1. Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), P. O. Box 3640, 76021 Karlsruhe, Germany
2. DFG Center for Functional Nanostructures (CFN), Karlsruhe Institute of Technology (KIT), 76049 Karlsruhe, Germany
3. Physics Institute and Institute of Solid State Physics, Karlsruhe Institute of Technology (KIT), 76049 Karlsruhe, Germany
4. Department of Chemistry, University of Basel, Switzerland, Fax: +41‐61‐267‐1016