Vibronically Coherent Exciton Trapping in Monolayer WS.

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 WS 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.