1. Department of Materials Science, Martin-Luther University, Halle-Wittenberg, D-06099 Halle/S, Germany, Center of Nanoscience, Ludwig-Maximilians University Munich, Schellingstrasse 4, D-80799 Munich, Germany, and Max Plank Institute for Chemical Physics of Solids, Nöthnitzer Strasse 40, D-01187 Dresden, Germany