Tailoring the nonlinear optical performance of two-dimensional MoS2 nanofilms via defect engineering

Abstract

Defect engineering plays a key role in determining the catalytic and optical properties of two-dimensional (2D) materials such as molybdenum disulfide (MoS₂) in their practical applications in optical and photonic devices. Here, we report a direct strategy for the fabrication of wafer-scale 2D MoS₂ nanofilms with tunable sulfur (S) vacancies and crystallinity by a modified solvothermal method via a polyelectrolyte-assisted annealing process. Our results demonstrate that the S vacancies in MoS₂ nanofilms can induce saturable absorption (SA) in MoS₂ by introducing new energy bands within the band gap of MoS₂, and the crystallinity has a significant effect on the two-photon absorption (TPA) coefficient of MoS₂ nanofilms. The SA responses in MoS₂ will gradually dominate the nonlinear optical (NLO) behavior of MoS₂ with a lower saturable intensity along with increasing the S vacancies. The TPA coefficient of the MoS₂ nanofilms with increased crystallinity is improved to (4.3 ± 0.5) × 10² cm GW⁻¹ on increasing the crystallinity of MoS₂ films, over four times larger than that of their counterpart with relatively low crystallinity. Additionally, the damage threshold of MoS₂ nanofilms after polyelectrolyte-assisted annealing treatment is greatly improved to ∼74.1 GW cm⁻² compared to ∼32.6 GW cm⁻² of their counterpart with few S vacancies and relatively low crystallinity, due to the increased crystallinity and partial oxidation of MoS₂. This work sheds light on how the defects tailor the nonlinear optical properties of 2D MoS₂ nanofilms and affords an effective strategy for defect engineering via a polyelectrolyte-assisted annealing process, which can be applied to other 2D transition metal dichalcogenides.