1. Institute of Nanoscience and Nanotechnology, NCSR Demokritos, 15310 Athens, Greece
2. Department of Education, School of Pedagogical and Technological Education, 14121 Athens, Greece
3. Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, USA
4. Department of Materials Science and Engineering, University of Ioannina, 45110 Ioannina, Greece