Janus Ga4S4Cl2X2 (X = F, Br, I) monolayers: first-principles investigation of in- and out-of-plane piezoelectric properties

Abstract

With the rapid development of device miniaturization and self-powered detection, detecting two-dimensional materials with significant polarization to improve the electromechanical conversion efficiency has become a research hotspot. Here, a Janus material structure Ga₄S₄Cl₂X₂ (X = F, Br, I) was proposed and its piezoelectric properties were explored by first-principles. The results show that the in-plane piezoelectric coefficient d₁₁ of Ga₄S₄Cl₂F₂, Ga₄S₄Cl₂Br₂, and Ga₄S₄Cl₂I₂ are 10.01, 13.78, and 15.17 pm V⁻¹, respectively. At the same time, because of the inherent asymmetry of the structure, the non-zero out-of-plane piezoelectric coefficient d₃₁ of Ga₄S₄Cl₂F₂, Ga₄S₄Cl₂Br₂, and Ga₄S₄Cl₂I₂ are 0.43, 0.16, and 0.43 pm V⁻¹, respectively. Therefore, it can be found that the Ga₄S₄Cl₂F₂ and Ga₄S₄Cl₂I₂ monolayers have the strongest vertical polarization, which is consistent with the trend of the change in the dielectric constant and the electric dipole moment, the built-in electric field strength, and its excellent flexibility. These results lay a solid theoretical foundation for the potential applications of the novel Janus two-dimensional Ga₄S₄Cl₂X₂ monolayers in nanodevices, microelectromechanical systems, and sensitive biological probes.