1. Department of Materials Science and Engineering, Aalto University School of Chemical Technology, P.O. Box 16200, FI-00076 Aalto, Finland
2. Department of Physics, University of Jyväskylä, P.O. Box 35, FI-40014 Jyväskylä, Finland
3. Department of Micro- and Nanosciences, Aalto University School of Electrical Engineering, P.O. Box 13500, FI-00076 Aalto, Finland