[1] NCSR Demokritos, Inst Microelect, Aghia Paraskevi 15310, Greece
[2] Univ Leeds, Dept Phys & Astron, Leeds LS2 9JT, W Yorkshire, England
[3] Univ Durham, Ctr Mol & Nanoscale Elect, Durham DH1 3LE, England
[4] Natl Tech Univ Athens, Dept Appl Sci, Zografos 15780, Greece