1. School of Physics, Peking University, Beijing 100871, People’s Republic of China
2. Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-St. 2, D-12489 Berlin, Germany
3. Max Planck Institute for Solid State Research, Heisenbergstrasse 1, 70569 Stuttgart, Germany
4. Laboratory of Physical Chemistry, ETH Zurich, CH-8093 Zurich, Switzerland
5. Department of Physics and Astronomy, London Centre for Nanotechnology, Thomas Young Centre, University College London, Gower Street, London WC1E 6BT, United Kingdom
6. Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom