Mid-IR probing unveils vanadium doping-induced unsaturation of defect states in monolayer MoS2

Long-lived hot carriers in transition metal dichalcogenides (TMDCs) are essential for efficient light–matter interactions, underpinning applications in optoelectronics, photodetection, and quantum technologies. Vanadium doping introduces tunable defect states that significantly modify the electronic structure and carrier dynamics, yet their precise role in electron–hole recombination remains unresolved. Here, we employ mid-infra-red (mid-IR) transient absorption spectroscopy to probe carrier trapping mechanisms in pristine and Vanadium-doped MoS2 monolayers. Under near-resonant excitation of A and B excitons, followed by mid-IR probing at 0.31 and 0.62 eV, we observe distinct non-radiative decay pathways. While pristine MoS2 exhibits fluence-dependent defect state saturation, Vanadium doping introduces additional relaxation channels that suppress this effect, leading to nearly pump fluence- and energy-independent carrier lifetimes. Our results provide direct experimental evidence of doping-mediated suppression of defect state saturation, offering new insight into electron–phonon interactions and defect-assisted recombination in TMDCs. These findings establish mid-IR transient spectroscopy as a powerful tool for quantifying and engineering defect states, paving the way for optimized doping strategies in next-generation TMDC-based optoelectronic devices.