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The negative Poisson's ratio (NPR) of graphene is studied via molecular dynamics (MD) simulations when epoxy groups are orderly arranged on its surface. When two epoxy groups are located on the same side of the zigzag bridge in a representative cell of graphene, a ripple area appears on the graphene surface. A truss structure model of the ripple is presented to reveal the nature of the NPR phenomenon due to the de-wrinkling effect during stretching. Based on this, the wrinkle region is defined as a periodic distribution unit, and it is evenly distributed along the X- and Y-direction in the 5 nm × 5 nm graphene, then it is found that graphene exhibits a remarkable negative Poisson's ratio phenomenon. In addition, the results show that the oxygen atoms proportion of graphene significantly influence its NPR, and the NPR is the most remarkable when the proportion of oxygen atoms is 4.2 %. [Display omitted] • NPR of graphene caused by epoxy group was studied by theoretical and MD method. • A truss structure model was presented to reveal the nature of NPR phenomenon. • The designed graphene containing epoxy groups exhibits remarkable NPR phenomenon. • NPR of graphene containing epoxy groups depends on the proportion of oxygen atoms. • GO with 4.2 % oxygen atoms has the most remarkable NPR. [ABSTRACT FROM AUTHOR]

The Poisson's ratio of the in-plane pristine armchair and zigzag graphene under uniaxial tensile loading is studied by molecular dynamics simulations, which indicates that Poisson's ratio strongly depends on the tensile strain. At the critical strain, the Poisson's ratio will transform from 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. The distributions of 5-8-5 defects adjacent to each other in the X-direction and Y-direction can significantly reduce the critical strain value. Furthermore, by increasing the percentage of 5-8-5 defects in graphene, the critical strain value can be reduced to about 50%, and the 5-ring defect plays a vital 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. The results confirm that the critical strain that induces NPR is 0. In a word, the NPR of in-plane pristine graphene can be induced by tensile strain, 5-8-5 defects can regulate the critical strain value of NPR, and penta-graphene is a perfect NPR material. [Display omitted] • Poisson's ratio will transform positive to negative at critical strain. • NPR originates from competition between the variation rates of bond length and bond angle. • The critical strain that induces NPR of graphene has significant size effects. • The distributions and percentage of 5-8-5 defects significantly reduce the critical strain. • The critical strain that induces NPR for a penta-graphene consisting entirely of 5-rings is 0. [ABSTRACT FROM AUTHOR]