Interfacial Oxidation of Metals on Graphene

In this work, we report the graphene-promoted formation of an interfacial oxide layer when certain metals are deposited on graphene. We probe interfacial oxide formation through the observation that several metals, when 10-12 nm in thickness and deposited on graphene on a transparent substrate, show a change in optical contrast compared to that in areas where the metal directly contacts the substrate. Aluminum shows this effect, while platinum and nickel do not exhibit such a pronounced optical contrast change. To understand this phenomenon, we characterize the Al-graphene, Ti-graphene, and Ni-graphene interfaces using techniques including X-ray photoelectron spectroscopy depth profiling, X-ray reflectivity, and Raman spectroscopy. These techniques show the presence of oxide at the buried metal-graphene interface for the cases of aluminum and titanium deposition, and we discuss how this explains the change in optical contrast. We show that this process is sensitive to the background vacuum level during deposition. In the case of nickel, we did not observe the presence of an oxide. Building upon these findings, we propose structures for Al-graphene, Ti-graphene, and Ni-graphene interfaces. We propose a model based on the metal work function and interaction with graphene that can guide the metals for which interfacial oxidation is to be expected, and we discuss the role of the deposition conditions in controlling the extent of oxide formation. These observations provide important implications for various devices using graphene as either the channel or the contact. Depending on whether a metal-graphene interfacial oxide is desirable and its functionality, these findings will afford guidance for their fabrications in the future.