1. Scientific Center of Materials Science, Department of Chemistry, Philipps-Universität Marburg, Hans-Meerwein-Strasse, D-35032 Marburg, Germany, Department of Physics, Martin-Luther-University Halle-Wittenberg, Heinrich Damerow-Strasse 4, D-06120 Halle, Germany, and Max Planck Institute of Microstructure Physics, Weinberg 2, D-06120 Halle, Germany