1. S3 Centre, Istituto Nanoscienze, CNR, Via Campi 213/a, 41125 Modena, Italy
2. Dipartimento di Fisica, Università di Roma La Sapienza, Piazzale Aldo Moro 5, I-00185 Roma, Italy
3. Nanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan
4. The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Mihogaoka 8-1, Osaka, Ibaraki 567-0047, Japan
5. Center for Life NanoScience, Istituto Italiano di Tecnologia, viale Regina Elena 291, 00161 Rome, Italy
6. CNR-SPIN, Area della Ricerca di Tor Vergata, Via del Fosso del Cavaliere 100, I-00133 Rome, Italy
7. University of Vienna, Faculty of Physics, Strudlhofgasse 4, A1090 Vienna, Austria