Asymmetric 2D MoS₂ for Scalable and High-Performance Piezoelectric Sensors

Piezoelectricity in two-dimensional (2D) transition-metal dichalcogenides (TMDs) has attracted significant attention due to their unique crystal structure and the lack of inversion centers when the bulk TMDs thin down to monolayers. Although the piezoelectric effect in atomic-thickness TMDs has been reported earlier, they are exfoliated 2D TMDs and are therefore not scalable. Here, we demonstrate a superior piezoelectric effect from large-scale sputtered, asymmetric 2D MoS₂ using meticulous defect engineering based on the thermal-solvent annealing of the MoS₂ layer. This yields an output peak current and voltage of 20 pA and 700 mV (after annealing at 450 °C), respectively, which is the highest piezoelectric strength ever reported in 2D MoS₂. Indeed, the piezoelectric strength increases with the defect density (sulfur vacancies), which, in turn, increases with the annealing temperature at least up to 450 °C. Moreover, our piezoelectric MoS₂ device array shows an exceptional piezoelectric sensitivity of 262 mV/kPa with a high level of uniformity and excellent performance under ambient conditions. A detailed study of the sulfur vacancy-dependent property and its resultant asymmetric structure-induced piezoelectricity is reported. The proposed approach is scalable and can produce advanced materials for flexible piezoelectric devices to be used in emerging bioinspired robotics and biomedical applications.