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-TaS₂ undergoes an incommensurate charge density wave (I-CDW) at temperature T_I-CDW~76 K, however, the onset of CDW in atomically thin layers is not clear. We have explored the CDW instability in exfoliated 2H-TaS₂ using low temperature polarized Raman spectroscopy and first principles calculation. We have emphasized CDW associated modes, M₁~125 cm^-1 (E₁), M₂~158 cm^-1 (A₁), and M₃~334 cm^-1 (E₂), in the ultrathin layer of a single unit cell with thickness d~3 nm. The asymmetric Fano line shape of M₂ and its anomalous behavior suggests strong electron-phonon coupling, which mainly drives the CDW instability in 2H-TaS₂. Our observations provide key evidence that CDW can persist even in one unit cell with a T_I-CDW~200 K, which is much higher than bulk 2H-TaS₂, and the results are supported by density functional theory.