Symmetry breaking and structure instability in ultra-thin 2H-TaS2 across charge density wave transition

Abstract: Ultra-thin 2D materials have shown complete paradigm shift of understanding of physical and electronic properties because of confinement effects, symmetry breaking and novel phenomena at nanoscale. Bulk 2H-TaS2 undergoes an incommensurate charge density wave (I-CDW) transition temperature, TI-CDW - 76 K, however, onset of CDW in atomically thin layers is not clear. We explored the evidence of CDW instability in exfoliated atomically thin 2H-TaS2 using low temperature Raman spectroscopy. We have emphasized on CDW associated modes, M1 - 125 cm-1, M2 -158 cm-1, and M3 -334 cm-1, with thickness - 3 nm (one unit-cell). The asymmetric (Fano) line shape of M2 suggests evidence of strong electron-phonon coupling, which mainly drives the CDW instability. Our observations provide key evidence that the CDW can persists even in one-unit cell with a TI-CDW well above - 200 K, which is higher than bulk 2H-TaS2.