1. Consiglio Nazionale delle Ricerche, Istituto per la Microelettronica e Microsistemi (CNR-IMM), Rome 00133, Italy
2. Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, GR-11635 Athens, Greece
3. Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas (FORTH-IESL), GR-70013 Heraklion, Greece