First-principles investigations on the structure, stability and electronic properties of monolayer P₂X₃ (X = S, Se): Piezoelectricity, ferroelectricity and negative Poisson's ratio

In this work, we systematically explore the geometric, stability, mechanical, electronic, and optical properties of two-dimensional (2D) P₂X₃ (X = S, Se) by using density-functional theory (DFT) based first-principles calculations. Our calculated results demonstrate that monolayer P₂X₃ exhibit high dynamical, thermodynamic, and mechanical stability. Interestingly, both P₂S₃ and P₂Se₃ show auxeticity with negative Poisson's ratios, and their noncentrosymmetric structures give rise to piezoelectricity. The structural asymmetry also induces ferroelectricity, and the calculated values of spontaneous polarization for P₂S₃ and P₂Se₃ are 232.04 and 64.92 pC/m, respectively. Analyses on electronic structures suggest that P₂X₃ are indirect band gap semiconductors, and their carrier mobilities are dominated by ionized impurity and polar optical phonon scattering at low and high carrier concentrations, respectively. The calculated optical absorption coefficients suggest that P₂X₃ prefer to absorb near-ultraviolet light. With high flexibility and unique electronic properties, the 2D P₂X₃ systems are anticipated to serve as promising candidates for diverse applications in high-performance mechanical and optoelectronic devices.