Anisotropic Mechanical Properties of Orthorhombic SiP₂ Monolayer: A First-Principles Study

In recent years, the two-dimensional (2D) orthorhombic SiP₂ flake has been peeled off successfully by micromechanical exfoliation and it exhibits an excellent performance in photodetection. In this paper, we investigated the mechanical properties and the origin of its anisotropy in an orthorhombic SiP₂ monolayer through first-principles calculations, which can provide a theoretical basis for utilizing and tailoring the physical properties of a 2D orthorhombic SiP₂ in the future. We found that the Young's modulus is up to 113.36 N/m along the a direction, while the smallest value is only 17.46 N/m in the b direction. The in-plane anisotropic ratio is calculated as 6.49, while a similar anisotropic ratio (~6.55) can also be observed in Poisson's ratio. Meanwhile, the in-plane anisotropic ratio for the fracture stress of the orthorhombic SiP₂ monolayer is up to 9.2. These in-plane anisotropic ratios are much larger than in black phosphorus, ReS₂, and biphenylene. To explain the origin of strong in-plane anisotropy, the interatomic force constants were obtained using the finite-displacement method. It was found that the maximum of interatomic force constant along the a direction is 5.79 times of that in the b direction, which should be considered as the main origin of the in-plane anisotropy in the orthorhombic SiP₂ monolayer. In addition, we also found some negative Poisson's ratios in certain specific orientations, allowing the orthorhombic SiP₂ monolayer to be applied in next-generation nanomechanics and nanoelectronics.