Nanoindentation Experiments For Single-layer Molybdenum Disulfide With The Point Defect:A Molecular Dynamic Study

Due to its specific atomic layer structure,molybdenum disulfide is equipped with good lubricity and resiliency,with which it usually serves as solid lubricant.However,as the number of layers decreases,single-layer molybdenum disulfide(SLMoS2)shows direct gap semiconducting property;therefore,it is considered as one of the new generation of candidate materials to replace the traditional silicon.Although SLMoS2 possesses promising application potential because of its unique electronic and electrical properties,knowledge of the mechanical properties of SLMoS2,especially with inherent defects,is still lacking.In this thesis,we investigate the mechanical properties of defect-free and vacancy sulfur(VS)defect SLMoS2 by simulating nanoindentation experiments with classical molecular dynamics method.This thesis is organized as follows:So as to simulate the nanoindentation process exactly,the cycloidal self-suspension SLMoS2 thin film,equipped with the property of uniform biaxial tensile is fixed by the border.Through loading and unloading tests during the elastic deformation part and plastic deformation part,we gain the deformation and failure mechanism of SLMOS2.We observe that the phase transition is arisen by the sudden decline of the dimension in the layer of S-S before it loses efficacy.Meanwhile,we have found that SLMoS2's maximum stress and the Young modulus increase as the diameter of pressure head increases.As the pressure head's loading speed increases,the maximum load and its corresponding depth of critical pressure rise dramatically.Subsequently,we introduce the most common Vs hole defects during the preparation of SLMoS2 disulfide into the nanoindentation model of SLMoS2 disulfide so as to explore structure defect's impact on its mechanical property.We find that phase transition begins from cumulative hole far away from the center of thin film and accelerates the new phase propagating process.At the same time,maximum load shows a descending trend as the concentration of Vs hole defects increases.Besides,we find that vacancy of the top single sulfur atom has abnormal enhancement effect on SLMoS2's in-plane elastic stiffness;on the contrary,vacancy of the underlying single sulfur atom leads to weakening effect,while lack of the sulfur atom pair shows continued weakening trend,slower than that of underlying single sulfur atom vacancy.In addition,we have conducted an atomistic study on the attributes of the thermally agitated ripples in SLMoS2 membranes with constrained boundary conditions.The effects of aspect ratio,types and shapes of boundary conditions,and chirality,on the ripples' attributes are investigated.The results show that the height fluctuation in all of SLMoS2 membranes increases with the increasing temperature,which is consistent with the elastic theory of membrane.It is revealed that the aspect ratio has a major influence on the vibrational mode of the thermally agitated ripples for rectangular SLMoS2 membranes,the types of boundary conditions is shown to have minor influence on the vibrational mode and height fluctuation for the rectangular SLMoS2 membranes.Most of the effect of chirality is concentrated on the height fluctuation or ripple height for the slender SLMoS2 nanoribbons due to the different number of ripples.Unlike the vibrational motion of graphene nanodrums,only high-order resonant mode is thermally agitated in SLMoS2 nanosheets with circular boundary conditions.