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

二维层状材料在各种应用中扮演着重要的角色，其中之一是作为固体润滑剂，将摩擦和磨损降至最低。许多二维层状材料只有在适当的环境条件下才会表现出超低的摩擦力，而理想的环境可能会因材料而异。对这种环境敏感性的一种解释来自于这样一个事实，即二维材料不仅有光滑的最顶层表面，而且还有最顶层开始或停止的表面台阶边缘。这样的表面位置可能会与环境中的分子发生化学反应，如水和碳氢化合物，这反过来会影响摩擦。这个项目发展了对无处不在的台阶边缘及其与环境气体的反应在确定二维材料的摩擦力方面的关键作用的基本理解。这种对表面化学的基础研究可以产生新的润滑剂类别，并影响其他应用，如先进的电子学，其中2D材料的边缘起着强大的作用。二维材料的共同特征是原子平整的层，层间相互作用较弱，导致层之间的低剪切阻力，这一直被认为是超级润滑性的起源。这张简单的图片并没有解释石墨在真空中的润滑失效，二硫化钼在潮湿空气中的性能差，以及硼酸在干燥空气中的润滑性差。这项工作的前提是，几何平坦的片层是超润滑性所必需的，但不是充分的；要获得超低摩擦，既需要基面的低波纹势面，也需要基面边缘位置的适当钝化。这一假设将通过明智的实验设计和最先进的表面分析方法以及可以模拟滑动界面上的化学反应的分子动力学模拟来验证。这项研究将解决以下关键问题：(I)二维层状材料的摩擦对环境中的湿度敏感的内在原因是什么，为什么不同的材料表现出非常不同的湿度相关性；(Ii)石墨阶梯边摩擦响应的分子来源是什么；以及(Iii)在不同环境下，导致二维材料界面断裂和层间滑移的因素是什么。这笔赠款支持两个机构之间的研究人员交流计划，强调参与人数不足的群体。

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.