Two-phonon Raman bands of bilayer graphene: revisited

We present complete calculations of the two-phonon Raman bands of bilayer graphene, including all overtone and combination modes, within a density-functional tight-binding model. Based on our results, we assign unambiguously the observed two-phonon Raman bands to two-phonon modes, thus resolving the existing controversies. In particular, we show that both overtone and combination modes have essential contribution to the 2D band, bringing about specific modifications of the band shape. We argue that a mid-range two-phonon Raman band, previously assigned to the 2ZO mode, should be assigned to the TOZO' mode. We find that the Raman band, usually assigned to the LOLA mode, has significant contribution from the TOZO mode. The predicted Raman bands can be used for assignment of the observed ones in the Raman spectra of bilayer graphene for the needs of sample characterization for future technological applications.