Influence of heating on the measured friction behavior of graphene evaluated under ultra-high vacuum conditions

Abstract

Atomic scale friction measurements of exfoliated graphene on a silicon oxide substrate were conducted under ultra-high vacuum (UHV) conditions using an atomic force microscope. Two groups of samples were prepared: one that was heated for 3 h at 800 °C in the UHV chamber and one that was not heated in the UHV chamber. The heated sample showed much lower friction, a lower pull-off force during unloading, and no hysteresis between loading and unloading friction measurements on the heated sample compared with the unheated sample. Additionally, a significantly higher tilt of the friction loop was observed in the unheated sample compared with no or very little tilt of friction loops in the heated sample. Interpretation of lateral forces using the Prandtl–Tomlinson model showed a significantly higher energy corrugation on the unheated sample, but similar lateral contact stiffnesses. Furthermore, no hysteresis in either the energy corrugation or lateral contact stiffness was observed in either sample, suggesting that friction hysteresis is less likely correlated with the energy corrugation and lateral contact stiffness. These observations suggest that surface contamination was present on the unheated sample, which is removed or reduced to undetectable levels on the heated sample. Furthermore, the study distinguishes features in the friction behavior induced by surface contamination with those associated with graphene's intrinsic properties.