1. Department of Microtechnology and Nanoscience, Chalmers University of Technology, Fysikgränd 3, S-41296 Göteborg, Sweden, Nano-Science Center (Niels Bohr Institute & Department of Chemistry), University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark, and Forschungszentrum Karlsruhe, Institut für Nanotechnologie, 76021 Karlsruhe, Germany