Janus $β$-PdXY (X/Y = S, Se, Te) Materials with high Anisotropic Thermoelectric Performance

Abstract: Two-dimensional (2D) materials have garnered considerable attention as an emerging thermoelectric (TE) material owing to their unique density of state (DOS) near the Fermi level. We investigate the TE performance of Janus $\beta$-PdXY (X/Y=S, Se, Te) monolayer materials as a function of carrier concentration and mid-temperature range (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 dynamical stability. The transport calculation results reveal the highly anisotropic TE performance for both n and p-type Janus $\beta$-PdXY monolayers. Meanwhile, the coexistence of low phonon group velocity and converged scattering rate leads to lower lattice thermal conductivity (K_l) of 0.80 W/m K, 0.94 W/m K, and 0.77 W/m K along y-direction for these Janus materials. While the high TE power factor is attributed to the high Seebeck coefficient (S) and electrical conductivity, which is due to the degenerate top valance bands of these Janus monolayers. The combination of lower K_l and high-power factor at 300K (800 K) leads to an optimal figure of merit (ZT) as 0.68 (2.21), 0.86 (4.09) and 0.68 (3.63) for p-type Janus PdSSe, PdSeTe and PdSTe monolayers. To capture rational electron transport properties, the effects of acoustic phonon scattering ($\tau$_ac), impurity scattering ($\tau$_imp), and polarized phonon scattering ($\tau$_polar) are included in the temperature-dependent electron relaxation time. These findings indicated that the Janus $\beta$-PdXY monolayers are promising candidates for TE conversion devices.