1. Department of Earth Sciences, University College London 1 , London WC1E 6BT, United Kingdom
2. Thomas Young Centre, University College London 2 , London WC1E 6BT, United Kingdom
3. London Centre for Nanotechnology, University College London 3 , London WC1E 6BT, United Kingdom
4. Dipartimento di Fisica Ettore Pancini, Università di Napoli Federico II 4 , Monte S. Angelo, I-80126 Napoli, Italy