[en] The time scale for carrier relaxation in semiconductors is on the same order of magnitude as the life time of shallow core hole states (a few femtoseconds). Resonant Inelastic soft X-ray scattering (RIXS) which involves (virtual) excitations of core levels consequently contains information about the time development of the electronic structure on this time scale. In many cases one can treat the scattering in an absorption (SXA) followed-by-emission (SXE) picture, where simply the rates for various processes can be compared with the intermediate core hole state decay rate as an internal open-quotes clockclose quotes. By variation of x (0 < x < 1) in La_xSr_1-xTiO₃, the amount of Ti d electrons in the system can be controlled. SrTiO₃ (x=0) is an insulator with an empty Ti d band. With increasing x, electrons are doped into the Ti d-band, and LaTiO₃ (x=1) is a Mott Hubbard insulator with a Ti 3d¹ configuration. In this work the authors demonstrate that the rate for Ti 2p core hole screening in La_xSr_1-xTiO₃ is doping dependent. The screening rate increases with the availability of Ti 3d electrons, and they estimate it to be 3.8 x 10¹³/sec in La_0.05Sr_0.95TiO₃

[en] In recent years silicon nanostructures have gained great interest because of their optical luminescence, which immediately suggests several applications, e.g., in optoelectronic devices. Nanostructures are also investigated because of the fundamental physics involved in the underlying luminescence mechanism, especially attention has been drawn to the influence of the reduced dimensions on the electronic structure. The forming of stable and well-defined nanostructured materials is one goal of cluster physics. For silicon nanostructures this goal has so far not been reached, but various indirect methods have been established, all having the problem of producing less well defined and/or unstable nanostructures. Ion implantation and subsequent annealing is a promising new technique to overcome some of these difficulties. In this experiment the authors investigate the electronic structure of ion-implanted silicon nanoparticles buried in a stabilizing SiO₂ substrate. Soft X-ray emission (SXE) spectroscopy features the appropriate information depth to investigate such buried structures. SXE spectra to a good approximation map the local partial density of occupied states (LPDOS) in broad band materials like Si. The use of monochromatized synchrotron radiation (MSR) allows for selective excitation of silicon atoms in different chemical environments. Thus, the emission from Si atom sites in the buried structure can be separated from contributions from the SiO₂ substrate. In this preliminary study strong size dependent effects are found, and the electronic structure of the ion-implanted nanoparticles is shown to be qualitatively different from porous silicon. The results can be interpreted in terms of quantum confinement and chemical shifts due to neighboring oxygen atoms at the interface to SiO₂

[en] Fluorescence yield (FY) soft X-ray absorption (SXA) spectra from electrolytes, aqueous complexes and biomolecules in liquid environment are presented in order to demonstrate the capabilities of a newly designed liquid sample cell which is compatible with ultra high vacuum environments. Changes of the electronic structure are observed in sodium electrolytes as a function of concentration and solvent

[en] The electronic structure of single wall carbon nanotubes (SWNTs) in intact, undissolved buckypaper has been studied using STM and STS at 23 K. STS allows to distinguish between metallic, narrow gap semiconducting, and wide gap semiconducting SWNTs. In a statistical analysis we find a distribution ratio of 34±6%:15±4%:51±7%, respectively. These ratios indicate that metallic SWNTs are preferentially generated in the buckypaper production process

[en] In coherent soft x-ray scattering from magnetically ordered surfaces there are contributions to the scattering from the magnetic domains, from the surface roughness, and from the diffraction associated with the pinhole aperture used as a coherence filter. In the present work, we explore the interplay between these contributions by analyzing speckle patterns in diffusely scattered x rays from the surface of magnetic thin films. Magnetic contrast from the surface of anti ferro magnetically ordered LaFeO3 films is caused by magnetic linear dichroism in resonant x-ray scattering. The samples studied possess two types of domains with their magnetic orientations perpendicular to each other. By tuning the x-ray energy from one of the two Fe-L3 resonant absorption peaks to the other, the relative amplitudes of the x-ray scattering from the two domains is inverted which results in speckle pattern changes. A theoretical expression is derived for the intensity correlation between the speckle patterns with the magnetic contrast inverted and not inverted. The model is found to be in good agreement with the x-ray-scattering observations and independent measurements of the surface roughness. An analytical expression for the correlation function gives an explicit relation between the change in the speckle pattern and the roughness, and magnetic and aperture scattering. Changes in the speckle pattern are shown to arise from beating of magnetic scattering with the roughness scattering and diffraction from the aperture. The largest effect is found when the surface roughness scatter is comparable in intensity to the magnetic scatter

[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] General aspects related to lensless imaging of magnetic materials using X-rays are discussed. For holographic and iterative approaches, the requirements regarding coherence and oversampling are compared. The polarization dependence in resonant magnetic scattering is discussed with respect to the possibility of interference in holograms and speckle patterns. (orig.)

No abstract available

No abstract available

[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