  Graphene is of interest in the development of next-generation electronics due
to its high electron mobility, flexibility and stability. However, graphene
transistors have poor on/off current ratios because of the absence of a
bandgap. One approach to introduce an energy gap is to use hydrogenation
reaction, which changes graphene into insulating graphane with sp3 bonding.
Here we show that an electric field can be used to control
conductor-to-insulator transitions in microscale graphene via a reversible
electrochemical hydrogenation in an organic liquid electrolyte containing
dissociative hydrogen ions. The fully hydrogenated graphene exhibits a lower
limit sheet resistance of 200 Gohm/sq, resulting in graphene field-effect
transistors with on/off current ratios of 10^8 at room temperature. The devices
also exhibit high endurance, with up to one million switching cycles. Similar
insulating behaviours are also observed in bilayer graphene, while trilayer
graphene remains highly conductive after the hydrogenation. Changes in the
graphene lattice, and the transformation from sp2 to sp3 hybridization, is
confirmed by in-situ Raman spectroscopy, supported by first-principles
calculations.
