1. Department of Physics and Nanotechnology, Aalborg University, DK-9220 Aalborg East, Denmark,
2. Department of Micro- and Nanotechnology, Technical University of Denmark, DTU Nanotech, Building 345 East, DK-2800 Kongens Lyngby, Denmark
3. Department of Applied Physics, Aalto University, P.O. Box 11100, FI-00076 AALTO, Finland