Air-Stable Lithiation 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. Herein, the nanoprobe-controlled fabrication of Li_xMoS₂-based multilayers is demonstrated, exhibiting a direct bandgap and strong photoluminescence emission from tightly bound excitons and trions. The fabrication of Li_xMoS₂ multilayers is facilitated by the newly developed Li-ion platform, featuring tip-induced Li intercalation, doping patterning with a spatial resolution of 517 nm, air stability, and rewritability. Ultralow frequency 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. The intercalation result is different from existing observations of transforming MoS₂ multilayers into metallic phases. This work not only provides a highly controllable Li-ionic engineering platform for studying Li-material interactions and developing novel ionic electronics but also offers an intriguing direct-bandgap semiconductor for optoelectronic applications.