1. Department of Physics, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133 Milano, Italy
2. Institute of Materials Science (ICMUV), University of Valencia, Catedrático Beltrán 2, E-46980 Valencia, Spain
3. CNR-ISM, Division of Ultrafast Process in Materials (FLASHit), Area della Ricerca di Roma 1, Monterotondo Scalo, Italy
4. Cambridge Graphene Centre, University of Cambridge, 9 JJ Thomson Avenue, Cambridge CB3 0FA, U.K.
5. Université du Luxembourg, 162 A, avenue de la Faencerie, Luxembourg City L-1511, Luxembourg
6. IFN-CNR, Piazza Leonardo da Vinci 32, I-20133 Milano, Italy