Impact of plasma-induced defect states on the optical properties of bound excitons in monolayer WS<sub>2</sub>

Abstract

Monolayer transition metal dichalcogenides (TMDCs) exhibit exceptional properties including atomic-scale thickness, direct bandgap, and strong spin-orbit coupling, rendering them great potential for spintronics, optoelectronics, and other fields with promising applications. Invariably, materials contain various structural defects, either formed during preparation and growth or induced by subsequent treatments. These defects can significantly alter their physicochemical properties. Consequently, controlling and comprehending defects is an important approach to tailoring the properties of these materials. Herein, we employed Ar<sup>+</sup> plasma bombardment on mechanically exfoliated monolayer WS<sub>2</sub> to introduce defects of different densities by changing the bombardment duration. The Photoluminescence (PL) and Raman spectroscopic measurements at different temperatures and powers were utilized to investigate the optical properties of the defects. Furthermore, time-resolved photoluminescence was employed to unveil the dynamics of free and trapped excitons. The bombardment can introduce different defect types in typical 2D TMDCs such as MoS<sub>2</sub> and WS<sub>2</sub>. Single sulfur vacancies are frequently generated, while other defects like double sulfur vacancies or metal atom vacancies can also occur. Exciton effects dominate the optical properties of monolayer TMDCs due to reduced screening and large effective mass. At low temperatures, bound exciton emissions arise from trapped states. Our measurements revealed two types of defect-bound excitons from the PL spectra at around 1.85eV (X<sup>B1</sup>) and 1.55eV (X<sup>B2</sup>). Meanwhile, the Raman peaks of the samples before and after treatment exhibited no obvious changes, indicating the lattice structure remained. After the Ar<sup>+</sup> bombardment, the intensity of the free neutral exciton was significantly reduced to 1/6 of untreated WS<sub>2</sub>, owing to the free exciton population and the increased non-radiative centers. The exciton dynamics of these two bound excitons were considerably slower compared to the neutral exciton, showing the typical dynamics of defect-bound excitons. Furthermore, comparing the PL under vacuum and atmospheric conditions, the intensities of the two bound excitons exhibited opposing behaviors. In an atmospheric environment, neutral excitons, and bound exciton X<sup>B1</sup> possessed higher intensities. In the vacuum, the strength of neutral exciton and X<sup>B1</sup> decreased quickly, while the intensity of deep-level bound exciton X<sup>B2</sup> increased. In summary, we observed two bound exciton states arising from specific vacancy states in monolayer WS<sub>2</sub> after Ar<sup>+</sup> bombardment. Their energies are lower than the neutral exciton by 200 meV and 500 meV, with a splitting energy around 300 meV. The detailed evolution of the relative spectral weight with temperature and excitation power is presented. This work provides insights into the generation, control, and characteristic spectra of defects in 2D materials.