Ultrahigh thermoelectric performance of Janus {\alpha}-STe2 and {\alpha}-SeTe2 monolayers

Combined with first-principles calculations and semiclassical Boltzmann transport theory, Janus {\alpha}-STe2 and {\alpha}-SeTe2 monolayers are investigated systematically. Janus {\alpha}-STe2 and {\alpha}-SeTe2 monolayers are indirect semiconductors with band gaps of 1.20 and 0.96 eV. It is found they possess ultrahigh figure of merit (ZT) values of 3.9 and 4.4 at 500 K, much higher than that of the pristine {\alpha}-Te monolayer (2.8). The higher ZT originates from Janus structures reduce lattice thermal conductivities remarkably compared with pristine {\alpha}-Te monolayer. The much higher phonon anharmonicity in Janus monolayers leads to significant lower lattice thermal conductivity. It is also found electronic thermal conductivity can play an important role in thermoelectric efficiency for the materials with quite low lattice thermal conductivity. This work suggests the potential applications of Janus {\alpha}-STe2 and {\alpha}-SeTe2 monolayers as thermoelectric materials and highlights Janus structure as an effective way to enhance thermoelectric performance.