1. Department of Physics, Tampere University of Technology, P.O.
Box 692, FI-33101 Tampere, Finland
2. Department of Micro and Nanosciences, Aalto University, P.O. Box 13500, FI-00076 Aalto, Finland
3. Department of Applied Physics and Nanomicroscopy Center, Aalto University, P.O.
Box 15100, FI-00076 Aalto, Finland
4. COMP Centre of Excellence, Department of Applied Physics, Aalto University, FI-00076 Aalto, Finland