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Motivated by the recent experimental synthesis of atomic-thick SnTe [Liu et al., Science 353(6296), 274 2016] exhibiting a layered orthorhombic phase similar to SnSe, we carried out systematic investigations on its electronic, thermoelectric, and phonon transport properties based on a combination of density functional theory and Boltzmann transport theory. Our results indicate that the monolayer is dynamically stable with a band gap of 1.05 eV. A considerable figure of merit (ZT) is predicted to be 2.9 for n-type doping and 2.2 for p-type doping along the armchair direction at a moderate carrier concentration of 1020 cm−3. The electronic band structure and the Fermi surface with multi-valleys lead to band convergence and anisotropic transport behavior. The synergistic optimization of Seebeck coefficient and electrical conductivity is achieved in anisotropic monolayer SnTe, due to the independence of carrier relaxation time and directional effective mass. A maximum power factor of 37 mW/(mK2) can be achieved for the n-type SnTe monolayer along the armchair direction, almost two times as high as that in the zigzag direction. However, the anisotropy of intrinsic lattice thermal conductivity is relatively low and strong phonon anharmonicity is found due to the coexistence of weak bonding and resonant bonding.

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A lead-free SnTe compound shows good electrical properties but also high thermal conductivity, resulting in a low figure of merit ZT. We demonstrate a significant enhancement of the thermoelectric properties of SnTe by (Ge, Mn) co-doping. (Ge, Mn) co-doped samples (SnGe)MnTe with x = 0, 0.03, 0.06, 0.09, 0.12, 0.15, 0.18 and 0.2 were prepared for this investigation. The substitution of Ge for Sn in SnTe promotes the solubility of Mn in a SnTe-based phase up to 20 at%, which further enlarges the band gap and gives rise to enhanced valence band convergence as compared with Mn doping, leading to a notably increased Seebeck coefficient and a power factor. All alloys retain p-type conduction and hole carrier concentration increases with increasing Mn content. The solute Ge and Mn atoms as well as the second phase of Ge in a SnTe-based system enhance phonon scattering and thus reduce thermal conductivity. The synergistic role that Ge and Mn play in regulating the electron and phonon transport of SnTe yields a maximum figure of merit ZT of 1.22 at 873 K for the sample (SnGe)MnTe.