Collaborative Research: Friction on 2D Materials -- Understanding the Critical Role of Edge Chemistry

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Two-dimensional layered materials play important roles in a variety of applications, one of which is as a solid lubricant that minimizes friction and wear. Many two-dimensional layered materials exhibit ultra-low friction only under the "proper" environmental conditions, and the ideal environment may vary from material to material. One explanation for this environmental sensitivity comes from the fact that two-dimensional materials have not only the slippery topmost surface, but also surface step edges where the topmost layer starts or stops. Such surface sites may react chemically with molecules in the environment, like water and hydrocarbons, which will in turn affect friction. This project develops a fundamental understanding of the critical role of the ubiquitous step edges and their reactions with environmental gases in determining friction of two-dimensional materials. This basic research on surface chemistry can lead to new classes of lubricants and impact other applications, such as advanced electronics, where the edge of the 2D materials play a strong role.The common feature of two-dimensional materials is atomically-flat layers with weak inter-lamellar interactions, leading to low shear resistance between layers, which has long been believed to be the origin of super-lubricity. This simple picture does not explain lubrication failure of graphite in vacuum, poor performance of molybdenum disulfide in humid air, and poor lubricity of boric acid in dry air. The work is based on the premise that geometrically-flat lamellae are necessary for super-lubricity, but not sufficient; attaining ultra-low friction requires both the low-corrugation potential surface of the basal plane and proper passivation of edge sites of the basal plane. This hypothesis will be tested through judicious experimental design and state-of-the-art surface analysis methods as well as molecular dynamics simulations that can model chemical reactions at sliding interfaces. This research will address the following key questions: (i) what is the intrinsic reason that friction of two-dimensional layered materials is sensitive to humidity in the environment and why do different materials exhibit very different humidity dependence; (ii) what is the molecular origin of the frictional response of graphite step edges; and (iii) what are the factors governing interface fracture and inter-lamellar slip of two-dimensional materials in various environments. The grant supports a researcher exchange program between two institutions emphasizing the engagement of underrepresented groups.