Strain-Induced Activation of Symmetry-Forbidden Exciton-Phonon Couplings for Enhanced Phonon-Assisted Photoluminescence in MoS2 Monolayers
Abstract
Phonon-assisted photoluminescence (PL) in molybdenum-based two-dimensional dichalcogenides is typically weak due to the dormant phonon coupling with optically inactive momentum-dark (intervalley) excitons, unlike in tungsten-based dichalcogenides where such processes are more prominent. Despite this inefficiency, we revisit excitons in MoS2 using rigorous finite-momentum Bethe-Salpeter equation calculations to identify ways to enhance phonon-assisted recombination channels. Our ab-initio results, complemented by group-theoretic analyses, reveal that while unstrained MoS2 exhibits no phonon-assisted PL emissions at cryogenic temperatures due to forbidden A phonon modes, biaxial strain opens a pathway to significantly intensify this emission by activating hole-phonon A-mediated scattering channels. By calculating allowed exciton-phonon matrix elements and scattering rates, we demonstrate how strain redistributes oscillator strengths toward radiative recombination. These findings provide a promising route to improving PL emission efficiency in various metal dichalcogenide monolayers through strain engineering and offer valuable insights for further exploration of exciton-phonon dynamics, including time-resolved spectroscopic studies.
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