Chemical state

About: Chemical state is a research topic. Over the lifetime, 2378 publications have been published within this topic receiving 78183 citations.

Negligible Sr segregation on SrTiO3(001)-(13×13)-R33.7° reconstructed surfaces

Abstract: We compare the extent of Sr segregation on buffered-HF (BHF)-etched and ( 13×13)-R33.7° reconstructed SrTiO3(001) surfaces, and reveal that Sr segregation is suppressed on the reconstructed surface. Scanning tunneling microscopy and photoemission studies of both surfaces show distinct differences in terms of atomic arrangements, electronic structures, and chemical states. The emission-angle-dependent Sr 3d core-level spectra indicate that the amount of Sr is low at the surface of the reconstructed surfaces, while the amount increases near the surface of the BHF-etched surface. Our investigations would facilitate the preparation of a compositionally well-defined SrTiO3 surface that is of significant importance to accelerate oxide electronics research.

Chemical speciation via X‐ray emission spectra

Abstract: Since the early days of X-ray spectrometry, X-ray emission and fluorescence spectra have been used to investigate chemical speciation, e.g. the dependence on the formal oxidation state. Laboratory wavelength-dispersive spectrometers have adequate resolution for these measurements. However, almost all studies have employed empirical methods to interpret the spectra. We aim to place such methods on a quantitative basis by means of efficient ab initio calculations of the X-ray emission line shapes based on a self-consistent, real-space Green's function approach, as implemented in the X-ray spectroscopy code FEFF8.2. Calculations are presented for the phosphorus K-M2, 3, and the chromium L-series emission lines for a selection of simple compounds. These lines exhibit changes depending on the oxidation state and on the neighboring atoms in the compounds that can be observed with instruments available in many XRF laboratories. The calculated spectra, as modified by convolution with a model monochromator response function, are compared with measured spectra. Simulated and measured spectra are found to be in reasonable agreement, and show that the approach has the potential to yield quantitative information about the chemical state. Copyright © 2006 John Wiley & Sons, Ltd.

Chemical bonding of nitrogen formed by nitridation of crystalline and amorphous aluminum oxide studied by X-ray photoelectron spectroscopy

Abstract: Numerous efforts have already been made to optimize nitridation of crystalline sapphire (c-Al2O3) substrates whereas very little attention has been paid to nitridation of amorphous aluminum oxide layers (a-AlOx). An extensive analysis of the reaction of amorphous aluminum oxide films with nitrogen species is thus needed to clarify the mechanisms of nitrogen incorporation into such layers and to control their properties. In this work X-ray photoelectron spectroscopy was used to determine the chemical state of nitrogen formed by nitrogen plasma treatment of c-Al2O3 and 15 nm thick a-AlOx layers grown by atomic layer deposition on Si and sapphire substrates. The results show that the nitridation proceeds significantly different for c-Al2O3 and a-AlOx samples, which we correlate with the initial stoichiometry of both materials. At the surface of sapphire O vacancies were found, which are necessary for the formation of AlN-type bonding via diffusion limited replacement of oxygen by nitrogen. This process was slow and involved formation of oxinitride AlN–O. After 80 min of nitridation only ∼3.4 at% of N was incorporated. In contrast, in a-AlOx layers Al vacancies were present before nitridation. This opened a new, more effective path for nitrogen incorporation via accumulation of N in the cation-deficient lattice and creation of the Al(NOy)x phase, followed by AlN and AlN–O formation. This scenario predicts more effective nitrogen incorporation into a-AlOx than c-Al2O3, as indeed observed. It also explains our finding that more N was incorporated into a-AlOx on Si than on sapphire due to supply of oxygen from the sapphire substrate.