1. Institut de Physique des Nanostructures, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland
2. Departments of Chemistry and Physics & Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada
3. Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany