1. Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, GR-11635 Athens, Greece
2. Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas, GR-70013 Heraklion, Crete, Greece
3. Section of Electronic Physics and Systems, Department of Physics, National and Kapodistrian University of Athens, GR-15784 Athens, Greece
4. Consiglio Nazionale delle Ricerche, Istituto per la Microelettronica e Microsistemi (CNR-IMM), 00133 Rome, Italy
5. Department of Materials Science Technology, University of Crete, GR-70013 Heraklion, Crete, Greece
6. Department of Physics, University of Crete, GR-70013 Heraklion, Crete, Greece