First-principles investigations on the structural, mechanical, electronic and optical properties of auxetic monolayer penta-SiX2 (X = N, P, As, Sb).

Using density functional theory based first-principles calculations, we have systematically investigated structural, mechanical, electronic and optical properties of monolayer penta-SiX2 (X = N, P, As, Sb), which exhibit dynamic, thermal, and mechanical stability. Owing to their unique buckled structure, penta-SiX2 systems exhibit strong auxeticity with negative Poisson's ratio (NPR), and intriguingly, the NPR of penta-SiN2 is omnidirectional that applies to all crystal directions. The Poisson's ratios can be effectively tuned through biaxial strain, with the NPR value of penta-SiN2 increased from -0.25 (without strain) to -0.51 under a tensile strain of 4%. Electronic structure calculations show that penta-SiX2 systems are indirect band gap semiconductors, with band gaps ranging from 2.07 to 5.07 eV. Meanwhile, it is revealed that penta-SiX2 exhibit high electron/hole mobilities, beneficial for carrier transport in devices. We further demonstrate that penta-SiX2 structures exhibit high absorption coefficients in visible and ultraviolet light regions. Our findings not only highlight the potential of penta-SiX2 for applications in nanomechanics and optoelectronics, but also shed light on the exploration and design of two-dimensional pentagonal materials with auxetic behaviors.