[en] Full text: The synthesis of the subnanometer-diameter MoS₂ single wall nanotubes (SWNTs) was reported recently. The structure reported are (3,3) tubes which in turn form bundles containing interstitial iodine. With a resulting diameter of 9.6 Angstroms, the individual tubes are extremely compact with bond angles significantly deviating from bulk MoS₂. So far, the bonding and electronic structure in these SWNTs is not understood theoretically. We investigate the electronic structure of the MoS₂ SWNTs synthesized by D. Mihailovich et al. as described in. Nanotubes with and without interstitial iodine were studied in comparison to crystalline bulk material by combining soft x- ray absorption and soft x-ray fluorescence spectroscopy at the sulfur 2p_3/2 resonance. We observed a significant modification of the density both unoccupied and occupied electronic states in the SWNTs. Furthermore, an influence of the interstitial iodine on the electronic structure locally at the sulfur site is clearly visible. To understand the influence of iodine on the electronic structure of the MoS₂ we calculate the electronic structure and the resulting spectra for simple model clusters. The calculations are carried out with the StoBe code, based on a solution of the Kohn-Sham density functional theory equations. From our model calculations, we propose that the iodine bonds at least partially covalent to the sulfur atoms in the outer layer of the SWNTs

[en] Full text: Nanoscaled structures play an important role in optoelectronic technologies. After the discovery of the strong, size dependent photoluminiscence (PL) in the visible spectral range of porous silicon, further research focused on Si nanoparticles which are more stable under ambient conditions. A synthesis technique providing control over the nanoparticle size distribution and spacing has been developed in Max Planck Institute for Microstructure Physics (Halle). The process is based on the growth of a SiO/SiO₂ superlattice with subsequent high temperature annealing in N₂ atmosphere, which leads to the formation of Si particles in a SiO₂ matrix. Si nanoparticles as small as 1.5 nm in diameter can be produced with a narrow size distribution in this way. Resonant and low temperature PL measurements indicate the existence of electronic quantum confinement with a strongly enhanced ratio of direct to phonon-assisted transitions at low temperature due to momentum broadened band edge states. An increase of the PL peak energy up to 0.5 eV compared to bulk crystalline Si has been observed. We investigate changes in the electronic structure of Si nanoparticles in a SiO₂ matrix dependent on the size of the particles by soft x-ray absorption and soft x-ray fluorescence spectroscopy. Selective excitation based on the presence of a Si 2p core level energy shift of about 1 eV per oxygen neighbor allows us to probe different oxide species. As a result, the structure of the oxide interface region between a Si nanoparticle and the surrounding SiO₂ matrix could be studied via the resulting local electronic structure. We observe the presence of a pure crystalline Si core with a relatively sharp interface region to surrounding oxides. Only few suboxides with a stoichiometry Si₂O₂ and/or Si₂O₃ are present. Results on the electronic structure within the Si core as a function of nanocrystal size are presented

[en] The dissolution parameters of calcium carbonates play a key role in agriculture as they regulate the plant nutrients uptake and buffer the pH-value of the soil. The combination of μXRF, μXAS and μXRD mapping in the hard X-ray regime for the examination of both short- and long range order in natural limestones revealed that their dissolution properties depend both on the crystallinity of the mineral phases and their composition. The results particularly show that in many cases the presence of magnesium inhibits dissolution of the phases. (paper)

[en] A new hard X-ray beamline for CATalysis and ACTinide research has been built at the synchrotron radiation facility ANKA. The beamline design is dedicated to X-ray spectroscopy, including ‘flux hungry’ photon-in/photon-out and correlative techniques with a special infrastructure for radionuclide and catalysis research. The CAT-ACT beamline will help serve the growing need for high flux/hard X-ray spectroscopy in these communities. The design, the first spectra and the current status of this project are reported. (paper)

[en] Recent results obtained in our laboratories on interlayer exchange coupling of Fe films across interlayers of iron silicides, Fe_1-xSi_x 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 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