Graphene on metal surfaces

The article reviews work on graphene monolayers adsorbed on metal surfaces. Graphene layers on metals have been prepared by surface segregation of carbon and by decomposition of hydrocarbons. The films are often not rotationally aligned to the metal surface. However, for a number of hexagonally close-packed surfaces perfectly ordered epitaxial overlayers have been obtained, with domains larger than the terraces of the metal substrate. In most cases the well-ordered overlayers display moiré structures with large periodicities, resulting from the lattice mismatch between graphene and the underlying metal. These structures are connected with a buckling of the graphene layer indicating local variations of the binding to the metal. For the metal-graphene spacings values between approximately 2.1 and 3.8 Å have been found, depending on the metal. Reasons for these strong variations are not yet clear, but there are indications that the systems fall into two classes that differ qualitatively with respect to the metal/graphene interaction. These variations are also reflected by the electronic structure. There are metal-graphene systems in which the π band is significantly downshifted in energy compared to the free-standing graphene, and a band gap of order eV has opened at the K¯ point of the Brillouin zone. In other systems, the electronic structure of free-standing graphene is almost intact. The perfectness of the epitaxial moiré phases offers promising applications, e.g., as templates for nanostructures.