Scanning tunneling microscopy (STM) images and scanning tunneling spectroscopy (STS) spectra of galena (PbS) and hematite (a-Fe203) were calculated using ab-initio methods in order to interpret experimental images and spectra that were taken in previous studies. These calculations have helped to understand which states of the mineral surfaces were imaged depending on the bias voltage and tip-sample separation. The computational results also gave insight in electron transfer processes that take place during surface adsorption/oxidation/reduction processes. In this context, different oxidation (using O₂ and ferric iron as oxidants) and gold adsorption/reduction mechanisms on galena were evaluated at an atomic level. On hematite, the main emphasis was determining the differences in the local electronic structure of specific sites above the surface and the electronic structure of the bulk. Hereby, step sites turned out to have an increased local density of states at certain electron binding energies that are absent on flat surfaces. states can explain the highly increased reactivity of step sites as compared to terraces. X-ray photoelectron spectra (XPS) were calculated to compare the photoelectron peaks of the calculated specific surface structures (that do not have a bulk equivalent) with experimentally obtained XPS spectra. Most of the calculated peak chemical shifts coincided with those that were found in experiments and that were previously interpreted in terms of known bulk structures. Therefore, it can be inferred that the conventional way of interpreting XPS spectra might be incomplete if specific surface structures are neglected. In order to understand step velocities on a gypsum (010) surface, step energies of different step directions were calculated using an ab-initio approach. An approximately linear relationship was found between the calculated step energies and the experimentally determined step velocities. / Ph. D.

L’objectif principal de cette thèse est d’utiliser le calcul ab initio fondé sur la théorie de la densité fonctionnelle (DFT) pour calculer et comprendre la transition HS-BS de la molécule FePhen adsorbée sur un substrat métallique. Il s’agit d’abord de décrire les interactions entre FePhen et un substrat métallique ferromagnétique, comme le Co, ou un substrat paramagnétique comme l’or ou le cuivre. Nous avons calculé la barrière de potentiel nécessaire pour basculer la molécule de l’état BS à l’état HS en utilisant la méthode ’Nudged Elastic Band’ (NEB) et développé une méthode basée sur les principes de la méthode NEB, pour déterminer la trajectoire d’énergie minimale (TEM) de la molécule adsorbée sur un substrat métallique. Nous avons calculé le couplage ferromagnétique entre la molécule et le cobalt en fonction du nombre de couches intermédiaires de cuivre. Nous avons étudié en particulier (1) le rôle joué par les interactions van der Waals, (2) la modélisation de la microscopie à effet tunnel (STM) et (3) les propriétés de transport. Nous avons calculé les images STM en utilisant l’approximation de Tersoff et Hamann, qui montrent un bon accord avec les résultats expérimentaux. Nous vons étudié les propriétés de transport de la molécule FePhen adsorbée sur une surface métallique, en utilisant le code Smeagol basé sur la méthode des fonctions de Green hors-équilibre, et le formalisme de Landauer. / The main objective of this PhD thesis is to use ab initio methods based on DFT to calculate and understand the mechanism of spin crossover phenomena in FePhen molecule adsorbed on a metallic substrate. We studied the structural, electronic and magnetic properties of the free and adsorbed FePhen molecule on a ferromagnetic metal substrate, such as cobalt, or a paramagnetic substrate such as gold or copper. We calculated the energy barrier required for the molecule to switch from low-spin to high-spin states using the ’Nudged Elastic Band’ (NEB) method. We also computed the ferromagnetic coupling between two magnetic layers, the magnetic FePhen molecule and the cobalt substrate according to the number of non-magnetic intermediate layer of copper. The focus is mainly on (1) the role played by van der Waals interactions, (2) the modeling of scanning tunneling microscopy (STM) and (3) the transport properties. We calculated the STM images using the Tersoff-Hamann approximation, which showed a good agreement with recent experimental STM images. We studied the transport properties of the adsorbed molecule FePhen on a metallic surface, using the Smeagol code which is based on the non-equilibrium Green’s function and Landauer formalism.