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
3. Department of Chemistry; Ludwig-Maximilians-Universität (LMU), Butenandtstrasse 5-13 (D); D-81377 Munich Germany
4. Nanosystems Initiative Munich (NIM) and Center for Nanoscience; Schellingstraße 4; 80799 Munich Germany