1. Max-Planck Institute of Colloids and Interfaces, Golm/Potsdam, D-14424, Germany, Center for Nanoscience and Photonics and Optoelectronics Group, Physics Department, Ludwig-Maximilians Universität of München, Amalienstr. 54, Munich, D-80799, Germany, and IRC/Department of Materials, Queen Mary University of London, Mile End Road, E1 4NS, London, U.K.