Effect Of External Electric Field And Grain Boundary On The Nanofriction Properties Of Two-dimensional Transition Metal Dichalcogenides

Selenide molybdenum（MoSe₂） and molybdenum disulfide（MoS₂）have excellent performance on solid friction lubrication,exploring the friction performance of MoS₂and MoSe₂will promote the application in micro-nano electromechanical systems as solid lubricating materials.With the rapid development of industry,micro/nano electromechanical systems will be widely used in complex industrial environments,so it is significantly important to explore the influence of electric field and material defects on the friction performance for MoSe₂ and MoS₂.In this paper,single-layer MoSe₂ and single-layer polycrystalline MoS₂ were prepared by chemical vapor deposition（CVD）.Based on advanced scanning probe technology,the friction performance was studied for monolayer MoSe₂ under the external electric field,and which was also researched for monolayer polycrystalline MoS₂included the grain boundary at different angles.Exploring the influence of electric field and grain boundary on the friction performance of MoS₂ and MoSe_2,which is conducive to deeply understand the friction mechanism and provides a new ideas to regulate nano-friction forces in the future.The main contents of this article are as follows:（1）In order to explore the friction behavior and mechanism of MoSe₂ under electric field,the in-situ system for testing friction force and surface potential under the external electric field was established on atomic force microscope（AFM）.The experiment showed that the friction coefficient of MoSe₂ increases with the sample bias,and the adhesion and electrostatic force between the probe and sample increase positively with the bias voltage,and approximately follow the parabolic law.（2）Based on Amonton’s law and electrostatic force model,the friction mechanism under the electric field was discussed with electrostatic force.In addition,by characterizing the surface potential at different charging times,it was found that the surface potential and frictional force changed with the charging time.And the tendency was consistent with charging/discharging rules of capacitor.This may be explained with the electrostatic adsorption induced by the charge accumulation.（3）To test the adhesion of grain boundary through the force curve function,it is found that the adhesion forces at grain boundary were larger than that at the area without grain boundary,but angles have little effect on adhesion,which may be due to the polarity particles adsorbed at grain boundary strengthen the interaction between tip and MoS₂ surface.Meanwhile,the experimental results showed that the friction coefficients of the grain boundary region were larger than that of the non-grain boundary region,and increase with grain boundary angles.（4）To calculate the friction dissipation during the rubbing process by friction ring.It is found that and friction dissipations in the grain boundary regions are larger than those in the non-grain boundary regions,and which increases with grain boundary angles.There are discrepancies in bending stiffness of grain boundaries at different angles,which leads to different pucker effect when the probe slides through grain boundaries.Higher normal load will cause more out-of-plane deformation,causing surface pucker effect.Therefore,friction dissipation increases with grain boundary angles and normal load.