Edge functionalization of finite graphene nanoribbon superlattices

The effect of chemical functionalization on the electronic properties of graphene nanoribbon superlattices with zigzag and armchair terminations is investigated using the density functional theory. The calculated positive binding energies imply that all the considered structures are stable before and after chemical modifications. The superlattices with armchair edges are characterized by a wide energy gap while those with zigzag edges have a narrow energy gap. The energy gap in superlattices with armchair edges nearly independent of their length while it strongly decreases and almost closes in superlattices with zigzag edges. The energy gap is comparably sensitive to the width variations in both types of the superlattices. It was found that the electric dipole moment increases with increasing the width in the case of armchair while it oscillates in zigzag superlattices. The electric dipole moment can be enhanced by chemical functionalization with COOH and NH2 groups. The effect of this functionalization is moderate for the energy gap, but the adsorption of tetracyanoquinodimethane transforms the system from insulator (Eg = 2.66 eV) to a narrow band gap semiconductor (Eg = 0.25 eV). The adsorption energy of tetracyanoquinodimethane and tetrathiafulvalene molecules can be enhanced by chemical functionalization which makes graphene superlattices useful for sensor applications and wastewater treatment.