1. School of Physics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
2. School of Chemical Engineering, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
3. BAM Federal Institute for Materials Research and Testing, 12200 Berlin, Germany
4. Department of Physics and IESL-FORTH, University of Crete, 70013 Heraklion, Crete, Greece