The electronic structure of some transition metal compounds, specifically, Ca-doped LaMnO3, fundamental Mn oxides (MnO, Mn2O3, Mn3O4, and MnO2), and Fe-doped ZnO is studied using a combination of soft X-ray spectroscopy and atomic multiplet calculations. X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) are used as experimental tools to probe the unoccupied and occupied partial density of electronic states,respectively.<p> Ca-doped LaMnO3 perovskites have attracted great attention due to their colossal magnetoresistance and a wide range of magnetic and structural transitions. The magnetic and charge transport properties of these perovskites are directly related with Mn 3d-occupancy or Mn-valency and therefore, an investigation of the Mn-valence at Ca-doped LaMnO3 system is important. In this system, the Mn-valency is generally considered as a mixture of Mn3+ and Mn4+. But my research suggests the presence of Mn2+ at the surface of Ca-doped LaMnO3 samples. It is observed that increasing Ca-doping decreases Mn2+ concentration, and conversely, increases Mn3+ concentration. High temperature annealing at 1000 °C in air leads to the full reduction of surface Mn2+. Mechanisms for these observations are proposed in this study.<p> Mn oxides (MnO, Mn2O3, Mn3O4, and MnO2) are often used as reference standards for determining the Mn-valency in Mn-related complex systems and therefore a detailed understanding of their electronic structure is necessary. The Mn L2,3 XAS and O K XAS are measured for the four Mn oxides consisting of three common Mn oxidation states (Mn2+ in MnO, Mn3+ in Mn2O3, mixture of Mn2+ and Mn3+ in Mn3O4, and Mn4+ in MnO2). A significant energy shift with a systematic trend is observed in measured Mn L2,3 and O K absorption edges. These energy shifts are identified as a characteristic shift for different Mn oxidation states. Mn L2,3 Resonant Inelastic X-ray Scattering (RIXS) spectroscopy is demonstrated as a powerful tool in describing low energy excitations, e.g. d-d excitations and charge-transfer excited states in Mn oxides. For the first time, a RIXS study of Mn2O3,Mn3O4, and MnO2 is accomplished. Atomic multiplet calculations are used to successfully reproduce the energy positions and intensity variations of d-d excitation peaks observed in the experiment, and thus to describe the experimental RIXS spectra.<p> Finally, the local electronic structure of Fe implanted ZnO samples, a useful diluted magnetic semiconductor for spintronics, is investigated to shed light on the existing debate about the origin of ferromagnetism in these materials. Fe L2,3 XAS reveals that doped Fe ions are present in both Fe2+ and Fe3+ valence states. A combined theoretical and experimental study shows that doped ions are incorporated into Zn-sites of ZnO in tetrahedral symmetry. Fe L3- RIXS measurements demonstrate that a high Fe-ion dose of 8 × 107 cm-2 causes formation of FeO clusters, while low dose samples exhibit more free carriers.

The electronic structure of some transition metal compounds, specifically, Ca-doped LaMnO3, fundamental Mn oxides (MnO, Mn2O3, Mn3O4, and MnO2), and Fe-doped ZnO is studied using a combination of soft X-ray spectroscopy and atomic multiplet calculations. X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) are used as experimental tools to probe the unoccupied and occupied partial density of electronic states,respectively.<p> Ca-doped LaMnO3 perovskites have attracted great attention due to their colossal magnetoresistance and a wide range of magnetic and structural transitions. The magnetic and charge transport properties of these perovskites are directly related with Mn 3d-occupancy or Mn-valency and therefore, an investigation of the Mn-valence at Ca-doped LaMnO3 system is important. In this system, the Mn-valency is generally considered as a mixture of Mn3+ and Mn4+. But my research suggests the presence of Mn2+ at the surface of Ca-doped LaMnO3 samples. It is observed that increasing Ca-doping decreases Mn2+ concentration, and conversely, increases Mn3+ concentration. High temperature annealing at 1000 °C in air leads to the full reduction of surface Mn2+. Mechanisms for these observations are proposed in this study.<p> Mn oxides (MnO, Mn2O3, Mn3O4, and MnO2) are often used as reference standards for determining the Mn-valency in Mn-related complex systems and therefore a detailed understanding of their electronic structure is necessary. The Mn L2,3 XAS and O K XAS are measured for the four Mn oxides consisting of three common Mn oxidation states (Mn2+ in MnO, Mn3+ in Mn2O3, mixture of Mn2+ and Mn3+ in Mn3O4, and Mn4+ in MnO2). A significant energy shift with a systematic trend is observed in measured Mn L2,3 and O K absorption edges. These energy shifts are identified as a characteristic shift for different Mn oxidation states. Mn L2,3 Resonant Inelastic X-ray Scattering (RIXS) spectroscopy is demonstrated as a powerful tool in describing low energy excitations, e.g. d-d excitations and charge-transfer excited states in Mn oxides. For the first time, a RIXS study of Mn2O3,Mn3O4, and MnO2 is accomplished. Atomic multiplet calculations are used to successfully reproduce the energy positions and intensity variations of d-d excitation peaks observed in the experiment, and thus to describe the experimental RIXS spectra.<p> Finally, the local electronic structure of Fe implanted ZnO samples, a useful diluted magnetic semiconductor for spintronics, is investigated to shed light on the existing debate about the origin of ferromagnetism in these materials. Fe L2,3 XAS reveals that doped Fe ions are present in both Fe2+ and Fe3+ valence states. A combined theoretical and experimental study shows that doped ions are incorporated into Zn-sites of ZnO in tetrahedral symmetry. Fe L3- RIXS measurements demonstrate that a high Fe-ion dose of 8 × 107 cm-2 causes formation of FeO clusters, while low dose samples exhibit more free carriers.

Ce travail de thèse porte sur l'analyse de nanofils de ZnO dopés au cobalt par implantation ionique, en utilisant la fluorescence des rayons X, la spectroscopie d'absorption des rayons X et les techniques de diffraction des rayons X à l'échelle nanométrique sur la ligne de lumière ID22 de l'Installation Européenne de Rayonnement Synchrotron. Les nanofils sont obtenus par croissance catalysée sur des substrats de p-Si (100). Les nanofils de ZnO synthétisés ont été dopés avec du cobalt par d'implantation ionique. Pour la première fois, l'utilisation combinée des techniques de caractérisation par rayons X citées ci-dessus nous permet d'étudier l'homogénéité de la distribution des dopants, la composition, ainsi que l'ordre structurel à courte et grande distance de nanofils individuels. Les résultats de la nano-fluorescence des rayons X indiquent que le dopage au cobalt par implantation ionique dans les nanofils de ZnO est homogène, avec les concentrations désirées. La spectroscopie d'absorption de rayons X et l'analyse des données de diffraction de rayons X fournissent de nouvelles informations sur la distorsion du réseau cristallin produite par l'introduction de défauts structuraux par le processus d'implantation ionique. Ces résultats soulignent l'importance du recuit thermique après l'implantation pour récupérer la structure des nanofils de ZnO à l'échelle du nanomètre. Les mesures complémentaires de micro-photoluminescence et cathodo-luminescence corroborent ces résultats. En conclusion, les méthodes utilisées dans cette thèse ouvrent de nouvelles voies pour l'application de mesures multi-techniques basées sur le rayonnement synchrotron pour l'étude détaillée des nanofils semi-conducteurs à l'échelle nanométrique. / The PhD dissertation focuses on the investigation of single Co-implanted ZnO nanowires using X-ray fluorescence, X-ray absorption spectroscopy, and X-ray diffraction techniques with nanometer resolution at the beamline ID22 of the European Synchrotron Radiation Facility. The ZnO nanowires were grown on p-Si (100) substrates using vapor-liquid-solid mechanism. The synthesized ZnO nanowires were doped with Co via an ion implantation process. For the first time, the combined use of these techniques allows us to study the dopant homogeneity, composition, short- and large-range structural order of individual nanowires. The nano-X-ray fluorescence results indicate the successful and homogeneous Co doping with the desired concentrations in the ZnO nanowires by an ion implantation process. The nano-X-ray absorption spectroscopy and X-ray diffraction data analyses provide new insights into the lattice distortions produced by the structural defect formation generated by the ion implantation process. These findings highlight the importance of the post-implantation thermal annealing to recover the structure of single ZnO nanowires at the nanometer length scale. Complementary microphotoluminescence and cathodoluminescence measurements corroborrate these results. In general, the methodologies used in this work open new avenues for the application of synchrotron based multi-techniques for detailed study of single semiconductor nanowires at the nanoscale.