Phonon stiffening induced by copper intercalation in monolayer graphite on Ni(111)

The intercalation of copper underneath a graphite monolayer (MG) formed on Ni(111) has been investigated by means of HREELS, Auger spectroscopy and LEED. The softening of the graphite phonon modes in the case of the MG/Ni(111) system, caused by a strong interaction with the Ni substrate, is removed after intercalation of copper. The corresponding phonon dispersion curves are very similar to those of bulk graphite and the C (KVV) Auger spectra resemble very much those of classical graphite intercalated compounds. This demonstrates that the observed stiffening is a consequence of a smaller charge transfer from the copper atoms into the π-states of graphite.