Single-layer XBi 2 Se 4 (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 SnBi 2 Se 4 and PbBi 2 Se 4, 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/m 2 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, SnBi 2 Se 4 and PbBi 2 Se 4 have comparable electron and hole mobility of about ∼10 2  cm 2 /Vs and ∼10 3  cm 2 /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 (SnBi2Se 4 ) and 1.73 W/mK (PbBi 2 Se 4 ). Thus, they demonstrate high thermoelectric figures of merit, namely 0.31 (SnBi2Se 4 ) and 0.37 (PbBi 2 Se 4 ) 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.