Strong anisotropy of Sc₂X₂Se₂ (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 Al₂X₂Se₂ (X = Cl, Br, I), Sc₂I₂S₂, and Ir₂Cl₂O₂. 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 Sc₂X₂Se₂ (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 Sc₂Cl₂Se₂, Sc₂Br₂Se₂, and Sc₂I₂Se₂ monolayers. Additionally, the electronic and thermal transport properties of Sc₂X₂Se₂ 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: Sc₂I₂Se₂ > Sc₂Br₂Se₂ > Sc₂Cl₂Se₂. At 300 K, we obtained maximum ZT of 0.34, 0.77, and 1.97 for Sc₂Cl₂Se₂, Sc₂Br₂Se₂, and Sc₂I₂Se₂, respectively, by n-type doping in the x direction. These results demonstrate that monolayer Sc₂X₂Se₂ (X = Cl, Br, I) materials are promising thermoelectric materials, Sc₂I₂Se₂ 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.