Abstract
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.
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This work was supported by the National Science Foundation (Grant No. CMMI-1727571).
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Zhe CHEN. Dr. Chen earned his B.S. degree in mechanical engineering from Northwestern Polytechnical University and Ph.D. degree in mechanical engineering from Tsinghua University in China. He is now a postdoctoral researcher. His research interests are interfacial sliding properties and applications of two-dimensional layered materials as solid lubricants and lubricant additives.
Seong H. KIM. Prof. Kim joined the Faculty of Chemical Engineering in 2001 after completing a Ph.D. study in chemistry from Northwestern University and a postdoctoral research at University of California, Berkeley, USA. He earned his B.S. and M.S. degrees in chemistry from Yonsei University, Korea. The main theme of his research lies in surface science and nano-engineering, especially applying surface science fundamentals to important interfacial problems involved in tribology and lubrication, nanomanufacturing, and surface engineering.
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Chen, Z., Kim, S.H. Measuring nanoscale friction at graphene step edges. Friction 8, 802–811 (2020). https://doi.org/10.1007/s40544-019-0334-y
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DOI: https://doi.org/10.1007/s40544-019-0334-y