Graphene is a two-dimensional (2D) crystal made out of carbon atoms arranged on a hexagonal (honeycomb) structure, as shown schematically in Fig. 1 (a). Although it has been experimentally isolated only recently (2004)[1], graphene has been the object of theoretical investigations for more than 60 years, because it represents the simplest structure to understand the electronic properties of all other allotropes of carbon with sp2 hybridisation.