1. Solid State Physics Section, University of Athens, Panepistimiopolis Zografos, 15784 Athens, Greece
2. Department of Electrical and Computer Engineering, University of Thessaly, 38333 Volos, Greece
3. School of Nuclear Engineering, Purdue University, West Lafayette, IN 47906, USA
4. Department of Physics, V. N. Karazin Kharkiv National University, 4 Svobody sq., 61077 Kharkiv, Ukraine
5. Department of Materials, Imperial College London, London SW7 2BP, UK