1. Institute of Materials Chemistry, Vienna University of Technology, Veterinärplatz 1/GA, A-1210 Vienna, Austria, University Service Centre for Transmission Electron Microscopy, Vienna University of Technology, Wiedner Hauptstrasse 8-10/052, A-1040 Vienna, Austria, and Institute of Particle Technology, Friedrich-Alexander-University Erlangen-Nuremberg, Cauerstrasse 4, 91058 Erlangen, Germany