Oxygen passivation driven defect states healing in monolayer MoSe2 for ultra-high photoresponsivity

Heterostructure-based photodetectors often face performance limitations due to strong interlayer coupling and restricted band modulation. An alternative approach is engineering doped 2D TMDs, particularly through hole doping, to achieve tunable electronic and optoelectronic properties. In this work, we report oxygen passivation of selenium vacancies in MoSe2 (OP-MoSe2) and compare it with vacancy-rich MoSe2 (VSe-MoSe2) in terms of structural, chemical, and optoelectronic characteristics, demonstrating its application in weak-light photodetection. Raman spectroscopy, PL spectroscopy, and X-ray photoelectron spectroscopy confirm that oxygen passivation improves structural quality and optical performance. Low-temperature PL reveals reduced inhomogeneous broadening and biexcitonic features, indicating modulation of excitonic interactions. First-principles calculations show oxygen preferentially occupies vacancy sites, effectively passivating them and suppressing defect-related states. Consequently, OP-MoSe2 photodetectors achieve outstanding performance under 530 nm illumination, with responsivity of 0.74 × 105 A/W, detectivity of ~1014 Jones, at 89 nW/cm2, and a low noise-equivalent power of 0.087 fW/Hz1/2. Furthermore, we demonstrate weak-light tracking of moving objects at varying speeds, mimicking security surveillance conditions. These results establish oxygen-passivated MoSe2 as a promising platform for high-performance, low-light photodetection. This work underscores oxygen passivation as an effective defect engineering strategy for high-performance, ultra-sensitive 2D photodetectors.