[1] Univ Basque Country, CSIC, CFM, Paseo Manuel de Lardizabal 5, Donostia San Sebastian 20018, Basque Country, Spain
[2] DIPC, Manuel Lardizabal Pasealekua 4, Donostia San Sebastian 20018, Basque Country, Spain
[3] Univ Basque Country, Fis Aplikatua Saila 1, UPV EHU, Europa Plaza 1, Donostia San Sebastian 20018, Basque Country, Spain
[4] Univ Roma La Sapienza, Dipartimento Fis, Piazzale Aldo Moro 5, I-00185 Rome, Italy
[5] Fdn Ist Italian Tecnol, Graphene Labs, Via Morego, I-16163 Genoa, Italy
[6] CALTECH, Dept Appl Phys & Mat Sci, Steele Lab, Pasadena, CA 91125 USA
[7] Univ Paris 06, UPMC, Sorbonne Univ, IMPMC,UMR CNRS 7590,MNHN,IRD, 4 Pl Jussieu, F-75005 Paris, France
[8] Sorbonne Univ, Inst Nanosci Paris, UMR7588, CNRS, F-75252 Paris, France
[9] Univ Basque Country, Dept Fis Mat Condensada, UPV EHU, Bilbao 48080, Basque Country, Spain