1. Department of Chemistry and Pharmacy, Laboratory of Materials Science and Nanotechnology, CR-INSTM, University of Sassari, via Vienna 2, 07100 Sassari, Italy
2. Department of Physics, University of Cagliari, sp 8, km 0.700, 09042 Monserrato, Italy
3. Department of Chemical and Geological Sciences and INSTM, University of Cagliari, sp 8, km 0.700, 09042 Monserrato, Italy