1. Institute of Materials Chemistry, Vienna University of Technology, Veterinärplatz 1/GA, A-1210 Vienna, Austria, MKS Instruments Deutschland GmbH, Schatzbogen 43, D-81829 München, Germany, and University Service Centre for Transmission Electron Microscopy, Vienna University of Technology, Wiedner Hauptstrasse 8-10/137, A-1040 Vienna, Austria