On chemical bonding and electronic structure of graphene metal contacts
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The nature of chemical bonding at graphene-metal interfaces is intriguing from a fundamental perspective and has great relevance for contacts to novel spintronics and high-frequency electronic devices. Here, we use near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in conjunction with Raman spectroscopy and first-principles density functional theory to examine chemical bonding and perturbation of the -electron cloud at graphene-metal interfaces. Graphene-metal bonding has been contrasted for graphene interfaced with single-crystalline metals, polycrystalline metal foils, and with evaporated metal overlayers and is seen to be strongest at the last noted interface. Strong covalent metal-d-graphene- hybridization and hole doping of graphene is observed upon deposition of Ni and Co metal contacts onto graphene/SiO2and is significantly stronger for these metals in comparison to Cu. Of single-crystalline substrates, the most commensurate (111) facets exhibit the strongest interactions with the graphene lattice. First-principles electronic structure simulations, validated by direct comparison of simulated spectra with NEXAFS measurements, suggest that metal deposition induces a loss of degeneracy between the - and -graphene sublattices and that spin-majority and spin-minority channels are distinctly coupled to graphene, contributing to splitting of the characteristic * resonance. Finally, the electronic structure of graphene is found to be far less perturbed by metal deposition when the cloud is pinned to an underlying substrate; this remarkable behaviour of "sandwich" structures has been attributed to electronic accessibility of only one face of graphene and illustrates the potential for anisotropic functionalization. 2013 The Royal Society of Chemistry.