1. Department of Chemical Sciences; Università di Padova; Via Marzolo 1 35131 Padova Italy
2. Department of Electrical Engineering; Università di Padova; Via Gradenigo 7 35131 Padova Italy
3. Faculty of Physics; Univeristy of Belgrade; 11001 Belgrade, P.O. Box 368 Serbia
4. Institute of Physics; Univeristy of Belgrade; 11081 Belgrade, P.O. Box 68 Serbia