Spatially Resolved Bound and Free Trion Dynamics in Defect-Engineered WS2 Heterojunctions

Abstract

Understanding how structural defects influence exciton dynamics in two-dimensional semiconductors is crucial for tailoring their optoelectronic properties toward fast electronic sensing or catalytic applications. In this work, we engineer defects in monolayer WS2 by selectively introducing point defects using focused ion beam (FIB) lithography, thereby creating a lateral heterostructure composed of pristine and defect-rich regions. We systematically investigate the impact of different defect concentrations on excitonic features and recombination dynamics using steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectroscopies. Our results reveal the coexistence of two distinct trion species with very different lifetimes: defect-bound trions and free-trions. Indeed, TRPL measurements exhibit biexponential decay dynamics, where the fast component is attributed to neutral exciton recombination, while the slower component corresponds to defect-bound charged excitons, also referred to as trions. Controlled defect implantation by FIB irradiation induces localized states that significantly modify trion recombination kinetics. We observe that increasing defect density leads to longer defect-bound trion lifetimes. In contrast, at the pristine-bombarded heterojunction interface, carrier diffusion promotes the formation of free trions associated with delocalized electrons, resulting in faster recombination dynamics. These results highlight that FIB lithography is a powerful tool for defect-engineering and for controlling excitonic recombination dynamics in 2D semiconductors.