Although graphene is well known for super-lubricity on its basal plane, friction at its step edge is not well understood and contradictory friction behaviors have been reported. In this study, friction of mono-layer thick graphene step edges was studied using atomic force microscopy (AFM) with a Si tip in dry nitrogen atmosphere. It is found that, when the tip slides over a ‘buried’ graphene step edge, there is a resistive force during the step-up motion and an assistive force during the step-down motion due to the topographic height change. The magnitude of these two forces is small and the same in both step-up and step-down motions. As for the ‘exposed’ graphene step edge, friction increases in magnitude and exhibits more complicated behaviors. During the step-down motion of the tip over the exposed step edge, both resistive and assistive components can be detected in the lateral force signal of AFM if the scan resolution is sufficiently high. The resistive component is attributed to chemical interactions between the functional groups at the tip and step-edge surfaces, and the assistive component is due to the topographic effect, same as the case of buried step edge. If a blunt tip is used, the distinct effects of these two components become more prominent. In the step-up scan direction, the blunt tip appears to have two separate topographic effects elastic deformation of the contact region at the bottom of the tip due to the substrate height change at the step edge and tilting of the tip while the vertical position of the cantilever (the end of the tip) ascends from the lower terrace to the upper terrace. The high-resolution measurement of friction behaviors at graphene step edges will further enrich understanding of interfacial friction behaviors on graphene-covered surfaces.

中文翻译：

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在石墨烯台阶边缘测量纳米级摩擦

尽管石墨烯因其基面上的超润滑性而闻名，但对其台阶边缘的摩擦尚不甚了解，并且已经报道了相互矛盾的摩擦行为。在这项研究中，使用原子力显微镜（AFM）在干燥的氮气气氛中使用Si尖端研究了单层厚石墨烯台阶边缘的摩擦。已经发现，当尖端在“埋入的”石墨烯台阶边缘上滑动时，由于地形高度变化，在上升运动期间存在阻力，而在下降运动期间存在辅助力。这两个力的大小很小，并且在上移和下移运动中都相同。至于“裸露”的石墨烯台阶边缘，摩擦的幅度会增加，并且表现出更复杂的行为。在尖端在暴露的台阶边缘上的下降运动期间，如果扫描分辨率足够高，则可以在AFM的横向力信号中检测到电阻分量和辅助分量。电阻性成分归因于尖端和台阶边缘表面上的官能团之间的化学相互作用，而辅助成分归因于形貌效应，与掩埋台阶边缘的情况相同。如果使用钝头，则这两个组件的独特效果会更加突出。在升压扫描方向上，钝化的尖端似乎具有两个单独的形貌效应，这是由于在台阶边缘处的基板高度变化以及尖端的垂直位置时尖端的倾斜导致尖端底部接触区域的弹性变形。悬臂（尖端的末端）从下部露台上升到上部露台。