Air-stable lithiation engineering of ${\mathrm{MoS}}_2$ for direct-bandgap multilayers

Abstract

Due to its sizable direct bandgap and strong light-matter interactions, the preparation of monolayer ${\mathrm{MoS}}_2$ has attracted significant attention and intensive research efforts. However, multilayer ${\mathrm{MoS}}_2$ 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 ${\mathrm{MoS}}_2$ can exhibit a monolayer-like direct-gap behavior. Here, we demonstrate the nanoprobe fabrication of ${\mathrm{Li}}_x{\mathrm{MoS}}_2$-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 ${\mathrm{MoS}}_2$ 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 ${\mathrm{MoS}}_2$ multilayers into metallic phases.