1. Department of Chemistry, University of Zurich, Winterthurerstrasse 190, Zurich 8057, Switzerland
2. Molecular Surface Science Group, Empa, Swiss Federal Laboratories for Materials Science and Technology, Dübendorf 8600, Switzerland
3. Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, Lausanne 1015, Switzerland