La chimie de surface de petites nanoparticules métalliques ( ~ 1 nm), principalement de ruthénium ou d'alliages de ruthénium, a été étudiée par une approche théorique au niveau DFT. Cela est appuyé par le développement d'outils d'analyse de propriétés structurales, électroniques et thermodynamiques de ces nanoparticules. Une première partie est consacrée à l'étude des propriétés structurales de nanoparticules métalliques. La variété de morphologie des nanoparticules ainsi que la nécessité de pouvoir générer des modèles appropriés sont mises en évidence. En particulier, l'affinement de la génération de modèles structuraux théoriques est rendu possible via l'implémentation de méthodes de modélisation de nanoparticules génériques couplées à l'utilisation de la méthode de Monte Carlo inversé permettant se rapprocher au plus près de la réalité expérimentale. L'application à ces nanoparticules de descripteurs électroniques ou morphologiques, tels que le d-band center ou le nombre de coordination généralisé, est par la suite proposée en relation avec leur capacités d'adsorption, et plus généralement dans le cadre du principe de Sabatier. Un descripteur électronique de la liaison chimique (COHP) est appliqué aux différentes nanoparticules, pour mettre en évidence les différences entre structures aussi bien que la nature des interactions au sein du cœur métallique, ainsi qu'entre ce cœur et les espèces de surface. Enfin, l'adsorption d'espèces à la surface de ces modèles est étudiée. L'adsorption d'un seul ligand à la surface d'une nanoparticule modèle est utilisée comme sonde de détermination de sites d'adsorption préférentiels, puis des taux d'adsorption plus élevés sont considérés dans le but d'étudier l'influence de celui-ci sur l'adsorption de ligands surnuméraires, ainsi que pour rendre compte de l'influence des ligands de surface sur la morphologie du cœur métallique. Pour cela, les propriétés thermodynamiques des systèmes adsorbés ont été modélisées par prise en compte de l'influence de la pression et de la température sur la stabilité relative des diverses structures via une modélisation de thermodynamique ab initio. Enfin, cette même approche à été utilisée pour étudier la co-adsorption de ligands H2 et CO à la surface de nanoparticules de ruthénium et de rhénium dans le cas particulier de la synthèse de Fischer-Tropsch, permettant notamment de proposer un intermédiaire thermodynamiquement favorable pour cette réaction. Une étude préliminaire de cette réaction, d'un fort intérêt chimique et sociétal, conclut ce manuscrit. L'utilisation combinée des approches structurale, électronique et thermodynamique permet alors d'avoir un point de vue élargi sur certains aspects de la chimie de ces nanoparticules de ruthénium. / Surface chemistry of small metallic nanoparticles ( ~ 1 nm), mainly ruthenium or ruthenium alloys, has been studied at the DFT level via a theoretical approach. This study is supported by the development of analytical tools, that allow to investigate structural, electronic and thermodynamical properties of those nanoparticles. A first part is dedicated to the structural properties of metallic nanoparticles. Morphological diversity is highlighted as well as the necessity of being able to desing reliable models. The refinement of structural models is made possible via the combined use of generic nanoparticles structure design and of the reverse Monte Carlo method in order to fit experiments. Electronic or morphologic descriptors such as d-band center or generalized coordination number are applied to those nanoparticles, in relationship with their adsorption possibilities and, to a larger extent, with the Sabatier principle. An electronic descriptor of the chemical bond (COHP) is applied to the considered nanoparticles in order to show differences between structures, as well as the interactions within the metallic core and between the core and surface species. Finally, adsorption of surface species is studied. A single ligand probe is used to spot favorable adsorption sites, then higher coverages are considered so as to test its influence on the adsorption of extra ligands, and to investigate the effect of surface ligands on the metallic core morphology. To do this, thermodynamical properties of adsorbed systems have been modeled by taking into account the effect of pressure and temperature on the nanoparticles relative stabilities via ab initio thermodynamics. The same approache was eventually applied to H2/CO coadsorbed at ruthenium and rhenium nanoparticles surface, in the context of the Fischer-Tropsch synthesis, allowing to propose a thermodynamically favorable intermediate for this reaction. Preliminary study of this reaction, of high chemical and societal interest, conclude this manuscript. The combined use of structural, electronic and thermodynamical approaches widens the overview on some aspects of ruthenium nanoparticles chemistry

<p>Cette thèse est consacrée à l'étude des effets isotopiques dans les atomes neutres et ions négatifs. En particulier, nous ciblons notre recherche sur le calcul ab initio des déplacements isotopiques (DI) sur les électroaffinités des éléments des blocs p des deuxième et troisième périodes (B à F et Al à Cl). Ces derniers sont les systèmes les plus susceptibles d'être l'objet d'études expérimentales de haute précision.</p><p><p><p>Le premier chapitre se concentre sur une étude didactique du problème atomique et des effets isotopiques. Nous concluons par une description détaillée des motivations de notre thèse.</p><p><p><p>Le second chapitre présente le modèle Hartree-Fock (HF) et son extension multi-configurationelle (MCHF). Nous y énonçons le théorème de Brillouin et sa généralisation à un ansatz MCHF. Pour ce faire, nous formulons de manière originale le principe d'invariance d'une fonction d'onde CAS (Complete Active Set) par rapport aux rotations d'états d'orbitales. De cette formulation, nous caractérisons la famille des solutions CAS n'interagissant pas avec une fonction d'état de configuration (CSF) particulière et démontrons sa multiplicité. Finalement, nous appliquons notre technique d'analyse à l'étude de modèles concrets et prédisons l'apparition de minima locaux correspondant à chacune de ces solutions GBT. Introduisant le concept de quasi-symétrie de la fonctionnelle d'énergie, nous expliquons l'origine de fortes perturbations du "coeur" atomique dans des modèles particuliers.</p><p><p><p>Les troisième et quatrième chapitres fournissent les outils méthodologiques de base utilisés dans la deuxième partie de notre thèse qui présente des résultats quantitatifs originaux.</p><p><p><p>Le cinquième chapitre traite des DI et structures hyperfines des termes les plus bas de S, S-, Cl, Cl-, Si et Si-.</p><p><p><p>Dans le sixième chapitre, nous rapportons un profond désaccord entre théorie et expérience au sujet de la structure hyperfine de transitions de l'azote dans le infrarouge lointain. Nous montrons que les simulations basées sur nos valeurs de constantes isotopiques sont compatibles avec les spectres enregistrés moyennant une réassignation des raies faibles à des signaux de "cross-overs". Sur cette base, nous déduisons un nouvel ensemble de constantes hyperfines pour les états considérés, en bon accord avec nos valeurs théoriques, en nous basant uniquement sur les données expérimentales.</p><p><p><p>Le septième chapitre est une étude globale des configurations de plus basse énergie du C et C- (i.e. tous les états liés de ce dernier). Par une étude détaillée de nos incertitudes, nous obtenons des estimations très fiables et de grande précision pour un ensemble de propriétés. En particulier, nous présentons les valeurs de structure fine et hyperfine du C-, ainsi que les probabilités de transitions intra-configurationelles fournissant une base solide pour l'étude spectroscopique de ce système.</p><p><p><p>Dans le huitième chapitre, nous étudions la périodicité du déplacement spécifique de masse sur l'électroaffinité dans le Tableau Périodique des Eléments. Nous avançons les contributions dominantes qui interviennent dans cette grandeur et analysons les principales limitations des techniques de calcul actuelles dans ce contexte.</p><p><p><p>Nous présentons nos conclusions générales et les perspectives de notre travail dans le neuvième chapitre.</p><p><p><p> / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

In this thesis we describe the extension and implementation of the Sternheimer- GW method to a first-principles pseudopotential framework based on a planewaves basis. The Sternheimer-GW method consists of calculating the GW self-energy operator without resorting to the standard expansion over unoccupied Kohn- Sham electronic states. The Green's function is calculated by solving linear systems for frequencies along the real axis. The screened Coulomb interaction is calculated for frequencies along the imaginary axis using the Sternheimer equa- tion, and analytically continued to the real axis. We exploit novel techniques for generating the frequency dependence of these operators, and discuss the imple- mentation and efficiency of the methodology. We benchmark our implementation by performing quasiparticle calculations on common insulators and semiconductors, including Si, diamond, LiCl, and SiC. Our calculated quasiparticle energies are in good agreement with the results of fully-converged calculations based on the standard sum-over-states approach and experimental data. We exploit the methodology to calculate the spectral func- tions for silicon and diamond and discuss quasiparticle lifetimes and plasmaronic features in these materials. We also exploit the methodology to perform quasiparticle calculations on the 2-dimensional transition metal dichalcogenide system molybdenum disulfide (MoS₂). We compare the quasiparticle properties for bulk and monolayer MoS2 , and identify significant corrections at the GW level to the LDA bandstructure of these materials. We also discuss changes in the frequency dependence of the electronic screening in the bulk and monolayer systems and relate these changes to the quasiparticle lifetimes and spectral functions in the two limits.

Des films minces d’oxydes de cuivre (Cu2O, Cu4O3 et CuO) ont été déposés à température ambiante sur des substrats en verre et en silicium par pulvérisation magnétron réactive. Une attention particulière a été portée à l’influence des conditions de synthèse (débit d’oxygène et pression totale) sur la structure et l’orientation préférentielle des dépôts. La pression totale est le paramètre principal influençant la texture des films de Cu2O et de Cu4O3. En revanche l’orientation préférentielle des films de CuO est contrôlée par le débit d’oxygène. Pour des films de Cu2O et de Cu4O3, un phénomène de croissance épitaxique locale (CEL) a été mis en évidence. Il résulte de l’utilisation d’une première couche qui joue le rôle d’une couche de germination lors du processus de croissance. Ainsi, les films peuvent croître avec une texture donnée indépendamment de leurs conditions de synthèse. Cet effet de CEL a été mis à profit pour élaborer des films biphasés (Cu2O + Cu4O3) qui présentent une microstructure originale. L’augmentation de la transmittance optique et du gap optique de films de Cu2O a été rendue possible par des traitements thermiques dans l’air qui permettent de diminuer la densité de défauts dans les films. Finalement, les propriétés optiques et la structure électronique des oxydes de cuivre qui ont été calculées par la méthode GW sont en accord avec des résultats expérimentaux obtenus par absorption optique, photoémission et spectroscopie de perte d’énergie des électrons. / Copper oxide (Cu2O, Cu4O3 and CuO) thin films have been deposited on unmatched substrates by sputtering at room temperature. The influence of oxygen flow rate and total pressure on the film structure and preferred orientation has been studied. The total pressure is a relevant parameter to control the texture of Cu2O and Cu4O3 films, while the oxygen flow rate is effective to tune the preferred orientation of CuO films. Local epitaxial growth, where epitaxial relationship exists in columns of sputtered films, has been observed in Cu2O and Cu4O3 films by using a seed layer. The seed layer will govern the growth orientation of top layer via the local epitaxy, independently of the deposition conditions of top layer. Unusual microstructure that both phases have the vertically aligned columnar growth has been evidenced in biphase Cu2O and Cu4O3, which may relate to the local epitaxial growth of Cu2O. The lower resistivity than that in single phase films has been observed in this biphase film. Annealing in air can increase the transmittance of Cu2O films in visible region by the reduction of the impurity scattering, while the optical band gap is enlarged due to the partial removal of defect band tail. The optical properties and electronic structure of copper oxides calculated by GW approach with an empirical on-site potential for Cu d orbital, are in good accordance with experimental results from optical absorption, photoemission and electron energy loss spectroscopies

Designing novel nanostructures which exploit the self-assembly capabilities of biomolecules yields a promising approach to control matter at the nanoscale. Here, the homochiral molecular self-assemblies of the methionine and tyrosine amino acids on the monocrystalline Ag(111) and Cu(111) surfaces are characterized by means of scanning tunneling microscopy (STM) and spectroscopy (STS), helium atom scattering (HAS), x-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) in ultrahigh vacuum (UHV). On Ag(111), methionine self-assembles into supramolecular chains following the <110> substrate axis, forming regular nanogratings with tunable periodicity. Within the nanowires, a zwitterionic dimerization scheme is revealed. STS shows that the biomolecular nanostructures act as tunable one-dimensional quantum resonators for the surface state electrons. Zero-dimensional electronic confinement is achieved by positioning single iron atoms in the molecular trenches. This shows a novel approach to control the dimensionality of surface state electrons. The nanogratings were exploited to steer the spontaneous one-dimensional ordering of cobalt and iron atoms. For T > 15 K, the metal species self-align into homogeneously distributed chains in between the biomolecular trenches with ~25 Å interatomic distace. For Co, the dynamics of the self-alignment was monitored, revealing a reduced mobility in comparison with isolated Co atoms on bare Ag(111). On Cu(111), the self-assembly of methionine is influenced by the substrate reactivity and its temperature during molecular deposition. For T < 273 K, the biomolecules assemble in anisotropic extended clusters oriented with a -10° rotation off the <110> substrate orientations, whereas above 283 K a regularly ordered 1D phase arises with a +10° rotation off these high-symmetry axis. XPS reveals a structural transformation triggered by a thermally activated deprotonation of the zwitterionic ammonium group. On Ag(111), tyrosine self-assembles above a critical temperature into linear structures primarily following the substrate crystalline symmetry. A zwitterionic non-covalent molecular dimerization is demonstrated, NEXAFS data providing evidence of a non-flat adsorption of the phenyl ring. This recalls the geometrical pattern of methionine on Ag(111) and supports a universal self-assembling scheme for amino acids on close-packed noble metal surfaces, the different mesoscopic ordering being determined by the side chain reactivity. / Science, Faculty of / Chemistry, Department of / Graduate

This thesis deals with theoretical studies of electronic properties of organic conjugated molecules. The first chapter introduces different classes of organic conjugated molecules which possess high hole mobility, large quadratic non-linear response and low band gap. In this chapter, we further describe different photo-physical processes and the basic principles of various opto-electronic devices. The second chapter provides an introduction to various many-body techniques, which are employed in studying ground and excited state properties of organic conjugated systems. First, we describe the Hartree-Fock theory and the Density Functional (DFT) method. These are followed by full Configuration-Interaction (CI) methods and various semi-empirical methods (CNDO, INDO and NDDO). The INDO method is used in subsequent chapters to obtain the ground and excited state properties of organic conjugated molecules. In addition, we describe the restricted CI (SCI and SDCI) and the Density Matrix Renormalization Group (DMRG) methods. The third chapter of this thesis deals with a time evolution study to ascertain the role of the triplet state in the green emission of the ethyl-hexyl substituted poly-fluorene (PF2/6) films. To understand this phenomenon, we have modeled various non-radiative processes like (i) Inter-System Crossing (ISC), (ii) electron-hole Recombination (e-hR) and (iii) Triplet Quenching (TQ). These studies conclusively prove the contribution of triplet states to the 500 nm EL peak. In chapter four, we describe the origin of the unusual EL in tri-p-tolylamine (TTA) based hole conductors. In order to model this phenomenon, we have performed SCI calculations on TTA, its radical ions and allied hole conductors (TAPC and TPD). These calculations indicate that the unusual EL is due to low-lying charge-transfer (CT) state, which is stabilized by charge-dipole and charge-induced-dipole interactions. In chapter five, we turn our attention to the calculation of ground and excited state properties of a class of donor-acceptor (DA) system using ab-initio DFT and INDO methods. In these systems, DFT calculations along with INDO-SCI calculation, show strong intramolecular charge transfer interaction between the D and the A units. We have further calculated various properties like permanent dipole moments, oscillator strengths, Stoke’s shifts in various solvents etc. In chapter six, we focus on studying linear and non-linear optical properties of first generation nitrogen based dendrimers, using DMRG method. A novel scheme which includes the weights of the dipole allowed states in the computation of the density matrix is developed to obtain accurate dipole allowed excited states as well as the linear and nonlinear optical responses. Chapter seven deals with non-linear optical properties of weak donor-acceptor (DA) complexes formed between methyl substituted phenylenes (donor) and Chloranil or DDQ (acceptors). We have calculated the ground and the low-lying excited states of these DA complexes using INDO-SDCI method. The first hyperpolarizability (β) response coefficients are calculated using the Correction Vector (CV) technique, which are further used to obtain macroscopic depolarization ratios. By comparing the theoretical results with experimental findings, it can be shown that the slipped parallel configuration with a slight twist is the most preferred geometry of these weak DA complexes in solution.

Graphene – a two-dimensional honeycomb lattice of sp2-bonded carbon atoms – has been attracting a great deal of research interest since its first experimental realization in 2004, due to its various novel properties and its potential for applications in futuristic nanodevices. Being the fundamental building block for carbon allotropes of other dimensionality, it can be stacked to form 3d graphite or rolled into 1d nanotube. Graphene is the thinnest known material in the universe, and one of the strongest materials ever measured in terms of its in-plane Young modulus and elastic stiffness. The charge carriers in graphene exhibit giant mobility as high as 20 m2/Vs, have almost zero effective mass, and can travel for micrometers without scattering even at ambient conditions. Graphene can sustain current densities six orders of magnitude higher than that of copper, shows record thermal conductivity and stiffness, is impermeable to gases, and renders easy accessibility to optical probes. Electron transport in graphene is described by a Dirac-type equation, which allows the investigation of “relativistic” quantum phenomena in a benchtop experiment. This results in the observation of a number of very peculiar electronic properties from an anomalous quantum Hall effect to Kien paradox and the absence of localization. All these enticing features make this material an excellent candidate for application in various electronic, photonic and optoelectronic devices. For instance, its ballistic ambipolar transport and high carrier mobility are the most useful traits for making ultrafast and low-power electronic devices. Its high surface area shouldmake it handy in manufacturing tough composite materials. The extreme thinness of graphene could also lead to more efficient field emitters that release electrons in the presence of strong electric fields. Its robustness and light weight are useful for micromechnical resonators. The tunability of its properties could make it possible to build so-called spin-valve transistors, as well as ultra-sensitive chemical detectors. Many of such applications of graphene require tuning of its properties, which can be achieved by varying the number of layers or/and by doping. There are several ways to dope graphene: (i)electrochemically gated doping, (ii)molecular charge-transfer doping, and (iii) substitutional doping by atoms like Boron or Nitrogen.Moreover, for graphene, a zero band gap semiconductor in its pristine form, to become a versatile electronic device material it is mandatory to find means to open up a band gap and tune the size of the band gap. Several strategies have been adopted to engineer such a band gap in graphene in a controlled way. Some of these are based on the ability to control the geometry of graphene layers, some use graphene-substrate interactions, while others are based on chemical reactions of atoms or molecules with the graphene layer. Motivated by these considerations, in this thesis we present a systematic and thorough study of the structural, electronic and vibrational properties of graphene and their dependence on the number of layers, and on doping achieved electrochemically, molecularly and substitutionally, using first principles density functional theory (DFT). In Chapter 1, we give an introduction to the hitherto beguiling world of graphene. Here, we briefly discuss the structure, novel properties and potential applications of graphene, and the motivation for this thesis. In Chapter 2, an overview of the DFT formalism adopted here is given. We clearly state the theorems of the formalism and the approximations used when performing calculations. We succinctly explain how the various quantities like total energies, forces, stresses etcetera are calculated within this formalism. We also discuss how phonon frequencies, eigenvectors, electron-phonon couplings are obtained by using density functional perturbation theory (DFPT), which calculates the full dynamical matrices through the linear response of electrons to static perturbations induced by ionic displacements. Calculations are done first using a fully ab-initio approach within the standard Born-Oppenheimer approximation, and then time-dependent perturbation theory is used to explore the effects of dynamic response. In Chapter 3, using such first-principles density-functional theory calculations, we determine the vibrational properties of ultra-thin n(1,2,...,7)-layer graphene films and present a detailed analysis of their zone-center phonons. We present the results (including structural relaxations, phonons, mode symmetries, optical activities) for bulk Graphite, single-layer graphene and ultrathin n-layer graphene films. and discuss the underlying physics of our main results together with a pictorial representation of the phonon modes. We demonstrate that a low-frequency (∼ 112 cm−1 ) optical phonon with out-of-plane displacements exhibits a particularly large sensitivity to the number of layers, although no discernible change in the interlayer spacing is found as n varies. Frequency shifts of the optical phonons in bilayer graphene are also calculated as a function of its interlayer separation and interpreted in terms of the inter-planar interaction. The surface vibrational properties of n-layer graphene films are presented in Chapter 4, which renders a detailed and thorough analysis of all the surface phonon modes by determining, classifying and identifying them accurately. The response of surface modes to the presence of adsorbed hydrogen molecules is determined. As an illustrative adsorbate, hydrogen is chosen here mainly because of its huge importance in fuel cell technology and as a molecular sensor. We demonstrate that a doubly degenerate surface phonon mode with low-frequency (~ 35cm−1)exhibits a particularly large sensitivity to the adsorption of hydrogen molecules, as compared to other surface modes. Futhermore, we show that a low-frequency (108.8 cm−1)bulk-like phonon with out-of-plane displacements is also very sensitive and gets upshifted by as much as 21 cm−1 due to this adsorption. In Chapter 5, we determine the adiabatic frequency shift of the and phonons in a monolayer graphene as a function of both electron and hole doping. The doping is simulated here to correspond to electrochemically gated graphene. Compared to the results for the E2g -Γ phonon (Raman G band), the results for the phonon are dramatically different, while those for the phonon are not so different. Furthermore, we calculate the frequency shifts, as a function of the charge doping, of the (K + ΔK) phonons responsible for the Raman 2D band –a key finger print of graphene, where [ΔK] is determined by the double resonance Raman process. Doping graphene with electron donating or accepting molecules is an interesting approach to introduce carriers into it, analogous to electrochemical doping accomplished in graphene when used in a field-effect transistor. In Chapter 6, we use first-principles density-functional theory to determine changes in the electronic structure and vibrational properties of graphene that arise from the adsorption of aromatic molecules such as aniline and nitrobenzene. Identifying the roles of various mechanisms of chemical interaction between graphene and the adsorbed molecules, we bring out the contrast between electrochemical and molecular doping of graphene. Our estimates of various contributions to shifts in the Raman active modes of graphene with molecular doping are fundamental to the possible use of Raman spectroscopy in (a)characterization of the nature and concentration of carriers in graphene arising from molecular doping, and (b) graphene-based chemical sensors. Graphene doped electrochemically or through charge-transfer with electron-donor and acceptor molecules, shows marked changes in electronic structure, with characteristic signatures in the Raman spectra. Substitutional doping, universally used in tuning properties of semiconductors, could also be a powerful tool to control the electronic properties of graphene. In Chapter 7, we present the structure and properties of boron and nitrogen doped graphenes, again using first-principles density functional theory. We demonstrate systematic changes in the carrier-concentration and electronic structure of graphenes with B/N-doping, accompanied by a stiffening of the G-band and change of the defect related D-band in the Raman spectra. Such n/p -type graphenes obtained without external fields or chemical agents should find device applications.

Après la découverte de la magnéto-résistance géante (GMR) par Albert Fert et Peter Grünberg, l'électronique a connu une véritable avancée avec la naissance d'une nouvelle branche appelée spintronique. Cette discipline, encore jeune, consiste à exploiter le spin des électrons dans le but notamment de stocker de l'information numérique. La plupart des dispositifs exploitant cette propriété quantique des électrons consistent en une alternance de fines couches magnétiques et non magnétiques sur un substrat semi-conducteur. L'un des outils de choix pour la caractérisation de ces structures, inventé en 1988 par Kaiser et Bell, est le microscope à émission d'électrons balistiques (BEEM). A l'origine, ce microscope, dérivé du microscope à effet tunnel (STM), était dédié à l'imagerie d'objets (nanométriques) enterrés ainsi qu'à l'étude de la barrière de potentiel (barrière Schottky) qui se forme à l'interface d'un métal et d'un semi-conducteur lors de leur mise en contact. Avec l'essor de la spintronique, le BEEM est devenu une technique de spectroscopie essentielle mais encore fondamentalement incomprise. C'est en 1996 que le premier modèle réaliste, basé sur le formalisme hors équilibre de Keldysh, a été proposé pour décrire le transport des électrons dans cette microscopie. Il permettait notamment d'expliquer certains résultats expérimentaux jusqu'alors incompris. Cependant, malgré son succès, son usage a été limité à l'étude de structures semi-infinies via un méthode de calcul appelée décimation de fonctions de Green. Dans ce contexte, nous avons étendu ce modèle au cas des films minces et des hétéro-structures du type vanne de spin : partant du même postulat que les électrons suivent la structure de bandes du matériaux dans lesquels ils se propagent, nous avons établi une formule itérative permettant le calcul des fonctions de Green du système fini par la méthode des liaisons fortes. Ce calcul des fonctions de Green a été encodé dans un programme Fortran 90, BEEM v3, afin de calculer le courant BEEM ainsi que la densité d'états de surface. En parallèle, nous avons développé une autre méthode, plus simple, qui permet de s'affranchir du formalisme hors équilibre de Keldysh. En dépit de sa naïveté, nous avons montré que cette approche permettait l'interprétation et la prédiction de certains résultats expérimentaux de manière intuitive. Cependant, pour une étude plus fine, le recours à l'approche “hors équilibre” reste inévitable, notamment pour la mise en évidence d'effets d'épaisseur, lés aux interfaces inter-plans. Nous espérons que ces deux outils puissent se révéler utiles aux expérimentateurs, et notamment pour l'équipe Surfaces et Interfaces de notre département. / After the discovery of Giant Magneto-Resistance (GMR) by Albert Fert and Peter Grünberg, electronics had a breakthrough with the birth of a new branch called spintronics. This discipline, while still young, exploit the spin of electrons, for instance to store digital information. Most quantum devices exploiting this property of electrons consist of alternating magnetic and nonmagnetic thin layers on a semiconductor substrate. One of the best tools used for characterizing these structures, invented in 1988 by Kaiser and Bell, is the so-called Ballistic Electron Emission Microscope (BEEM). Originally, this microscope, derived from the scanning tunneling microscope (STM), was dedicated to the imaging of buried (nanometer-scale) objects and to the study of the potential barrier (Schottky barrier) formed at the interface of a metal and a semiconductor when placed in contact. With the development of spintronics, the BEEM became an essential spectroscopy technique but still fundamentally misunderstood. It was in 1996 that the first realistic model, based on the non-equilibrium Keldysh formalism, was proposed to describe the transport of electrons during BEEM experiments. In particular, this model allowed to explain some experimental results previously misunderstood. However, despite its success, its use was limited to the study of semi-infinite structures through a calculation method called decimation of Green functions. In this context, we have extended this model to the case of thin films and hetero-structures like spin valves: starting from the same postulate that electrons follow the band structure of materials in which they propagate, we have established an iterative formula allowing calculation of the Green functions of the finite system by tight-binding method. This calculation of Green’s functions has been encoded in a FORTRAN 90 program, BEEM v3, in order to calculate the BEEM current and the surface density of states. In parallel, we have developed a simpler method which allows to avoid passing through the non-equilibrium Keldysh formalism. Despite its simplicity, we have shown that this intuitive approach gives some physical interpretation qualitatively similar to the non-equilibrium approach. However, for a more detailed study, the use of “non-equilibrium approach” is inevitable, especially for the detection of thickness effects linked to layer interfaces. We hope these both tools should be useful to experimentalists, especially for the Surfaces and Interfaces team of our department.

Durant l'irradiation en réacteur la fission des atomes d'actinides entraine la création de grandes quantités de défauts, qui affecte les propriétés physiques et chimiques des matériaux dans le réacteur, en particulier les matériaux combustibles ou de structure. Une des méthodes non destructives pouvant être utilisées pour caractériser les défauts induits par irradiation, vides ou contenant les produits de fission, est la spectroscopie d'annihilation de positons (SAP). Cette technique expérimentale consiste à détecter le rayonnement généré lors de l'annihilation du paire électron-positon dans un échantillon et en déduire les propriétés de la matière étudiée. Les positons peuvent être piégés dans les défauts de type lacunaire dans les solides, et en mesurant leur temps de vie et les distribution de moment du rayonnement d'annihilation, on peut obtenir des informations sur les volumes libres et les environnements chimiques des défauts. Dans ce travail, des calculs de structure électronique des caractéristiques d'annihilation de positons ont été effectués en utilisant la théorie de la fonctionnelle de la densité à deux composants (TCDFT). Pour calculer les distributions de moment rayonnement d'annihilation, nous avons implémenté les méthodes nécessaires dans le code de calcul libre ABINIT. Les résultats théoriques ont été utilités pour contribuer à l'identification des défauts d'irradiation dans deux céramiques nucléaires, le carbure de silicium (SiC) et le dioxyde d'uranium (UO2). / During in-pile irradiation the fission of actinide nuclei causes the creation of large amounts of defects, which affect the physical and chemical properties of materials inside the reactor, in particular the fuel and structural materials. Positron annihilation spectroscopy (PAS) can be used to characterize irradiation induced defects, empty or containing fission products. This non-destructive experimental technique involves detecting the radiation generated during electron-positron annihilation in a sample and deducing the properties of the material studied. As positrons get trapped in open volume defects in solids, by measuring their lifetime and momentum distributions of the annihilation radiation, one can obtain information on the open and the chemical environments of the defects. In this work electronic structure calculations of positron annihilation characteristics were performed using two-component density functional theory (TCDFT). To calculate the momentum distributions of the annihilation radiation, we implemented the necessary methods in the open-source ABINIT program. The theoretical results have been used to contribute to the identification of the vacancy defects in two nuclear ceramics, silicon carbide (SiC) and uranium dioxide (UO2).

The structural, electronic, energetic and optical properties of the compound Li2B4O7 containing the substitutional defects Cu or Ag were investigated by means of calculations of first principles at the Density Functional Theory level using the LAPW method implemented in the computer code Wien2k. The isolated Cu and Ag defects are considered in four charge states (q = -1, 0, +1, +2) with objective to simulate situations of the capture of an electron or a hole. In all cases, the atomic positions are computationally relaxed, Cu – O and Ag – O chemical bonds nature carefully analyzed and local structure around the defects determined. It is found that the defects vastly perturbs its neighborhood and the Cu and Ag themselves exhibit significant off-site dislocation from initial Li position in their Cu1+ and Ag1+ charge states, which becomes especially more pronounced for the Cu0 and Ag0 defects. Only the Cu2+ and Ag2+ centers stabilize at the substitutional Li site. Resulting defect formation energies demonstrate that the Cu1+, Cu0, Ag1+, and Ag0 centers are the most stable ones. Electronic structure calculations reveal that the Cu and Ag ions introduce their d- and s-states within the gap and their energies and occupation depend strongly on the charge state of the defect. Experimental optical absorption spectra are well reproduced by the Cu1+ and Ag1+ defects spectra, leading to the conclusion that in the as-grown material just Cu1+ and Ag1+ centers are formed. In the case of irradiated compound, present study predicts formation of the interstitial Cu0 defects, whose presence should significantly change the optical absorption and emission of the material, as well as demonstrates the presence of new absorption peaks associated with the interstitial Ag0 and substitutional Ag2+ centers, which reasonably describe the experimental spectrum. / As propriedades estruturais, eletrônicas, energéticas e ópticas do composto Li2B4O7 contendo os defeitos substitucionais Cu ou Ag foram investigadas por meio de cálculos de primeiros princípios ao nível da Teoria do Funcional da Densidade, utilizando-se o método LAPW implementado no código computacional Wien2k. Os efeitos de troca-correlação foram considerados usando-se o potencial modificado de Becke-Johnson (mBJ) e a aproximação do gradiente generalizado (GGA) na parametrização PBEsol. As células contendo os defeitos (Cu ou Ag) foram estudadas em quatro estados de carga diferentes (q = -1, 0, +1, +2) com o objetivo de simular situações de captura de elétrons ou de buracos. Em todos os casos, as posições atômicas foram relaxadas, a natureza das ligações Cu – O e Ag – O cuidadosamente analisada e as estruturas locais em torno dos defeitos determinadas. Verificou-se que os defeitos perturbam amplamente suas vizinhanças e que os centros Cu1+ e Ag1+ exibem um deslocamento significativo a partir da posição inicial do Li, que se torna ainda mais acentuado para os centros Cu0 e Ag0. Apenas os defeitos Cu2+ e Ag2+ se estabilizam no sítio substitucional do Li. Os resultados das energias de formação dos defeitos demonstraram que os centros Cu1+, Cu0, Ag1+ e Ag0 são os mais estáveis. Cálculos de estrutura eletrônica revelaram que as impurezas introduzem os estados d e s dentro do gap, cujas energias e ocupações dependem fortemente do estado de carga do defeito. Os espectros ópticos experimentais foram bem reproduzidos pelos espectros dos centros Cu1+ e Ag1+ calculados, levando-se à conclusão de que somente estes centros são formados no material não irradiado. Durante a irradiação do composto, o presente estudo prevê a formação dos centros intersticiais Cu0, fato que deve alterar significativamente a absorção e emissão óptica do material, bem como demonstra a presença de novos picos de absorção associados aos centros intersticiais Ag0 e substitucionais Ag2+, os quais descrevem razoavelmente bem o espectro experimental. / São Cristóvão, SE