Janus β-PdXY (X/Y = S, Se, Te) materials with high anisotropic thermoelectric performance

Abstract

Two-dimensional (2D) materials have garnered considerable attention as emerging thermoelectric (TE) materials owing to their unique density of states (DOS) near the Fermi level. We investigate the TE performance of Janus β-PdXY (X/Y = S, Se, Te) monolayer materials as a function of carrier concentration and temperature in the mid-range from 300 to 800 K by combining density functional theory (DFT) and semi-classical Boltzmann transport theory. The phonon dispersion spectra and AIMD simulations confirm their thermal and dynamic stability. The transport calculation results reveal the highly anisotropic TE performance of both n and p-type Janus β-PdXY monolayers. Meanwhile, the coexistence of low phonon group velocity and a converged scattering rate leads to a lower lattice thermal conductivity (K_l) of 0.80 W mK⁻¹, 0.94 W mK⁻¹, and 0.77 W mK⁻¹ along the y-direction for these Janus materials, while the high TE power factor is attributed to the high Seebeck coefficient (S) and electrical conductivity, which are due to the degenerate top valence bands of these Janus monolayers. The combination of lower K_l and a high-power factor at 300 K (800 K) leads to an optimal figure of merit (ZT) of 0.68 (2.21), 0.86 (4.09) and 0.68 (3.63) for p-type Janus PdSSe, PdSeTe and PdSTe monolayers, respectively. To evaluate rational electron transport properties, the effects of acoustic phonon scattering (τ_ac), impurity scattering (τ_imp), and polarized phonon scattering (τ_polar) are included in the temperature-dependent electron relaxation time. These findings indicated that the Janus β-PdXY monolayers are promising candidates for TE conversion devices.