Realistic metal-graphene contact structures.

The contact resistance of metal-graphene junctions has been actively explored and exhibited inconsistencies in reported values. The interpretation of these electrical data has been based exclusively on a side-contact model, that is, metal slabs sitting on a pristine graphene sheet. Using in situ X-ray photoelectron spectroscopy to study the wetting of metals on as-synthesized graphene on copper foil, we show that side-contact is sometimes a misleading picture. For instance, metals like Pd and Ti readily react with graphitic carbons, resulting in Pd- and Ti-carbides. Carbide formation is associated with C-C bond breaking in graphene, leading to an end-contact geometry between the metals and the periphery of the remaining graphene patches. This work validates the spontaneous formation of the metal-graphene end-contact during the metal deposition process as a result of the metal-graphene reaction instead of a simple carbon diffusion process.