Tailoring the phase transition and electron-phonon coupling in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>1</mml:mn><mml:msup><mml:mi>T</mml:mi><mml:mo>′</mml:mo></mml:msup><mml:mtext>−</mml:mtext><mml:mi>Mo</mml:mi><mml:msub><mml:mi>Te</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math> by charge doping: A Raman study
Suvodeep Paul, Saheb Karak, Manasi Mandal, Ankita Ram, Sourav Marik, R. P. Singh, Surajit Saha
Abstract
Transition-metal dichalcogenides are a class of widely studied two-dimensional layered materials which exist in various polymorphs. The $1{T}^{\ensuremath{'}}$ phase of $\mathrm{Mo}{\mathrm{Te}}_{2}$ is of prime importance as it has been reported to show quantum spin-Hall (QSH) behavior with a fairly large band gap of \ensuremath{\sim}60 meV, in contrast to most QSH materials known. It is noteworthy that though the monolayer $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{Te}}_{2}$ was initially predicted to show the QSH behavior, recent theoretical studies claim that the few-layered counterparts also exhibit higher-order topological behavior. Besides, $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{Te}}_{2}$ also undergoes a hysteretic phase transition to the ${T}_{d}$ phase (which is a type-II Weyl semimetal) by breaking the inversion symmetry of the crystal. While the phase transition between these two topological phases is of utmost importance, its study has been mostly restricted to bulk single-crystal flakes, thereby not sufficiently exploring the effect of dimensionality. We have studied the phase transition in $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{Te}}_{2}$ as a function of flake thickness. Altough our Raman studies show a suppression of the phase transition in the thin (thickness $<10\phantom{\rule{4pt}{0ex}}\mathrm{nm}$) flakes [similar to the report by He et al., Phys. Rev. B 97, 041410 (2018)], we have experimentally demonstrated the possibility of stabilizing the desired phase ($1{T}^{\ensuremath{'}}$ or ${T}_{\mathrm{d}}$) at room temperature by charge doping. Further, we have observed clear signatures of electron-phonon coupling in $\mathrm{Mo}{\mathrm{Te}}_{2}$, which evolves as a function of flake thickness and charge doping.