Penta-PdX2 (X = S, Se, Te) monolayers: promising anisotropic thermoelectric materials

Abstract

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.