1. Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain
2. National Center for Scientific Research “Demokritos”, 15310 Athens, Greece
3. Departamento de Física, Universidad Autónoma de Barcelona, Bellaterra, 08193 Barcelona, Spain
4. ICREA, Passeig Lluis Companys 23, 08010 Barcelona, Spain