1. Nanoscience and Materials Division, INRIM, Strada delle Cacce, 91 Torino, Italy
2. Department of Science and Technological Innovation, University of Eastern Piedmont “A. Avogadro”, Viale T. Michel 11, 1512 Alessandria, Italy
3. Physics Dept. and NIS Interdepartmental Centre, University of Torino, via P. Giuria 1, 10125 Torino, Italy