1. Department of Information Engineering, University of Padova, Padova 35131, Italy
2. Department of Electrical and Computer Engineering, University of California, Santa Barbara, California 93106, United States
3. Materials Department, University of California, Santa Barbara, California 93106, United States
4. Formerly with Intel Corporation, Santa Clara, California 95054, United States
5. Department of Physics and Astronomy, University of Padova, via Marzolo 8, Padova 35131, Italy