Thermoelectric materials can be used to convert waste heat into electrical energy, which is considered to be a cleaner form of energy that reduces carbon dioxide and greenhouse gas emissions. In this work, we study the thermoelectric properties of penta-PdX₂ (X = S, Se, Te) using first-principles calculations together with Boltzmann transport theory. The dynamic and thermal stability of penta-PdX₂ is confirmed based on phonon dispersion and ab initio molecular dynamics (AIMD) simulations. The transport coefficients, such as thermal conductivity (k_l), power factor (PF) and thermoelectric figure of merit (ZT), exhibit strong anisotropy in both x and y directions. The lower thermal conductivity of penta-PdX₂ can be attributed to the lower group velocity and larger phase space. The thermal conductivity of penta-PdX₂ along the same direction (x or y direction) adheres to the trend of PdS₂ > PdSe₂ > PdTe₂. The anisotropy of PF and k_l of penta-PdX₂ monolayers inevitably leads to anisotropy of ZT. The largest ZT values of penta-PdX₂ (X = S, Se) for p-type are 0.85 and 1.18, respectively, while the maximum ZT value of penta-PdTe₂ reaches 2.42. The predicted ZT values of penta-PdX₂ are larger than those of the commercial TE material Bi₂Te₃ (about 0.8) and some other transition metal di-chalcogenides, indicating that penta-PdX₂ (X = S, Se, Te) monolayers are potential anisotropic thermoelectric materials.

In this paper, the thermoelectric (TE) properties of Janus MXY monolayers (M = Pd, Pt; X, Y = S, Se, Te) are systematically studied using first principles and the Boltzmann transport theory. The thermal conductivity (k), Seebeck coefficient (S), power factor (PF), and TE figure of merit (ZT) are calculated accurately for various carrier concentrations. The lattice thermal conductivities of these six materials sequentially decrease in the order PtSSe, PtSTe, PtSeTe, PdSSe, PdSTe, and PdSeTe. PdSeTe and PtSeTe monolayers have a high ZT close to one at 300 K. In addition, we predicted the TE properties at high temperatures and found that the maximum ZT (2.54) is achieved for a monolayer of PtSeTe at 900 K. The structural and electronic properties of these six Janus transition-metal dichalcogenide (TMD) monolayers were systematically studied from first principles. Our results show that all six materials are semiconductors with bandgaps between 0.77 eV and 2.26 eV at the Heyd-Scuseria-Ernzerhof (HSE06) level. The present work indicates that the Janus MXY TMD monolayers (M = Pd, Pt; X, Y = S, Se, Te) are potentially TE materials.