1. Department of Applied Physics, National Technical University of Athens, 15780 Zografou, Greece
2. Institute for Nuclear and Particles Physics, N.C.S.R. “Demokritos,” 15341 Aghia Paraskevi, Greece
3. Institute of Nanoscience and Nanotechnology, N.C.S.R. “Demokritos,” 15341 Aghia Paraskevi, Greece