[1] CNR NNL CNR NANO, Inst Nanosci, Natl Nanotechnol Lab, I-73100 Lecce, Italy
[2] Univ Roma La Sapienza, Dept Phys, I-00185 Rome, Italy
[3] ISMN CNR, Inst Study Nanostruct Mat, I-00016 Rome, Italy
[4] Univ Salento, Dept Math & Phys, Lecce, Italy
[5] Italian Inst Technol, Ctr Biomol Nanotechnol CNB, Lecce, Italy