1. Institute of Bio- and Nanosystems IBN 3 and Center of Nanoelectronic Systems for Information Technology, Research Center Jülich, D-52425 Jülich, Germany, Department of Chemistry and Biochemistry, University of Berne, CH-3012-Berne, Switzerland, Institute of Nanotechnology, Research Center Karlsruhe, P.O. Box 3640, D-76021 Karlsruhe, Germany, and Institut für Theorie der Kondensierten Materie, Universität Karlsruhe, 76021 Karlsruhe, Germany