▎ 摘　　要

Free-standing (FS) two-dimensional (2D) materials are commonly found under an adhesive boundary condition, i.e., a small portion near the boundary is adhered to the sidewall of a cylindrical hole in a substrate via van der Waals (vdW) interaction, but its influence on mechanical properties characterization via indentation testing has never been considered. Based on the analyses of both experimental and numerical results, we found that the separation of the adhesive boundary of monolayer graphene prepared by chemical vapor deposition (CVD) in indentation tests is indeed possible. The adhesive boundary effect on indentation response can be clearly shown from indentation loading -reloading curves, including a nominal material softening in the first loading response and a nominal material stiffening in the consecutive reloading responses, caused by the coupling effect of the vdW interactions with an AFM tip and with a substrate near the boundary of the FS region. Thus, the conventional assumption of a clamped boundary in the existing FS indentation models is inappropriate. Finally, a tentative approach to eliminate the adhesive boundary effect on the indentation response is suggested, in which the intrinsic property of a CVD grown graphene might be obtained via a large AFM tip after several loading-unloading-reloading cycles. (C) 2019 Elsevier Ltd. All rights reserved.