Large trion binding energy in monolayer WS${}_2$ via strain-enhanced electron-phonon coupling

Abstract

Transition metal dichalcogenides and related layered materials in their monolayer and a few layers thicknesses regime provide a promising optoelectronic platform for exploring the excitonic- and many-body physics. Strain engineering has emerged as a potent technique for tuning the excitonic properties favorable for exciton-based devices. We have investigated the effects of nanoparticle-induced local strain on the optical properties of exciton, $X^0$, and trion, $X^{\text{-}}$, in monolayer WS${}_2$. Biaxial tensile strain up to 2.0% was quantified and verified by monitoring the changes in three prominent Raman modes of WS${}_2$: E${}_{2g}^1$($\Gamma$), A${}_{1g}$, and 2LA(M). We obtained a remarkable increase of 34 meV in $X^{\text{-}}$ binding energy with an average tuning rate of 17.5 $\pm$ 2.5 meV/% strain across all the samples irrespective of the surrounding dielectric environment of monolayer WS${}_2$ and the sample preparation conditions. At the highest tensile strain of $\approx$2%, we have achieved the largest binding energy $\approx$100 meV for $X^{\text{-}}$, leading to its enhanced emission intensity and thermal stability. By investigating strain-induced linewidth broadening and deformation potentials of both $X^0$ and $X^{\text{-}}$ emission, we elucidate that the increase in $X^{\text{-}}$ binding energy is due to strain-enhanced electron-phonon coupling. This work holds relevance for future $X^{\text{-}}$-based nano-opto-electro-mechanical systems and devices.

arXiv comment7 figures

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