Graphene, a new class of two-dimensional carbon nanostructure, has attracted tremendous attention from both the experimental and theoretical scientific communities in recent years1. This unique nanostructure holds great promise for potential applications in many technological fields such as nanoelectronics, sensors, nanocomposites, batteries, supercapacitors and hydrogen storage1. However, a lack of an efficient approach to producing processable graphene sheets in large quantities has been a major obstacle to exploiting most proposed applications.