1. Department of Colloid Chemistry, Max Planck Institute of Colloids and Interfaces, Research Campus Golm, D-14424 Potsdam, Germany, Institute of Particle Technology, Technical University of Braunschweig, Volkmaroder Strasse 5, D-38104 Braunschweig, Germany, Department of Materials, Swiss Federal Institute of Technology (ETH) Zürich, Wolfgang-Pauli-Strasse 10, CH-8093 Zürich, Switzerland