Density functional theory prediction of thermoelectric properties of two-dimensional Janus HfXY (X≠Y, X/Y=Cl, Br, I) monolayers with high carrier mobilities

Two-dimensional (2D) materials have garnered significant attention due to their exceptional thermoelectric properties. In this study, 2D Janus HfXY (X≠Y, X/Y=Cl, Br, I) monolayer materials are comprehensively examined using ab initio methods to ascertain their potential as promising thermoelectric (TE) materials. Our predictions reveal that these Janus materials exhibit favorable dynamic, thermal and mechanical stabilities. These materials are predicted to exhibit semiconductor characteristics, showcasing indirect HSE06 band gaps ranging from 1.30 eV to 1.44 eV when accounting for the spin-coupling effect. Furthermore, the calculated lattice thermal conductivities are 22.38 W/mK for Janus HfBrCl, 11.86 W/mK for Janus HfBrI, and 7.79 W/mK for Janus HfClI monolayers. These figures notably reside below the thermal conductivities found in the extensively employed transition metal dichalcogenides (TMDCs). These Janus materials are predicted to have higher electron carrier mobilities ranging from 1220 cm2/sV to 4289 cm2/sV, surpassing the majority of other 2D materials. The enhanced electron carrier mobility result in good n-type power factor parameters, ranging from 58.20 mW/mK2 to 247.40 mW/mK2 between 300 K and 600 K. Such merits of lower thermal conductivities and higher power factor parameters endow these monolayers with the large figure of merit (zT) values for n-doping types along x (y) directions: 0.87 (0.88) for Janus HfBrCl, 1.62 (1.80) for Janus HfBrI, and 1.67 (2.15) for Janus HfClI at 300 K. Furthermore, these zT values can further increase along with external temperature increasing. These calculated thermoelectric figure of merit values surpass those of Hafnium-based TMDCs. Our study highlights the promising prospects of 2D Janus HfXY (X≠Y, X/Y=Cl, Br, I) monolayers as strong contenders for applications in thermoelectric conversion devices.