Broad-range modulation of light emission in two-dimensional semiconductors by molecular physisorption gating

In the monolayer limit, transition metal dichalcogenides become direct-bandgap, light-emitting semiconductors. The quantum yield of light emission is low and extremely sensitive to the substrate used, while the underlying physics remains elusive. In this work, we report over 100 times modulation of light emission efficiency of these two-dimensional semiconductors by physical adsorption of O₂ and/or H₂O molecules, while inert gases do not cause such effect. The O₂ and/or H₂O pressure acts quantitatively as an instantaneously reversible "molecular gating" force, providing orders of magnitude broader control of carrier density and light emission than conventional electric field gating. Physi-sorbed O₂ and/or H₂O molecules electronically deplete n-type materials such as MoS₂ and MoSe₂, which weakens electrostatic screening that would otherwise destabilize excitons, leading to the drastic enhancement in photoluminescence. In p-type materials such as WSe₂, the molecular physisorption results in the opposite effect. Unique and universal in two-dimensional semiconductors, the effect offers a new mechanism for modulating electronic interactions and implementing optical devices.