  The potential of graphene-based materials consisting of one or a few layers
of graphite for integrated electronics originates from the large
room-temperature carrier mobility in these systems (approx. 10,000 cm2/Vs).
However, the realization of electronic devices such as field-effect transistors
will require controlling and even switching off the electrical conductivity by
means of gate electrodes, which is made difficult by the absence of a bandgap
in the intrinsic material. Here, we demonstrate the controlled induction of an
insulating state - with large suppression of the conductivity - in bilayer
graphene, by using a double-gate device configuration that allows an electric
field to be applied perpendicular to the plane. The dependence of the
resistance on temperature and electric field, and the absence of any effect in
a single-layer device, strongly suggest that the gate-induced insulating state
originates from the recently predicted opening of a bandgap between valence and
conduction bands.
