1. Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, People's Republic of China
2. London Centre for Nanotechnology, University College London, London WC1H 0AJ, U.K.
3. Department of Chemistry, University College London, London WC1H 0AJ, U.K.
4. School of Physics, Peking University, Beijing 100871, People's Republic of China