  A striking feature of bilayer graphene is the induction of a significant band
gap in the electronic states by the application of a perpendicular electric
field. Thicker graphene layers are also highly attractive materials. The
ability to produce a band gap in these systems is of great fundamental and
practical interest. Both experimental and theoretical investigations of
graphene trilayers with the typical ABA layer stacking have, however, revealed
the lack of any appreciable induced gap. Here we contrast this behavior with
that exhibited by graphene trilayers with ABC crystallographic stacking. The
symmetry of this structure is similar to that of AB stacked graphene bilayers
and, as shown by infrared conductivity measurements, permits a large band gap
to be formed by an applied electric field. Our results demonstrate the critical
and hitherto neglected role of the crystallographic stacking sequence on the
induction of a band gap in few-layer graphene.
