1. Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom
2. School of Physics, Peking University, Beijing 100871, P. R. China
3. Max Planck Institute for Solid State Research, Heisenbergstrasse 1, Stuttgart 70569, German
4. Thomas Young Centre, Department of Physics and Astronomy, and London Centre for Nanotechnology, University College London, Gower Street, London WC1E 6BT, United Kingdom
5. Departments of Chemistry and Materials, Imperial College London, Imperial College Road, London SW7 2AZ, United Kingdom