1. Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany, Center for Functional Materials, Department of Physical Chemistry, Åbo Akademi University, Porthansgatan 3-5, FIN-20500 Turku, Finland, Institute of Biophysics and Nanosystems Research, Austrian Academy of Sciences, Schmiedlstr. 6, A-8042 Graz, Austria