First-principles study of the effect of multi-Dirac phonons on the thermoelectric properties of the Janus BiSbC ₃ monolayer

Dirac phonons have attracted considerable attention due to their distinctive edge or surface states. However, their impact on the thermoelectric transport properties of semiconductors remains largely unexplored. Here, we systematically investigate the thermoelectric transport properties of the Janus BiSbC₃ monolayer and its Dirac phonon effects. The phonon dispersion exhibits distinct multi-Dirac phonon characteristics, with an electronic band gap of 0.88 eV (1.45 eV for HSE06). The Janus BiSbC₃ monolayer exhibits an ultralow lattice thermal conductivity (<i>κ</i>_l) of 4.79 W m^-1 K^-1 at 300 K, of which 26.09% is directly attributable to the multi-Dirac phonon channels. Simultaneously, it achieves excellent electron transport performance: the electron mobility (<i>μ</i>) is 11 848 cm² V^-1 s^-1 and the electron relaxation time (<i>τ</i>) is 326.84 × 10^-14 s, demonstrating great potential for achieving high thermoelectric performance. Under n-type doping, the thermoelectric figure of merit (<i>ZT</i>) reaches 3.52 at 700 K. Further analysis indicates that multi-Dirac phonons in the Janus BiSbC₃ monolayer make a notable contribution, accounting for 16.32% of the maximum <i>ZT</i> (<i>ZT</i>_max) at 300 K. These findings highlight the crucial role of multi-Dirac phonons in modulating the thermoelectric transport properties of the Janus BiSbC₃ monolayer. Our study clarifies how multi-Dirac phonons influence the thermoelectric transport properties of 2D materials, providing a theoretical basis and design strategy for developing high-efficiency thermoelectric materials through phonon engineering.