This dissertation research is focused on first principles studies of graphene and single organic molecules for nanoelectronics applications. These nanosized objects attracted considerable interest from the scientific community due to their promise to serve as building blocks of nanoelectronic devices with low power consumption, high stability, rich functionality, scalability, and unique potentials for device integration. Both graphene electronics and molecular electronics pursue the same goal by using two different approaches: top-down approach for graphene devices scaling to smaller and smaller dimensions, and bottom-up approach for single molecule devices. One of the goals of this PhD research is to apply first-principles density functional theory (DFT) to study graphene/metal and molecule/metal contacts at atomic level. In addition, the DFT-based approach allowed us to predict the electronic characteristics of single molecular devices. The ideal and defective graphene/metal interfaces in weak and strong coupling regimes were systematically studied to aid experimentalists in understanding graphene growth. In addition, a theory of resonant charge transport in molecular tunnel junctions has been developed. The first part of this dissertation is devoted to the study of atomic, electronic, electric, and thermal properties of molecular tunnel junctions. After describing the model and justifying the approximations that have been made, the theory of resonant charge transport is introduced to explain the nature of current rectification within a chemically asymmetric molecule. The interaction of the tunneling charges (electrons and holes) with the electron density of the metal electrodes, which in classical physics is described using the notion of an image potential, are taken into account at the quantum-mechanical level within the tight binding formalism. The amount of energy released onto a molecule by tunneling electrons and holes in the form of thermal vibration excitations is related to the reorganization energy of the molecule, which is also responsible for an effective broadening of molecular levels. It was also predicted that due to the asymmetry of electron and hole resonant energy levels with respect to the Fermi energy of the electrodes, the Joule heating released from the metallic electrodes is also non-symmetric and can be used for the experimental determination of the type of charge carriers contributing to the molecular conductance. In the second part of the dissertation research ideal and defective graphene/metal interfaces are studied in weak and strong interface coupling regimes. The theoretical predictions suggest that the interface coupling may be controlled by depositing an extra metallic layer on top of the graphene. DFT calculations were performed to evaluate the stability of a surface nickel carbide, and to study graphene/carbide phase coexistence at initial stages of graphene growth on Ni(111) substrate at low growth temperatures. Point defects in graphene were also investigated by DFT, which showed that the defect formation energy is reduced due to interfacial interactions with the substrate, the effect being more pronounced in chemisorbed graphene on Ni(111) substrate than in physisorbed graphene on Cu(111) substrate. Our findings are correlated with recent experiments that demonstrated the local etching of transfered graphene by metal substrate imperfections. Both graphene and molecular electronics components of the PhD dissertation research were conducted in close collaboration with several experimental groups at the University of South Florida, Brookhaven National Laboratory, University of Chicago, and Arizona State University.

Les argiles sont utilisées dans de nombreuses applications industrielles pour lesquelles l’étude des interactions entre l’eau et les matériaux argileux sont primordiales. Les mécanismes de rétention et de transport de l'eau et des ions à la surface des argiles peuvent être modélisés à l'échelle atomique grâce à des méthodes classiques comme la Dynamique Moléculaire. Ces méthodes nécessitent de paramétrer au préalable les interactions entre les atomes du système. L'objectif principal de cette étude est d'améliorer la description de ces systèmes via la paramétrisation d'un nouveau champ de force polarisable entièrement basée sur des calculs issus de la méthode de la Théorie de la Fonctionnelle de la Densité. Les propriétés structurales, thermodynamiques et dynamiques de la pyrophyllite, du talc, et de la Na-, Ca-, Sr- et Cs-montmorillonite (sèches et hydratées) ont été bien reproduites. Notamment, la structure des couches tétraédriques et celle des espaces interfoliaires sont en très bon accord avec les données expérimentales. / The wide use of clay minerals in industrial applications is partly due to their remarkable properties of water retention at the mineral surface. Retention and transport mechanisms of water molecules and ions at the surface of clays can be modeled at the atomic scale via different classical methods such as molecular dynamics. These methods require to parametrize in advance the interaction between the atoms of the system. The goal of this study is to improve the description of these systems via the parametrization of a new polarizable force field entirely based on density functional theory calculations.The structure, the thermodynamics and the dynamics properties of pyrophyllite, talc and Na-, Ca-, Sr- and Cs-montmorillonite are well reproduced. The atomic structure of sheets and interfoliar space are in good agreement with experimental results.

The need for highly efficient energy storage devices has been steadily increasing due to growing energy demands. Research in electrochemical energy storage in the form of batteries has consequently become crucial. Currently, the most commercialized battery technology is the lithium ion battery (LIB). Concerns over the relatively limited global lithium supply, however, have led to the development of sodium ion batteries (SIBs). Solid-state nuclear magnetic resonance (ssNMR) spectroscopy is an ideal technique for analyzing battery materials as it can potentially distinguish between different ions within the material. The most typical cathode for commercial LIBs are the family of NMC layered oxides with the general form Li[NixMnyCo1-x-y]O2, which consist of Li layers between sheets of transition metals (TMs). The pj-MATPASS NMR technique, in conjunction with Monte Carlo simulations, was applied to investigate the ionic arrangement within TM layers of NMC622 (Li[Ni0.6Mn0.2Co0.2]O2), which revealed the presence of ion clustering in the pristine form of this material. This thesis also investigated the promising SIB cathode, Na3V2(PO4)2F3 (NVPF). NVPF has the capability to produce energy densities comparable to those of LIBs and is well understood from a structural standpoint, however ion dynamics within the material are still undetermined. A series of materials have, therefore, been synthesized with the general form, Na3V2-xGax(PO4)2F3 (where x = 0, 1, and 2), where diamagnetic Ga3+ was introduced into the structure to enable the establishment of a structural correlation with observed Na-ion dynamics. It, therefore, became possible to explore ionic site exchange using 23Na ssNMR. Density functional theory (DFT) calculations have also been performed alongside ssNMR to confirm chemical shift assignments and provide structural insight. Additionally, electron paramagnetic resonance (EPR) spectroscopy was also used to investigate the paramagnetic nature of NVPF and its variants. Insights into the ionic arrangement and very fast Na-ion dynamics within these materials were revealed. / Thesis / Master of Science (MSc) / The need for highly efficient energy storage devices, especially in the form of batteries, has been steadily increasing due to growing energy demands. Presently, the most commercialized types of batteries are lithium ion batteries (LIBs). Concerns over the relatively limited global lithium supply, however, have led to the development of sodium ion battery (SIB) alternatives. Various solid-state nuclear magnetic resonance (ssNMR) techniques have been employed in this thesis to investigate both LIB and SIB cathode materials. The LIB cathode Li[Ni0.6Mn0.2Co0.2]O2 was examined with a combination of ssNMR and Monte Carlo simulations, and it was found that ion clustering occurs in the pristine form of these materials. The promising family of SIB cathodes, Na3V2-xGax(PO4)2F3, was studied by a combination of ssNMR, ab initio calculations, and EPR, which allowed for a correlation to be established between the crystal structure and the fast ion dynamics within these materials.