Decoding unique electronic and phononic features in Janus systems beneficial for thermoelectricity

Abstract

Achieving high thermoelectric performance in two-dimensional (2D) transition metal dichalcogenides (TMDs) is crucial for energy utilization. In this study, we explored the electrical and thermal transport characteristics of ZrSe2 and its Janus derivatives, ZrSeS and ZrSeS0.5Te0.5, by combining first-principles calculations with the semi-classical Boltzmann transport theory. The chemical, dynamical, and mechanical stabilities of these monolayers are confirmed by cohesive energy calculation, phonon band structure analysis, and elastic constant determination, in turn indicating their experimental feasibility. The introduction of S and Te substitutions in ZrSe2 induces significant phonon softening, leading to a reduction in group velocity and enhanced anharmonicity. Contrary to conventional results, Janus ZrSeS exhibits lower lattice thermal conductivity than the ZrSeS0.5Te0.5 system due to increased scattering rate and high absolute Grüneisen parameter. ZrSeS0.5Te0.5 exhibits lower lattice thermal conductivity than pristine ZrSe2 monolayer owing to lower group velocity and strong phonon softening. Electronic transport analysis of Janus quasi-ternary system reveals a high-power factor in the p-type region, attributed to moderate Seebeck coefficient and the elevated electrical conductivity. Notably, ZrSeS0.5Te0.5 also demonstrates superior carrier mobility due to a conductive network formed by Zr–Se and Zr–Te bonds, facilitating efficient charge transport. The strong antibonding interaction between Zr and chalcogen atoms further enhances its thermoelectric efficiency. Our results demonstrate that ZrSeS0.5Te0.5 is a promising material for thermoelectric utilizations with a high thermoelectric figure of merit ZT ∼ 2.34 at a temperature of 1000 K for p-type, providing new insights into transport mechanisms in Janus quasi-ternary TMDs. The results suggest that phonon softening, conductive network formation, and optimal band alignment play key roles in enhancing thermoelectric efficiency. These findings pave an avenue for the fabrication of advanced thermoelectric materials utilizing Janus monolayers.