Study Of The Mechanical And Electrochemical Properties Of Two-dimensional MoS₂ Based On Scanning Probe Microscopy

Two-dimensional materials have promising applications in many fields such as micro and nanoelectromechanical systems and energy storage devices due to their special extremely thin layered structures and outstanding physical and chemical properties.Recent advances in scanning probe microscopy provide new opportunities to characterize nanoscale 2D materials for applications in mechanics and electrochemistry.However,it is difficult to achieve accurate measurements of the elastic modulus of two-dimensional materials due to characteristics such as their structure and atomic-level thickness,and the mechanism of ion diffusion at this scale has not been clearly addressed.Therefore,although molybdenum disulfide has been used in a variety of devices,little has been reported on its mechanical and electrochemical properties from a microscopic perspective.The details and results of this paper are as follows:1.Using chemical vapor deposition to prepare experimental samples,molybdenum disulfide with different layer numbers and single or polycrystalline morphologies can be obtained by changing the precursor ratio and reaction time.2.The intrinsic mechanical properties of two-dimensional molybdenum disulfide are studied by a dual-mode AFM technique,which allows the direct measurement of Young’s modulus distribution between the substrate and the sample,and the introduction of a finite thickness model to correct for the substrate effect.The theoretical values of Young’s modulus are calculated using the first-nature principle,and the comparative experiments demonstrate that dual-mode AFM is a reliable method for direct testing of Young’s modulus in two-dimensional materials,and that the method eliminates the need for tedious steps such as preparation of suspended two-dimensional materials,which avoids the shortcomings of conventional testing and results in much smaller testing errors than conventional methods.This work paves the way for future testing of mechanical properties of two-dimensional materials or thin-layered materials,and facilitates the organization and analysis of experimental data on material mechanics.3.To investigate the diffusion mechanism of lithium ions between molybdenum disulfide layers at the microscopic scale,electrochemical strain response and out-ofplane piezoelectric response were observed in the depressed and protruding regions in chemically embedded lithium disulfide molybdenum,respectively,and the out-of-plane piezoelectric response caused by the flexural electrical response was distinguished in the electrochemical response.An effective out-of-plane piezoelectric response was observed on the lithiated molybdenum disulfide due to the lithium embedding in the molybdenum disulfide nanosheets inducing inhomogeneous strain,and the inhomogeneous strain led to the generation of flexural electrical effects.New ideas are provided for the implementation of electrochemical performance characterization at the nanoscale for lithium-ion batteries.4.The final element of this work is the development of a fully coupled nonlinear electro-chemical-force finite element model and its application to molybdenum disulfide nanosheets to determine the fundamental mechanisms governing the generation of electrochemical strain signals.The Young’s modulus and diffusion coefficient values obtained in the first two research elements were also brought into the finite element model to achieve a sophisticated theoretical analysis of the electrochemical strain response mechanism.The developed finite element model combined with experiments to rigorously analyze the lithium-ion diffusion mechanism from the nanoscale can be used to evaluate the ion diffusion properties of new electrochemically active materials.