| Friction consumes more than 1/3 of the primary energy,and 80%of the machinery fails due to wear.It is important to control friction and reduce energy dissipation to reduce energy consumption.Active,dynamic and repeatable methods to regulate the friction are required for charged mechanical systems and micro/nano electromechanical devices operating under the electric fields.Molybdenum disulfide（Mo S2）has excellent mechanical and lubricating properties and can be used as an ideal lubrication coating for the lubrication of micro/nano devices to reduce interface friction and energy dissipation.However,there are few studies on friction characteristics of Mo S2 under the electric field,and the mechanism is not fully understood.In order to further reveal the effect of electric field on the nanofriction of Mo S2 and further seek a stale frictional control method,we studied the basic science problems based on the semiconductor properties:the tunning effects and mechanisms of（1）the carrier concentration under the electric field,（2）the interfacial charge density,（3）the surface functionalization on Mo S2 surface were investigated.On the basis of experimental study,the methods of controlling nanofriction on Mo S2 surface by tuning carrier concentration,electronic tight-binding and surface functionalization were proposed.The main research contents are as follows:Firstly,a carrier regulator is constructed and the tunning effects and mechanisms of the carrier concentration on nanofriction of Mo S2 under the electric field was studied.It is found that friction increases with the increase of carrier concentration and decreases with the dissipation of carrier concentration.Then,the carrier concentration of Mo S2under different gate voltages was calculated by surface potential measurements,and the corresponding relationship between the friction force and carrier concentration was established quantitatively.The relationship between friction force and carrier concentration was approximately linear.Raman spectroscopic results show that increasing the carrier concentration enhances the electron-phonon coupling of MoS2,resulting in more efficient energy dissipation and increased friction.Lowering the carrier concentration inhibits electron-phonon coupling and thus reduces friction.Finally,tuning friction in a dynamic,reversible,and controllable way was achieved by regulating the carrier concentration.Then,the effect of interfacial charge density on friction characteristics of Mo S2 was studied.It is found that the charge of Mo S2 is transferred to substrate and accumulated at Mo S2/Si O2 interface under negative gate voltage,which results in a significant reduction of nanofriction on Mo S2 surface.Combined with first-principle calculation,it is found that the enhanced interfacial charge density can enhance the bonding strength between Mo S2and substrate,reduce the out-of-plane deformation and contact quality of Mo S2 during sliding process,and thus reduce the nanofriction of Mo S2 and achieve ultra-low friction state（friction coefficient is less than 0.005）.The ultra-low friction state on atomically thin Mo S2 is patterned further by controllably regulating position,time,and electric field during the rubbing process.In addition,the effect of charge density on interface adhesion and friction was studied by accumulating higher charge density on silica surface by means of triboelectrification.The results show that the increase of interfacial charge density can enhance the adhesion and friction of the interface,and the effect mechanism of the increase of interfacial charge density on the interface bonding strength is revealed experimentally.Finally,the regulation and mechanism of surface functionalization on nanofriction of Mo S2 was investigated under electric field.The controllable functionalization of the surface of Mo S2and WS2 was realized by applying electric field to the sample using c-AFM.The results of surface potential test and Raman spectroscopy indicate that the surface of Mo S2 can be hydrogenated under electric field.Then,the nanofriction of Mo S2 surface can be controlled by controlling the degree of hydrogenation.Through density functional theory,we revealed the mechanism of hydrogenation on friction of Mo S2.The significant increase in nanofriction results from the increased energy corrugation at the contact interface because of the strong local charge concentrated at the hydrogenated region. |
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