Direct observation of the work function evolution of graphene-two-dimensional metal contacts

The work function evolution of a graphene monolayer under two-dimensional metal electrodes was studied by combining in situ metal deposition and ultraviolet photoelectron spectroscopy under an ultra-high vacuum system. The process of the charge transfer at the graphene-metal interfaces was investigated. The transfer of electrons from metal to graphene and then from graphene to metal, with deposition of the metal, was observed. The work function of the graphene-metal contact shifted, and was finally stopped at the theoretical work function of the metal when the metal turned from a two-dimensional film into a bulk material. Meanwhile, the energy barrier of the metal-graphene interface could be tailored by altering the metal thickness. This research makes it possible to use the graphene device in future applications.