基于滑动势能面的二维材料原子尺度摩擦行为的量化计算

近年来,二维材料优异的摩擦特性成为人们关注的焦点,然而目前缺乏理论上对其摩擦力进行快速、有效、精确的计算预测方法.本文提出采用密度泛函理论计算真实体系的滑动势能面,利用得到的"数值型势能面"替代传统的解析势函数,并结合Prandtl-Tomlinson模型,量化求解具有复杂形状势能面的真实二维材料体系的摩擦行为.基于该方法,揭示了原子力显微镜实验中观察到的石墨烯Moire纹超晶格结构的双周期"黏-滑"摩擦现象;理论预测了二维材料异质结构的层间超低摩擦现象,相对于同质材料,其静摩擦力和滑动摩擦力均成数量级降低,发现势能面起伏和驱动弹簧刚度均会影响层间相对滑动路径,进而对层间的摩擦行为产生影响.该方法同样可拓展到其他van der Waals作用主导的界面摩擦体系.

Quantitative calculation of atomic-scale frictional behavior of two-dimensional material based on sliding potential energy surface

The excellent tribological characteristics of two-dimensional (2D) materials have received great attention, however, how to effectively predict their frictions is still lacking. Here, we propose to obtain the sliding potential energy surface by density functional theory calculations, instead of simplified potential energy function. Thus it is able to solve the frictional behaviors of 2D materials with irregular complex potential energy surfaces. Firstly, we reveal the mechanism of dual-scale stick-slip behavior between a tip and a graphene/Ru(0001) heterostructure. With a dual-wavelength potential energy surface, we observe a similar frictional behavior to those captured in atomic force microscopy experiments, in which a significant long-range stick-slip sawtooth modulation emerges with a period coinciding with the Moiré superlattice structure. Secondly, we discuss the interlayer frictions of 2D materials, including graphene/graphene, fluorinated graphene/fluorinated graphene, MoS₂/MoS₂, graphene/MoS₂ and fluorinated graphene/MoS₂. With sliding potential energy surface obtained by density functional theory calculations, the interlayer friction is estimated according to the Prandtl-Tomlinson model calculation method. Compared with the friction between homostructures, the friction between heterostructures is lowered by orders of magnitude, which could be attributed to its ultralow sliding potential barrier. The stick-slip instability could be observed in homostructure, while heterostructure exihibits smooth friction loops. The 2D sliding path between the layers is recorded in the sliding process, showing its dependence on both the potential energy barrier and the spring constant. The sliding path shift increases with the increase of potential energy barrier and the decrease of spring constant in the y direction. This method is also applicable to tribological systems with dominated interfacial van der Waals interaction.