1. Department of Physics and Astronomy; Uppsala University; 75120 Uppsala Sweden
2. Institut des Nanosciences de Paris; UPMC Univ. Paris 06; CNRS UMR 7588, 75005, Paris France
3. Department of Physical and Chemical Sciences; University of L'Aquila; Via Vetoio, 67010 Coppito, L'Aquila Italy
4. CNR-IOM; Laboratorio TASC; ss. 14 km. 163.5, Basovizza, 34149 Trieste Italy
5. Department of Chemical and Pharmaceutical Sciences; University of Trieste; 34127 Trieste Italy
6. CNR-ISM; Basovizza Area Science Park; 34149 Trieste Italy