Direct observation of ultrafast defect-bound and free exciton dynamics in defect-engineered WS$_2$ monolayers

TL;DR

This study directly observes ultrafast defect-bound and free exciton dynamics in defect-engineered WS$_2$ monolayers, revealing their formation, interaction, and conversion processes within hundreds of femtoseconds, advancing understanding of defect-related optoelectronic phenomena.

Contribution

The paper provides the first direct ultrafast spectroscopic observation of defect-bound exciton dynamics and their coherent coupling with free excitons in WS$_2$ monolayers with engineered defects.

Findings

Both free and defect-bound excitons form within 300 fs.

Defect-bound excitons have shorter lifetimes than free excitons.

Ultrafast interconversion occurs within ~150 fs, indicating coherent coupling.

Abstract

Defects in two-dimensional transition metal dichalcogenides (TMDCs) broadly affect their optical and electronic properties. Directly capturing the ultrafast processes of exciton trapping and defect-bound exciton formation is crucial for understanding and advancing defect-mediated optoelectronics and quantum technologies. However, the weak transient optical absorption of defect-bound excitons has limited their experimental observation to date. Here, we report the direct observation of the ultrafast dynamics of defect-bound excitons in monolayer WS$_2$ crystals with a high density of mono-sulfur vacancies (V$_S$) and W-site defect complexes (S$_W$V$_S$) resulting from synthesis by alkali metal halide-assisted chemical vapor deposition. The dynamics of excitons bound to these defects, along with their coherent interactions with free excitons, are elucidated using ultrafast optical spectroscopy. Using above band-edge photoexcitation, we find that both free and defect-bound excitons simultaneously form within 300 fs from hot carrier relaxation. The defect-bound excitons exhibit shorter lifetimes than free excitons, leading to a population difference of the corresponding excitonic states and free exciton trapping within a 1--100 ps window. Band-edge photoexcitation of free and defect-bound exciton states reveals ultrafast interconversion within ~150 fs (comparable to our temporal resolution), indicating possible coherent coupling between these states. We further demonstrate efficient up-conversion of defect-bound excitons to free excitons with photon energies up to ~300 meV below the free exciton resonance. These findings provide insights into the ultrafast dynamics of defect-bound excitons in TMDCs and their coupling with free excitons, which are relevant to defect-engineered optoelectronic, quantum photonic, and valleytronic applications.