We present the first successful attempt at calculating cluster full-potential x-ray absorption near-edge structure (XANES) spectra, based on the finite difference method. By fitting XANES simulations onto experimental spectra we are able to perform electron population analysis. The method is tested in the case of Ti K-edge absorption spectrum in TiO 2 , where the amount of charge transfer between Ti and O atoms and of the screening charge on the photoabsorber is obtained taking into account both dipolar and quadrupolar transitions. [S0031-9007(99)08724-4] The close interplay between experimental measurements and theoretical analysis has been and continues to be one of the most fruitful approaches to our present understanding of the electronic structure of condensed matter. This has become particularly true following the advent of synchrotron radiation experiments with their highly sophisticated detection techniques, since they provide a wealth of information impossible to exploit completely without the assistance of an adequate theoretical analysis.The study of the occupied and unoccupied electronic states of matter by x-ray emission and absorption is one such instance. Traditionally band structure calculations in periodic systems have been of great help in this interaction process, especially for the occupied part of the states or the unoccupied part very near to the Fermi energy (10-15 eV). Nowadays one can perform very sophisticated full-potential self-consistent band calculations that can be used to advantage to analyze experimental data. However there are severe limitations regarding their applicability to systems of physical interest: (a) They can be applied only to periodic crystals; (b) band programs are not geared for calculating empty states far above the Fermi level (apart from an early attempt [1] that has remained isolated); (c) the charge relaxation around the core hole is difficult to implement (supercell calculations have to be done but it is not at all easy to reach self-consistency).In this respect short range cluster calculations [2] are far more flexible and superior, since they can be applied to the vast majority of interesting cases (in material science, e.g., nanostructured materials, biology and coordination chemistry, catalysis and industrial processes, disordered systems, absorbates, etc.), and the energy range over which absorption spectra can be calculated is virtually unlimited. Incidentally even for periodic systems a long range band calculation is not necessary, for the simple physical reason that the finite lifetime of the excited photoelectron in the final state limits the size of the region sampled around the photoabsorber.Unfortunately up to now cluster calculations have been based on multiple scattering theory with optical potential restricted to the muffin-tin approximation. This is a very limiting feature, especially at low photoelectron kinetic energies [the x-ray absorption near-edge structure (XANES) part of an absorption spectrum within about 50 eV from the edge], sinc...

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The growth and structural properties of GaN/AlN core-shell nanowire heterostructures have been studied using a combination of resonant x-ray diffraction, Raman spectroscopy and high resolution transmission electron microscopy experiments. For a GaN core of 20 nm diameter on average surrounded by a homogeneous AlN shell, the built-in strain in GaN is found to agree with theoretical calculations performed using a valence force field model. It is then concluded that for an AlN thickness up to at least 12 nm both core and shell are in elastic equilibrium. However, in the case of an inhomogeneous growth of the AlN shell caused by the presence of steps on the sides of the GaN core, plastic relaxation is found to occur. Consistent with the presence of dislocations at the GaN/AlN interface, it is proposed that this plastic relaxation, especially efficient for AlN shell thickness above 3 nm, is promoted by the shear strain induced by the AlN inhomogeneity.

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We report on the growth of AlxGa1-xN nanowires by plasma-assisted molecular beam epitaxy for x in the 0.3-0.8 range. Based on a combination of macro- and micro-photoluminescence, Raman spectroscopy, x-ray diffraction and scanning electron microscopy experiments, it is shown that the structural and optical properties of AlGaN NWs are governed by the presence of compositional fluctuations associated with strongly localized electronic states. A growth model is proposed, which suggests that, depending on growth temperature and metal adatom density, macroscopic composition fluctuations are mostly of kinetic origin and are directly related to the nucleation of the AlGaN nanowire section on top of the GaN nanowire base which is used as a substrate.

The pre-edge region of Ti N-edge polarized XANES spectra in TiO2-mtile is investigated by full-potential calculations based on the finite-difference method. Both dipolar and quadrupolar transitions are considered. The use of"non muffin-tin" potential allows a clear interpretation of the pre-edge features. The results are consistent with Full-potential LAPW band structure calculations, and are also compared with multiple-scattering calculations.

We have performed a real-time in situ x-ray scattering study of the nucleation of GaN nanowires grown by plasma-assisted molecular beam epitaxy on AlN(0001)/Si(111). The intensity variation of the GaN diffraction peak as a function of time was found to exhibit three different regimes: (i) the deposition of a wetting layer, which is followed by (ii) a supralinear regime assigned to nucleation of almost fully relaxed GaN nanowires, eventually leading to (iii) a steady-state growth regime. Based on scanning electron microscopy and electron microscopy analysis, it is proposed that the granular character of the thin AlN buffer layer may account for the easy plastic relaxation of GaN, establishing that three-dimensional islanding and plastic strain relaxation of GaN are two necessary conditions for nanowire growth.

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