FRICTION BEHAVIOUR OF HEXAGONAL-SHAPED GRAPHENE ISLANDS GROWN ON COPPER BY CHEMICAL VAPOUR DEPOSITION (CVD)

摘要

Single asperity friction experiments on graphene, as well as other atomically thin films, have shown that single layers of atoms can dramatically reduce friction when compared to the substrate material. In particular, graphene has been shown to be a remarkable (dry) lubricant on a number of surfaces including copper, nickel, silicon, and mica. However, because few layer graphene (FLG) films have atomic dimensions in one direction their mechanical properties are not isotropic. In this study, we examine high-quality chemical vapour deposited (CVD) graphene on polycrystalline surfaces using silicon atomic force microscopy (AFM) tips. Precise control over the deposition conditions allowed for growth of isolated, single-domain graphene islands that contain a very low defect density and are approximately 5-40 μm across. Compared to the surrounding copper surface, friction is reduced by a factor ranging from 1.5 to 7 over loads from the adhesive minimum up to 80 nN. Friction measurements also show a layer-dependence on friction, resulting from the formation of pucker around the sliding tip. Furthermore, substantial hysteresis in the load dependence of friction is observed when scanning on an individual graphene island while contact between the AFM tip and graphene film is maintained throughout the experiment. The observed hysteresis depends on sliding time and maximum applied normal force, indicating that there is a relatively weak adhesive interaction between graphene and the copper foil and that friction depends on the sliding history of the graphene-tip contact.