En esta tesis se ha estudiado en una primera parte el efecto del hidrógeno sobre el sistema bimetálico B2 FeAl con defectos puntuales. En este caso se determinó la ubicación de la impureza en un sitio intersticial octaédrico y los resultados se compararon con cálculos de densidad funcional y con algunos resultados experimentales de la literatura. En una segunda parte se estudio el efecto de diferentes adsorbatos (H y CO) en la superficie (110) del compuesto intermetálico PdGa. Se determinaron las geometrías de adsorción y el enlace químico. Además, se computaron las frecuencias de vibración metal-adsorbato. Finalmente, se estudio el efecto del hidrógeno sobre las superficies de bajo índice (100), (111) y (1 1 1) de PdGa; determin ándose geometrías de adsorción, enlace químico y frecuencias de vibración. Todos los cálculos fueron realizados utilizando la teoría del funcional de la densidad (DFT). La optimización de la geometría, la estructura electrónica y frecuencias de vibración se obtuvieron con el código VASP. Los cambios en el enlace químico del hidr´ogeno adsorbido/ absorbido y el CO adsorbido se computaron con el código SIESTA. Se analizó el rol de los orbitales s, p y d en el mecanismo de enlace para todos los sistemas metálicos así como el efecto del hidrógeno y CO sobre la fuerza cohesiva metal-metal. / In this thesis, it has been studied the hydrogen e↵ect on the bimetallic system B2 FeAl with single defects, as a first part. The impurity location has been determined as an octahedral interstitial site and the results were compared with density functional calculations and some experimental data from literature. In a second part, it has been studied the e↵ect of di↵erent adsorbates (H and CO) on the (110) surface of the intermetallic compound PdGa. The adsorption geometries and chemical bond were computed. In addition, the metal-adsorbate vibrational frequencies were calculated. Finally, the hydrogen e↵ect on the low-index surface (100), (111) and (1 1 1) of PdGa has been studied. The adsorption geometries, chemical bond and vibrational frequencies were also computed. All the calculations have been performed using density functional theory (DFT). The optimized geometry, electronic structure and vibrational frequencies have been obtained using the VASP code. The changes in the chemical bond of the adsorbed/ absorbed hydrogen and CO adsorbed have been computed with the SIESTA code. The role of s, p and d orbitals in the bonding mechanism for all metallic systems has been analyzed as well as the hydrogen and CO e↵ect on the metal-metal cohesive interaction.

En la presente tesis se ha realizado un estudio teórico de la formación de nanoestructuras de carbono sobre superficies de catalizadores empleando un método abinitio basado en la teoría DFT (VASP). Para caracterizar distintas formas de interacción entre los adsorbatos y los substratos, se han elaborado varios modelos de superficies y de nanopartículas. Se logró discernir la fase inicial en la obtención de átomos de carbono, a partir del estudio de la reacción de reformado de metano en seco, que luego constituirían las estructuras de carbono tipo grafeno. Una vez obtenido el carbono de la síntesis, se procedió a estudiar su adsorción sobre el substrato, considerando diferentes superficies con defectos y a distintos cubrimientos. Para poder comprender las particula-ridades que gobiernan el proceso de crecimiento del grafeno sobre Pd, se procedió a repetir el estudio sobre Pt y Ni por ser representantes de interacciones grafeno-metal con propieda-des completamente distintas. En este punto se procedió a completar el análisis con las monocapas de grafeno ya forma-das sobre el metal. Los resultados mostraron que las fuerzas de Van der Waals juegan un papel dominante en la adsorción de grafeno en Pd. Los resultados teóricos explican algunas de las particularidades encontradas con imágenes de Microscopia de Alta Resolución, como ser la deformación de las partículas de Pd, el crecimiento de grafeno paralelo a la superficie del metal y la disociación del grafeno de la partícula. Para finalizar vale mencionar que los resultados de espectros vibracionales en la adsorción de especies intermedias de la reacción (CHx, CO) muestran buen acuerdo con resultados experimentales de Espectroscopia Infrarroja. / In this theoretical thesis, ab-initio calculus have been performed in order to study the formation of carbon nanos-tructures on surfaces of catalysts by using a method based on DFT theory (VASP). To characterize different forms of interaction between adsorbates and substrates have been developed several models of surfaces and nanoparticles. We succeded in discern the initial phase in obtaining carbon atoms, from a study of the reaction of dry reforming of me-thane, which constitute the graphene-like carbon structures. Once the carbon is obtained from the synthesis, we procee-ded to study its adsorption on the substrate by considering different surfaces with defects and at various coverages. To understand the characteristics that govern the process of growth of graphene on Pd, we proceeded to repeat the study on Pt and Ni, in order to be representatives of graphene-metal interactions, with completely different properties. At this point we proceeded to complete the analysis of the graphene monolayer already formed on the metal. The results showed that the Van der Waals forces play a dominant role in the adsorption of graphene on Pd. The theoretical results explain some of the features found with images of High Resolution Microscopy, such as the deformation of the Pd particles, the growth of graphene parallel to the metal surface and the dissociation of graphene from the particle. To end, it is worth mentioning that the results of vibrational spectra on the adsorption of intermediates of the reaction (CHx, CO) show a good agreement with experimental Infrared Spectros-copy results.

El objetivo de esta tesis es estudiar en forma teórica las propiedades estructurales, energéticas, termodinámicas y magnéticas de la aleación FeRh utilizando cálculos de primeros principios, debido a su notable transición de fase de primer orden desde el estado antiferromagnético (AFM) al ferromagnético (FM) a temperatura cercana a la del ambiente. Para ello se analizó el caso del FeRh bulk extendiendo el estudio a las superficies aisladas en las direcciones cristalográficas [001], [110] y [111] y luego a los films soportados en epitaxis sobre el óxido MgO(001). Los resultados estructurales y magnéticos indican que la transición metamagnética es acompañada por un aumento del 2.4% en el volumen de la celda, de un 9.6% en el momento magnético de espín del Fe y lo más importante, de la aparición del momento magnético del Rh. A pesar de lo originalmente publicado en la literatura, la fase AFM muestra una distorsión de celda cúbica a tetragonal y a ortorrómbica, resolviendo las inestabilidades observadas en las curvas de las estructuras de bandas de fonones de la estructura cúbica correspondiente. La estabilidad de la configuración AFM ortorrómbica se corroboró con simulaciones de dinámica molecular. A partir de los espectros fonónicos se obtuvieron las propiedades térmicas del sistema. Los estudios se complementan con el análisis de las propiedades termoeléctricas de la aleación. Para los films ultradelgados la estabilidad relativa de los crecidos en las direcciones [001] y [110] es similar. Por el contrario, los films generados en la dirección [111] presentan valores menos favorables en términos energéticos. En particular, para los films FeRh(001), el análisis de energía de anisotropía magnetocristalina indica que la magnetización perpendicular al plano resulta ser la más favorable, salvo para la terminación de films en Rh, configuración AFM, donde se observa una proyección de la magnetización en el plano. En el caso de los films soportados, resultan más favorables las terminaciones en Fe con menores energías de adhesión. En función de todo lo analizado se puede concluir que los films terminados en Rh presentan en la mayoría de los casos comportamientos inestables y hasta impredecibles dependiendo de la cantidad de capas de Rh que conforman el film. / The objective of this thesis is to perform a theoretical study of the structural, energetic, thermodynamic and magnetic properties of the FeRh alloy using first principle calculations, due to its remarkable first-order phase transition from the antiferromagnetic state (AFM) to the ferromagnetic (FM) at a temperature close to that of the environment. It is for this reason that the case of FeRh bulk was analyzed, extending the study to the isolated surfaces in the crystallographic directions [001], [110] and [111] and then to the films supported in epitaxis on MgO(001) oxide. The structural and magnetic results indicate that the metamagnetic transition is accompanied by a 2.4% increase in cell volume, a 9.6% in the magnetic spin moment of Fe atom and, most importantly, the appearance of the Rh magnetic moment. Despite what was originally published in the literature, the AFM phase shows a distortion of cubic cell to tetragonal and orthorhombic, resolving the instabilities observed in the phonon band structures of the corresponding cubic structure. The stability of the orthorhombic AFM configuration was corroborated with molecular dynamics simulations. The thermal properties of the system were obtained from the phonon spectra. The studies are complemented by the analysis of the thermoelectric properties of the alloy. For ultra-thin films, the relative stability of the films grown in [001] and [110] directions are similar. On the contrary, films generated in the [111] direction present less favorable energy values. In particular, for FeRh(001) films, the magnetocrystalline anisotropy energy analysis indicates that the magnetization perpendicular to the plane x is found to be the most favorable, except for films -Rh terminated, AFM configuration, where a projection of the magnetization is observed in the plane. In the case of supported films, those -Fe terminated with lower adhesion energies are more favorable. Based on everything analyzed, it can be concluded that –Rh terminated films present in most cases unstable and even unpredictable behaviors depending on the number of Rh layers that make up the film.

Después del descubrimiento del grafeno y de sus extraordinarias propiedades electrónicas, la funcionalización química de este material se ha vuelto un foco de especial interés en la química física contemporánea. El objetivo del trabajo que se presenta en esta Tesis es estudiar la interacción entre grafeno y adsorbatos atómicos y moleculares. Esta interacción es modelada con el uso de cálculos ab initio y los resultados son contrastados con otros cálculos ab initio y, en lo posible, con resultados experimentales. El modelado mecano-cuántico es el método computacional más correcto usado en el cálculo de energías y en la predicción de estructuras de equilibrio del estado sólido y de sistemas moleculares. Específicamente, en el marco teórico de la teoría del Funcional de la Densidad se emplearon dos modelos: de cluster y de slab, que utilizan bases localizadas y ondas planas, respectivamente. Complementariamente se realizaron cálculos con la técnica MP2. Primeramente se consideró la interacción de un átomo de hidrógeno con grafeno en el rango de distancias donde predominan las fuerzas dispersivas, pasando luego a estudiar la interacción presente en el sistema Al/grafeno. Para ello se analizó detalladamente la adsorción de un átomo de aluminio sobre una hoja de grafeno perfecta y luego cuando presentan una monovacancia y una divacancia, teniendo en cuenta la magnetización inducida en el grafeno debido a la presencia del defecto. Finalmente se consideró la adsorción de un dímero Al2 sobre una monovacancia y una divacancia. / Since the discovering of graphene and its extraordinary electronic properties, the chemical functionalization of this material has been object of special interest in the contemporary chemical physics. The goal of the work presented in this Thesis is to study the interaction between graphene and atomic/molecular adsorbates. This interaction has been modeled by using ab-initio calculations and the results are tested with other similar calculations and, as possible, with experiments. The quantum-mechanical modeling is the more correct computational method to use in the calculation of energies and in the prediction of the geometrical structures in equilibrium for the solid state and chemical systems. Specifically, two models based on the cluster and slab approaches have been employed in the framework of Density Functional Theory, where localized basis sets and plane waves are used, respectively. Complementary calculations with the MP2 technique have been performed. First, the interaction of one hydrogen atom with graphene in the range of distances where the dispersive forces are predominant has been considered, and afterwards, the interaction present in the Al/graphene system was studied. For that purpose the adsorption of one aluminum atom on a perfect graphene sheet was analyzed in detail and, subsequently, the situation with a monovacancy and a divacancy, taking into account the magnetization induced on graphene due to the presence of such defect. Finally the adsorption of Al2 dimer on the monovacancy and the divacancy was considered.

Palladium-107 is one of the radionuclides of interest for the safety assessment of deep geological disposal of used nuclear fuel. Bentonite clay is used in the engineered barrier system of a deep geological repository for used nuclear fuel because of its low permeability, large swelling capacity, retention properties and thermal stability. With montmorillonite being the main component of the bentonite clay, this study focuses on using the Density Functional Theory (DFT) simulation to explore the sorption mechanism of palladium on montmorillonite in a Na-Ca-Cl saline environment. The method of substituting Se(IV) species with Pd species and phlogopite edge surfaces with the basal and edge surfaces of montmorillonite is justified with benchmark simulation results. The presence of PdCl+, PdCl2, PdCl3-, and PdCl42- complexes in the Na-Ca-Cl saline environment is confirmed by the calculated negative free energy. The most favourable palladium chloride species for sorption reactions onto montmorillonite in the saline environment is PdCl42-, followed by PdCl3- , PdCl2 and PdCl+. / Thesis / Master of Applied Science (MASc)

L’utilisation des combustibles fossiles (énergies non renouvelables) est responsable de l’augmentation de la concentration en gaz à effet de serre dans l’atmosphère. Parmi les solutions de remplacement envisagées, l’hydrogène apparaît comme le vecteur énergétique le plus séduisant. Son stockage dans des intermétalliques permet d’obtenir des capacités massiques et volumiques (e.g. 140 g/L) supérieures à celles obtenues en voie liquide ou sous pression (respectivement 71 et 40 g/L). Nous avons élaboré des composés à base de Mg et de terres rares (e.g. Y, Ce et Gd) dérivant des phases de Laves cubiques AB2. Leurs propriétés physico-chimiques ont été étudiées (hydruration, électrochimie, magnétisme, …). Les conditions de sorption (P et T) se sont révélées particulièrement favorables (i.e. absorption à température ambiante et pression atmosphérique). Par ailleurs, afin d’améliorer la cinétique de sorption du magnésium métallique, les composés précédemment élaborés ont été utilisés comme catalyseurs. Ainsi, GdMgNi4 a été co-broyé avec du magnésium et les vitesses d’absorption et de désorption du composite sont supérieures à celles obtenues pour les composites Mg+Ni ou Mg+V qui sont des références. Une approche théorique (DFT) a permis de modéliser la structure électronique des composés ternaires (i.e. TRMgNi4) et ainsi de prédire ou de confirmer les résultats expérimentaux. Enfin nous avons étudié de nouveaux intermétalliques riches en terre rare (TR4MgNi) dont les capacités d’absorption en hydrogène sont élevées (2H/M). / The use of fossil fuels (non-renewable energy) is responsible for increasing the concentration of greenhouse gases in the atmosphere. Among the considered alternatives, hydrogen is seen as the most attractive energy vector. The storage in intermetallics makes it possible to obtain mass and volume capacities (e.g. 140 g/L) higher than those obtained by liquid form or under pressure (respectively 71 and 40 g/L). We have synthesised Mg and Rare Earth based compounds (RE = Y, Ce and Gd), derived from the cubic Laves phases AB2. Their physical and chemical properties have been studied (hydrogenation, electrochemistry, magnetism, ...). The conditions of sorption (P and T) are particularly favorable (i.e. absorption at room temperature and atmospheric pressure). Besides, to improve the sorption kinetics of metallic magnesium, the compounds developed previously were used as catalysts. Thus, GdMgNi4 was milled with magnesium and the speeds of absorption and desorption of the mixture are found higher than those obtained for the composites Mg+Ni or Mg+V, which are reference systems. A theoretical approach (DFT) was used to model the electronic structure of the ternary compounds (i.e. REMgNi4) and thus to predict or confirm the experimental results. Finally we have studied new intermetallics rich in rare earth (RE4MgNi) whose hydrogen absorption capacities are high (2H/M).

Cette thèse est entièrement consacrée à l'étude par la théorie de la fonctionnelle de la densité des composés pérovskites halogénés, matériaux prometteurs pour de nombreux domaines applicatifs. Le caractère « hybride » de ce type de matériau est illustré par différentes études de leurs propriétés structurales et électroniques. Le composé tridimensionnel MAPbBr₃ est tout d'abord présenté. Les propriétés électroniques fondamentales telles que les structures de bandes, les états de densité projetée ou les fonctions d'ondes sont discutées. De plus, l'importance du couplage spin-orbite est mise en évidence. L'analyse des symétries est appliquée pour comprendre la symétrie des états électroniques et pour interpréter les propriétés optiques des différents matériaux. Les reconstructions structurales à la surface des cristaux entraînent l'apparition d'effets Rashba-Dresselhaus. Par ailleurs les défauts de surface et leurs passivations sont également étudiés. Des travaux sur les matériaux sans plomb potentiellement moins toxiques sont proposés dans un second temps. Ces études ont pour but d'analyser leurs potentiels pour le photovoltaïque du point de vue des structures électroniques. Différentes stratégies de substitution sont envisagées allant du simple remplacement du plomb à d'autres alternatives plus élaborées comme les pérovskites doubles ou les pérovskites de basse dimensionnalité. / This actual work is entirely devoted to the study of halide perovskite materials, promising materials in many fields of application, by means of the Density Functional Theory. The "hybrid" feature of this type of material is illustrated through various studies of their structural and electronic properties. The three-dimensional compound CH₃NH₃PbBr₃ is firstly presented. Basic electronic properties such as band structures, projected density of states or wave functions are discussed. In addition, the importance of spin-orbit coupling is highlighted. Symmetry analysis is applied to understand and interpret the optical properties of different materials. Structural reconstructions on the surface of the crystals lead the Rashba-Dresselhaus effects. In addition, surface defects and their passivations are also studied. Studies on lead-free materials that are potentially less toxic are proposed in a second step. These studies aim to analyze their potentials for photovoltaic devices from the point of view of electronic structures. Different substitution strategies, ranging from the simple replacement of lead to other more elaborate alternatives such as double perovskites or low-dimensional perovskites are investigated as well.

Neste trabalho foi feito o estudo teórico de complexos de rutênio de fórmula: cis-[RuCl2(NN)(PP)], [Ru(NS)(bipy)(PP)]+, [Ru(pic)(bipy)(PP)]+ e [RuCl(bCN)(bipy)(PP)]+, onde NN = 2,2\'-bipiridina (bipy), 4,4\'-dimetil-2,2\'-bipiridina (Mebipy) e 1,10-fenantrolina (phen); PP = 1,1-Bis(difenilfosfino)metano (dppm), 1,2-Bis(difenilfosfino)etano (dppe), 1,3-Bis(difenilfosfino)propano (dppp), 1,4-Bis(difenilfosfino)butano (dppb) e 1,1\'-Bis(difenilfosfino)ferroceno (dppf), NS = 2-mercaptopiridina (pyS), 2-mercaptopirimidina (prm), 4,6-dimetil-2-mercaptopirimidina (dmpm); pic = 2-picolinato; bCN = benzonitrila em nível da teoria DFT utilizando o funcional B3LYP. Os complexos tiveram suas estruturas otimizadas, as quais foram comparadas às estruturas obtidas experimentalmente por difração de raios X e apresentaram grande concordância com estas. A análise de decomposição de carga revelou que os ligantes NN, PP, NS, pic, bCN e Cl doam elétrons para o centro metálico tornando-o mais negativo, isso reflete na energia dos orbitais HOMO, que por sua vez pode ser relacionado com os valores dos potencias de oxidação dos complexos. Pode-se observar que quanto maior o valor do potencial de oxidação, menor a energia do HOMO. Além disso, nos complexos cis-[RuCl2(NN)(PP)] os valores de potenciais de oxidação são mais anódicos porque os dois átomos de cloro somados doam mais para o rutênio que os ligantes NS e pic e bCN. Os espectros eletrônicos foram calculados e as bandas observadas em 420 e 290 nm nos complexos [Ru(NS)(NN)(PP)]+ puderam ser atribuídas a transições dos tipos MLCT e LLCT devido a análise da composição dos orbitais. As bandas de baixa intensidade observadas nos complexos cis-[RuCl2(NN)(PP)] em torno de 460 nm, mostraram ser transições do tipo MLCT do HOMO-2 para o LUMO, sendo que esses orbitais estão concentrados nos átomos de rutênio e nos ligantes NN, respectivamente. Os espectros vibracionais calculados estão de acordo com os dados experimentais. As bandas observadas em torno de 1380, 1160 e 760 cm-1 foram atribuidas aos estiramentos vC-S e as bandas em 1715 cm-1 ao estiramento νC=O; no ligante livre essa última banda é observada em 1654 cm-1. A banda referente ao estiramento vC≡N, que aparece em 2229 cm-1 na benzonitrila não coordenada, nos complexos [RuCl(bCN)(NN)(PP)]+ aparece em 2250 cm-1. / Ruthenium complexes with formulas: cis-[RuCl2(NN)(PP)], [Ru(NS)(bipy)(PP)]+, [Ru(pic)(bipy)(PP)]+ e [RuCl(bCN)(bipy)(PP)]+, where NN = 2,2\'-bipyridine (bipy), 4,4\'-dimethyl-2,2\'-bipyridine (Mebipy) e 1,10-phenanthroline (phen); PP = 1,1-Bis(diphenylphosphino)methane (dppm), 1,2- Bis(diphenylphosphino)ethano (dppe), 1,3- Bis(diphenylphosphino)propane(dppp), 1,4-Bis(diphenylphosphino)buthane (dppb) e 1,1\'-Bis(diphenylphosphino)ferrocene (dppf), NS = 2-mercaptopyridine (pyS), 2-mercaptopyrimidine (prm), 4,6-dimethyl-2-mercaptopyrimidina (dmpm); pic = 2-picolinate; bCN = benzonitrile were studied in this thesis applying DFT theory with B3LYP functional. The complexes had their structures optimized and comparison with experimental structures determined by X ray crystallography show that they compare well. Charge decomposition analysis revealed that NN, PP, NS, pic, bCN and Clligands donate eletrons to the metal center makig it more negative, what reflects in the HOMO energies, that can be related to oxidation potencials of the complexes. The higher the oxidation potential of the complexes the lower the energy of HOMO. In complexes cis-[RuCl2(NN)(PP)] the oxidation potentials were more anodic because the two chlorine ligands togheterdonate more than NS, pic or bCN ligands. Electronic spectra of the complexes [Ru(NS)(NN)(PP)]+ were calculated and the bands around 420 and 290 nm could be assigned to transitions of the type MLCT, LLCT based on the analysis of the orbitals composition. The bands of low intensity around 460 nm observed in the spectra of cis-[RuCl2(NN)(PP)] were also MLCT transitions from HOMO-2 to LUMO, these orbitals are concentrated in ruthenium and NN ligands, respectively. The calculated vibrational spectra are in good agreement with experimental data, bands around 1380, 1160 e 760 cm-1 were assigned to vC-S stretching and bands around 1715 cm-1 to νC=O, in pic not coordinated this band appears at 1654 cm-1. In freebenzonitrile vC≡N band is observed at 2229 cm-1, but in complexes [RuCl(bCN)(NN)(PP)]+ this band is upshifted to 2250 cm-1.

Nesta tese investigamos a partir de cálculos de primeiros princípios, dispositivos e componentes de dispositivos baseados em grafeno. Abordamos os campos da nanoeletrônica e da spintrônica. Dentro da nanoeletrônica investigamos: (i) propriedades de transporte de um nanotransistor de bicamada de grafeno na presença de um gate duplo. Demonstramos que sobre a ação de um campo elétrico externo, mesmo utilizando um gate da ordem de 10 nm, à temperatura ambiente e 4.5K uma corrente nula nunca é exibida. Esses resultados são explicados por um regime de tunelamento; (ii) propriedades eletrônicas e de transporte de multicamadas de grafeno em função do número de camadas e tipo de empilhamento entre elas. Mostramos que a estrutura eletrônica do sistema depende fortemente desse novo grau de liberdade de empilhamento. Na presença de um campo elétrico externo aplicado perpendicular ao sistema, o empilhamento do tipo Bernal nunca exibe um gap de energia, ao contrário do empilhamento romboédrico que exige um gap ajustável através da intensidade do campo. Mostramos também que é possível diferenciar os tipos de empilhamentos através da resistência do sistema e variando-se a temperatura; (iii) dentro das componentes de um nanotransistor mais realista, estudamos as propriedades eletrônicas e estruturais de: (a) bicamadas de grafeno sobre um substrato de nitreto de boro hexagonal. Neste sistema o limite de voltagens que podem ser aplicadas depende fortemente do número de camadas de h-BN e da direção do campo, onde quanto menos camadas maior é a voltagem que pode ser aplicada; (b) heteroestruturas compostas de bicamadas de grafeno, nitreto de boro hexagonal e cobre. Demonstramos que para uma aplicação direta em um dispositivo a configuração com uma bicamada de grafeno depositada sobre um substrato de h-BN e este conjunto sobre a superfície de cobre é a mais favorável. Nessa configuração é possível tanto controlar o gap na bicamada como a dopagem do sistema, sem a abertura de canais de condução através do dielétrico (h-BN). Dentro do campo da spintrônica estudamos: (i) propriedades de transporte das nanofitas de grafeno (GNR) (3,0) pristinas e dopadas com boro e nitrogênio. Para as GNR pristinas mostramos com os eletrodos em um alinhamento de spin anti-paralelo o sistema apresenta um comportamento de filtro de spin, onde para tensões de bias positivos/negativos somente o canal up/down conduz. Para as GNR dopadas com boro e nitrogênio, mostramos que as correntes para os diferentes canais de spin são não degeneradas ao longo de todo o intervalo de tensões aplicadas, apresentando desse modo um comportamento de filtro de spin; (ii) finalmente estudamos as propriedades de transporte de uma junção túnel magnética, composta de GNR intercaladas por uma nanofita de nitreto de boro hexagonal. Mostramos que esse sistema pode ser utilizado tanto como filtros de spin como elementos para dispositivos de magnetoresistência gigante, onde para este último a sua eficiência é muito mais pronunciada. / In this thesis we investigated by first principle calculations, devices and components of devices based on graphene. We covered the fields of nanoelectronics and spintronics. On the field of nanoelectronics we investigated: (i) the transport properties of a dual gate bilayer graphene nanotransistor. We showed that under the action of an external electrical field, even with a gate length of 10 nm, at room temperature and 4.5K a zero current is never exhibited. These results could be explained by a tunneling regime; (ii) the electronic and transport properties of few layer graphene, as function of the number and type of stacking of the layers. We show that the electronic structure strong deppends of the stacking order. On the presence of a external electrical field applied to the system, the Bernal stacking never presents a gap, contrary to the rombohedrical one, that posses a tuneable energy gap. Also we showed that is possible to differentiate the types of stacking by the resistance of the system and varying the temperature;(iii) for the components of a more realistic nanodevice, we study the structural and electronic properties of: (a) bilayer graphene over a hexagonal boron nitride substrate. We show that the voltages that could be applied to the system strongly depends of the number 0 layers and the direction of the field, where with more layers, smaller is the field; (b) heterostructures composed with bilayer graphene, hexagonal boron nitride and cooper. We show that for a direct application on a device, the better configuration is with a bilayer graphene over the hexagonal boron nitride, and this set over a cooper. In this configuration is possible to control both the gap and the doping of the system, without the creation of conducting channels through the dielectric (h-BN). On the field of spintronics, we study: (i) the transport properties (3,0) graphene nanoribbons pristines and doped with nitrogen and boron. For the pristine GNR we show that for the electrodes in an anti-parallel alignment the system presents a spin filter behavior, where for positive/negative bias the transport is only by up/down channel. For the GNR doped with nitrogen and boron we show that the current is non-degenerated in all range of voltages applied, presenting a spin filter behavior; (ii) finally, we study the transport properties of a magnetic tunnel junction, consisting of a GNR intercalated with a hexagonal boron nitride nanoribbon. We show that such system could be used both as a spin filter as a device that uses the the giant magnetoresistance effect, where for the last the system if more efficient.

Este trabalho foi desenvolvido com o propósito de investigar as reações de polimerização por metátese via abertura de anel (ROMP) assistidas pelos catalizadores desenvolvidos pelo grupo do Prof. Dr. Benedito dos Santos de Lima Neto do Instituto de Química de São Carlos (IQSC-USP), usando métodos computacionais de mecânica quântica. Portanto, este projeto foi uma combinação de resultados teóricos e experimentais. No capítulo 1, foi elucidado o mecanismo principal da reação de ROMP de norborneno por meio do iniciador catalítico [RuCl2(PPh3)2(piperidina)]. O estudo mostrou que a primeira etapa da reação implica numa associação entre o complexo iniciador e o diazoetanoato de etila (EDA), resultando na formação de uma ligação metalcarbeno. Então, essa espécie perde duas moléculas de trifenilfosfina nos passos subsequentes, sendo que a etapa determinante da velocidade da reação está associada com a saída da segunda molécula de PPh3 devido ao sinergismo eletrônico envolvendo este ligante e a molécula de piperidina. Logo após a etapa determinante da velocidade, ocorre a coordenação de uma molécula de norborneno e um similar efeito trans-sinérgico entre este monômero e o ligante piperidina ativa a catálise. No capítulo 2 está descrito o estudo do mecanismo de uma série de complexos de fórmula [RuCl2(PPh3)2(amina)] (amina = pirrolidina, piperidina, azepano, azocano). Estas aminas cíclicas possuem volumes moleculares que aumentam no sentido da pirrolidina ao azocano. A troca dessas aminas afeta substancialmente o comportamento termodinâmico desses complexos através da combinação de efeitos eletrônicos e estéricos. A investigação realizada para os compostos dessa série levou ao mesmo mecanismo elucidado para o complexo contendo piperidina e com os mesmos processos mais relevantes que são as saídas dos ligantes trifenilfosfinas. Notou-se que a formação do isômero PBQ fica comprometida ao passo que a amina no complexo [RuCl2(PPh3)2(amina)] vai ficando mais volumosa devido ao maior espaço ocupado ao redor do metal na posição apical da pirâmide de base quadrada (PBQ) e que representa uma tensão considerável na estrutura do complexo que tende a se converter, preferencialmente, à geometria bipirâmide trigonal (BpT), como acontece com o complexo iniciador [RuCl2(PPh3)2(azocano)]. Observou-se também que a saída da primeira fosfina é influenciada pelo efeito estérico provocado pela amina. Em contrapartida, a saída da segunda já é um processo regido eletronicamente e é influenciado pela capacidade doadora da amina trans-posicionada ao ligante PPh3 em um dos intermediários que é formado durante a reação. No capítulo 3 está descrita a investigação da reação de ROMP com o complexo iniciador [RuCl2(PPh3)2(piperidina)] usando as moléculas diazoetanoato de etila (EDA), diazoetanoato de benzila (BDA) e diazoetanoato de terc-butila (TDA) como fontes de carbeno para gerar a espécie in situ [Ru(=CHR)Cl2(PPh3)2(piperidina)]. Curiosamente, a troca dos carbenos também não afetou o mecanismo proposto inicialmente para a série das aminas. Os perfis termodinâmicos traçados para esses diferentes carbenos mostram que o complexo contendo o carbeno etanoato de benzila-1-ilideno (EBI) foi o que mais se diferiu, seguindo uma tendência termodinâmica distinta para a saída dos ligantes trifenilfosfina. Para as demais etapas, os três complexos apresentaram um comportamento bastante similar. O estudo dessa série mostrou-se ser mais intricado do que a série das aminas, pois esses carbenos podem afetar tanto eletrônico quanto estericamente a estrutura dos compostos em diferentes etapas do mecanismo que ainda não foram investigadas minunciosamente. / This work was developed with the purpose of investigating the ring-opening metathesis polymerization (ROMP) with catalysts developed in the research group of Prof. Dr. Benedito dos Santos Lima Neto of the Institute of Chemistry of São Carlos (IQSC-USP) by computational methods of quantum mechanics. Therefore, this project was a combination of theoretical and experimental results. In Chapter 1, the main mechanism of ROMP reaction of norbornene by the initiator complex [RuCl2(PPh3)2(piperidine)] was elucidated In this investigation, the first reaction step involves an association between the initiator complex and ethyl diazoacetate (EDA), resulting the formation of metalcarbene bond. Then, this species loses two triphenylphosphine ligands in the subsequent steps and the rate reaction is associated with the loss of the second molecule due to the electronic PPh3 synergism involving this ligand and the piperidine molecule. Immediately after the rate reaction step, the coordination of norborneno occurs and a similar trans synergistic effect between the monomer and piperidine ligand actives the catalysis. Chapter 2 describes the study of the mechanism of complexes type [RuCl2(PPh3)2(amine)] (amine = pyrrolidine, piperidine, azepane, azocane). These cyclic amines have molecular volumes that increase from pyrrolidine towards azocane. The exchange of amines affects substantially the thermodynamic behavior of these complexes by a combination of electronic and steric effects. The four compounds showed the same mechanism elucidated for the complex containing piperidine. It was noted that the formation of the PBQ isomer is avoided while the amine in the complex [RuCl2(PPh 3)2(amine)] becomes more voluminous. It was also observed that the loss of the first phosphine is influenced by the steric effect caused by the amine. In contrast, the loss of the second (phosphine) is already a process electronically-governed, and is influenced by the donor capacity of the amine ligand trans-positioned to PPh3 in one of the intermediate that is formed during the reaction. The Chapter 3 describes the investigation of the ROMP reaction using the initiator complex [RuCl2(PPh3)2(piperidine)] with the molecules ethyl diazoacetate (EDA), benzyl diazoacetate (BDA) and tert-butyl diazoacetate (TDA) to generate the carbene species [Ru(=CHR)Cl2(PPh3)2(piperidine)] in situ. Interestingly, the exchange of carbenes did not affect the mechanism originally proposed for the series of amines complexes. The complex containing the carbene ethanoate benzyl-1-ylidene (EBI) was what showed the most different thermodynamic profile from the others, following a distinct thermodynamic tendency for the loss of triphenylphosphine ligands. For the remaining steps, the three complexes showed a very similar behavior. The study of this series proved to be more intricate, because these carbenes may affect as much as sterically electronic structure of the compounds at different stages of the mechanism.