1. Institute of Nanotechnology, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany
2. Institute of Physical Chemistry, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany
3. Department of Physics, Nanoscience Center, University of Jyväskylä, 40014 Jyväskylä, Finland
4. Department of Chemistry, Nanoscience Center, University of Jyväskylä, 40014 Jyväskylä, Finland