1. Department of Engineering, University Roma Tre, Via Vito Volterra 62, Rome 00146, Italy
2. CNR Nanotec, Istituto di Nanotecnologia, Via Monteroni, Lecce 73100, Italy
3. IHP-Leibniz Institut für Innovative Mikroelektronik, Im Technologiepark 25, 15236 Frankfurt (Oder), Germany