1. Department of Applied Physics, Royal Institute of Technology, Albanova University Centre, Roslagstullsbacken 21, 106 91 Stockholm, Sweden
2. Institute of Semiconductor and Solid State Physics, Johannes Kepler University Linz, 4040 Linz, Austria
3. Dipartimento di Fisica, Sapienza Università di Roma, Piazzale A. Moro 1, I-00185 Roma, Italy
4. Kavli Institute of Nanoscience, Delft University of Technology, Lorentzweg 1, 2628CJ Delft, The Netherlands