1. Department
of Physics and Earth Sciences, University of Parma, Viale G.P.
Usberti 7/A, 43124 Parma, Italy
2. Department
of Materials Science, University of Milano-Bicocca, Via R. Cozzi 55, 20125 Milano, Italy
3. Center
for Nanotechnology Innovation@NEST, Istituto Italiano di Tecnologia, Piazza S. Silvestro 12, 56127 Pisa, Italy