The Investigation Of Local Structure And Electronic Structure Of DMS By XAS

In this thesis, firstly we reviewed the principle of XAS, experimental equipment and data analyzing method. Secondly we introduced the application of XAS combined multiple-scattering theory on studying atomic local structure and electronic structure of Dilute magnetic semiconductors (DMSs) system.In the past decade, following the first report by Ohno, diluted magnetic semiconductor (DMS) materials attracted a considerable interest due to their potential promising applications in spintronic devices. However, the understanding of this new class of materials is a real challenge because of their anomalous magnetic behavior and the occurrence of room temperature ferromagnetism (RTF). Several manuscripts are available in the literature and sometime controversial interpretations have been proposed. Some contributions addressed the RTF origin to the presence of magnetic clusters or secondary phases, while other associated the observed RTF to intrinsic properties of the systems. At the same time, a consensus exists on the presence in these materials of a conventional super-exchange or a double-exchange interaction, i.e., a mechanism that does not support the existence of a long-range magnetic order at concentrations of magnetic cations of a few percent. Several theoretical frameworks capable to treat this fundamental aspect have been also proposed. Dietl et al. calculated the Curie temperature for various p-type semiconductors by means of the Zener model. They suggested that both ZnO and GaN are suitable DMS materials and the RTF is possibly due to the interplay of the magnetic ion doping with the hole content in the host semiconductor. RTF behavior in insulators can be also explained, as proposed by Coey et al., via a ferromagnetic exchange mechanism mediated by shallow donor electrons forming bound magnetic polarons (BMPs). In particular, a spin-orbit-lattice coupling have been taken into account to introduce a new mechanism that consider the magnetoelastic effect of 3dn ions with Jahn-Teller orbital occupancy as the origin of the lattice-assisted spin ordering. In this framework, a previous experimental investigation can be reasonably understood.1. The local structure of Co in Co-doped ZnO single crystal systemSingle crystal is a material'form with few impurities and vacancy and has perfect lattice structure, thus it can be as an ideal candidate system for fundamental research. In this investigation, we synthesized Zn0.95Co0.05O single crystal by hydrothermal method and perform the X-ray absorption spectroscopy (XAS) measurement at Co K-edge. By comparing the XANES between our sample and several cobalt oxides, we demonstrated that Co is incorporated into ZnO matrix well and occupied the Zn site.2. Determination for location of oxygen vacancy in Co-doped ZnO systemsSince a number of investigations have demonstrated that oxygen vacancies (Ov) play a significant role in mediating the RTF behavior, different experimental researches provided the evidence of their existence. However, no experimental data clarify the exact location that is an important issue addressed by several theoretical calculations. In this contribution, we investigated by X-ray absorption spectroscopy (XAS) combined with ab initio calculations the local atomic structure of Co and Zn in the Zn0.9Co0.1O system before and after an annealing treatment. The analysis indicates that an oxygen atom in the second shell surrounding the cobalt atom disappears after the annealing treatment. Moreover, from the Co L3,2-edge XANES we may also recognize that the Ov affects significantly the occupancy of Co-3d states. DOS calculations based on the density functional theory (DFT) strongly support this claiming.3. The evidence of matrix lattice distortion in Zn1-xCoxO nanocrystalsIn this work we investigated the doping effect on the local structure of Co, Zn and O in Zn1-xCoxO (x=0.02, 0.2) nanocrystals by X-ray absorption spectroscopy (XAS) combined with ab initio calculations. We identified the presence of distorted CoO4 structure and show that the distortion propagates homogeneously in the matrix with increasing the doping concentration. The presence of a distorted matrix lattice plays an important role in the mechanism of room temperature ferromagnetism (RTF) observed in diluted magnetic semiconductor (DMS) materials, in agreement with a new mechanism recently introduced. Information about the electronic structure of the bottom of the conduction band have been also achieved by O K-edge XANES spectra that addresses also a significant contributions of the matrix lattice distortion to the distribution of O-2p states. 3. The atomic local structure and electronic structure in ZnS nanoparticles system which can be as DMS'matrix.In previous investigations, several group have reported that room temperature ferromagnetism is possible the universal property of nanoparticles. This phenomenon could be attributed to the complex atomic local structure and electronic structure in the surface of nanoparticles. A room temperature solid-state structural transformation was observed in 3 nm ZnS nanoparticles by means of wide angle X-ray scattering (WAXS). In this work, the same system has been investigated by employing the X-ray absorption near edge structure (XANES) at Zn and S K-edge combined with multiple-scattering (MS) calculation. The results show that the structure of nanoparticles form has larger structure disorder relative to its bulk form, moreover this disorder is not only with a"short-range"character but also with a"medium-range"character. Specially, by means of constructing theoretical models, we indicate that there is possible coexistence of sphalerite and wurtzite lattice structure in nanoparicles sample. Additionally, the electronic structure also has been researched and shows that the distribution of the Zn-4p and S-3p empty states is wider in nanoparticles. In consequence, it would be responsible for its distinct properties.4. Atomic Structure and Electronic Reconstruction at Nanoparticle Surface of a Mott Insulator Controlled by CappingMott insulator oxide nanopaticles have attracted more attention due to its distinct properties. In this investigation, we report a result that, differ from semiconductors, the unique electronic state in the NiO surface can be formed due to"electron-electron"interaction and, furthermore the capping can control this new surface state i.e., the lowest electron-addition state in the Mott insulator nanoparticles. We investigate the atomic local structures and electronic structure of 6 NiO nanoparticles with different surface environment by means of the O K-edge X-ray absorption near edge structure (XANES). Spectra are interpreted based on the multiple scattering (MS) calculation. Results show that capping different molecules to the surface structure not only dramatically influences the reconstruction of near surface structure (constructed from Oh to Td symmetry) but also strongly changes the structure of the conduction band by giving rise to or eliminating a electron-addition state with the Zhang-Rice (ZR) state character lying within the gap. This additional state is due to the excessive oxygen attributed to the non-stoichiometry in the structure reconstructed process. Results of this study are important for designing the nano-device with different performance.