  The absence of a band gap in graphene restricts its straight forward
application as a channel material in field effect transistors. In this letter,
we report on a new approach to engineer a band gap in graphene field effect
devices (FED) by controlled structural modification of the graphene channel
itself. The conductance in the FEDs is switched between a conductive "on-state"
to an insulating "off-state" with more than six orders of magnitude difference
in conductance. Above a critical value of an electric field applied to the FED
gate under certain environmental conditions, a chemical modification takes
place to form insulating graphene derivatives. The effect can be reversed by
electrical fields of opposite polarity or short current pulses to recover the
initial state. These reversible switches could potentially be applied to
non-volatile memories and novel neuromorphic processing concepts.
