1. Institute of Applied Physics, University of Muenster 1 , 48149 Muenster, Germany
2. Institute of Physics and Center for Nanotechnology (CeNTech), University of Muenster 2 , 48149 Muenster, Germany
3. Center for Soft Nanoscience, University of Muenster 3 , 48149 Muenster, Germany
4. Kirchhoff-Institute for Physics, Heidelberg University 4 , 69120 Heidelberg, Germany