1. Department of Physics, Stockholm University, Stockholm, Sweden
2. Institute of Physics, Chemnitz University of Technology, 09126 Chemnitz, Germany
3. Center for Microtechnologies, Chemnitz University of Technology, 09126 Chemnitz, Germany
4. Fraunhofer Institute for Electronic Nanosystems, 09126 Chemnitz, Germany
5. Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany
6. Department of Molecular Sciences and Nanosystems, Ca' Foscari University of Venice, 30172 Venice, Italy