A. Zimina

Molecular and electronic structure in NaCl electrolytes of varying concentration: Identification of spectral fingerprints

Near edge x-ray absorption spectra at the Na K edge in aqueous NaCl electrolytes are presented as a function of concentration. The spectra are modeled by electronic structure calculations. We find and identify the orbital origin of two spectral fingerprints with sensitivity to the Na+-H2O distance and the Na+-Cl- distance in the electrolyte. Interionic interaction is found to be crucial for the description of the electrolytes at high concentrations.

Electronic structure and chemical environment of silicon nanoclusters embedded in a silicon dioxide matrix

Using photon-in photon-out soft x-ray spectroscopy, the electronic structure of silicon nanoclusters embedded in an electrically insulating SiO2 host matrix is investigated as a function of nanocluster size. We find the nanoclusters to be of a core-shell structure with a crystalline Si core and a thin transition layer of a suboxide. Effects of electronic quantum confinement are detected in the Si cores. We find that the influence of confined excitonic states manifests itself predominantly in the unoccupied electronic states.

Exchange coupling of ferromagnetic films across metallic and semiconducting interlayers

Recent results obtained in our laboratories on interlayer exchange coupling of Fe films across interlayers of iron silicides, Fe1−xSix with x = 0.5– 1, are reviewed. Samples are prepared by molecular beam epitaxy and characterized by means of low-energy electron diffraction and cross-sectional transmission electron microscopy. Coupling across interlayers of iron silicide with x ≈ 0.5 is found to be oscillatory with a strength of the order of 1 mJ m−2, and across well ordered Si interlayers (nominally x = 1) the coupling is exponentially decaying. In the latter case the maximum coupling turns out to be surprisingly strong (}6

DEPTH-RESOLVED SOFT X-RAY EMISSION SPECTROSCOPY OF Si-BASED MATERIALS

>> 6 mJ m−2), in particular considering the fact that the electrical resistivity is found to be large. Current–voltage curves for currents across the interlayers are characteristic of electron tunnelling. Soft-x-ray emission and near-edge x-ray absorption spectroscopy further support a semiconducting nature for the nominally pure Si interlayers.

Filled and empty states of disordered GaN studied by x-ray absorption and emission

We discuss a soft X-ray emission (SXE) valence band (VB) spectroscopy method for the study of the electronic structure and chemical phase composition of solids in a near-surface region with depth resolution. The depth information is obtained by variation of the energy of the incident electron beam used to excite the SXE spectra. As the information depth can be varied from about 1 nm to 1 μm in silicon, this method is suitable for the investigation of materials of modern micro- and nanoelectronics. VB → core level (Si 2p or Al 2p) transitions in Si-based materials are used to demonstrate the technique. It was found that the contribution of the signal from the near-surface region (< 1.5 nm) can be substantial (up to 50%) when the primary electron energy does not exceed the Si L2,3 threshold by more than 150 eV. The technique is applied to Al impurities in a Si matrix, produced by ion implantation. The electronic structure at the Al sites and depth distribution of the Al impurity change markedly after postimplantation annealing. The observed electronic structure after annealing is in agreement with electronic structure calculations for substitutional Al impurities in a crystalline Si lattice.

Absorption spectroscopic study of the conduction band of titanium dichalcogenides

X-ray absorption and emission spectroscopies are used to study the effects of short-ranged ordering on the electronic states of disordered GaN. Nanocrystalline samples with crystallites as small as 3nm exhibit an electronic structure resembling a broadened version of that in crystalline GaN. The electronic structure is even more heavily broadened in amorphous GaN films containing oxygen impurities or excess gallium. The oxygen containing films show an additional peak in the density of states just above the conduction band edge, and a downward shift of the valence band edge. The signature of molecular nitrogen trapped within the films is evident in both the absorption and emission spectra.

Electronic structure of doped fullerenes and single wall carbon nanotubes

The L2, 3 spectra of titanium in the layered compound TiSe2 and intercalated compounds Fe1/2TiSe2, Cr1/3TiSe2, and Fe1/4TiTe2 are studied. Theoretical calculations of the electronic structure of these compounds are performed. The experimental data and calculations suggest that the intercalation of the Cr and Fe atoms into the TiSe2 matrix brings about a partial filling of the Ti 3d states and the spin polarization of the Cr 3d and Fe 3d states. Chemical bonds are formed through the hybridization of the d orbitals of intercalated atoms with the Ti 3d-Se 4p states of the matrix.

Electronic Structure of Subnanometer Diameter MoS2−Ix Nanotubes

Soft x-ray absorption and emission spectroscopy is being used to study and compare the electronic structure of (a) potassium doped single wall carbon nanotubes (interstitial dopant), (b) La@C82 (endohedral dopant) and (c) (C59N)2 (on-cage dopant).