2D Janus AlSXY (X, Y = Cl, Br, I; X ≠ Y) monolayers with significant piezoelectric and high-temperature thermoelectric properties

Based on first-principles calculations, the new tetragonal Janus AlSXY monolayers (X,Y = Cl, Br, I; X ≠ Y) are predicted, and the electronic structure, thermoelectric (TE) and piezoelectric properties are also explored. It is found that the system exhibits high electrical conductivity and excellent power factor due to the high carrier mobility in the<i>Y</i>-axis direction under n-doping. Notably, the high Grüneisen parameters and low the phonon velocity lead to ultralow lattice thermal conductivity (1.35 W mK^-1for AlSClBr, 1.07 W mK^-1for AlSClI, and 1.06 W mK^-1for AlSBrI). At 700 K, the optimal TE figure of merit reach 4.54 (AlSClBr), 8.27 (AlSClI), and 6.63 (AlSBrI) along<i>Y</i>-direction, surpassing previously reported 2D layered TE materials. Furthermore, the three monolayers show strong piezoelectric responses, where the piezoelectric strain coefficient<i>d</i>₃₁of AlSClI reaches 0.32 pm V^-1, indicating the potential applying value in high-performance TE and piezoelectric field.