1. Faculty of Physics, University of Duisburg-Essen, Lotharstrasse. 1, 47057 Duisburg, Germany
2. Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Carl-Benz-Str. 199, 47057 Duisburg, Germany
3. Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany
4. Center of Excellence for Advanced Materials and Sensing Devices, Institute of Physics, Bijenička cesta 46, HR-10000 Zagreb, Croatia
5. Physics of Interfaces and Nanomaterials, MESA+ Research Institute, University of Twente, Drienerlolaan 5, 7522 NB Enschede, The Netherlands
6. Ames Laboratory—U.S. Department of Energy, Ames, Iowa 50011, USA
7. Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA