Abnormality in fracture strength of polycrystalline silicene

Silicene, a silicon-based homologue of graphene, arouses great interest in nano-electronic devices due to its outstanding electronic properties.However, its promising electronic applications are greatly hindered by lack of understanding in themechanical strength of silicene. Therefore, in order to design mechanically reliable deviceswith silicene, it is necessary to thoroughly explore themechanical properties of silicene.Due to current fabricationmethods, graphene is commonly produced in a polycrystalline form; the samemay hold for silicene.Here we performmolecular dynamics simulations to investigate themechanical properties of polycrystalline silicene. First, an annealing process is employed to construct amore realisticmodeling structure of polycrystalline silicene. Results indicate that amore stable structure is formed due to the breaking and reformation of bonds between atoms on the grain boundaries.Moreover, as the grain size decreases, the efficiency of the annealing process,which is quantified by the energy change, increases. Subsequently, biaxial tensile tests are performed on the annealed samples in order to explore the relation between grain size andmechanical properties, namely in-plane stiffness, fracture strength and fracture strain etc. Results indicate that as the grain size decreases, the fracture strain increaseswhile the fracture strength shows an inverse trend. The decreasing fracture strengthmay be partly attributed to the weakening effect fromthe increasing area density of defectswhich acts as the reservoir of stress-concentrated sites on the grain boundary.The observed crack localization and propagation and fracture strength are well-explained by a defect-pileupmodel.