1. Centre for Molecular and Nanoscale Electronics, University of Durham, Durham DH1 3LE, UK, Oxonica Ltd., Begbroke Science and Business Park, Yarnton, Oxfordshire, OX5 PF, UK, Institute of Microelectronics, NCSR Demokritos, 15310 Aghia Paraskevi, Greece, and Department of Applied Sciences, National Technical University of Athens, 15780 Zografou, Greece