1. Department of Electrical and Computer Engineering and Micro and Nanotechnology Lab, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
2. Department of Electrical Engineering, Stanford University, Stanford, California 94305, USA
3. Department of Physics, Mathematics and Computer Science, University of Modena and Reggio Emilia, Via Campi 213/A, I-41125 Modena, Italy
4. Department of Electrical, Electronic and Information Engineering “Guglielmo Marconi”, University of Bologna, Viale Risorgimento 2, I-40136 Bologna, Italy