Spontaneous Polarization Suppression of Exciton-Exciton Annihilation in Rhombohedral-Stacked Bilayer Molybdenum Disulfide.

Abstract

Rapid exciton-exciton annihilation (EEA) in two-dimensional semiconductors limits access to high-density excitonic regimes essential for efficient optoelectronic operation under strong excitation. Here, we show that EEA is strongly suppressed in rhombohedral (3R)-stacked MoS₂ bilayers relative to nonpolar 2H bilayers, and that this suppression is quantitatively consistent with repulsive dipole-dipole interactions between layer-polarized excitons induced by spontaneous polarization. Using ultrafast pump-probe spectroscopy, we measure an EEA rate of γ_EEA = (5.03 ± 0.99) × 10^-3 cm² s^-1 in 3R bilayers, which is approximately 18.2-fold smaller than that in monolayers and 2.9-fold smaller than that in nonpolar 2H bilayers. Despite the higher exciton diffusivity recently reported for 3R relative to 2H bilayers, the reduced EEA rate in 3R indicates a rate-limited regime governed by the close-encounter annihilation probability rather than diffusion. A rate-limited annihilation model incorporating a dipole-dipole repulsive potential captures the observed ratio γ_EEA,3R/γ_EEA,2H ≈ 0.35 for an exciton-exciton encounter distance of ∼1.3 nm, consistent with the bilayer exciton Bohr radius. These results indicate that spontaneous polarization in 3R-stacked bilayers can suppress nonlinear excitonic losses and provide a route toward high-density excitonics.