Distinct surface and bulk charge density waves in ultrathin 1T-TaS₂

We employ variable temperature low-frequency Raman spectroscopy to study the nearly commensurate (NC) to commensurate (C) charge density wave (CDW) transition in 1T -TaS2 ultrathin flakes protected from oxidation. We identify additional modes originating from C-phase CDW phonons that are distinct from those seen in bulk 1T -TaS2. We attribute these to CDW modes from the surface layers. By monitoring individual modes with temperature, we find that surfaces undergo a separate, low-hysteresis NC-C phase transition that is decoupled from the transition in the bulk layers. This indicates the activation of a secondary phase nucleation process in the limit of weak interlayer interaction, which can be understood from energy considerations.