Highly radiative emission of room temperature-localized excitons enabled by charge-neutralized 0D quantum wells in 2D semiconductors

Abstract

Nondiffusing localized excitons (X_L) in two-dimensional semiconductors present a robust platform for mediating light-matter interactions, with potential applications in both photovoltaics and light-emitting devices. However, at room temperature, high thermal energy hinders X_L formation, while excess charges diminish the quantum yield (QY) through nonradiative decay. Here, we present high-QY X_L emission in ambient conditions by removing excess charges and inducing efficient exciton funneling into a Au nanohole. Specifically, by evaporating an H₂O barrier between the n-type MoS₂ and the Au substrate, we induce a grounding effect on electrons. Dominantly populating excitons are then funneled and bound to the nanohole through the strain-induced zero-dimensional quantum well effect. We confirm the exciton confinement efficiency of ~98% using a drift-diffusion model, enabling bright X_L emission at the nanoscale. Using tip-induced gigapascal-scale pressure, we control X_L dynamics and QY in a reversible manner. Our approach provides an innovative strategy for X_L-based nanophotonic devices.

Fig. 1.. Schematic illustrations of the experimental design for dominant radiative decay of X_L at the nanohole.

(A) Illustration of effectively funneled and bound excitons at the nanohole, with dominant radiative decay, facilitated by the electron quenching process in the strained-MoS₂ ML. Energy band diagrams of the strain-induced 2D crystal before (B) and after (C) thermal annealing. AFM height profiles of the MoS₂ ML on the Au film before (D) and after (E) thermal annealing, confirming the removal of the H₂O layer.

Fig. 2.. Hyperspectral PL imaging of the strained MoS₂ ML at the 1D nanogap.

X₀ PL peak intensity (I_PL) images of the strained MoS₂ on the 1D nanogap before (A) and after (B) thermal annealing, illustrating funneling dynamics of excitons and electrons under the 1D potential confinement. PL spectra of the MoS₂ ML on a Au substrate before (C) and after (D) thermal annealing. PL spectra of the strained MoS₂ ML at the Au nanogap before (E) and after (F) thermal annealing. The PL spectra are fitted to a Lorentzian function, where the black line represents the fit to the raw spectra, while the blue and green lines correspond to the fits for the neutral exciton (X₀) and trion (X−). The black dots indicate the raw data points. a.u., arbitrary units.

Fig. 3.. Robust X_L emission at room temperature via dominant radiative decay of effectively funneled excitons bound at the nanohole.

(A) X− PL intensity image of the strained MoS₂ ML at the nanohole before thermal annealing, with an illustration of electron funneling into the 0D potential well. (B) X_L PL intensity image of the strained MoS₂ ML at the nanohole after thermal annealing, with an illustration of electron quenching into the Au substrate. Scale bar, 500 nm. (C) Line profiles of X₀, X−, and X_L PL intensities before (left) and after (right) thermal annealing, derived from the center line of the nanohole. Corresponding PL spectra of the strained MoS₂ ML before (D) and after (E) thermal annealing, demonstrating the robust X_L emission after thermal annealing. (F) Polarization-dependent PL intensities of X_L and X₀, exhibiting linearly polarized emission of X_L. (G) Excitation power-dependent PL intensities of X_L and X₀, showing a saturation behavior of X_L.

Fig. 4.. Simulated exciton funneling dynamics at the nanohole and experimental results of tip-induced modulation of exciton confinement and X_L emission.

(A) Simulation results showing the bandgap-energy modification [Δu(r), top], the drift-to-diffusion flow ratio of excitons [−S(r)∇u(r)/∇S(r), middle], and the exciton density [n(r), bottom] with as a function of the distance r from the center for the strain-induced MoS₂ ML on the nanohole. Schematic illustration of tip-induced modulation of exciton behavior under tip-induced pressure (P_press) at the nanohole is shown before (B) and after (D) thermal annealing. PL intensity of excitonic emissions (X–, X₀, and X_L) as a function of tip displacement (Δz) before (C) and after (E) thermal annealing, attributed to modifications in u, E_b, J_μ, and number of electrons (n_e–). Simulated spatial profiles of Δu(r) (F) and −S(r)∇u(r)/∇S(r) (G) under tip-induced pressure (P_press). (H) Simulated tip-induced modulation of the exciton transport ratio dominated by drift [J_μ/(J_μ + J_D)] and the redistributed exciton density at r = 0 [Δn(0)] as a function of applied strain (ε).