1. Laboratory for Surface Science and Technology, Department of Materials, and Materials Research Center, ETH Zurich, Wolfgang-Pauli-Str. 10, CH-8093 Zurich, Switzerland, Nanotechnology Group, ETH Zurich, Tannenstrasse 3, CH-8092 Zurich, Switzerland, and Dipartimento di Chimica Inorganica ed Analitica, Università degli Studi di Cagliari, Cittadella Universitaria di Monserrato, I - 09100 Cagliari, Italy