Suppressing Diffusion-Mediated Exciton Annihilation in 2D Semiconductors Using the Dielectric Environment

TL;DR

This study demonstrates that changing the substrate's dielectric environment can significantly suppress exciton-exciton annihilation in 2D semiconductors, enhancing their potential for efficient optoelectronic devices.

Contribution

It reveals how substrate dielectric properties influence exciton interactions, providing a method to control exciton-exciton annihilation in 2D materials.

Findings

Replacing SiO2 with Al2O3 or SrTiO3 reduces annihilation rate constants by one or two orders of magnitude.

The exciton diffusion coefficient in MoS2 is approximately 0.06 cm2/s, with a diffusion length of 350 nm.

The effective annihilation radius decreases as the substrate's refractive index increases.

Abstract

Atomically thin semiconductors such as monolayer MoS2 and WS2 exhibit nonlinear exciton-exciton annihilation at notably low excitation densities (below ~10 excitons/um2 in MoS2). Here, we show that the density threshold at which annihilation occurs can be tuned by changing the underlying substrate. When the supporting substrate is changed from SiO2 to Al2O3 or SrTiO3, the rate constant for second-order exciton-exciton annihilation, k_XX [cm2/s], is reduced by one or two orders of magnitude, respectively. Using transient photoluminescence microscopy, we measure the effective room-temperature exciton diffusion coefficient in chemical-treated MoS2 to be D = 0.06 +/- 0.01 cm2/s, corresponding to a diffusion length of LD = 350 nm for an exciton lifetime of {\tau} = 20 ns, which is independent of the substrate. These results, together with numerical simulations, suggest that the effective exciton-exciton annihilation radius monotonically decreases with increasing refractive index of the underlying substrate. Exciton-exciton annihilation limits the overall efficiency of 2D semiconductor devices operating at high exciton densities; the ability to tune these interactions via the dielectric environment is an important step toward more efficient optoelectronic technologies featuring atomically thin materials.