Thermoelectric properties of layered ternary telluride ${\mathrm{Nb}}_3{\mathrm{SiTe}}_6$

We report the study of the thermoelectric properties of layered ternary telluride ${\mathrm{Nb}}_3{\mathrm{SiTe}}_6$. The temperature dependence of the thermoelectric power (TEP) evolves from nonlinear to linear when the thickness of the devices is reduced, consistent with the suppression of electron-phonon interaction caused by quantum confinement. The magnitude of TEP strongly depends on the hole density. It increases with decreasing hole density when the hole density is low, as observed in ionic-liquid-gated thin flakes. However, the device with the largest hole density possesses the highest TEP. Theoretical analysis suggests that the high TEP in the device with the largest hole density can be ascribed to the phonon-mediated intervalley scatterings. The highest TEP reaches $\sim 230\mu \mathrm{V}/\mathrm{K}$ at 370 K while the electrical resistivity of the device is maintained below $1.5\mathrm{m}\mathrm{\Omega }\mathrm{cm}$. Therefore, a large power factor PF $\sim 36\mu \mathrm{W}\mathrm{c}{\mathrm{m}}^{-1}{\mathrm{K}}^{-2}$ comparable to the record values reported in $p$-type materials is obtained.

Figure 1

(a) The crystal structure of ${\mathrm{Nb}}_3{\mathrm{SiTe}}_6$. (b) Single-crystal x-ray diffraction result along the ($0k0$) plane for bulk ${\mathrm{Nb}}_3{\mathrm{SiTe}}_6$.

Figure 2

(a) Temperature dependence of resistivity ${\rho }_{xx}$ in devices N1, N2, and N3. The triangle denotes the transition temperature of the resistivity upturn observed in device N3. (b) Temperature dependence of thermoelectric power (TEP) S in devices N1, N2, and N3.

Figure 3

(a) Temperature dependence of hole density extracted from the Hall resistivity measurements in devices N1, N2, and N3. (b) Temperature dependence of the inverse mobility variation in device N1. Solid line is the best fit $\mathrm{\Delta }\frac{1}{\mu \left(T\right)}\propto T^{1.44}$. Inset: Temperature dependence of the inverse mobility variation in devices N2 and N3. Solid lines are linear fit of $\mathrm{\Delta }\frac{1}{\mu \left(T\right)}$.

Figure 4

Gate voltage dependence of the conductivity and TEP in device N5 at 300 K.

Figure 5

(a) First-principles calculated band structure of ${\mathrm{Nb}}_3{\mathrm{SiTe}}_6$ with spin-orbit coupling. The green (bottom) and blue (top) dashed lines indicate the chemical potential in devices N1 and N2, respectively. (b) PF vs temperature in device N1. Data extracted from previous studies of ${\mathrm{CsBi}}_4{\mathrm{Te}}_6$ [24], ${\mathrm{Bi}}_{2-x}{\mathrm{Sb}}_x{\mathrm{Te}}_3$ [24], $\mathrm{ZrT}{\mathrm{e}}_5$ [25], SnSe [12], nano ${\mathrm{Bi}}_{0.4}{\mathrm{Sb}}_{1.6}{\mathrm{Te}}_3$ [13], and $\mathrm{Y}{\mathrm{b}}_{0.94}{\mathrm{Fe}}_4{\mathrm{Sb}}_{12.07}$ [26] are also presented. The dashed line is a guiding line indicating the linearly increasing trend of the PF with temperature in device N1.