1. Laboratory of Interfaces and Nanosize Systems, Institute of Chemistry, Eötvös Loránd University, Pázmány Péter stny. 1/a, H-1117 Budapest, Hungary, Dipartimento di Fisica, Università degli Studi di Trento, 14 via Sommarive, I-38050 Povo (Trento), Italy, and Frankfurt Institute for Advanced Studies, Ruth-Moufang strasse 1, D-60438 Frankfurt, Germany