Induced inhomogeneity in graphene work function due to graphene - TiO2/Ag/glass substrate interaction

Important issues in optoelectronic devices based on exfoliated graphene are the electronic noise due to the interaction between graphene flakes and the substrate, and the accumulation of electrical charge at the flake edges. These effects, in addition to the distribution of flakes in the composites, being random and locally showing multiple overlaps, cause significant Inhomogeneity in the measured work function of the graphene-based materials. To shed light on the above effects, we investigated both graphene flakes and single-layer graphene by means of scanning Kelvin probe microscopy in ambient conditions. The graphene materials were deposited on rough substrates of silver with a top layer of titanium dioxide. The roughness caused the random disorder of interest for characterization, while the thin dielectric layer of titanium dioxide helped in keeping the electric field localized and provided better contrast in the surface potential images. It is demonstrated that proper engineering of the substrate can minimize the work function inhomogeneity. Additionally, SKPM allows for local mapping of graphene flakes overlaps and thus effective thickness, for possible quality control in produced devices.