High thermoelectric performances of monolayer GeTe allotropes

Aiming at finding wide-temperature-zone thermoelectric (TE) materials, five kinds of monolayer GeTe allotropes including the newly designed γ-, δ-, and ɛ-GeTe monolayers and the usual α- and β-GeTe ones are constructed. By using the density functional theory and the nonequilibrium Green’s function method, all their electronic properties and TE transport properties are comparatively investigated. It is found that the room-temperature figure of merit ZT of the γ-GeTe (ɛ-GeTe) along the armchair (zigzag) direction can amount to 4.5 (3.5), which is further increased to 7.15 (5.91) at 700 K. These ZT values are much higher than the other IV-VI compounds usually with ZT < 3, indicating that the armchair γ-GeTe and the zigzag ɛ-GeTe we designed here can be used as superior wide-temperature-zone and high-performance TE materials in the temperature range from 300 K to 700 K. Moreover, with the increase of temperature, the ZT peaks will become wider in width and move towards the position of zero chemical potential, which will make the GeTe-based TE devices work at low bias voltages more efficiently. This work should be an important reference on the way to the stage of Z T ⩾ 4 , which will motivate more explorations into the high-performance TE materials working in a wider temperature scope.