  By comparing the growth of Cu thin films on bare and graphene-covered
Ru(0001) surfaces, we demonstrate the role of graphene as a surfactant allowing
the formation of flat Cu films. Low-energy electron microscopy, X-ray
photoemission electron microscopy and X-ray absorption spectroscopy reveal that
depositing Cu at 580 K leads to distinct behaviors on both types of surfaces.
On bare Ru, a Stranski-Krastanov growth is observed, with first the formation
of an atomically flat and monolayer-thick wetting layer, followed by the
nucleation of three-dimensional islands. In sharp contrast, when Cu is
deposited on a graphene-covered Ru surface under the very same conditions, Cu
intercalates below graphene and grows in a step-flow manner: atomically-high
growth fronts of intercalated Cu form at the graphene edges, and extend towards
the center of the flakes. Our findings suggest potential routes in metal
heteroepitaxy for the control of thin film morphology.
