1. Institute for Nanotechnology, Karlsruhe Institute of Technology (KIT), D-76344 Eggenstein-Leopoldshafen, Germany
2. Center for Functional Nanostructures (CFN), Karlsruhe Institute of Technology (KIT), Wolfgang-Gaede-Strasse 1a, 76131 Karlsruhe, Germany
3. KIT-TUD Joint Research Laboratory Nanomaterials,
Institute of Materials Science, Technische Universität Darmstadt (TUD), Petersenstrasse 32, D-64287 Darmstadt, Germany