1. Department of Physics, University of Ioannina, P.O. Box 1186, 451 10 Ioannina, Greece
2. Institut für Chemie neuer Materialien, Universität Osnabrück, D-49069 Osnabrück, Germany
3. University Research Center of Ioannina (URCI) - Institute of Materials Science and Computing, 451 10 Ioannina, Greece