1. Colloid Chemistry Department, Max Planck Institute of Colloids and Interfaces, Am Mühlenberg 1, 14476 Potsdam, Germany
2. Institute of Physics, University of Münster, Heisenbergstr. 11, 48149 Münster, Germany
3. Center for Nanotechnology, University of Münster, Heisenbergstr. 11, 48149 Münster, Germany
4. Kirchhoff-Institute of Physics, University of Heidelberg, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany