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(lll) ha s been measured in the TK direction of the Brillonin zone by means of high- resolution electron energy-loss spectroscopy (HREELS). The phonon dispersio n 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 interca lation, in contrast, the corresponding surface dispersion curves are very s imilar to those of bulk graphite. Calculations of the surface phonon disper sion based on a force constant model revealed that the force constants rela ted to vertical motion in the MC are very much more affected after intercal ation than those related to horizontal vibrations. This demonstrates that t he stiffening observed after Cu and Ag intercalation is caused by a weaker interaction of the graphite layer with the Ni substrate. (C) 2000 Elsevier Science B.V. All rights reserved.