Thermoelectric properties of two-dimensional topological insulators TlBi and TlSb

A comprehensive investigation into the thermoelectric (TE) properties of the two-dimensional TlBi and TlSb topological insulators has been conducted by means of first-principles calculations and nonequilibrium Green’s function methods. The TlBi (TlSb) monolayers of bandgap 0.413 eV (0.127 eV) show the high (moderate) TE figure of merit ZT≈2.6 (0.6) along the armchair and zigzag directions. In addition, the two ZT peaks along the two directions of the TlSb are almost identical, which are different from the case of the TlBi with the right ZT peak appearing at different chemical potentials. In the TlBi (TlSb) nanoribbons, the influences of the edge states and the bulk states on the TE properties have been fully revealed. The appearance of the edge states in the bulk bandgap enables the nanoribbons to behave as metals, which seriously suppresses the Seebeck coefficients and then the ZTs. As a consequence, the total ZTs become much smaller than the separate ZTs contributed by the edge and bulk states and smaller than the ZTs of the corresponding monolayers. The mechanism by which edge states influence TE properties provides a helpful reference for understanding the TE properties of analogous two-dimensional topological insulators.