标题： 分子在轨道上：二维材料上有机纳米晶体的摩擦各向异性和优先滑动方向 显示英文标题

标题： Molecules on rails: friction anisotropy and preferential sliding directions of organic nanocrystallites on two-dimensional materials

摘要： 二维（2D）材料被视为纳米机械系统的超薄固体润滑剂。到目前为止，它们在纳米尺度上的摩擦特性已经通过标准的摩擦力显微镜进行了研究。然而，横向操控纳米颗粒是研究摩擦与两个接触表面晶体学依赖关系的更合适方法。尽管如此，这样的实验仍然缺乏。在本研究中，我们结合基于原子力显微镜（AFM）的横向操控和分子动力学模拟，以研究通过范德华外延生长在石墨烯和六方氮化硼上的有机针状纳米晶体的运动。我们观察到，当被AFM探针推动时，纳米针碎片并不沿着原来的推动方向移动。相反，它们优先沿着针的生长方向在二维材料上滑动，这些方向作为共格方向上的隐形轨道。此外，当通过在优选滑动方向上施加扭矩来旋转纳米晶体时，我们发现AFM悬臂的扭转信号增加。我们结合模拟证明，显著的摩擦各向异性和优选滑动方向是由复杂的外延关系决定的，并且源于有机纳米晶体和二维材料之间的共格和非共格状态。 显示英文摘要

摘要： Two-dimensional (2D) materials are envisaged as ultra-thin solid lubricants for nano-mechanical systems. So far, their frictional properties at the nanoscale have been studied by standard friction force microscopy. However, lateral manipulation of nanoparticles is a more suitable method to study the dependence of friction on the crystallography of two contacting surfaces. Still, such experiments are lacking. In this study, we combine atomic force microscopy (AFM) based lateral manipulation and molecular dynamics simulations in order to investigate the movements of organic needle-like nanocrystallites grown by van der Waals epitaxy on graphene and hexagonal boron nitride. We observe that nanoneedle fragments -- when pushed by an AFM tip -- do not move along the original pushing directions. Instead, they slide on the 2D materials preferentially along the needles' growth directions, which act as invisible rails along commensurate directions. Further, when the nanocrystallites were rotated by applying a torque with the AFM tip across the preferential sliding directions, we find an increase of the torsional signal of the AFM cantilever. We demonstrate in conjunction with simulations that both, the significant friction anisotropy and preferential sliding directions are determined by the complex epitaxial relation and arise from the commensurate and incommensurate states between the organic nanocrystallites and the 2D materials.