1. Physics Department E20, Technical University of Munich, James-Franck-Str. 1, 85748 Garching, Germany
2. Institute of Physics, Chinese Academy of Sciences, 100190 Beijing, P.R. China
3. Department of Applied Natural Sciences, Lübeck University of Applied Sciences, Mönkhofer Weg 239, 23562 Lübeck, Germany
4. Institute of Nanotechnology (INT), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Karlsruhe, Germany
5. Department of Chemistry, University of Basel, St Johannsring 19, CH-4056 Basel, Switzerland