1. nanotech@surfaces Laboratory, Empa, Swiss Federal Laboratories for Materials Testing and Research, Feuerwerkerstrasse 39, 3602 Thun, Switzerland, and Ueberlandstrasse 129, 8600 Dübendorf, Switzerland, International School for Advanced Studies, INFM/CNR/DEMOCRITOS, and International Centre for Theoretical Physics, 34014 Trieste, Italy, and Department of Organic Chemistry, University of Zurich, Winterthurerstr. 190, 8057 Zurich, Switzerland