Vacancy Controlled Contact Friction in Graphene

Defects-controlled friction in graphene is of technological importance in many applications, but the underlying mechanism remains a subject of debate. Here it is shown that, during the controlled oxidation in oxygen plasma and subsequent reduction induced by high-energy photons, the contact friction in chemical vapor deposition grown graphene is dominantly determined by the vacancies formed instead of the bonding with add-atoms. This effect is attributed to the vacancy-enhanced out-of-plane deformation flexibility in graphene, which tends to produce large puckering of graphene sheet near the contact edge and thus increases the effective contact area. Modified graphene with large contact friction has a large density of defects, but remains a good electrical conductor, in which the carrier transport is strongly affected by quantum localization effects even at room temperature. It is also found that the oxidation process in graphene is substrate-sensitive. Comparing to monolayer graphene on SiO2 substrate, the oxidation process progresses much faster when the substrate is SrTiO3, while bilayer graphene exhibits great oxidation resistance on both substrates. The collection of observations provides important information for tailoring the mechanical, electrical, and chemical properties of graphene through selected defects and substrates.