Formation of charged defect excitons and reconfigured exciton relaxation pathways in chromium-doped monolayer MoS2.

Substitutional doping provides an effective route for tailoring the optical and electronic properties of two-dimensional transition metal dichalcogenides. Here, we investigate the influence of Cr-induced defect states on the ultrafast carrier dynamics of monolayer MoS2 (MS) using femtosecond transient absorption (TA) spectroscopy. Cr incorporation introduces distinct defect-related excitonic resonances that strongly modify the relaxation pathways of photoexcited carriers. Our measurements reveal enhanced bandgap renormalization and a progressive transfer of spectral population from the A exciton into Cr-induced defect states, indicating efficient carrier localization. At elevated excitation densities, a new defect-related transition emerges which exhibits distinct spectral evolution, relaxation dynamics, and superlinear population growth, consistent with charged defect-bound excitonic states. Interestingly, carrier trapping into Cr-induced states suppresses density-dependent exciton-exciton annihilation and prolongs excitonic lifetimes. Furthermore, the defect states exhibit exceptionally large saturation fluences (up to ∼640 µJ cm-2), enabling substantially enhanced carrier-storage capacity. These findings establish dopant-induced defect engineering as a powerful approach for tuning ultrafast carrier relaxation and nonlinear optical response in monolayer MS, paving the way for advanced nonlinear photonic and optoelectronic devices.