1. Laboratory for Nanotechnology, Department of Microsystems Engineering, University of Freiburg, Freiburg, 79110, Germany
2. Walter Schottky Institut and Physics Department, Technische Universität München, Garching, 85748, Germany
3. Karlsruhe Nano and Micro Facility (KNMF) and Institute of Nanotechnology (INT), Karlsruhe Institute of Technology (KIT), Eggenstein-Leopoldshafen, 76344, Germany
4. Department of Materials and Earth Sciences, Technical University Darmstadt, Darmstadt, 64287, Germany