[1] Fdn Res & Technol Hellas FORTH, IESL, Iraklion 70013, Crete, Greece
[2] Univ Sheffield, Dept Mat Sci & Engn, Kroto Res Inst, Sheffield S3 7HQ, S Yorkshire, England
[3] Univ Crete, Dept Mat Sci & Technol, Khania, Greece
[4] Univ Crete, Dept Phys, Khania, Greece