1. Department of Physics and Nanoscience Center, University of Jyväskylä, P.O. Box 35, FI-40014 Jyväskylä, Finland
2. Department of Chemistry and Nanoscience Center, University of Jyväskylä, P.O. Box 35, FI-40014 Jyväskylä, Finland
3. University of Tartu, Institute of Physics, W. Ostwaldi tn 1, 50411 Tartu, Estonia