▎ 摘　　要

The mechanical properties of nanomaterials can be strongly affected by their crystal structures and defect configurations. Here, the inplane stiffness of polycrystalline graphene obtained by chemical vapor deposition (CVD) has been investigated by using bulge tests on suspended graphene membranes. In particular, the influence of grain boundaries (GBs) on the in-plane stiffness of graphene membranes was studied by controlling the density of GBs within graphene membranes. The GBs were visualized by sequential growth of C-13- and C-12-graphene along with detection of Raman peak shifts for C-13 and C-12 in graphene, which enabled the nondestructive evaluation of the GB density (defined as the ratio of the total GB length within a graphene membrane to the diameter of the membrane). Single-crystal graphene membranes without any GBs had an average Young's modulus of 0.95 +/- 0.12 TPa (corresponding to an average in- plane stiffness of 318 +/- 40 N/m), comparable to that obtained from mechanically exfoliated graphene. An increased GB density within the membranes softened the graphene membranes, resulting in a lower in-plane stiffness. This phenomenon was most clearly observed when the lateral sizes of grains