Prof. Dr. Ulrich Köhler

Experimental Physics
Ruhr-Universität Bochum

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  • Propanoate grafting on (H,OH)-Si(0 0 1)-2×1
    Bournel, F. and Gallet, J.-J. and Köhler, U. and Ellakhmissi, B.B. and Kubsky, S. and Carniato, S. and Rochet, F.
    Journal of Physics Condensed Matter 27 (2015)
    We have examined the reactivity of water-covered Si(0 0 1)-2?×1, (H,OH)-Si(0 0 1)-2?×1, with propanoic (C2H5COOH) acid at room temperature. Using a combination of spectroscopic techniques probing the electronic structure (XPS, NEXAFS) and the vibrational spectrum (HREELS), we have proved that the acid is chemisorbed on the surface as a propanoate. Once the molecule is chemisorbed, the strong perturbation of the electronic structure of the hydroxyls, and of their vibrational spectrum, suggests that the molecule makes hydrogen bonds with the surrounding hydroxyls. As we find evidence that surface hydroxyls are involved in the adsorption reaction, we discuss how a concerted or a radical-mediated reaction (involving the surface silicon dangling bonds) could lead to water elimination and formation of the ester. © 2015 IOP Publishing Ltd.
    view abstract10.1088/0953-8984/27/5/054005
  • Benzaldehyde on water-saturated Si(001): Reaction with isolated silicon dangling bonds versus concerted hydrosilylation
    Pierucci, D. and Naitabdi, A. and Bournel, F. and Gallet, J.-J. and Tissot, H. and Carniato, S. and Rochet, F. and Köhler, U. and Laumann, D. and Kubsky, S. and Silly, M.G. and Sirotti, F.
    Journal of Physical Chemistry C 118 (2014)
    Despite strong similarities due to the common presence of silicon monohydrides and isolated silicon dangling bonds (silicon radicals), the water-saturated Si(001)-2 × 1 surface and the hydrogen-terminated Si(001)-2 × 1 surface show very different reactivities with respect to benzaldehyde. By using real-time scanning tunneling microscopy, synchrotron radiation photoemission, X-ray absorption, and high-resolution electron energy loss spectroscopies in combination, we demonstrated that benzaldehyde reacts with the silicon dangling bonds of water-saturated Si (001). As we found no evidence for the abstraction of a nearby H leading to the formation of a new dangling bond, the formation of a stable radical adduct is a plausible explanation. This observation contrasts with the H-terminated case for which benzaldehyde grafting occurs via a radical chain reaction that can propagate after abstraction of a nearby H by the radical adduct. Also at odds with the H-terminated case, a second chemisorption channel is observed [i.e., a concerted hydrosilylation reaction between a surface monohydride (SiH) and the carbonyl moiety] without any participation of the silicon dangling bond. We discuss how the presence of hydroxyls on water-saturated Si(001)-2 × 1 could make its reactivity markedly different from that of H-terminated Si(001)-2 × 1. © 2014 American Chemical Society.
    view abstract10.1021/jp4077678
  • Elucidating elementary processes at Cu/ZnO interfaces: A microscopical approach
    Zychma, A. and Wansing, R. and Schott, V. and Köhler, U. and Wöll, C. and Muhler, M. and Birkner, A.
    Physica Status Solidi (B) Basic Research 250 (2013)
    Despite its enormous importance for heterogeneous catalysis, and in particular methanol synthesis, detailed information about the Cu/ZnO interface is still far from being complete. Here we present an overview of recent work carried out using different types of microscopical methods from which the complexity of the problem becomes apparent. In addition to results from transmission electron microscopy (TEM) and scanning electron microscopy (SEM) data also obtained using scanning probe techniques, in particular scanning tunneling microscopy (STM) and atomic force microscopy (AFM) are presented. Special attention is paid to the influence of elevated temperatures on the Cu/ZnO interface. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201248513
  • Surface morphology and atomic structure of thin layers of Fe3Si on GaAs(001) and their magnetic properties
    Noor, S. and Barsukov, I. and Özkan, M.S. and Elbers, L. and Melnichak, N. and Lindner, J. and Farle, M. and Köhler, U.
    Journal of Applied Physics 113 (2013)
    The structural and magnetic properties of ultrathin near-stoichiometric Fe3Si layers on GaAs(001) are investigated after using scanning tunneling microscopy (STM) analysis to optimize the deposition process. This includes atomic resolution imaging of the surface as measured by STM revealing the atomic ordering and characteristic defects in the topmost layers. Emphasis is laid on connections between the layer morphology and its magnetic properties, which are analysed by in situ MOKE, FMR, and SQUID magnetometry. Upon nucleation, the Fe3Si islands behave like superparamagnetic nanoparticles where we find a quantitative agreement between the size of the nanoparticles and their superspin. At higher coverage, the Fe3Si layers show ferromagnetic behaviour. Here, we investigate the superposition of the magnetocrystalline and the uniaxial anisotropies where the latter can be excluded to be caused by shape anisotropy. Furthermore, an unexpected increase of the magnetic moment towards low coverage can be observed which apart from an increased orbital moment can be attributed to an increased step density. © 2013 American Institute of Physics.
    view abstract10.1063/1.4795163
  • The thermally induced interaction of Cu and Au with ZnO single crystal surfaces
    Köhler, U. and Kroll, M. and Löber, T. and Birkner, A. and Schott, V. and Wöll, C.
    Physica Status Solidi (B) Basic Research 250 (2013)
    The reaction of Cu-clusters with a polar and a mixed terminated single crystalline ZnO-substrate upon thermal treatment in UHV is studied in comparison with Au-films. Scanning tunneling microcopy and spectroscopy in combination with photoemission experiments reveal the geometrical and chemical changes in the Cu-cluster system on ZnO(0001)-Zn and ZnO(10$\overline {1} $0) upon annealing up to 770K. On ZnO($10\overline {1} 0$) the Cu-clusters show a roof like outline with Cu(111) side facets. The data points to a Cu(110) interface with the ZnO-substrate. The interface of the Cu-clusters and the ZnO-substrate was investigated by the controlled removal of clusters using STM-tip manipulation exposing the "footprints" of the clusters. On both investigated ZnO surfaces an entrenching of the Cu-clusters during annealing was found which partly explains the observed decrease of the amount of Cu visible above the ZnO-substrate level upon annealing. Even at higher annealing temperature the main body of the cluster surface is still pure copper. No large scale oxidation or brass formation was found. Scanning tunneling spectroscopy shows an increased density of occupied states at the cluster perimeter which is possibly relevant as an active site in catalysis. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201248447
  • Thermal behavior of MOCVD-grown Cu-clusters on ZnO(1010)
    Kroll, M. and Löber, T. and Schott, V. and Wöll, C. and Köhler, U.
    Physical Chemistry Chemical Physics 14 (2012)
    Scanning tunnelling microscopy (STM) and X-ray photoelectron spectroscopy (XPS, AES) were used to study MOCVD of Cu-clusters on the mixed terminated ZnO(1010) surface in comparison to MBE Cu-deposition. Both deposition methods result in the same Cu cluster morphology. After annealing to 670 K the amount of Cu visible above the oxide surface is found to decrease substantially, indicating a substantial diffusion of Cu atoms inside the ZnO-bulk. The spectroscopic data do not show any evidence for changes in the Cu oxidation state during thermal treatment up to 770 K. © the Owner Societies 2012.
    view abstract10.1039/c2cp22901c
  • Easily made and handled carbon nanocones for scanning tunneling microscopy and electroanalysis
    Sripirom, J. and Noor, S. and Köhler, U. and Schulte, A.
    Carbon 49 (2011)
    Well-formed carbon nanocones at the ends of micrometer-diameter carbon fibers (CFs) were fashioned into functional tips for scanning tunneling microscopy (STM) and miniaturized voltammetric sensors. Sharpening of single graphite filaments was achieved by simple DC electrochemical etching in 0.1 N NaOH. Operated as STM tips, pointed CFs resolved in air the contour and surface morphology of a nanoscopic Au line pattern and imaged in vacuum a Si (1 1 1) surface with clear atomic resolution. Subjecting already etched CFs to tip-sparing insulation with electrodeposited paint produced conical carbon ultramicroelectrodes (UMEs) with effective radii down to about 900 nm. Comparative cyclic voltammetry trials in alkaline, neutral and acidic solutions showed that the conical carbon UME's had a wider practical potential window for electroanalytic applications than, for instance, Pt disk UMEs. The CF-based conical sensors described here are exceptionally easy to make with simple laboratory equipment and perform well in STM topography imaging and voltammetry. The inherent simplicity of sensor production widens the field of potential users, and offers clear advantages over existing types of UMEs, in particular those based on carbon nanotubes, which are especially hard to handle in an optical microscope setting. © 2011 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.carbon.2011.02.007
  • Isolated silicon dangling bonds on a water-saturated n+-doped Si(001)-2 × 1 surface: An XPS and STM study
    Gallet, J.-J. and Bournel, F. and Rochet, F. and Köhler, U. and Kubsky, S. and Silly, M.G. and Sirotti, F. and Pierucci, D.
    Journal of Physical Chemistry C 115 (2011)
    Using Si 2p core-level X-ray photoelectron spectroscopy we have measured the upward band bending at the surface of n+-doped water-saturated Si(001)-2 × 1 and inferred the macroscopic negative surface charge density of the surface. These macroscopic results are in excellent accord with the microscopic view provided by dual-bias scanning tunneling microscopy showing that the isolated silicon dangling bonds (∼1.2 × 10-2 defects per Si atom) bear indeed a negative charge. Noting the structural analogy between isolated dangling bonds on water-saturated Si(001) and H-terminated Si(001), in the final, prospective section of the paper, we raise the question of the possible role that these defects could play in radical chain reactions with π-bonded molecules, in relationship with the hydride/hydroxyl patterns that are resolved in the scanning tunneling images. © 2011 American Chemical Society.
    view abstract10.1021/jp201262x
  • Thermal expansion of RFeAsO (R=La,Ce,Pr,Sm,Gd)
    Klingeler, R. and Wang, L. and Köhler, U. and Behr, G. and Hess, C. and Büchner, B.
    Journal of Physics: Conference Series 200 (2010)
    We present measurements of the thermal expansion coefficient α of polycrystalline RFeAsO (R = La,Ce,Pr,Sm,Gd). Anomalies at the magnetic ordering transitions indicate a significant magneto-elastic coupling and a negative pressure dependence of TN. The structural transitions are associated by large anomalies in α. Rare earth magnetic ordering in CeFeAsO, PrFeAsO, and SmFeAsO yields large positive anomalies at low temperatures. © 2010 IOP Publishing Ltd.
    view abstract10.1088/1742-6596/200/1/012088
  • molecular beam epitaxis

  • nanostructures

  • scanning tunneling microscopy

  • semiconductors

  • thin films

  • x-ray photoelectron spectroscopy

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