1. Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), D-76021 Karlsruhe, Germany
2. DFG Center for Functional Nanostructures (CFN), D-76049 Karlsruhe, Germany
3. Department of Chemistry, University of Basel, CH-4056 Basel, Switzerland
4. Physics Institute and Institute for Solid State Physics, Karlsruhe Institute of Technology (KIT), D-76049 Karlsruhe, Germany