1. Faculty of Physics and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg−Essen, Lotharstr. 1, 47057 Duisburg, Germany
2. Institute for Electrical Measurement Science and Fundamental Electrical Engineering, TU Braunschweig, Hans-Sommer-Straße 66, 38106 Braunschweig, Germany