1. Institute of Applied Physics, Wiedner Hauptstrasse 8-10, Vienna University of Technology, 1040 Vienna, Austria.
2. Institute of Materials Chemistry, Getreidemarkt 9, Vienna University of Technology, 1060 Vienna, Austria.
3. Chair of Solid State Physics, University of Erlangen-Nürnberg, Staudtstrasse 7, 91058 Erlangen, Germany.