1. Institute of Applied Physics, Karlsruhe Institute of Technology (KIT), D-76128 Karlsruhe, Germany.
2. Deutsche Forschungsgemeinschaft (DFG)–Center for Functional Nanostructures (CFN), KIT, D-76128 Karlsruhe, Germany.
3. Institute of Nanotechnology, KIT, D-76021 Karlsruhe, Germany.