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Au clusters containing exactly 55 atoms are especially stable since
they form a highly symmetric cuboctahedron. Such clusters can be
prepared chemically* and stabilized by appropriate ligands.
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1,4nm
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A fundamental question then is whether such clusters already exhibit
a metallic behavior, i.e. occupied electronic states at the Fermi
energy are observed, or rather behave like large molecules. This question
had been addressed preciously and controversial answers were given
depending on the applied experimental technique. Especially, the role
of the ligands remained unclear. |
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Using photoelectron spectroscopic methods (XPS, UPS), we could demonstrate
quite recently that the (PPh3)12Cl6
ligand shell actually dominates the electronic properties of the 55
Au atoms leading to a non-metallic behavior of the cluster compound.
Controlled radiation-induced damage of the ligands allowed us to systematically
reduce the influence of the shell on the electronic properties of
individual Au clusters. As a result, one is able to switch the Au
clusters to a metallic behavior as evidenced by a finite density of
states at the Fermi level.
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Simulation
of a Au55 cluster protected by a
(PPh3)12Cl6 ligand shell yellow:
Au; green: Cl; orange: P; blue: phenyl (Ph) rings (Courtesy: Dr. H.
Kuhn and M. Leis, Fachbereich Chemie, Universität Essen) |
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*G.
Schmid, Clusters and Colloids. From Theory to Applications (VCH
Weinheim, 1994); Physics and Chemistry of Metal Cluster Compounds,
edited by L. de Jongh (Kluwer, Dordrecht, 1994).
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