1. NanoEngineering Group, Faculty of Mechanical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel
2. NEST, Instituto Nanoscienze-CNR and Scuola Normale Superiore, Piazza S. Silvestro 12, I-56127 Pisa, Italy
3. Dipartimento di Fisica “Enrico Fermi”, Università di Pisa, Largo Bruno Pontecorvo 3, I-56127 Pisa, Italy
4. NEST, Istituto Nanoscience-CNR, Piazza S. Silvestro 12, I-56127 Pisa, Italy