Nanotribological Properties Of MoS₂ Based On Atomic Force Microscopy

MoS2 nanomaterials has been widely used in optoelectronic devices electrochemical energy storage devices and micro/nano sensors owing to excellent energy storage,catalytic,photovoltaic and lubricating properties.The interface parameters play a major role in the interface interaction of nanomaterials,such as friction,adhesion and critical shear stress.The surface and interface properties have a great influence on the stability and reliability of micro/nano devices.It is of great significance to accurately regulate the interfacial properties of nanomaterials by constructing a theoretical model that more suitable for describing nanoscale friction phenomena,and to improve the tribological stability of micro/nano devices by researching the relationship among nanomaterials friction properties,material parameters and external loads.The research aspects of thesis are as follows.(1)Based on the Amonton's law and the fundamental of atomic force microscopy(AFM),the rapid lateral force calibration method is proposed.The silicon tips are calibrated by single crystal silicon wafer and mica chip.Compared with the lateral force calibration factor calculated by traditional Two-Step method,the maximum calibration deviations are 1.66%and 6.39%for rapid lateral force calibration method with silicon wafer and mica chip.The proposed method overcomes the disadvantages,such as high calibration cost,complicated calibration process and large transfer error in traditional lateral force calibration method.This method not only meets the need of fast calibrating in the friction test of nanomaterials,but also ensures the calibration accuracy.(2)For MoS2 nanosheets prepared by mechanical exfoliation method,the morphology and thickness are characterized by AFM and Raman spectroscopy.The results show that the Raman peak distances vary with the number of layers for MoS2 nanosheets at the range of 18-25 cm-1.The friction coefficients 0.001-0.007 are measured by lateral force mode for MoS2 nanosheets,and they vary with the number of layers.The monolayer MoS2 nanosheet has obvious anisotropy when friction occurs at atomic level.Researching the friction coefficient and anisotropy of MoS2 nanosheets are of great significance for the development of ultra-lubricating technology,real-time crystal orientation detection and high-precision ultrasonic motor technology.(3)Based on Johnson-Kendall-Roberts(JKR)and interfacial friction theories,the B-JKR model is proposed to describe the relationship between the critical shear stress and the mechanical parameters such as frictional force,normal force and adhesive force.The critical shear stress is experimentally measured by the later force mode and force map mode of AFM.The mean critical shear stresses are measured as 0.24 GPa for monolayer MoS2 and 0.69 GPa for graphene.MoS2 nanosheet and graphene are of the lower critical shear stress and friction coefficient than those of the bulk materials,which are good agreement with the regularities in nanotribology.The critical shear stress can be used to not only quantitatively characterize the frictional properties,but also analyze the friction at different loading stages.The critical shear stress map could play an important role in sliding contact of nanomaterials for the drive part design and nanotribological reliability of nanoelectron mechanical system.