1. Department of Applied Physics, Aalto University, Otakaari 1, FI-02150 Espoo, Finland
2. Department of Electrical Engineering and Automation, Aalto University, PO Box 13500, 00076 Aalto, Finland
3. Department of Chemistry, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
4. Nanoscopic Physics, Institute of Condensed Matter and Nanosciences, Université Catholique de Louvain, 1348 Louvain-la-Neuve, Belgium