Design of Self-AssembledMonolayer in Tungsten DiselenideBilayer for Exciton Dissociation

Transition metal dichalcogenides (TMDs) have emerged as promising candidates for next-generation self-powered photodetectors due to their distinct optoelectronic properties, including strong light–matter interactions. However, their high exciton binding energies impede efficient exciton dissociation, hindering viable photodetector applications. This study, based on first-principles calculations, introduces a design approach featured by the asymmetrically enclosed structure of the TMD bilayer, i.e., two different self-assembled monolayers (SAMs) asymmetrically attached to each side of a tungsten diselenide bilayer by varying electron-donating and electron-withdrawing groups in SAMs. Compared to the electron-donating and electron-withdrawing tendencies, we demonstrate that the surface work function of the SAM is a crucial macroscopic parameter in fine-tuning the band offset without trap formation with a large degree of freedom. Optimizing the work function achieves trap-free exciton dissociation, establishing a type-II band alignment and a sufficient built-in electric field within the bilayer. This design approach offers not only a design strategy for two-dimensional (2D) self-powered photodetectors but also a guide to interface engineering of TMDs utilizing SAMs for integration into low-power applications.