interação spin-órbita. Estudamos também estes nanofios na presença de defeitos intrínsecos (vacâncias e antissítios) e impurezas do grupo III (Al, Ga, In e Tl). Utilizamos cálculos de primeiros pincípios dentro do formalismo da teoria do funcional da densidade (DFT). Observamos que os nanofios são mais estáveis na estrutura rock salt, e alinhados ao longo da direção (001). As propriedades eletrônicas desse nanofios são influenciadas por três efeitos: o confinamento quântico, a interação spin-órbita, e a estequiometria planar. O confinamento quântico aumenta a eficiência termoelétrica do PbSe e PbTe em comparação ao observado para o bulk, chegando a um aumento de até duas ordens de grandeza, fazendo com que os nanofios de PbSe e PbTe tenham um grande potencial para serem utilizados em dispositivos termoelétricos. Estudamos a influência de defeitos intrínsecos e da dopagem de impurezas do grupo III nas principais propriedades do PbSe e PbTe, onde mostramos que essa influência é diferente no bulk e no nanofio. Defeitos intrínsecos e impurezas do grupo III que alteram a densidade de estados eletrônicos (DOS) nas proximidades do topo da banda de valência ou do fundo da banda de condução, observamos um aumento da ficiência termoelétrica dos nanofios de PbSe e PbTe. Porém aqueles que introduzem níveis no gap de energia fazem com que a eficiencia termoelétrica diminua. O aumento (diminuição) da eficiência termoelétrica está associado(a) ao menor (maior) valor da comdutividade térmica eletrônica. De maneira geral, mostramos que nanofios de PbSe e PbTe são materiais muito promissores para a aplicação em dispositivos termoelétricos, eletrônicos, óticos, etc.