Surface phonon dispersion in graphite and in a lanthanum graphite intercalation compound

Abstract

Using high-resolution electron energy-loss spectroscopy the surface-phonon dispersion of graphite has been determined in the \ensuremath{\Gamma}K direction over the whole energy range and the whole Brillouin zone. Born--von Karman model calculations are used to describe the dispersion relations. An unexpected result is the splitting of the ZA and ZO mode at the K point. Following a previously introduced procedure to form in situ rare-earth graphite intercalation compounds (GIC), which for lanthanum results in an intermediate carbide phase, we prepared this carbidic phase and the final GIC-like phase. The carbide shows five dispersionless features that may be attributed to Einstein modes of graphite islands. The phonon dispersion of the final phase shows the same modes as graphite shifted in energy: softening of the optical and stiffening of the acoustical phonons occurs. This is described within a Born--von Karman model by weakening the nearest-neighbor interaction and strengthening the second-nearest-neighbor interaction. The evolution of the phonon dispersion gives a first hint that the GIC-like phase may develop in two stages: first a monolayer graphene on top of the carbide and then the very thin GIC layer.