Thermoelectric properties of M2BS2 (M = Ti, Zr, Hf) monolayers: An Ab initio study

The thermoelectric properties of two-dimensional M2BS2 (M = Ti, Zr, Hf) materials were investigated in this study by using first-principles calculations. The phonon dispersion indicate that the high-frequency branches are dominated by B vibrations, while the mid- and low-frequency branches are primarily influenced by M and S atoms. All three compounds are confirmed to be dynamically stable. The lattice thermal conductivity is primarily contributed by acoustic and low-frequency optical phonons, with its overall magnitude determined by relaxation times, group velocities, and Grüneisen parameters. The resulting thermal conductivities follow the sequence kl(Ti2BS2) > kl(Hf2BS2) > kl(Zr2BS2), reaching 2.35 W·m-1·K-1, 2.14 W·m-1·K-1, and 2.05 W·m-1·K-1 at room temperature, respectively. Monolayer Hf2BS2 maintains relatively high Seebeck coefficients and power factors under either doping polarity, achieving a peak thermoelectric figure of merit of 1.74 in the n-type configuration. These findings provide a strong theoretical foundation for designing novel, high-performance thermoelectric device materials.