Strong anisotropy of Sc 2 X 2 Se 2 (X = Cl, Br, I) monolayers contributes to high thermoelectric performance.

As a novel type of anisotropic two-dimensional material, extensive attention has been paid to the thermoelectric (TE) properties of FeOCl-type monolayers, such as Al 2 X 2 Se 2 (X = Cl, Br, I), Sc 2 I 2 S 2 , and Ir 2 Cl 2 O 2 . Recently, theoretical works based on first-principles calculations have been powerful driving forces in field of TE research. In this work, we perform an investigation into the TE properties of Sc 2 X 2 Se 2 (X = Cl, Br, I) monolayers based on density functional theory (DFT). A study on the stability, including AIMD simulation and phonon calculation, shows the stable structure of Sc 2 Cl 2 Se 2 , Sc 2 Br 2 Se 2 , and Sc 2 I 2 Se 2 monolayers. Additionally, the electronic and thermal transport properties of Sc 2 X 2 Se 2 monolayers are anisotropic along the x and y directions. Moreover, the combination of excellent Seebeck coefficient and ultralow lattice thermal conductivity contributes to outstanding ZT values, and the ZT values follow the order: Sc 2 I 2 Se 2 > Sc 2 Br 2 Se 2 > Sc 2 Cl 2 Se 2 . At 300 K, we obtained maximum ZT of 0.34, 0.77, and 1.97 for Sc 2 Cl 2 Se 2 , Sc 2 Br 2 Se 2 , and Sc 2 I 2 Se 2 , respectively, by n-type doping in the x direction. These results demonstrate that monolayer Sc 2 X 2 Se 2 (X = Cl, Br, I) materials are promising thermoelectric materials, Sc 2 I 2 Se 2 has more desirable properties along the x direction, and n-type doping can significantly enhance the ZT values. Our work lays a foundation for exploring the TE transport properties of FeOCl-type monolayers.