RESUMEN
A supposedly nonmagnetic 5d^{1} double perovskite oxide is investigated by a combination of spectroscopic and theoretical methods, namely, resonant inelastic x-ray scattering, x-ray absorption spectroscopy, magnetic circular dichroism, and multiplet ligand-field calculations. We found that the large spin-orbit coupling admixes the 5d t_{2g} and e_{g} orbitals, covalency raises the 5d population well above the nominal value, and the local symmetry is lower than O_{h}. The obtained electronic interactions account for the finite magnetic moment of Os in this compound and, in general, of 5d^{1} ions. Our results provide direct evidence of elusive Jahn-Teller distortions, hinting at a strong electron-lattice coupling.
RESUMEN
In order to optimize the performance of devices based on porphyrin thin films it is of great importance to gain a physical understanding of the various factors which affect their charge transport and light-harvesting properties. In this work, we have employed a multi-technique approach to study vacuum deposited zinc octaethyl porphyrin (ZnOEP) thin films with different degrees of long-range order as model systems. An asymmetrical stretching of the skeletal carbon atoms of the porphyrin conformer has been observed and attributed to ordered molecular stacking and intermolecular interactions. For ordered films, a detailed fitting analysis of the X-ray absorption near edge structure (XANES) using the MXAN code establishes a symmetry reduction in the molecular conformer involving the skeletal carbon atoms of the porphyrin ring; this highlights the consequences of increased π-π stacking of ZnOEP molecules adopting the triclinic structure. The observed asymmetrical stretching of the π conjugation network of the porphyrin structure can have significant implications for charge transport and light harvesting, significantly influencing the performance of porphyrin based devices.
RESUMEN
We report a detailed study of the K-edge X-ray absorption spectra of four transition metal phthalocyanines (MPc, M = Fe, Co, Cu and Zn). We identify the important single and multiple scattering contributions to the spectra in the extended energy range and provide a robust treatment of thermal damping; thus, a generally applicable model for the interpretation of X-ray absorption fine structure spectra is proposed. Consistent variations of bond lengths and Debye Waller factors are found as a function of atomic number of the metal ion, indicating a variation of the metal-ligand bond strength which correlates with the spatial arrangement and occupation of molecular orbitals. We also provide an interpretation of the near edge spectral features in the framework of a full potential real space multiple scattering approach and provide a connection to the local electronic structure.
RESUMEN
The bonding environment of oxygen implanted in GaN is studied using near edge X-ray absorption fine structure spectroscopy at the O-K-edge. The 70 keV oxygen ions form a 200 nm-thick subsurface layer that is highly defective or amorphous depending on the implantation fluence which ranges from 1 x 10(15) cm-2 to 1 x 10(17) cm(-2). The information depth of the fluorescence photons varies from 50 to 63 nm, depending on the angle of incidence. The spectra are simulated using the FEFF8 code and assuming various models, e.g., O interstitial, O substitutional in N sites, Ga and N vacancies, and various polymorphs of Ga2O3. The lattice disorder is modelled by displacing atoms from their equilibrium positions by adding to their Cartesian coordinates random numbers that belong to normal distributions. The simulation results reveal that at the low fluence limit, the O atoms occupy interstitial sites preferentially in the empty channels aligned to the c-axis and in the atomic planes containing the Ga atoms. When the fluence is equal to 1 x 10(16) cm(-2) the O atoms substitute N while at 1 x 10(17) cm(-2) they form mixed GaO(x)N(y) phases with the N/O ratio decreasing with increasing depth, i.e., as we approach the peak of the implanted O profile.
RESUMEN
We describe a soft x-ray appearance potential spectroscopy apparatus, which uses a windowless hyperpure Ge detector operated in the photon counting mode. Direct comparisons of recorded spectra with the self-convolution of x-ray absorption spectra and with ab initio simulations in the multiple scattering framework are reported and discussed.
RESUMEN
A SrF(2) ultrathin barrier layer on Si(001) is used to form a sharp interface and block reactivity and intermixing between the semiconductor and a Yb(2)O(3) overlayer. Yb(2)O(3)/Si(001) and Yb(2)O(3)/SrF(2)/Si(001) interfaces grown in ultra high vacuum by molecular beam epitaxy are studied by photoemission and x-ray absorption fine structure. Without the fluoride interlayer, Yb(2)O(3)/Si(001) presents an interface reacted region formed by SiO(x) and/or silicate compounds, which is about 9 Å thick and increases up to 14-15 Å after annealing at 500-700 °C. A uniform single layer of SrF(2) molecules blocks intermixing and reduces the oxidized Si region to 2.4 Å after deposition and to 3.5 Å after annealing at 500 °C. In both cases we estimate a conduction band offset and a valence band offset of â¼ 1.7 eV and 2.4 eV between the oxide and Si, respectively. X-ray absorption fine structure measurements at the Yb L(III) edge suggest that the Yb oxide films exhibit a significant degree of static disorder with and without the fluoride barrier. Sr K edge measurements indicate that the ultrathin fluoride films are reacted, with the formation of bonds between Si and Sr; the Sr-Sr and Sr-F interatomic distances in the ultrathin fluoride barrier film are relaxed to the bulk value.