1. Department of Physics, University of Ioannina, P.O. Box 1186, Ioannina 451 10, Greece
2. Department of Materials Science and Technology, University of Crete, Heraklion 71003, Greece
3. FORTH, Institute of Electronic Structure and Laser, Heraklion 71110, Crete, Greece
4. Max Planck Institute for Polymer Research, 55128 Mainz, Germany
5. University Research Center of Ioannina (URCI) - Institute of Materials Science and Computing, Ioannina 451 10, Greece