Strain-Induced Activation of Symmetry-Forbidden Exciton-Phonon Couplings for Enhanced Phonon-Assisted Photoluminescence in MoS$_2$ Monolayers

TL;DR

This study demonstrates that applying biaxial strain to MoS$_2$ monolayers activates forbidden exciton-phonon interactions, significantly enhancing phonon-assisted photoluminescence, which could improve optoelectronic device performance.

Contribution

The paper reveals how strain engineering activates symmetry-forbidden exciton-phonon couplings in MoS$_2$, enabling stronger phonon-assisted photoluminescence through ab-initio calculations and group-theoretic analysis.

Findings

Strain activates previously forbidden phonon-assisted emission channels.

Biaxial strain redistributes oscillator strength toward radiative recombination.

Enhanced PL emission observed at cryogenic temperatures with strain application.

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 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.