Layer-dependent Raman spectroscopy of ultrathin ${\mathrm{Ta}}_2{\mathrm{Pd}}_3{\mathrm{Te}}_5$

Two-dimensional topological insulators (2DTIs) or quantum spin Hall insulators are attracting increasing attention due to their potential applications in next-generation spintronic devices. Despite their promising prospects, realizable 2DTIs are still limited. Recently, ${\mathrm{Ta}}_2{\mathrm{Pd}}_3{\mathrm{Te}}_5$, a semiconducting van der Waals material, has shown spectroscopic evidence of quantum spin Hall states. However, achieving controlled preparation of few to monolayer samples, a crucial step in realizing quantum spin Hall devices, has not yet been achieved. In this work, we fabricated few to monolayer ${\mathrm{Ta}}_2{\mathrm{Pd}}_3{\mathrm{Te}}_5$ and performed systematic thickness- and temperature-dependent Raman spectroscopy measurements. Our results demonstrate that Raman spectra can provide valuable information to determine the thickness of ${\mathrm{Ta}}_2{\mathrm{Pd}}_3{\mathrm{Te}}_5$ thin flakes. Moreover, our angle-resolved polarized Raman (ARPR) spectroscopy measurements show that the intensities of the Raman peaks are strongly anisotropic due to the quasi-one-dimensional atomic structure, providing a straightforward method to determine its crystalline orientation. Our findings may stimulate further efforts to realize quantum devices based on few or monolayer ${\mathrm{Ta}}_2{\mathrm{Pd}}_3{\mathrm{Te}}_5$.

Figure 1

(a) Schematic drawing of the crystal structure of ${\mathrm{Ta}}_2{\mathrm{Pd}}_3{\mathrm{Te}}_5$. (b) Optical image of exfoliated ${\mathrm{Ta}}_2{\mathrm{Pd}}_3{\mathrm{Te}}_5$ thin flakes on an Au/${\mathrm{SiO}}_2$/Si substrate, with the number of layers indicated. (c) Relative optical reflectivity of 1–5 L extracted from the green channel image. The blue dotted line is a linear fit of the optical reflectivity versus the number of layers.

Figure 2

(a) Typical Raman spectrum of bulk ${\mathrm{Ta}}_2{\mathrm{Pd}}_3{\mathrm{Te}}_5$ measured with an excitation wave length of 532 nm, with the calculated Raman peaks indicated by red lines. (b) Atomic displacements of the ${\mathrm{A}}_g^8$ and ${\mathrm{B}}_{3g}^1$ Raman modes.

Figure 3

(a) Raman spectra of 1–10 L ${\mathrm{Ta}}_2{\mathrm{Pd}}_3{\mathrm{Te}}_5$ with the excitation wave length set to 532 nm and the polarization parallel to the $c$ axis of ${\mathrm{Ta}}_2{\mathrm{Pd}}_3{\mathrm{Te}}_5$. (b) Raman intensity ratio of the peak at 138 ${\mathrm{cm}}^{-1}$ with respect to Si as a function of the number of layers. (c) Temperature-dependent Raman spectra of the 10 L sample in the range from 80 K to 300 K. (d) Raman spectra of the 1 L and 2 L samples measured at 80, 180, 230, and 280 K, respectively.

Figure 4

(a) Schematic diagram of the ARPR measurement setup. (b), (c) Angular dependence of Raman intensities on the 10 L sample in the parallel-polarized (b) and cross-polarized (c) configuration, respectively. (d), (e) Normalized Raman intensities (red circles) of the ${\mathrm{A}}_g^8$ mode versus the azimuth angle in the parallel-polarized (d) and cross-polarized (e) configurations, respectively. (f), (g) Normalized Raman intensities (red circles) of the ${\mathrm{B}}_{3g}$ mode versus the azimuth angle in the parallel-polarized (f) and cross-polarized (g) configurations, respectively. The fitted results are indicated by green dotted lines in (d)–(g). The excitation wave length is 532 nm.