1. Institute for Nanoelectronics, Technische Universität München, Arcisstrasse
21, D-80333 Munich, Germany
2. Max Planck Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany, and Department
of Chemistry, Ludwig-Maximilians-Universität (LMU), Butenandtstrasse 5-13 (D), D-81377 Munich, Germany