Effect of hydrogenation on the band gap of graphene nano-flakes

•Density functional theory calculations were carried out for hydrogen on graphene•Effects of hydrogenation on the band gap of graphene were examined.•The band gap showed a minimum at a finite coverage.•Mechanism of specific band gap feature was discussed.

The effects of hydrogenation on the band gap of graphene have been investigated by means of density functional theory method. It is generally considered that the band gap increases with increasing coverage of hydrogen atom on the graphene. However, the present study shows that the band gap decreases first with increasing hydrogen coverage and reaches the lowest value at finite coverage (γ = 0.3). Next, the band gap increases to that of insulator with coverage from 0.3 to 1.0. This specific feature of the band gap is reasonably explained by broken symmetry model and the decrease of pi-conjugation. The electronic states of hydrogenated graphene are discussed.