Suppression of the commensurate charge density wave phase in ultrathin $1T\text{−}{\mathrm{TaS}}_2$ evidenced by Raman hyperspectral analysis

Using temperature-dependent and low-frequency Raman spectroscopy, we address the question of how the transition from bulk to a few atomic layers affects the charge density wave (CDW) phases in $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$. We find that for crystals with thickness larger than $\approx 10$ nm the transition temperatures between the different phases as well as the hysteresis that occurs in the thermal cycle correspond to the ones expected for a bulk sample. However, when the crystals become thinner than $\approx 10$ nm, the low-temperature commensurate CDW phases can be suppressed down to the experimentally accessible temperatures ($\sim 80$ K) upon cooling at moderate rates $\left(\sim 5\mathrm{K}\mathrm{mi}{\mathrm{n}}^{-1}\right)$. In addition, even the near commensurate CDW phase is not accessible in few-layer flakes below $\approx 4$ nm for even slower cooling rates $\left(\sim 1\mathrm{K}\mathrm{mi}{\mathrm{n}}^{-1}\right)$. We employ Raman hyperspectral imaging to statistically confirm these findings and consider the interlayer coupling and its dynamics to play significant roles in determining the properties of CDW systems consisting of a few unit cells in the vertical direction.

Figure 1

(a) Schematic of $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$ crystal. (b) The structure of the CCDW phase; Ta atoms in a Star of David pattern. (c) Different phases in $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$ as a function of temperature.

Figure 2

(a) Raman spectra taken at the indicated temperatures with a 532-nm laser on a bulk sample. (b) Enlarged view of the Raman spectra for the different CDW phases (black—CCDW, blue—NCCDW, red—ICCDW). (c) Raman intensity charts measured as a function of temperature with a 514-nm laser for a bulk sample in a cooling and warming cycle, respectively, at rates of $1\mathrm{K}\mathrm{mi}{\mathrm{n}}^{-1}$.

Figure 3

(a) Optical micrograph of area 1 and area 2. (b) Comparison at the indicated temperatures between the Raman spectra taken for areas 1 and 2 with a 514-nm laser. (c) Raman spectrum taken at different temperatures at the position indicated in the optical image of the flake as area 3. Inset: optical micrograph of area 3 and area 4. (d) Raman spectra for the indicated areas. (e) Optical micrograph of the flake with areas 5 and 6. (f) Averaged Raman spectra of indicated areas as extracted from hyperspectra collected by scanning the sample with a 532-nm laser.

Figure 4

(a) Low-frequency Raman spectra of area 5 and area 6 taken at 80 K with a 532-nm laser. For comparison, CCDW (black) and ICCDW (orange) are spectra for the bulk, collected, respectively, at 80 and 403 K. The colored strips highlight the Raman bands and frequency ranges analyzed for the spectral deconvolution. (b) Weight distribution maps of the deconvoluted spectral components for areas 5 and 6 as extracted from hyperspectra collected at 80 K with a 532-nm laser. Top: incommensurate (IC) component. Bottom: commensurate (C) component.