Spin-Orbit-Coupling-Induced Topological Transition and Anomalously Strong Intervalley Scattering in Two-Dimensional Bismuth Allotropes with Enhanced Thermoelectric Performances.

The convergence of multivalley bands is originally believed to be beneficial for thermoelectric performance by enhancing the charge conductivity while preserving the Seebeck coefficients, based on the assumption that electron interband or intervalley scattering effects are totally negligible. In this work, we demonstrate that β-Bi with a buckled honeycomb structure experiences a topological transition from a normal insulator to a topological insulator induced by spin-orbit coupling, which subsequently increases the band degeneracy and is probably beneficial for enhancement of the thermoelectric power factor for holes. Therefore, strong intervalley scattering can be observed in both band-convergent β- and aw-Bi monolayers. Compared to β-Bi, aw-Bi with a puckered black-phosphorus-like structure possesses high carrier mobilities with 318 cm/(V s) for electrons and 568 cm/(V s) for holes at room temperature. We also unveil extraordinarily strong fourth phonon-phonon interactions in these bismuth monolayers, significantly reducing their lattice thermal conductivities at room temperature, which is generally anomalous in conventional semiconductors. Finally, a high thermoelectric figure of merit () can be achieved in both bismuth monolayers, especially for aw-Bi with an n-type value of 2.2 at room temperature. Our results suggest that strong fourth phonon-phonon interactions are crucial to a high thermoelectric performance in these materials, and two-dimensional bismuth is probably a promising thermoelectric material due to its enhanced band convergence induced by the topological transition.