Mechanical Properties And Negative Poisson's Ratio(NPR) Of Graphene By Molecular Dynamics Study

Graphene,defined as the two-dimensional nanomaterials of honeycomb lattice with carbon atoms,has broad application prospects in the fields of nanodevices,flexible electronics,nanocomposites due to its exceptional mechanical properties.Young's modulus is a crucial indicator to measure the mechanical properties of graphene.Therefore,to obtain reliable and accurate Young's modulus has great significance for understanding its mechanical properties in depth.Simultaneously,Negative Poisson's ratio materials,also called auxetic materials,that is,undergo a transversal expansion with vertical tension.The unusual stretch expansion endows excellent mechanical properties such as enhanced toughness,shear resistance,indentation resistance,vibration absorption,which enable them to hold potential application prospects in aerospace,fasteners,biomedicine,sensors,and other fields.Graphene,as the most typical 2D nanomaterials,the negative Poisson's ratio(NPR)under tension has attracted more and more attention of researchers.In this paper,based on the molecular dynamics(MD)simulations,the strain range dependence of Young's modulus and size effects of mechanical properties for pristine graphene are studied.Meanwhile,the negative Poisson's ratio(NPR)of pristine graphene and 5-8-5 defected graphene under strain-induced is investigated.In the first,the larger size range(100nm×100nm)pristine monolayer graphene is uniaxial loaded/unloaded,which proves that graphene is perfectly elastic prior to the failure strain.Therefore,Young's modulus of graphene is calculated by selecting different strain ranges in the elastic region,it is found that Young's modulus is strongly dependent on the strain range.When the selected strain ranges are increased from 0.5% to 8%,Young's modulus of the armchair and zigzag graphene is reduced by approximately 60 GPa and 150 GPa,respectively.Based on the Pearson correlation coefficient method,the linearity of the stress-strain curve during graphene stretching is studied.The elastic region of the tensile curve is divided into the linear elastic region and non-linear elastic region,meanwhile,the linear elastic limit strains for the armchair and zigzag graphene are defined to be about 2.5% and 1.5%,respectively,and they are independent of the size of graphene.On this basis,the chirality dependence and size effects of Young's modulus,failure strain,and fracture strength of pristine monolayer graphene are investigated.In addition,the Poisson's ratio of the in-plane pristine armchair and zigzag graphene under uniaxial tensile loading is studied,which indicates that Poisson's ratio strongly depends on the tensile strain.At the critical strain,the Poisson's ratio will transform positive to negative,and the critical strain of the zigzag is far less than that of armchair.The study on the representative cell of graphene shows that the intrinsic nature of strain-induced negative Poisson's ratio(NPR)is the competition between the variation rates of bond length and bond angle.Meanwhile,the critical strain that induces NPR has significant size effects,which depends on the proportion of representative cells at the width boundary in the whole graphene.What's more,5-8-5 defect has a significant effect on the NPR phenomenon of graphene.By introducing 5-8-5 defects into in-plane graphene,the critical strain value that induces NPR can be effectively regulated by adjusting the distribution modes and percentage of defects.And the 5-ring defect plays an important role in reducing the critical strain value.Based on this,a penta-graphene consisting entirely of 5-rings is constructed,and its Poisson's ratio is studied,we find the critical strain that induces NPR for pentagraphene could be reduced to 0,which could be seen as an ideal negative Poisson's ratio material.