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

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 d(11) 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 d(31) 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.