Irradiation-induced broadening of the Raman spectra in monolayer graphene

Broadening of the Raman scattering (RS) spectra was studied in monolayer graphene samples irradiated with various doses of ions followed by annealing of radiation damage at different temperatures. It is shown that the width Γ (full width at half maximum) of three main RS lines (G, D, and 2D) increases linearly with the increase of the density of irradiation-induced point defects N_d: ΔΓ = mN_d. The slope m is the same for one-phonon emitting G-line and D-line and almost double for a two-phonon emitting 2D-line. It is observed that the width of the D-line for slightly irradiated samples is larger than one half of the width of the 2D-line, which shows that, in the case of the D-line, elastic electron scattering on point defects leads to an additional linewidth. This difference decreases with the increase of N_d, which shows the decrease of the role of scattering on a single point defect because of the spreading of this mechanism over the whole disordered film. A theoretical model of the width of the D-line in disordered graphene is developed. The analytical expressions explain the experimental observations and allow one to determine the numerical coefficient in the in-plane transverse optical phonon dispersion in graphene.