1. Physics Department, National and Kapodistrian University of Athens, Panepistimiopolis, GR 15784 Zografos, Athens, Greece
2. National Center of Natural Sciences Demokritos, Institute of Nanomaterials and Nanotechnology, 15354 Agia Paraskevi, Athens, Greece
3. Physics Department, University of Patras, Panepistimiopolis Rio, 265 00 Patras, Greece