The thermoelectric properties of two-dimensional Janus BiSeX materials: A first-principles study

Using first-principles calculations combined with semi-classical Boltzmann transport theory, this study systematically investigates the crystal structure, stability, electronic properties, thermal transport, and thermoelectric performance of Janus BiSeX (X = F, Cl, Br, I) monolayers. The results reveal that these materials exhibit excellent mechanical, dynamical, and thermal stability. By performing bandgap analyses with the Perdew–Burke–Ernzerhof and HSE06 schemes, we further confirm that the studied materials possess a semiconducting nature, with HSE06 yielding bandgap values of 2.86, 2.45, 2.35, and 2.36 eV, respectively. At room temperature, the in-plane lattice thermal conductivity was obtained by solving the Boltzmann transport equation, in which machine-learned moment tensor potentials were incorporated with the ShengBTE package, yielding values of 10.23, 6.83, 6.61, and 5.11 W/m K, respectively, decreasing to 1.68 W/m K for BiSeI at 900 K. To clarify the differences in lattice thermal conductivity, we analyzed the phonon group velocity, lifetime, and Grüneisen parameter in detail. Electronic transport calculations reveal that p-type materials exhibit higher Seebeck coefficients and power factors due to their flatter valence bands. For p-type Janus BiSeI, the estimated thermoelectric ZT values are 0.53 at 700 K and 0.78 at 900 K. The p-type two-dimensional Janus BiSeX (X = F, Cl, Br, I) materials show promising thermoelectric performance in the medium-to-high temperature range, making them ideal candidates for thermoelectric applications.