Vibronically Coherent Exciton Trapping in Monolayer WS₂

Area of Science:

• Materials Science
• Quantum Physics
• Spectroscopy

Background:

• Defect engineering in transition metal dichalcogenide (TMD) monolayers is key for advanced applications like single-photon emission and sensing.
• Understanding exciton trapping dynamics at defect sites, mediated by optical phonons, is crucial but not fully understood.
• Current models often rely on incoherent multiphonon emission within the Born-Oppenheimer approximation.

Purpose of the Study:

• To investigate the ultrafast exciton trapping mechanism in defect-modified WS2 monolayers.
• To elucidate the role of exciton-phonon coupling in defect-mediated energy transfer.
• To challenge and refine existing models of exciton trapping in solid-state materials.

Main Methods:

• Utilized impulsive vibrational spectroscopy to probe exciton dynamics.
• Studied defect-modified WS2 monolayers synthesized via metal-organic chemical vapor deposition.
• Analyzed the persistence of phonon coherences during exciton trapping.

Main Results:

• Observed persistent phonon coherences (A' and E' modes) during ultrafast exciton trapping (~100 fs).
• Evidence suggests a conical intersection-mediated, non-adiabatic trapping process.
• Identified the E' mode as a vibrational coordinate that promotes exciton trapping.

Conclusions:

• Exciton trapping at defects in TMDs can occur via a vibronically coherent mechanism, beyond the Born-Oppenheimer approximation.
• This finding contrasts with traditional incoherent trapping models.
• Provides new mechanistic insights into exciton-phonon interactions for designing functional TMD materials.