Temperature effects on the nano-friction across exposed atomic step edges

Despite the practical and fundamental importance of friction, there are many issues that are still needed to be explored and revealed; one of them is how friction changes across an atomic surface step edge at different temperatures. In this article, a friction force microscope under ultrahigh vacuum conditions has been used to study the temperature dependence of nanoscale friction between a silicon tip and a freshly cleaved HOPG surface with exposed single- and double-layer step edges. For the upward scanning from the lower terrace to the upper terrace, a large resistive force which is linearly dependent on the normal force is observed. A similar resistive force but with a smaller magnitude together with an assistive force is observed for the downward scanning; however, the resistive force is found to be independent of the normal force while the assistive force increases with the normal force. Besides, the resistive force for the double-layer step edge is found to be twice as high as that of the single-step edge, while the assistive force seems to be less influenced by the height of the step. Finally, the experimental results reveal that temperature has a negligible effect on the friction coefficients at the step edges, which is inconsistent with the thermal activated process where friction should decrease with temperature. Based on the theoretical studies, this observation can be explained by a process where the temperature effect is very small compared with the edge Schwoebel–Ehrlich barrier. These findings may help in understanding the temperature effects on macroscopic friction having a lot of step edges at the interface.