Strong direct-bandgap photoluminescence of suspended few-layer MoS₂via interlayer decoupling

Two-dimensional transition metal dichalcogenides (2D TMDCs) have attracted considerable research interest as key materials for next-generation integrated photonic and optoelectronic devices. However, the atomic layer materials are vulnerable to environmental influences. In addition, their ultimate thinness limits the effective length of light-matter interaction, restricting their emission intensity. Although bulk and few-layer TMDCs exhibit better environmental robustness, they typically suffer from indirect bandgap transitions, resulting in reduced optoelectronic efficiency. In this work, we report an in situ processing strategy to induce direct-bandgap exciton emission from few-layer (2-4 layers) MoS₂. A combined approach of mild oxygen plasma treatment and subsequent laser irradiation is employed to modify the few-layer MoS₂. Following the treatments, we observed pronounced photoluminescence (PL) emission in the suspended few-layer MoS₂, in contrast to the PL quenching effect detected in substrate-supported areas. Such a large difference in PL intensity is attributed to thermally driven interlayer decoupling of the few-layer MoS₂, which occurs exclusively in the suspended regions due to their significantly elevated temperature. According to the molecular dynamics simulation study, plasma treatment is essential for interlayer decoupling by injecting oxygen ions into the van der Waals gaps. These oxygen ions can potentially form oxygen molecules under laser-induced heat, leading to the expansion of van der Waals gaps. These findings demonstrate the potential for spatially selective PL enhancement in few-layer MoS₂. As a proof of concept, high-contrast PL patterns in bilayer MoS₂ were prepared, showcasing its promising application in anti-counterfeiting labeling. Furthermore, this work provides high-performance light-emitting materials for diverse photonic and optoelectronic applications.