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 α-SnTe, β-SnTe, and the newly designed γ-, δ-, and ɛ-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 α-, γ-, δ-, and ɛ-SnTe monolayers, two ZT peaks are observed near zero chemical potential and four ZT peaks will be observed for the β-SnTe monolayer. At room temperature, the maximum ZTs of the α-, γ-, and δ-SnTe monolayers are in the range from 1.5 to 2.0, and those of the β- and ɛ-SnTe monolayers are greater than 4.0. As the temperature is increased to 700 K, the maximum ZTs of all the five SnTe monolayers can be greater than 4.0 with the maximum ZT of the β-SnTe (ɛ-SnTe) being 7.51 (8.39) in the X direction. This indicates that the β- and ɛ-SnTe monolayers can be used as the superior TE materials.