Photoluminescence Enhancement in Two-Dimensional Semiconductors via Spacer-Free Metallic Screening

Coulomb-bound electron-hole pairs, namely the excitons, in monolayer transition metal dichalcogenides (TMDCs) present unique opportunities for optoelectronic and quantum photonic device applications. Due to their two-dimensional confinement, these excitons are highly sensitive to their environment, with electric field lines extending beyond the TMDC monolayer. This sensitivity allows for the engineering of screening effects to modulate exciton radiative recombination and photoluminescence (PL) intensity with the aim of enhancing device efficiency. While previous studies have explored modifying the dielectric environment and introducing metal surfaces separated from TMDCs by a dielectric spacer layer to influence screening effects and PL, the case where a metal layer is in contact with the TMDC monolayer by only a van der Waals gap—has not been demonstrated until now. In this study, we demonstrate that this limit can be achieved by vertically stacking metals with appropriate work functions either above or below a monolayer semiconducting TMDC. Our findings reveal that PL intensity can be increased by up to two orders of magnitude in such metal-semiconductor junctions, attributed to the suppression of exciton-exciton interactions due to the strong screening provided by the metallic layer in a dielectric spacer-free environment. The van der Waals gapped interface minimizes free carrier transfer from the metal to the TMDC. Time-resolved PL measurements further indicate that the observed PL enhancement is due to reduced exciton-exciton annihilation, even at high generation rates, facilitated by the strong screening effect of the metal. These results highlight the potential for engineering optical emission from TMDCs through direct metal interfacing.