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 2 H -TaS 2 undergoes an incommensurate charge density wave (I-CDW) at temperature T I-CDW ti 76 K, however, the onset of CDW in atomically thin layers is not clear. We have explored the CDW instability in exfoliated 2 H - TaS 2 using low temperature polarized Raman spectroscopy and first principles calculation. We have emphasized CDW associated modes, M 1 ti 125 cm - 1 ( E 1 ), M 2 158 cm - 1 ( A 1 ), and M 3 <^>, 334 cm - 1 ( E 2 ), in the ultrathin layer of a single unit cell with thickness d - 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 2 H -TaS 2 . 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 2 H -TaS 2 , and the results are supported by density functional theory.