Graphene on Metallic Substrates: Suppression of the Kohn Anomalies in the Phonon Dispersion

The phonon dispersion of graphene is known to display two strong Kohn Anomalies (kinks) in the highest optical branch (HOB) at the high-symmetry points Gamma and K [Piscanec, S.; et al. Phys. Rev. Lett. 2004, 93, 185503]. The phonon slope around the Kohn anomalies is related to the electron phonon coupling (EPC) with the graphene it bands. we show that this EPC, which has strong impact, for example, on Raman scattering and electron transport, can be strongly modified due to interaction with a metallic substrate. For graphene grown on a Ni(111) surface, a total suppression of the Kohn anomaly occurs: the HOB around Gamma and K becomes completely flat. This is due to the strong hybridization of the graphene pi-bands with the nickel d bands that lifts the linear crossing of the pi bands at K. In addition, the out-of-plane modes are also found to be strongly affected by the binding to the substrate. For other metallic substrates, where the distance between the graphene sheet and the substrate is larger, hybridization is much less pronounced and the Kohn anomaly is only weakly perturbed. From experimental phonon dispersions, one can therefore draw conclusions about the interaction strength between graphene and its different substrates.