Electronic and optical properties of a nanographite ribbon in an electric field

Through the tight-binding model, the effect of electric field on electronic and optical properties of a zigzag (an armchair) H-terminated nanographite ribbon is studied. The effective electric field shifts the Fermi level (EF), modifies the energy dispersions, alters the subband spacing, produces the new edge state, changes the band gap, and causes the semiconductor–metal transitions. First, in the metallic zigzag ribbon, the degeneracy of flat bands at EF is lifted by the electric field. Meanwhile, an energy band gap Eg, depending on the ribbon width and the field strength, is induced. And while the field strength increases, the semiconductor–metal transition occurs in both the semiconducting armchair and the zigzag ribbons. Then, the effect of electric field on band structures is completely reflected in the features of density of states—the shift of peak position, the change of peak height and the alternation of band gap. Most importantly, the effective electric field has great influence on the low-energy absorption spectra. It can change frequency of the first peak, alter the peak height, and even produce the new peaks.