Single-layer XBi2Se4 (X = Sn Pb) with multi-valley band structures and excellent thermoelectric performance

Using the first-principle simulations and the Boltzmann transport equation, our study investigated the properties of single-layer SnBi2Se4 and PbBi2Se4, including stability, elasticity, electronic and thermoelectric transport properties. We discovered that both 2D materials have acceptable cleavage energies ranging from 0.27 to 0.28 J/m2 and that they are indirect semiconductors with narrow band gaps of 0.68 eV and 0.94 eV, respectively. Interestingly, the valence band maximum exhibits ‘multi-valley’ energy dispersion. Furthermore, SnBi2Se4 and PbBi2Se4 have comparable electron and hole mobility of about ∼102 cm2/Vs and ∼103 cm2/Vs, respectively resulting in high conductivity and a high thermoelectric power factor. Owing to low group velocities and strong phonon–phonon scattering rates, the materials exhibit low lattice thermal conductivities of 2.59 W/mK (SnBi2Se4) and 1.73 W/mK (PbBi2Se4). Thus, they demonstrate high thermoelectric figures of merit, namely 0.31 (SnBi2Se4) and 0.37 (PbBi2Se4) at 300 K, which rise further to 1.22 and 1.82, respectively, at 700 K. Our results suggest that these two single-layer materials are promising candidates for use in nanoelectronics and thermoelectric appliances.