Photoluminescence Evolution and Synaptic Behavior of Aged Monolayer MoS ₂ Flakes and Films: Insight into Defect Dynamics and Oxygen-Mediated Recovery

Photoluminescence (PL) in monolayer (1L) MoS2 is highly sensitive to surface chemistry and ambient exposure, which can introduce defect states that modify radiative and nonradiative pathways. Herein, we investigate the influence of long-term environmental aging (up to 1 year, ∼60% relative humidity, ∼24 °C) on the PL spectra of CVD-grown 1L MoS2, focusing on the influence of morphology on the evolution of PL emission. While both 1L flakes and continuous films suffer from aging to different degrees, MoS2 films show significantly higher PL reduction, broader exciton line widths, and an enhanced trion-to-exciton (A–/A0) intensity ratio compared to MoS2 flakes. This pronounced PL quenching arises from the higher grain boundary density in films, which acts as a reactive site for defect generation under ambient exposure. Complementary temporal photoresponse studies further validate this observation, revealing higher dark current and longer decay times in the aged films, consistent with increased sulfur vacancy (VS) concentration and defect-assisted trapping. Interestingly, the aged 1L MoS2 exhibit persistent photoconductivity and synaptic behavior, characterized by optically driven modulation of conductance and carrier relaxation. To reverse the aging-induced degradation, moderate-temperature air annealing (200–300 °C) was employed. Remarkably, the annealing leads to substantial PL recovery in both flakes and films. Spectral deconvolution reveals a narrowing and blue-shifting of the exciton emission peak, along with a reduction in the A–/A0 ratio, indicating oxygen-mediated passivation of sulfur vacancies. Density functional theory further explains the trend, showing that sulfur vacancies introduce midgap states that enable nonradiative recombination, whereas oxygen incorporation suppresses these states and re-establishes radiative recombination pathways. Overall, our results connect morphology-dependent defect dynamics to macroscopic optical/electrical aging signatures and identify simple air annealing as a scalable route to heal vacancy-type defects in 1L MoS2.