Interfacial Oxidation\nof Metals on Graphene

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