▎ 摘　　要

Although graphene is well known for super-lubricity of its basal plane, friction at its step edge is not well understood. In this study, friction of a single-layer graphene step edge was studied using atomic force microscopy (AFM) in vacuum and humid air conditions. At a 0.34 nm thick graphene step edge, friction varies drastically depending on whether it is exposed at the topmost surface or covered under other graphene layers. The friction response of the step edge buried under one layer of graphene can be fully explained with the topographic effect only; in contrast, the exposed step edge exhibits both topographic and chemical contributions to friction. Chemical characterizations suggest that the exposed graphene step edge is terminated with C-OH groups, which can interact with the AFM tip surface through hydrogen bonding interactions and thus increase friction. The chemical interactions at the exposed step edge significantly amplify the topographic effect. When the step edge is covered by more than one layer of graphene, friction is not sensitive to the 0.34 nm height change. This must be due to the stiffness of multilayer graphene and the height changes gradually at the step edge. These findings will advance fundamental knowledge of the frictional behaviors of graphene. (C) 2019 Elsevier Ltd. All rights reserved.