1. Contribution from the Department of Physics and Chemistry, University of Southern Denmark (Odense University), Campusvej 55, DK-5230, Odense M, Denmark, California NanoSystems Institute and Department of Chemistry and Biochemistry, The University of California, Los Angeles,405 Hilgard Avenue, Los Angeles, California 90095, and Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100, København Ø, Denmark