Molybdän- und Wolframsulfidcluster : Bausteine neuer Nanomaterialien?


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BERTRAM, Nils, 2006. Molybdän- und Wolframsulfidcluster : Bausteine neuer Nanomaterialien?

@phdthesis{Bertram2006Molyb-8891, title={Molybdän- und Wolframsulfidcluster : Bausteine neuer Nanomaterialien?}, year={2006}, author={Bertram, Nils}, address={Konstanz}, school={Universität Konstanz} }

deu application/pdf Bertram, Nils deposit-license Guided by the future vision of producing nano materials from clusters, we studied whether it is possible to synthesise fullerenes from materials other than Carbon. For this purpose the layered materials Molybdenum- and Tungsten Sulphide seem to be particularly suitable. Different classes of WnSm- and MonSm-clusters have been produced and characterised with respect to their geometric and electronic structure. The following questions were addressed:<br /><br />Which underlying growth patterns and magic numbers do exist?<br />Is it possible to produce fullerenes in the size range up to several hundred atoms or is a two-dimensional platelet structure preferred?<br />Which MnSm-clusters (M = Mo, W) are suited for deposition on surfaces and how does the substrate alter their properties?<br /><br />Different kinds of MnSm-clusters were produced in the gas phase with a Pulsed Arc Cluster Ion Source (PACIS) and studied by means of mass- and photoelectron spectroscopy. Certain clusters were then deposited onto a Ag(111)-surface and first results could be obtained using different surface analysis tools. The work was carried out in cooperation with G. Seifert et al. (TU Dresden) who provided corresponding density functional calculations. The MnSm-clusters under investigation can be divided into the following classes:<br /><br />1. Small MnSm-Clusters:<br />In order to shed light onto the formerly unknown growth patterns, small MnSm-clusters up to four metal atoms have been studied. In contrast to WnOm-clusters, MnSm-clusters can accommodate a far over-stochiometric amount of Sulphur atoms by adsorption of trisulfide groups. M4S6- exhibits a particularly large band gap of about 2 eV, analogous to W4O6. Its geometry is highly symmetric with the metal atoms forming a tetrahedron and the sulphur atoms occupying bridge positions, whereas the corresponding oxide prefers terminal bonds. The doubly magic properties indicate high stability and suggest Mo4S6- be a promising building block for nano materials. The structure of Mo4S12- corresponds to a fragment of the MoO3-solid phase. It could thus be shown that structural differences between Sulphide and Oxide can be overcome by artificially altering the cluster s stochiometry.<br /><br />2. Chevrel Clusters:<br />Mo3nSn+2- clusters containing a lower sulphur concentration than the bulk form nano chains on the basis of a Mo6S8 octahedron, the building block of Chevrel phases. These clusters have been synthesised as individual units in the gas phase for the first time. By adding Mo3S3-triangles one dimensional chains up to seven units can be observed in mass spectra. Photoelectron spectra show a very small HOMO-LUMO-Gap of 0.4 eV in the smallest observed unit Mo6S8- which, in agreement with theory, vanishes completely for the larger ones. Calculations for an infinite chain yield a pseudo band gap, resulting in a conducting metallic core and no states near the Fermi level in the surrounding Sulphur atoms. Hence this chain may be considered a nano cable.<br /><br />3. Fullerenes or Nano Platelets?<br />Neither molecular dynamics simulations nor experimental observations have yet shown indications of small fullerenes made of MS2-layers. This may be explained by the large amount of energy needed to bend a MS2-layer compared to a graphite layer and by premature saturation of the edges with Sulphur. However, it could be shown that clusters near the solid stochiometry consisting of 10 to 30 metal atoms prefer a two-dimensional growth mode. A series of platelet structures W10S30, W15S42 and W21S56 has been identified in mass spectra. The peaks can be shifted to larger masses by adsorption of additional Sulphur. As the smallest theoretically possible platelets containing one, three, or six sulphur atoms do not occur, we conclude that these structures consisting solely of edge atoms are not stable and that a solid-like coordination is required. As the series of platelets ends with W21S56 we suggest that the edges saturate with Sulphur before further metal atoms can bind to the structure.. Photoelectron spectra of the platelets reveal metallic behaviour.<br /><br />4. Deposited WnSm-Clusters:<br />W1S3-, W3S9- and W5S2- could for the first time be deposited onto an Ag(111)-surface. XPS data of W3S9- on Ag(111). A shift of the S 2p3/2 and the W 4f7/2 lines of -1.0 eV and -1.3 eV compared to the bulk material has been determined respectively. This indicates a superposition of relaxation due to final state effects and a small weakening of the W-S bonds due to charge transfer from the substrate. HREELS spectra of all three deposited cluster masses show excitations within the energy range of W-S vibrations. Taking into account the dipole formation of the cluster in contact with the surface which depends on the adsorption geometry, accompanying calculations are required to finally interpret the data. However, the stable W3S9--cluster exhibits a higher vibrational energy, showing a difference in behaviour between magic and non-magic in contact with surfaces. Molybdän- und Wolframsulfidcluster : Bausteine neuer Nanomaterialien? Bertram, Nils 2011-03-24T17:51:24Z Molybdenum- and Tungsten Sulphide Clusters 2011-03-24T17:51:24Z Building Blocks for Novel Nanomaterials? 2006

Dateiabrufe seit 01.10.2014 (Informationen über die Zugriffsstatistik)

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