1. Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain
2. Departamento de Física, Universidad Autónoma de Barcelona, Bellaterra, 08193 Barcelona, Spain
3. Department of Information Engineering, Polytechnic University of Marche, Via Brecce Bianche, 1, 60131 Ancona, Italy
4. INFN-Laboratori Nazionali di Frascati, via E. Fermi 40, 00044 Frascati, Italy
5. ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain