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Electron–phonon interactions strongly influence exciton dynamics, enabling control at the nanoscale over light‐matter interactions and coherent phenomena for emerging quantum sensing applications. In transition metal dichalcogenides, momentum‐forbidden dark excitons occupy the energy ground state and exhibit exceptional sensitivity to their local environment. Here, we optically characterize these dark excitonic states in WS2and WSe2monolayers by tuning their band alignment through strain. We unveil their pronounced localization, extended lifetimes, and high linear polarization, signatures of strong exciton–phonon coupling and environmental responsiveness. Multiple distinct emission peaks are consistent with phonon‐dressed excitonic replicas with an estimated Huang‐Rhys factor of about 5. Density functional theory calculations, combined with measured strain‐dependent emission energies, constrain the relevant energy scales and suggest possible phonon‐assisted pathways underlying these phonon‐dressed states. Because replica energies, relative spectral weights, polarization axis, and lifetime respond to environmental factors such as local strain, dielectric environment, and phonon populations, these phonon‐dressed dark excitons provide a materials platform for exciton‐based nanoscale sensing and coherent control in 2D semiconductors.
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Observation of Multi-Phonon Emission in Monolayer WS2 on Various Substrates
Transition metal dichalcogenides (TMDs) have unique absorption and emission properties that stem from their large excitonic binding energies, reduced-dielectric screening, and strong spin–orbit coupling. However, the role of substrates, phonons, and material defects in the excitonic scattering processes remains elusive. In tungsten-based TMDs, it is known that the excitons formed from electrons in the lower-energy conduction bands are dark in nature, whereas low-energy emissions in the photoluminescence spectrum have been linked to the brightening of these transitions, either via defect scattering or via phonon scattering with first-order phonon replicas. Through temperature and incident-power-dependent studies of WS2 grown by CVD or exfoliated from high-purity bulk crystal on different substrates, we demonstrate that the strong exciton–phonon coupling yields brightening of dark transitions up to sixth-order phonon replicas. We discuss the critical role of defects in the brightening pathways of dark excitons and their phonon replicas, and we elucidate that these emissions are intrinsic to the material and independent of substrate, encapsulation, growth method, and transfer approach.
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- Award ID(s):
- 2132098
- PAR ID:
- 10528096
- Publisher / Repository:
- MDPI
- Date Published:
- Journal Name:
- Nanomaterials
- Volume:
- 14
- Issue:
- 1
- ISSN:
- 2079-4991
- Page Range / eLocation ID:
- 37
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.3390/nano14010037
