Thermoelectric properties of 1 T monolayer pristine and Janus Pd dichalcogenides

In this paper, we investigate the stability and thermoelectric properties of 1 T PdSSe, PdSTe and PdSeTe Janus structures using density functional theory (DFT). All three systems are narrow gap semiconductors with indirect bandgaps of 0.94 eV, 0.33 eV and 0.34 eV respectively. Compared to transition metal dichalcogenide (TMD) monolayers, PdS2 and PdSe2 are semiconductors with wider indirect bandgaps of 1.29 eV and 0.69 eV respectively. Phonon dispersion calculations demonstrate that all pristine and Janus structures are mechanically stable despite the presence of negligible negative frequencies around the Gamma point in PdSTe and PdSeTe. Inspection of the lattice thermal conductivity (kappa(L)) shows that these structures are slightly anisotropic in the x and y directions except for PdSe2 which shows a higher degree of anisotropy. Influenced by the values of kappa(L), the thermal electronic conductivity (kappa(e)), the electronic conductivity (sigma) and the Seebeck effect (S), the figure of merit along the x (ZT(xx)) and y (ZT(yy)) directions register the largest values in the case of electron doping for the PdSe2 and PdSeTe 2D crystals. Interestingly, the figures of merit of the Janus structures are larger than their corresponding pristine PdX2 (X = S, Se) structures. Once synthesized, such information is crucial for the implementation of the PdXY (Y = Se, Te) structures in industrial applications.