1. School of Materials Science, Japan Advanced Institute of Science and Technology (JAIST), 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan
2. London Centre for Nanotechnology, University College London (UCL), London WC1H 0AH, United Kingdom
3. Department of Chemistry, UCL, London WC1H 0AJ, United Kingdom
4. Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, USA
5. IBM Research Division, Thomas J. Watson Research Center, Yorktown Heights, New York 10598, USA
6. Department of Physics and Astronomy, UCL, London WC1E 6BT, United Kingdom