▎ 摘　　要

Using molecular mechanics simulations, we investigate the deformation mechanism of graphene monolayer under free standing indentation. During indentation, the van der Waals (VDW) interaction between the indenter tip and graphene monolayer will cause: (i) a phase lag between the indentation force P and the indentation displacement delta (i.e., delta > 0 when P = 0); (ii) a different strain energy function than that in in-plane tension; (iii) a larger nonlinear deformation than its counterpart in in-plane tension. Thus, the elastic properties of graphene monolayer (including the second-order elastic stiffness E and the third-order nonlinear elastic constant c(m)(i)) determined by free standing indentation are different than those determined by in-plane stretching, especially for c(m)(i). The VDW effect rapidly decreases with the increase of the indentation load. Under a small load (i.e., in-plane strain epsilon