1. NanoLund and Division of Solid State Physics, Lund University, Box 118, 221 00 Lund, Sweden
2. Physics, Faculty of Science and Engineering, Åbo Akademi University, FI-20500 Turku, Finland
3. Department of Electronics and Nanoengineering, Aalto University, P.O. Box 13500, FI-00076 Aalto, Finland