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We present first-principles calculations of the vibrational properties of thetransition metal dichalcogenide 1T-TaS$_2$ for various thicknesses in thehigh-temperature (undistorted) phase and the low-temperature commensuratecharge density wave (CDW) phase. We also present measurements of the Ramanspectra for bulk, few-layer, and monolayer samples at temperatures well belowthat of the bulk transition to the commensurate phase. Through ourcalculations, we identify the low-frequency folded-back acoustic modes as aconvenient signature of the commensurate CDW wave structure in vibrationalspectra. In our measured Raman spectra, this signature is clearly evident inall of the samples, indicating that the commensurate phase remains the groundstate as the material is thinned, even down to a single layer. This is incontrast to some previous studies which suggest a suppression of thecommensurate CDW transition in thin flakes. We also use polarized Ramanspectroscopy to probe c-axis orbital texture in the low-T phase, which hasrecently been suggested as playing a role in the metal-insulator transitionthat accompanies the structural transition to the commensurate CDW phase.

Zone-center phonons of bulk, few-layer, and monolayer 1T-TaS$_2$: Detection of the commensurate charge density wave phase through Raman scattering