1. Institute of Nanoscience and Nanotechnology, NCSR Demokritos, Athens 15310, Greece
2. Department of Physics, University of Delaware, Newark, Delaware 19716, USA
3. SIMAU, Università Politecnica delle Marche, via Brecce Bianche, Ancona, Italy
4. ISM-CNR, Area della Ricerca RM1, Via Salaria Km 29,300, P.B. 10-00015, Monterotondo Scalo, Roma, Italy