1. Laboratory of Multifunctional Materials and Structures National Institute for Materials Physics Bucharest-Magurele 077125 Romania
2. Institute of Nanoscience and Nanotechnology National Centre for Scientific Research “Demokritos” Athens 15310 Greece
3. Department of Physics National and Kapodistrian University of Athens Athens 15772 Greece
4. Department of Physics University of Ioannina Ioannina 45110 Greece