  We have performed electron energy-loss spectroscopy (EELS) studies of
Ni(111), graphene/Ni(111), and the graphene/Au/Ni(111) intercalation-like
system at different primary electron energies. A reduced parabolic dispersion
of the \pi plasmon excitation for the graphene/Ni(111) system is observed
compared to that for bulk pristine and intercalated graphite and to linear for
free graphene, reflecting the strong changes in the electronic structure of
graphene on Ni(111) relative to free-standing graphene. We have also found that
intercalation of gold underneath a graphene layer on Ni(111) leads to the
disappearance of the EELS spectral features which are characteristic of the
graphene/Ni(111) interface. At the same time the shift of the \pi plasmon to
the lower loss-energies is observed, indicating the transition of initial
system of strongly bonded graphene on Ni(111) to a quasi free-standing-like
graphene state.
