  Opening, in a controllable way, the energy gap in the electronic spectrum of
graphene is necessary for many potential applications, including an efficient
carbon-based transistor. We have shown that this can be achieved by chemical
functionalization of bilayer graphene. Using various dopants, such as H, F, Cl,
Br, OH, CN, CCH, NH2, COOH, and CH3 one can vary the gap smoothly between 0.64
and 3 eV and the state with the energy gap is stable corresponding to the
lowest-energy configurations. The peculiarities of the structural properties of
bilayer graphene in comparison with bulk graphite are discussed.
