The perfect graphene is believed to be the strongest material. However, the useful strength of large-area graphene with engineering relevance is determined by its fracture toughness, rather than the intrinsic strength that governs the uniform breaking of atomic bonds in perfect graphene. Here, we report the in situ tensile testing of suspended graphene using a nanomechanical device to measure the fracture toughness of graphene. During tensile loading, the cracked graphene samples fracture at a breaking stress substantially lower than the intrinsic strength of graphene. Our combined experiment and modeling verify the applicability of the classic Griffith theory of brittle fracture to graphene. Also in this talk, we report the atomic structure and morphology of the grains and their boundaries in the CVD grown polycrystalline molybdenum disulfide atomic layers. The implications of the effects of defects such as grain boundaries on mechanical and electrical properties of two-dimensional atomic layers will be discussed.

The perfect graphene is believed to be the strongest material. However, the useful strength of large-area graphene with engineering relevance is determined by its fracture toughness, rather than the intrinsic strength that governs the uniform breaking of atomic bonds in perfect graphene. Here, we report the in situ tensile testing of suspended graphene using a nanomechanical device to measure the fracture toughness of graphene. During tensile loading, the cracked graphene samples fracture at a breaking stress substantially lower than the intrinsic strength of graphene. Our combined experiment and modeling verify the applicability of the classic Griffith theory of brittle fracture to graphene. Also in this talk, we report the atomic structure and morphology of the grains and their boundaries in the CVD grown polycrystalline molybdenum disulfide atomic layers. The implications of the effects of defects such as grain boundaries on mechanical and electrical properties of two-dimensional atomic layers will be discussed.