Alexander Kompch

University of Duisburg-Essen, Essen, North Rhine-Westphalia, Germany

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Publications (6)20.34 Total impact

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    ABSTRACT: Epitaxial Bi2Se3 films were grown by molecular beam epitaxy on Si(111)-Bi(sqrt(3)xsqrt(3)) R30° at temperatures between 200 °C and 250 °C. The surface and bulk morphology was characterized by high resolution low energy electron diffraction, X-ray diffraction, and atomic forcemicroscopy for various filmthicknesses between 6 and 90 nm. The films are atomically smooth without small angle mosaics or small angle rotational domains. The precise determination of lattice parameter reveals that films grown at higher temperature exhibit a smaller value for the vertical lattice parameter. The presence of random stacking faults in the film is reflected by a parabolic increase of the width of the diffraction peaks in X-ray diffraction.
    Thin Solid Films 01/2014; · 1.87 Impact Factor
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    ABSTRACT: We dope CdSe nanocrystals with Ag impurities and investigate their optical and electrical properties. Doping leads not only to dramatic changes but surprising complexity. The addition of just a few Ag atoms per nanocrystal causes a large enhancement in the fluorescence, reaching efficiencies comparable to core-shell nanocrystals. While Ag was expected to be a substitutional acceptor, nonmonotonic trends in the fluorescence and Fermi level suggest that Ag changes from an interstitial (n-type) to a substitutional (p-type) impurity with increased doping.
    Nano Letters 04/2012; 12(5):2587-94. · 13.03 Impact Factor
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    ABSTRACT: High quality Cr-doped ZnO nanoparticles from the gas phase were prepared and investigated with respect to their structural, optical and magnetic properties. The extended x-ray absorption fine structure and the x-ray absorption near edge structure of the particles verify that after nanoparticle preparation Cr is incorporated as Cr3+ ) at least partially on sites with a 4-fold oxygen configuration, most likely on a Zn site, into the wurtzite lattice. Despite the fact that Cr is known to act as an efficient non-radiative loss centre for near band gap emission (NBE), a pronounced NBE is obtained up to room temperature even for a nominal Cr concentration of 10 at.%. Annealing at 1000 degrees C results in a significant improvement of the photoluminescence efficiency and a reduced PL linewidth down to 2.9 meV at low temperatures while the structural and magnetic data indicate the formation of ZnCr2O4 clusters.
    Nanotechnology 05/2009; 20(13):135604. · 3.84 Impact Factor
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    ABSTRACT: One key requirement for the production of multinary oxide films by chemical vapor deposition (CVD) or nanocrystalline multinary oxides particles by chemical vapor synthesis (CVS) is the availability of precursors with high vapor pressure. This is especially the case for CVS where much higher production rates are required compared to thin films prepared by CVD. However, elements, which form low valent cations such as alkaline earth metals, are typically only available as solid precursors of low volatility, e.g., in form of beta-diketonates. This study describes laser flash evaporation as precursor delivery method for CVS of nanocrystalline perovskites. Laser flash evaporation exploits the nonequilibrium evaporation of solid metal organic precursors of low vapor pressure by absorption of the infrared radiation of a CO(2) laser. It is shown that stoichiometric, nanocrystalline particles consisting of SrZrO(3) and SrTiO(3) can be formed from corresponding mixtures of beta-diketonates which are evaporated nonselectively and with high rates by laser flash evaporation.
    Review of Scientific Instruments 01/2008; 78(12):123903. · 1.60 Impact Factor
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    ABSTRACT: ZnO and Cr doped ZnO nanoparticles were synthesized by chemical vapor synthesis (CVS) which is a modified chemical vapor deposition (CVD) process. The resulting powders consist of nanocrystalline particles and were characterized by X-ray diffraction (XRD), nitrogen adsorption (BET), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), element analysis, and extended X-ray absorption fine structure (EXAFS) spectroscopy. The grain size decreases with increasing dopant concentration. The lattice constants extracted by the Rietveld method from XRD data vary slightly with doping concentration. XRD and EXAFS data analysis show that the Chromium dopant atoms are incorporated into the wurtzite host lattice.
    Journal of the European Ceramic Society.
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    ABSTRACT: SnO2-based gas sensors have been subject of extensive academic and industrial research. The operation principle is based on the increase of material conductance in the presence of a reducing gas. Pure tin oxide suffers, however, from lack of good sensitivity and selectivity. An improvement in these two parameters has been observed by dispersing low concentrations of metallic particles (e.g. Pd, Pt, Au, Ag) in the sensing material (1, 2). However, the role of the metal nanoparticles in the sensor response is not fully understood: chemical and electronic mechanisms are proposed. The former is focused on the ability of the metal nanoparticles to dissociate oxygen, to spill over the SnO2 surface, to catalyse the combustion of the target gas or even to reduce the SnO2 matrix. The latter on the other hand considers that the conductance changes due to the variation of contact potential at the metal nanoparticle / SnO2 interface. One of the techniques ideal for studying the sensing mechanism is XAS (X-ray Absorption Spectroscopy). Though there are reports on XAS studies of doped and undoped SnO2 thin films and nanoparticles, only few of those reports deals with the utilization of XAS for understanding the sensing mechanism of sensors based on doped SnO2. In-situ XAFS studies have been reported to get insight into the CO and H2 sensing mechanism of polycrystalline SnO2 thin films with homogeneously dispersed Pt and Pd nanoparticles. From these studies it has been concluded that whereas Pt exhibits an electronic sensing mechanism towards CO, Pd shows a chemical sensing towards H2. No reports exist on the XAS studies of SnOx:Ag nanoparticles, proposed to be investigated in the present study (3-7). The SnOx:Ag nanoparticle layers for the present study are synthesized using an aerosol route. SnOx nanoparticles are synthesized by evaporating SnO in a tube furnace in the presence of a carrier gas, followed by cooling leading to nucleation and coagulation. Then size selection using a differential mobility analyser (DMA) and crystallization by in-flight annealing in a controlled O2 atmosphere is carried out. Ag nanoparticles are synthesized in a second tube furnace analogously. The two aerosol flows are then mixed according to the desired concentration. The distinct advantage of the aerosol route is a very high level of control over the particle size (monodisperse) and doping level due to the use of DMA's. The nanoparticles in the present study are, therefore, well characterized in size, structure and stoichiometry and optimised for their sensor response to ethanol. The SnOx:Ag nanoparticle layers proposed to be investigated in the present study exhibit enhanced ethanol sensing down to the ppb level as compared to pure SnOx nanoparticles. In the present study the nanoparticle samples were deposited in the form of a spot of 0.5-0.8 mm in diameter and of few microns thickness on a silicon substrate. We investigated correspding thin granular films consisting of nanoparticles and standard materials (Sn, SnO, SnO2, Ag, AgO, and Ag2O) at beamline X1 at the Sn and Ag K-edges (29.2 and 25.5 keV). Since the nanoparticle films are only be of a few microns in thickness and of low density, we used the fluorescence detection mode and grazing incidence geometry. The measurements at high energies for both absorption edges under grazing incidence enables the observation of XAS under in-situ conditions in a successive project. We wanted to answer the following questions: Is the measurement of such samples feasable and how does the local structure and oxidation state of Ag and Sn change in SnOx:Ag nanoparticles as a function of size of the Ag and SnOx nanoparticles, concentration of the Ag nanoparticles, and exposure to different gas atmospheres? Direct results of the analysis of the XANES and EXAFS part of XAS yield information about the oxidation state and, therefore, electronic structure of the sensor materials and the local structure, i.e. coordinaton numbers, coordination distances and higher moments of the distribution functions. These results will have important implications not only for understanding the basic science involved in enhanced sensing but also for technological applications as it may reveal possible ways of improving sensor characteristics further.