1. The Blackett Laboratory, Department of Physics, Imperial College London, London SW7 2AZ, United Kingdom
2. Chair of Physical Chemistry I, Department of Chemistry and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Universitätsstraße 5, 45141 Essen, Germany
3. Chair in Hybrid Nanosystems, Nanoinstitute Munich, Faculty of Physics, Ludwig-Maximilians-Universität München, 80539 München, Germany