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

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nanotechnology

Background:

• Exciton dynamics are crucial for the optoelectronic performance of 2D transition metal dichalcogenides (TMDCs).
• Sulfur vacancies in MOCVD-grown WS₂ monolayers create in-gap states, leading to nonradiative recombination via defect trapping (DT).
• At higher excitation levels, exciton-exciton annihilation (EEA) becomes a competing nonradiative pathway.

Purpose of the Study:

• To quantitatively disentangle the competing exciton decay mechanisms (DT and EEA) in MOCVD-grown WS₂ monolayers.
• To investigate the influence of defect concentration on exciton dynamics across various excitation regimes.
• To establish a framework for defect engineering to tailor TMDC optoelectronic properties.

Main Methods:

• Femtosecond broadband transient absorption spectroscopy.
• Steady-state quantum efficiency measurements.
• Rate-equation modeling incorporating DT and EEA.

Main Results:

• Demonstrated partial occupation of defect states and their influence on photo-induced band renormalization.
• Quantitatively extracted constants for DT (0.02 cm²/s) and EEA (0.1 cm²/s), revealing diffusion-limited behavior.
• Identified a critical defect-to-exciton density ratio (≈3.5) for EEA activation and observed defect saturation suppressing DT at high exciton densities.

Conclusions:

• Provided unprecedented quantitative insights into defect-modulated exciton decay in WS₂ monolayers.
• Established a critical density ratio for the onset of exciton-exciton annihilation.
• Highlighted the potential for controlled defect engineering to optimize TMDC optoelectronics.