Symmetry breaking and structural instability in ultrathin 2H−TaS2 across the charge density wave transition

Ultrathin two-dimensional materials have shown a complete paradigm shift for understanding of physical and electronic properties because of quantum confinement effects and symmetry breaking at nanoscale. Bulk $2H\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}₂$ undergoes an incommensurate charge density wave (I-CDW) at temperature ${T}_{\mathrm{I}\ensuremath{-}\mathrm{CDW}}\ensuremath{\sim}76$ K, however, the onset of CDW in atomically thin layers is not clear. We have explored the CDW instability in exfoliated $2H\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}₂$ using low temperature polarized Raman spectroscopy and first principles calculation. We have emphasized CDW associated modes, ${M}₁\ensuremath{\sim}125 \mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ (${E}₁$), ${M}₂\ensuremath{\sim}158 \mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ (${A}₁$), and ${M}₃\ensuremath{\sim}334 \mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ (${E}₂$), in the ultrathin layer of a single unit cell with thickness $d\ensuremath{\sim}3$ nm. The asymmetric Fano line shape of ${M}₂$ and its anomalous behavior suggests strong electron-phonon coupling, which mainly drives the CDW instability in $2H\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}₂$. Our observations provide key evidence that CDW can persist even in one unit cell with a ${T}_{\mathrm{I}\ensuremath{-}\mathrm{CDW}}\ensuremath{\sim}200$ K, which is much higher than bulk $2H\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}₂$, and the results are supported by density functional theory.