Modification of the surface phonon dispersion of a graphite monolayer adsorbed on Ni(111) caused by intercalation of Yb, Cu and Ag

The surface phonon dispersion of a monolayer of graphite (MG) on Ni(111) has been measured in the Γ̄K̄ direction of the Brillouin zone by means of high-resolution electron energy-loss spectroscopy (HREELS). The phonon dispersion relations of the MG/Ni(111) system and those obtained after intercalation of Yb are characterized by graphite-like phonon modes, softened due to the strong interaction with the Ni substrate. In the case of Cu and Ag intercalation, in contrast, the corresponding surface dispersion curves are very similar to those of bulk graphite. Calculations of the surface phonon dispersion based on a force constant model revealed that the force constants related to vertical motion in the MG are very much more affected after intercalation than those related to horizontal vibrations. This demonstrates that the stiffening observed after Cu and Ag intercalation is caused by a weaker interaction of the graphite layer with the Ni substrate.