Suppression of the commensurate charge density wave phase in ultrathin 1T−TaS2 evidenced by Raman hyperspectral analysis

Using temperature-dependent and low-frequency Raman spectroscopy, we address the question of how the transition from bulk to a few atomic layers affects the charge density wave (CDW) phases in $1T\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}$. We find that for crystals with thickness larger than $\ensuremath{\approx}10$ nm the transition temperatures between the different phases as well as the hysteresis that occurs in the thermal cycle correspond to the ones expected for a bulk sample. However, when the crystals become thinner than $\ensuremath{\approx}10$ nm, the low-temperature commensurate CDW phases can be suppressed down to the experimentally accessible temperatures ($\ensuremath{\sim}80$ K) upon cooling at moderate rates $(\ensuremath{\sim}5\phantom{\rule{0.16em}{0ex}}\mathrm{K}\phantom{\rule{0.16em}{0ex}}\mathrm{mi}{\mathrm{n}}^{\ensuremath{-}1})$. In addition, even the near commensurate CDW phase is not accessible in few-layer flakes below $\ensuremath{\approx}4$ nm for even slower cooling rates $(\ensuremath{\sim}1\phantom{\rule{0.16em}{0ex}}\mathrm{K}\phantom{\rule{0.16em}{0ex}}\mathrm{mi}{\mathrm{n}}^{\ensuremath{-}1})$. We employ Raman hyperspectral imaging to statistically confirm these findings and consider the interlayer coupling and its dynamics to play significant roles in determining the properties of CDW systems consisting of a few unit cells in the vertical direction.