Thermoelectric properties of five SnTe monolayer allotropes

Aiming at exploring materials of high thermoelectric (TE) performance, we systematically construct five SnTe monolayer allotropes including the $\alpha$-SnTe, $\beta$-SnTe, and the newly designed $\gamma$-, $\delta$-, and $\varepsilon$-SnTe. By using the first-principles calculations and nonequilibrium Green’s function method, their TE properties including the electrical conductance, the Seebeck coefficient, the power factor, the thermal conductance, and the figure of merit $ZT$ have been comparatively studied. For the $\alpha$-, $\gamma$-, $\delta$-, and $\varepsilon$-SnTe monolayers, two $ZT$ peaks are observed near zero chemical potential and four $ZT$ peaks will be observed for the $\beta$-SnTe monolayer. At room temperature, the maximum $ZT$s of the $\alpha$-, $\gamma$-, and $\delta$-SnTe monolayers are in the range from 1.5 to 2.0, and those of the $\beta$- and $\varepsilon$-SnTe monolayers are greater than 4.0. As the temperature is increased to 700 K, the maximum $ZT$s of all the five SnTe monolayers can be greater than 4.0 with the maximum $ZT$ of the $\beta$-SnTe ($\varepsilon$-SnTe) being 7.51 (8.39) in the X direction. This indicates that the $\beta$- and $\varepsilon$-SnTe monolayers can be used as the superior TE materials.