Temperature dependence of the Raman spectra of multilayer graphene nanoribbons fabricated by unzipping method

The temperature dependence of Raman spectra of multilayer-graphene nanoribbons (MLGNRs) fabricated by unzipping method was investigated in the temperature range from 300 K to 700 K. MLGNRs with the width of similar to 200 nm are isolated and individually measured. The frequency of G band is monotonically downshifted with increasing temperature. The change in the G band frequency with temperature is reversible in thermal cycles with heating and cooling. By linear fitting, the temperature coefficient is estimated to be about - 0.021 cm(-1)/K. This value is smaller than - 0.028 cm(-1)/K of carbon nanotubes and larger than -0.011 and - 0.016 cm(-1)/K of graphite and graphene, respectively, as reported previously. This means that MLGNRs are thermally stable compared with carbon nanotubes with curvatures, whereas the thermal stability of MLGNRs is lower than those of graphene and graphite. The better fitting to the G band frequency shift with temperature is obtained with nonlinear quadratic curve. From the theoretical analysis of the fitted quadratic curve, it is clarified that the downshift of G band frequency with increasing temperature is attributed to the anharmonic phonon interaction, especially 4-phonon process rather than 3-phonon process. Comparing with other nanocarbon materials reported so far, it is suggested that the strength of the anharmonic phonon interaction depends on the layer number and size of graphene.