1. Nanoelectronic Devices Laboratory, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
2. Department of Electrical, Management and Mechanical Engineering, University of Udine, 33100 Udine, Italy
3. Department of Physics, University of Basel, 4003 Basel, Switzerland
4. Laboratory for Micro- and Nanotechnology, Paul Scherrer Institut, 5232 Villigen, Switzerland