Ultrafast Exciton Trapping and Exciton-Exciton Annihilation in Large-Area CVD-Grown Monolayer WS2

Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have shown promise for a variety of optoelectronic applications due to a wide range of optical, electrical, and mechanical properties. Large-area chemical vapor deposition (CVD)-grown TMDC flakes could be useful in such devices. However, the defects present in large-area TMDC flakes can significantly influence carrier dynamics and transport properties. Here, the ultrafast carrier dynamics of monolayer tungsten disulfide (WS2) covering a large area of the substrate was explored using transient absorption spectroscopy. By monitoring the transient optical response, exciton trapping by oxygen-induced defects has been identified in monolayer WS2. We observe excitation-density-dependent exciton decay dynamics for both band-edge and above band-edge excitations due to exciton-exciton annihilation. Our results demonstrate the impact of defect states on carrier recombination in CVD-grown TMDCs, which could pave the way for utilizing such materials in optoelectronic device applications.