Quadruple-layer group-IV tellurides: low thermal conductivity and high performance two-dimensional thermoelectric materials.

Through first-principles calculations, we report the thermoelectric properties of two-dimensional (2D) hexagonal group-IV tellurides XTe (X = Ge, Sn and Pb), with quadruple layers (QL) in the Te-X-X-Te stacking sequence, as promising candidates for mid-temperature thermoelectric (TE) materials. The results show that 2D PbTe exhibits a high Seebeck coefficient (∼1996 µV K-1) and a high power factor (6.10 × 1011 W K-2 m-1 s-1) at 700 K. The lattice thermal conductivities of QL GeTe, SnTe and PbTe are calculated to be 2.29, 0.29 and 0.15 W m-1 K-1 at 700 K, respectively. Using our calculated transport parameters, large values of the thermoelectric figure of merit (ZT) of 0.67, 1.90, and 2.44 can be obtained at 700 K under n-type doping for 2D GeTe, SnTe, and PbTe, respectively. Among the three compounds, 2D PbTe exhibits low average values of sound velocity (0.42 km s-1), large Grüneisen parameters (∼2.03), and strong phonon scattering. Thus, 2D PbTe shows remarkable mid-temperature TE performance with a high ZT value under both p-type (2.39) and n-type (2.44) doping. The present results may motivate further experimental efforts to verify our predictions.