Air-stable lithiation engineering of MoS₂ for direct-bandgap multilayers

Due to its sizable direct bandgap and strong light-matter interactions, the preparation of monolayer MoS₂ has attracted significant attention and intensive research efforts. However, multilayer MoS₂ is largely overlooked because of its optically inactive indirect bandgap caused by interlayer coupling. It is highly desirable to modulate and decrease the interlayer coupling so that each layer in multilayer MoS₂ can exhibit a monolayer-like direct-gap behavior. Here, we demonstrate the nanoprobe fabrication of LiₓMoS₂-based multilayers exhibiting a direct bandgap and strong photoluminescence emission from tightly bound excitons and trions. The fabrication is facilitated by our newly developed Li-ion platform, featuring tip-induced Li intercalation, air stability and rewritability. Raman characterizations reveal that controlled Li intercalation effectively transforms multilayer MoS₂ into the stack of multiple monolayers, leading to a 26-fold enhancement of photoluminescence, compared to a monolayer. This intercalation result is different from existing observations of transforming MoS₂ multilayers into metallic phases.