1. Department of Physics, University of Ioannina, P.O. Box 1186, 45110 Ioannina, Greece
2. Institut für Chemie neuer Materialien, Universität Osnabrück, D-49069 Osnabrück, Germany
3. Institute of Materials Science and Computing, University Research Center of Ioannina (URCI), Ioannina, Greece