Defect-engineered competition between exciton annihilation and trapping in MOCVD WS2

TL;DR

The paper shows how defects in WS2 monolayers affect exciton decay processes, offering a way to control optoelectronic properties through defect engineering.

Contribution

The study provides a quantitative framework for defect-modulated exciton decay in MOCVD WS2 using combined spectroscopy and modeling.

Findings

Defect trapping and exciton annihilation compete based on defect-to-exciton density ratio (~3.5).

Defect saturation at high exciton densities suppresses defect trapping.

Rate constants for defect trapping and annihilation are 0.02 cm² s⁻¹ and 0.1 cm² s⁻¹, respectively.

Abstract

Exciton dynamics critically influence the optoelectronic performance of two-dimensional transition metal dichalcogenides (TMDCs). In large-scale WS2 monolayers grown via metal–organic chemical vapor deposition (MOCVD), intrinsic sulfur vacancies introduce in-gap states that promote nonradiative recombination through defect trapping (DT). Under elevated excitation conditions, the decay behaviour changes as exciton–exciton annihilation (EEA) emerges as a competing nonradiative process. To investigate these mechanisms across excitation regimes, we combine steady-state quantum efficiency measurements with femtosecond broadband transient absorption spectroscopy on samples with varying defect concentrations. These complementary measurements provide an unprecedented quantitative disentanglement of these decay pathways, a level of analysis not previously reported for MOCVD-grown monolayer WS2. The induced defect states are partially occupied, as first revealed by sub-bandgap excitation, and variations in defect density exert a pronounced influence on the photo-induced band renormalization. After establishing these DT-specific properties, we apply a rate-equation model including both DT and EEA to extract constants of 0.02 cm2 s−1 and 0.1 cm2 s−1, followed by an in-depth exploration of their fundamentally diffusion-limited behaviour. The competition between DT and EEA can be set by a critical defect-to-exciton density ratio (≈3.5), which serves as the threshold for EEA activation. Moreover, at high exciton densities, defect saturation suppresses DT, reshaping the decay landscape. Overall, our findings provide detailed insights into defect-modulated exciton decay mechanisms and establish a quantitative framework for tailoring the optoelectronic properties of TMDCs via controlled defect engineering.

Steady-state and ultrafast spectroscopy reveal how controlled defect introduction tunes defect trapping and exciton–exciton annihilation in MOCVD-grown WS2, establishing a quantitative framework for diffusion-limited exciton dynamics.

Figures