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 $2H\text{−}\mathrm{Ta}{\mathrm{S}}_2$ undergoes an incommensurate charge density wave (I-CDW) at temperature $T_{\mathrm{I}-\mathrm{CDW}}\sim 76$ K, however, the onset of CDW in atomically thin layers is not clear. We have explored the CDW instability in exfoliated $2H\text{−}\mathrm{Ta}{\mathrm{S}}_2$ using low temperature polarized Raman spectroscopy and first principles calculation. We have emphasized CDW associated modes, $M_1\sim 125 \mathrm{c}{\mathrm{m}}^{-1}$ ($E_1$), $M_2\sim 158 \mathrm{c}{\mathrm{m}}^{-1}$ ($A_1$), and $M_3\sim 334 \mathrm{c}{\mathrm{m}}^{-1}$ ($E_2$), in the ultrathin layer of a single unit cell with thickness $d\sim 3$ 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 $2H\text{−}\mathrm{Ta}{\mathrm{S}}_2$. Our observations provide key evidence that CDW can persist even in one unit cell with a $T_{\mathrm{I}-\mathrm{CDW}}\sim 200$ K, which is much higher than bulk $2H\text{−}\mathrm{Ta}{\mathrm{S}}_2$, and the results are supported by density functional theory.

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• Charge density waves
• Transition metal dichalcogenides
• Inelastic light scattering