Strain-Induced Activation of Symmetry-Forbidden Exciton-Phonon Couplings for Enhanced Phonon-Assisted Photoluminescence in MoS₂ Monolayers

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 MoS$_2$ 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 MoS$_2$ exhibits no phonon-assisted PL emissions at cryogenic temperatures due to forbidden A$^{\prime\prime}$ phonon modes, biaxial strain opens a pathway to significantly intensify this emission by activating hole-phonon A$^{\prime}$-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.