Effect of vacancy defects on phonon properties of hydrogen passivated graphene nanoribbons

The phonon properties of hydrogen-passivated armchair graphene nanoribbons (AGNRs) with different vacancy concentrations are investigated theoretically. We calculate the change in the phonon density of states (PDOSs) due to a broad range of vacancies and hydrogen passivation effects using forced vibrational method. A large downshift of prominent Raman active Г point LO mode phonons with an increase of vacancy concentration or decrease of ribbon widths are observed. We find an increasing peak intensities for the C–H stretching mode with the decrease of ribbon width or the increase of defect density. An inserted vacancy concentration of 10% and higher induce the broadening and distorting of the PDOS peaks significantly. The localization properties of phonon due to defects were also studied. The typical mode pattern of K point iTO mode phonons show the spatial localized vibrations persuaded by armchair edges or vacancies, which are in conceptually good agreement with the large D band of the Raman spectra comes from the armchair-edges or the imperfections of crystal. The typical displacement pattern for C–H stretching mode shows a random displacement of H atoms in contrast to C atoms. Our simulation results show the significant impact of vacancy defects on the vibrational properties of GNRs.