1. Department of Physics; Centre for Materials Science and Nanotechnology; University of Oslo; P.O. Box 1048, Blindern NO-0316 Oslo Norway
2. Department of Chemistry; Centre for Materials Science and Nanotechnology; University of Oslo; FERMiO, Gaustadalléen 21 NO-0349 Oslo Norway
3. Catalan Institute of Nanoscience and Nanotechnology; ICN2 (CSIC/ICN); Campus UAB, Bellaterra Barcelona 08193 Spain