First-principles Study Of Several Friction Phenomena In Two-dimensional Materials

Nanotribology studies the friction-behaviors and mechanism at the atomic and molecular scales,which focus on the subtle changes in contact surfaces and is affected by many factors.In recent years,miniaturization and integration of devices require an understanding of nanofriction.Therefore,studying the friction at the nanometer scale is of great significance to the scientific progress and economic development of modern society.For a long time,people have been focued on macroscopic friction,and the understanding of friction at the nanometer scale is still limited.On this basis,the first-principles calculations which based on density functional theory were employed to calculate the several nanofriction properties of typical two-dimensional materials by calculating the interface electronic structure and sliding barrier in the paper.The research results are helpful for people to understand the friction phenomenon at the nanoscale,which is helpful for the tuning of the interfacial friction.The research contents are summarized as follows:1.The interlayer nanofriction properties of the hexagonal boron nitride were investigated.The bilayer hexagonal boron nitride model has two different stacking modes:one is that B atom facing B atom or N atom facing N atom,which is called homologous atom stacking;the other is that B atom facing N atom,which is called heterogenous atom stacking.It is found that for the homologous stack,the friction is isotropic along the two sliding paths.However,for the heterogeneous one,the frictional force on the polar sliding path is greater than that of on the non-polar one,and thus the frictional property is anisotropic.The above results indicate that the interlayer friction properties of hexagonal boron nitride depend on the initial stacking mode and the sliding direction.We further explain the above phenomena through electronic structures.2.The effect of doping on interlayer nanofriction properties of graphene have been studied.In the calculation,the p-type/p-type and n-type/n-type doping systems were constructed by doping B atom and N atom into bilayer graphene respectively.Compared with the undoped system,p-type/p-type and n-type/n-type systems can reduce the nanofriction between graphene layers with different extents,but the p-type/n-type system increases the nanofriction between the graphene layers.On this basis,we also consider the influence of doping concentration and impurity location on the friction properties.The results show that:one and two doping atoms have the same influence on the friction for the p-type/p-type and n-type/n-type systems,and the only difference is that the reducing effect is more obvious for two atoms n-type/n-type doping,while the p-type/n-type system increases the friction;Our results also find that the doping position has little effect on the nanofriction properties of graphene layers.This study provides a method to tune nanofriction between graphene layers.3.The interlayer nanofriction properties of the new two-dimensional?2D?material Ca₂N were studied.Ca₂N is a layered electronic compound material and has strong interlayer adhesion,but it exhibits lower interlayer friction behavior than the layered lubricating materials?such as graphene,hexagonal boron nitride and molybdenum disulfide?which are weakly combined with traditional van der Waals?vdW?.We calculated the interlayer electronic structure distribution of Ca₂N by first-principles method,and found that the 2D conductive electron distribution between the layers of the system is uniform.This result successfully explained the ultra-low interlayer friction properties of Ca₂N.The above research not only broadens the scope of 2D solid lubricants,but also enriches the understanding of the ultra-low friction mechanism of 2D systems.