1. Institute of Science and Technology Austria, Am Campus 1, 3400 Klosterneuburg, Austria
2. Institute of Semiconductor and Solid State Physics, Johannes Kepler University, Altenbergerstrasse 69, 4040 Linz, Austria
3. Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland
4. Electron Microscopy Center, Empa, Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse
129, 8600 Dübendorf, Switzerland
5. IBM Research Zürich, CH-8803 Rüschlikon, Switzerland
6. Chair for Nanoelectronic Materials, Technical University Dresden, 01062 Dresden, Germany