First-principles calculation of in-plane and out-of-plane piezoelectric properties of two-dimensional Janus MoSSiX2 (X = N, P, As) monolayers

Abstract

Probing Janus materials with significant piezoelectric polarization to enhance electromechanical conversion efficiency has been a hot research topic in recent years. Here, a Janus MoSSiX₂ (X = N, P, As) structure is proposed, and its piezoelectric properties are explored using first-principles calculations. The result shows that the in-plane coefficients d₁₁ of MoSSiN₂, MoSSiP₂, and MoSSiAs₂ are 1.87, 4.59, and 5.35 pm V⁻¹, respectively. Meanwhile, the non-mirror symmetry leads to nonzero out-of-plane piezoelectric coefficients d₃₁ of 0.16, 0.16, and 0.35 pm V⁻¹ for MoSSiN₂, MoSSiP₂, and MoSSiAs₂. It is found that the outstanding piezoelectric coefficients of MoSSiAs₂ are attributed to the elastic flexibility. Meanwhile, the electronegativity difference ratio is in line with the trend of piezoelectric coefficient variations, and the large built-in electric field strength enables the MoSSiAs₂ monolayer to exhibit the strongest vertical polarization. The work establishes a robust theoretical foundation for supporting the potential utilization of Janus MoSSiX₂ monolayers in nanodevices and micro-electro-mechanical systems.