1. Physics Department, University of Calabria, I-87036 Arcavacata di Rende, CS, Italy
2. CNR Nanotec-Institute of Nanotechnology, S.S. Cosenza, I-87036 Rende CS, Italy
3. Institute for Microelectronics and Microsystems, CNR, I-00133 Rome, Italy
4. Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, China