1. Physical Chemistry, Department of Chemistry and Biochemistry, and Center for Nanoscience, Ludwig-Maximilians-Universität München, Butenandtstr. 11, Haus E, D-81377 Munich, Germany, Department of Physics and Center for Nanoscience, Ludwig-Maximilians-Universität München, Geschwister-Scholl-Platz 1, D-80539 Munich, Germany, and Department of Physics, University of Illinois at Urbana-Champaign, 1110 West Green Street, Urbana, Illinois 61801