1. Department of Physics, Nanoscience Center, University of Jyväskylä, FI-40014 Jyväskylä, Finland
2. Faculty of Information Technology, University of Jyväskylä, FI-40014 Jyväskylä, Finland
3. Department of Chemistry, Nanoscience Center, University of Jyväskylä, FI-40014 Jyväskylä, Finland