Complex Nanotwin Substructure of an Asymmetric Σ9 Tilt Grain Boundary in a Silicon Polycrystal.

Grain boundaries in materials have substantial influences on device properties, for instance on mechanical stability or electronic minority carrier lifetime in multicrystalline silicon solar cells. This applies especially to asymmetric, less ordered or faceted interface portions. Here, we present the complex atomic interface structure of an asymmetric Σ9 tilt grain boundary in silicon, observed by high resolution scanning transmission electron microscopy (HR-STEM) and explained by atomistic modeling and computer simulation. Structural optimization of interface models for the asymmetric Σ9 and related symmetrical Σ9 and Σ3 tilt grain boundaries, by means of molecular-statics simulations with empirical silicon potentials in combination with first-principles calculations, results in a faceted asymmetric interface structure, whose grain-boundary energy is so low that it is likely to exist. The simulated local atomic structures match the observed HR-STEM images very well.

Magnetic bond-order potential for iron.

We present a magnetic bond-order potential (BOP) that is able to provide a correct description of both directional covalent bonds and magnetic interactions in iron. This potential, based on the tight binding approximation and the Stoner model of itinerant magnetism, forms a direct bridge between the electronic-structure and the atomistic modeling hierarchies. Even though BOP calculations are computationally more demanding than those using common empirical potentials, the formalism can be used for studies of complex defect configurations in large atomic ensembles exceeding 10(5) atoms. Our studies of dislocations in α-Fe demonstrate that correct descriptions of directional covalent bonds and magnetism are crucial for a reliable modeling of these defects.

Core-hole effects on the ELNES of absorption edges in SrTiO3.

Near-edge structures of absorption edges in electron energy-loss spectra (ELNES) of SrTiO(3) were calculated and compared to experimental inelastic electron scattering data. The goal of this study was to investigate final-state effects on the electronic structure. Two theoretical approaches were applied: density-functional theory with a band-structure supercell method and a real-space multiple-scattering cluster approach. Within both techniques, the Z+1 approximation was used to model the core hole generated by the inelastic scattering process. For the band-structure calculations, supercells of (SrTiO(3))(n)(n=1,4,8,16) composition with three-dimensional periodic boundary conditions were applied. The influence of supercell size and shape on calculated site- and symmetry-projected local densities of unoccupied states is assessed quantitatively. Relevant convergence criteria are the length scale set by the spatial extension of the valence-electron screening cloud around the core hole, and the interaction energy of neighbouring core hole centres. For a sufficiently large supercell size, the Z+1 approximation yields a reasonable description of the local densities of unoccupied states probed by the energy losses of inelastically scattered electrons of the Ti L(3)-, O K- and Sr L(3)-absorption edges. The quantitative equivalence of ELNES information extracted from the multiple-scattering cluster calculations and the band-structure supercell calculations is demonstrated. Discrepancies between theoretical and experimental results are discussed.

Quantitative atomic-scale analysis of interface structures: transmission electron microscopy and local density functional theory.

Transmission electron microscopy (TEM) and local density functional theory (LDFT) are combined to analyze the microscopic structure of the rhombohedral twin interface in alpha-Al2O3. LDFT provides interfacial energetics and atomic and electronic structures for three competing models. With high-resolution TEM the atomic structure at the interface is imaged quantitatively along two orthogonal zone axes. Electron energy loss spectroscopy in TEM with nanoscale spatial resolution yields the interfacial electronic structure. Both experiments confirm the theoretically preferred model quantitatively.

Valence electron energy loss study of Fe-doped SrTiO3 and a sigma13 boundary: electronic structure and dispersion forces.

Valence electron energy loss spectroscopy in a dedicated scanning transmission electron microscope has been used to obtain the interband transition strength of a sigma13 tilt grain boundary in SrTiO3. In a first step the electronic structure of bulk SrTiO3 has been analysed quantitatively by comparing VEELS spectra with vacuum ultraviolet spectra and with ab initio density of states calculations. The electronic structure of a near sigma13 grain boundary and the corresponding dispersion forces were then determined by spatially resolved VEELS. Also the effects of delocalization of the inelastic scattering processes were estimated and compared with results from the literature.

Density-functional modelling of core-hole effects in electron energy-loss near-edge spectra.

A series of (MgO)n supercells (n = 1, 4, 8, 16, 32) with three-dimensional periodic boundary conditions is investigated by density-functional band-structure calculations. The influence of supercell size and shape on calculated electron energy-loss near-edge spectra is assessed quantitatively, employing the Z + 1 approximation for the representation of final-state effects. Relevant convergence criteria are the length scale set by the spatial extension of the valence-electron screening cloud around the core hole and the interaction energy of neighbouring core-hole centres. A sufficient supercell size provided, the Z + 1 approximation yields a highly satisfactory description of excitations from the 1s shell of light elements, such as Mg and O, compared to experimental data. For comparison, pseudopotentials for excited states were generated for Mg, both with a large core (1s, 2s, 2p orbitals) and a small core (1s orbital only) included into the pseudopotential. The corresponding calculations with frozen core holes lead to very good agreement with the results from the Z + 1 calculation for the 1s excitations. The explicit treatment of the subvalence shell (2s, 2p), however, is mandatory for the proper modelling of excitations from orbitals higher than 1s. This indicates that the core polarisability plays an important role in excitations from more extended shells.

Core-hole effect in the ELNES of alpha-Al2O3: experiment and theory.

The occurrence of the core-hole effect at the Al-K and Al-L1 edge in alpha-Al2O3 was studied by comparing experimental electron energy-loss near-edge structures (ELNES) and results of band-structure calculations with and without accounting for core-hole effects by the Z + 1 approximation. It was found that the theoretically calculated unoccupied p-like projected densities of states (PDOS) without Z + 1 approximation matches better to the experimental Al-L1 ELNES, whereas the PDOS with Z + 1 approximation matches better to the experimental Al-K ELNES. We conclude that the localisation of the initial state is an important prerequisite for the observability of the core-hole effect in the ELNES.