1. NanoEngineering Group, Faculty of Mechanical Engineering, Technion-Israel Institute of Technology, Haifa 3200003, Israel
2. Dipartimento di Fisica, Università di Pisa, Largo B. Pontecorvo 3, I-56127 Pisa, Italy
3. NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore, Piazza San Silvestro 12, I-56127 Pisa, Italy