Raman-active modes of 1T′−WTe2 under tensile strain: A first-principles prediction

Monolayer $1{T}^{\ensuremath{'}}\ensuremath{-}{\mathrm{WTe}}_{2}$ attracts rapidly growing interests aiming for promising applications in spintronics, dissipationless transport, and quantum computations. Due to one-dimensional W-W chains, $1{T}^{\ensuremath{'}}\ensuremath{-}{\mathrm{WTe}}_{2}$ exhibits unique anisotropic structure and promising properties, which can be modified by simply applying strains. Based on first-principles calculations, we systematically study the phonon dispersion curves as well as the Raman-active modes of $1{T}^{\ensuremath{'}}\ensuremath{-}{\mathrm{WTe}}_{2}$ under different tensile strains. We find that one branch of acoustic phonon softens at special $\mathit{q}$ points under a critical strain of ${\ensuremath{\varepsilon}}_{a}=11.55%$ along the $a$ axis (with $W\ensuremath{-}W$ chains) direction, or ${\ensuremath{\varepsilon}}_{b}=7.0%$ along the $b$-axis direction and ${\ensuremath{\varepsilon}}_{ab}=8.44%$ along the biaxial direction. Before reaching such critical strains, the Raman frequencies of ${A}_{g}^{1}, {A}_{g}^{3}$, and ${A}_{g}^{4}$ modes, contributing to the main peaks in Raman spectra of $1{T}^{\ensuremath{'}}\ensuremath{-}{\mathrm{WTe}}_{2}$, show anisotropic responses to different strains. The response to biaxial strains is found to be the most sensitive. We find that the frequency shift of ${A}_{g}^{3}$ mode shows parabolic characters of strained $1{T}^{\ensuremath{'}}\ensuremath{-}{\mathrm{WTe}}_{2}$, then we split it into two parts and it shows a Raman-shift transition at $\ensuremath{\sim}5$% strains. While for $\mathrm{the}\phantom{\rule{4pt}{0ex}}{A}_{g}^{1}$ and ${A}_{g}^{4}$ modes, the frequencies change linearly.