1. Graphene Based Nanotechnology; School of Science and Technology; University of Siegen; Hölderlinstr. 3 57076 Germany
2. Dipartimento di Ingegneria dell'Informazione; Università di Pisa; Via G. Caruso, 16 Pisa 56122 Italy
3. CNR-ICCOM; Consiglio Nazionale delle Ricerche; via Giuseppe Moruzzi 1 Pisa 56124 Italy
4. Faculty of Electrical Engineering and Information Technology; RWTH Aachen University; Otto-Blumenthal Str. 25 Aachen 52074 Germany