1. Department of Physics and Materials Science, University of Luxembourg, 162a Avenue de la Faïencerie, L-1511 Luxembourg, Luxembourg
2. Department of Electrical and Systems Engineering, University of Pennsylvania, 19104 Philadelphia, Pennsylvania, United States
3. Nanotech@Surfaces Laboratory, EMPA, Ueberlandstrasse 129, 8600 Dübendorf, Switzerland
4. Department of Physics, Umeå University, Linnaeus väg 24, SE-90187 Umeå, Sweden