1. Advanced Technology Institute, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom
2. London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, London WC1H 0AH, United Kingdom
3. Laboratoire de Chemie Physique, Université Paris-Sud 11, 91405 Orsay, France
4. Institute of Micro/Nanoelectronics, Peking University, Beijing, China