1. Department of Chemical Sciences, University of Padova, Via Marzolo 1, 35131 Padova, Italy
2. Department of Physics, University of Milan, Via Celoria 16, 20133 Milan, Italy
3. Instituto de Cibernetica, Matematica y Física, Calle E esq 15 Vedado, 10400 La Habana, Cuba
4. Department of Physics and Astronomy, Rice University, Houston, Texas 77005, United States
5. CNR Institute of Nanoscience, via Campi 213/A, 41125 Modena, Italy