# Vibronically Coherent Exciton Trapping in Monolayer WS2

## TL;DR

The paper shows that exciton trapping in WS2 monolayers involves coherent vibrational processes, challenging traditional models.

## Contribution

The study reveals a vibronically coherent exciton trapping mechanism in WS2 monolayers, beyond the Born–Oppenheimer approximation.

## Key findings

- Phonon coherences of Raman-active A’ and E’ modes persist during ultrafast exciton trapping.
- Trapping involves a conical intersection-mediated process with nonadiabatic transitions.
- E’ mode dephases faster in trapped excitons, suggesting a role in promoting trapping.

## Abstract

Defect engineering
in transition metal dichalcogenide
(TMD) monolayers
enables applications in single-photon emission, sensing, and photocatalysis.
These functionalities critically depend on defect type, density, spatial
distribution, relative energy, and the dynamics of exciton trapping
at the defect sites. The latter are mediated by coupling to optical
phonons through mechanisms not yet fully understood. Traditionally,
exciton or carrier trapping at defects in inorganic crystals has been
described by incoherent multiphonon emission within the Born–Oppenheimer
approximationan approach that underpins the widely used Shockley–Read–Hall
framework for nonradiative recombination. Here, we use impulsive vibrational
spectroscopy to investigate exciton trapping in defect-modified monolayers
of WS2 grown through metal–organic chemical vapor
deposition. We find that the phonon coherences of the Raman-active
A’ and E’ modes persist throughout the ultrafast (∼100
fs) exciton trapping process, indicating a continuous evolution of
the excitonic wave function. This observation is consistent with a
conical intersection-mediated trapping process, in which a potential
energy surface crossing between the free and trapped excitonic states
acts as a funnel to drive this nonadiabatic transition. Such a molecular-like,
vibronically coherent mechanism lies beyond the Born–Oppenheimer
approximation, in stark contrast to classical, incoherent trapping
models in solids. Moreover, the faster dephasing of the E’
mode in the trapped exciton state compared to the free exciton suggests
it acts as a vibrational coordinate that promotes the trapping process.
These findings provide mechanistic insights into exciton–phonon
interactions at defects in TMD monolayers and inform strategies for
engineering quantum and energy functionalities.