1. Department of Chemistry and London Centre for Nanotechnology, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom
2. Dipartimento FIM, Università di Modena e Reggio Emilia, Via G. Campi 213/a, 41125 Modena, Italy
3. Istituto Nanoscienze, CNR, Via G. Campi 213/a, 41125 Modena, Italy