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发挥着重要作用，其中之一是作为固体润滑剂，最大限度地减少摩擦和磨损。许多二维层状材料只有在“适当”的环境条件下才表现出超低摩擦力，理想的环境可能因材料而异。对这种环境敏感性的一种解释来自于这样一个事实，即二维材料不仅具有光滑的最高表面，而且具有最高层开始或停止的表面台阶边缘。这些表面位点可能与环境中的分子（如水和碳氢化合物）发生化学反应，这反过来又会影响摩擦力。这个项目发展了一个基本的理解的关键作用的无处不在的步骤边缘和它们的反应与环境气体在确定二维材料的摩擦。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，例如先进的电子产品，其中2D材料的边缘发挥着重要作用。二维材料的共同特征是原子级平坦层，层间相互作用弱，导致层间剪切阻力低，长期以来一直被认为是超润滑性的起源。这张简单的图片并不能解释石墨在真空中的润滑失效，二硫化钼在潮湿空气中的不良性能，以及硼酸在干燥空气中的不良润滑性。这项工作是基于这样的前提，即几何平坦的薄层对于超润滑性是必要的，但不是足够的;实现超低摩擦需要基面的低摩擦势表面和基面边缘部位的适当钝化。这一假设将通过明智的实验设计和国家的最先进的表面分析方法，以及分子动力学模拟，可以模拟在滑动界面的化学反应进行测试。本研究将解决以下关键问题：（i）二维层状材料摩擦对环境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依赖性：（ii）石墨台阶边缘摩擦响应的分子起源是什么;以及（iii）在各种环境中，控制二维材料界面断裂和层间滑移的因素是什么。该赠款支持两个机构之间的研究人员交流计划，强调代表性不足的群体的参与。

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.