1. Max-Planck Institute for Colloids and Interfaces, Golm/Potsdam D-14424, Germany, Institut de Chimie Séparative de Marcoule, FRE CEA/CNRS 2926, CEA Valrhô BP 17171, 30207 Bagnols sur Cèze Cedex, France, Center for Nanoscience and Photonics and Optoelectronics Group, Physics Department of Ludwig-Maximilians-Universität München, Amalienstr. 54, Munich D-80799, Germany, and Department of Materials, Queen Mary University of London, Mile End Road, E1 4NS London, U.K.