Oxygen-Passivated Sulfur Vacancies in Monolayer MoS₂ for Enhanced Piezoelectricity

Modern-day applications demand onboard electricity generation that can be achieved using piezoelectric phenomena. Reducing the dimensionality of materials is a pathway to enhancing the piezoelectric properties. Transition-metal dichalcogenides have been shown to exhibit high piezoelectricity. Monolayer MoS₂ possesses strong piezoelectricity that is otherwise negligible in its bulk form. The presence of sulfur vacancy defects in two-dimensional MoS₂ can starkly reduce piezoelectric output due to enhanced charge screening. Oxygen passivation offers thermodynamically favorable and superior vacancy passivation. Here, we demonstrate an in situ oxygen passivation of sulfur vacancies achieved by performing chemical vapor deposition in atmospheric pressure conditions, resulting in a dramatically enhanced piezoelectric output. We achieved an out-of-plane effective piezoelectric coefficient d₃₃^eff 0.54 pm/V for the MoS₂ monolayer with sulfur vacancies (S_V-MoS₂) and 0.94 pm/V where sulfur vacancies are passivated by oxygen (O_P-MoS₂). The piezoelectric device (PED) based on O_P-MoS₂ exhibits 26% higher output voltage than S_V-MoS₂ with the maximum peak-to-peak value of 0.95 V. Additionally, we show that the O_P-MoS₂-based PED can charge a 330 nF capacitor 30% faster than the S_V-MoS₂ PED for up to 50 mV in 0.5 s by repetitive finger tapping. The evolution of piezoelectricity in MoS₂ with sulfur vacancy defect manipulation promises an avenue for scalable defect engineering for next-generation applications in miniaturized self-powered electronics and sensors across computing, healthcare, and size-, weight-, and power-constrained environments.