  Electronic properties of the graphene layer sandwiched between two hexagonal
boron nitride sheets have been studied using the first-principles calculations
and the minimal tight-binding model. It is shown that for the ABC-stacked
structure in the absence of external field the bands are linear in the vicinity
of the Dirac points as in the case of single-layer graphene. For certain atomic
configuration, the electric field effect allows opening of a band gap of over
230 meV. We believe that this mechanism of energy gap tuning could
significantly improve the characteristics of graphene-based field-effect
transistors and pave the way for future electronic applications.
