Pseudo-atomic-orbital band theory applied to electron-energy-loss near-edge structures

H2 ADSORPTION ON LiB (001) SURFACE: A FIRST PRINCIPLES CALCULATION

The adsorption behavior of H2 on the LiB (001) surface was investigated with density functional theory (DFT) method. It was found that the site of H2 adsorbed on the Li-B bridge II was easier than the other four sites ( Li top, B top, hollow vertical and Li-B bridge I). H2 adsorbed on the Li-B bridge II site was a strong chemical adsorption. The adsorption energy was 2.190 eV, and the H , B atoms exhibited covalent characteristics, the H – H atoms have a little interaction, and the H2 was 0.331 Å below the surface of Li-B bridge II. The charge density, band structure, totals and partial density of states were calculated utilizing the first principle method. These calculations showed that the H interacted with the surface atoms, and partially saturated the dangling bonds with the surface atoms. The interaction between H and the surface atoms were mainly attributed to the H 1 s, B 2 s and B 2 p states. The calculated band gap was 0.075 eV and 0.199 eV before and after adsorption.

Core-hole effects on energy-loss near-edge structure

We present first-principles electron energy-loss near-edge structure calculations that incorporate electron-hole interactions and are in excellent agreement with experimental data obtained with X-ray absorption spectroscopy (XAS) and electron energy-loss spectroscopy (EELS). The superior energy resolution in XAS spectra and the new calculations make a compelling case that core-hole effects dominate core-excitation edges of the materials investigated: Si, SiO2, MgO, and SiC. These materials differ widely in the dielectric constant leading to the conclusion that core-hole effects dominate all core-electron excitation spectra in semiconductors and insulators. The implications of the importance of core-holes for simulations of core-electron excitation spectra at interfaces will be discussed.

Excitonic effects in core-excitation spectra of semiconductors

Core-electron excitation spectra are used widely for structural and chemical analysis of materials, but interpretation of the near-edge structure remains unsettled, especially for semiconductors. For the important Si L(2,3) edge, there are two mutually inconsistent interpretations, in terms of effective-mass excitons and in terms of Bloch conduction-band final states. We report ab initio calculations and show that neither interpretation is valid and that the near-edge structure is in fact dominated by short-range electron-hole interactions even though the only bound excitons are effective-mass-like.

Simulating the oxygen K-edge spectrum from grain boundaries in ceramic oxides using the multiple scattering methodology

In this paper we demonstrate the use of the multiple scattering methodology to interpret oxygen K-edge spectra from both the bulk and grain boundaries in a variety of ceramic oxides. The experimental electron energy loss spectra (EELS) used in this study, were obtained from a dedicated scanning transmission electron microscope (STEM). Using the STEM to obtain the spectra has the advantage that each spectrum can be acquired with atomic spatial resolution. While the energy resolution is limited to approximately 0.8 eV, and the angular integration in the microscope apertures precludes momentum resolved spectroscopy, this unprecedented spatial resolution allows the electronic structure at individual defect sites to be determined. Additionally, as the microscope can also provide an atomic resolution image of the defect, the relationship between the atomic structure of the defect and its local electronic structure can be determined. In practice, this is achieved by using the structure observed in the image to build the real space atomic cluster for multiple scattering simulations. Detailed interpretation of the simulations of oxygen K-edge spectra from bulk MgO, CaO, SrTiO3, TiO2, MnO2, Mn3O4, Mn2O3 and MnO are presented. In addition, the simulations from grain boundaries in TiO2 (undoped) and SrTiO3 (undoped and Mn doped) are discussed in relation to quantifying the changes in the local electronic structure that are a direct consequence of the defect structure. The simulations are used to make interpretations of the structure-property relationships at these grain boundaries.