1. Division of Solid State Physics and NanoLund; Lund University; P.O. Box 118 SE-22100 Lund Sweden
2. Department of Electronics and Nanoengineering; Aalto University; P.O. Box 13000 FI-00076 Aalto Finland
3. Engineered Nanosystems Group; Aalto University; P.O. Box 12200 FI-00076 Aalto Finland