1. Department of Physics, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark
2. Department of Sciences, Roma Tre University, 00146 Rome, Italy
3. Department of Engineering, Roma Tre University, 00146 Rome, Italy
4. Department of Physics, University of Rome “Tor Vergata”, 00133 Rome, Italy
5. Istituto Nazionale di Fisica Nucleare, Roma 2, 00133 Rome, Italy
6. Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark
7. IBM Research Division, T.J. Watson Research Center, Yorktown Heights, New York, New York 10598, United States
8. Center for Nanotechnology Innovation @NEST, Istituto Italiano di Tecnologia, Pisa 56127, Italy
9. Graphene Laboratories, Istituto Italiano di Tecnologia, Genova 16163, Italy