Ab initio calculation of seebeck effect of bulk and monolayer palladium dichalcogenides

Abstract

All forms of the heat engines waste a sizeable part of heat due to their low efficiency. Thermoelectric (TE) materials can make use of this waste heat to generate electricity. The figure of merit ZT of a material is crucial in determining the energy conversion efficiency. However, to-date there is no large-scale application of TE power generator (TEG) due to unavailability of environmental friendly and high ZT materials. In recent studies it was noted, reduction in the dimensionality of TE materials can decrease thermal conductivity and hence increase ZT . In this regard, 2-D monolayer (ML) materials are considered promising candidates for TEG. Palladium disulphide (PdS2) and palladium diselenide (PdSe2) are historically known as high Seebeck coefficient materials but there is still insufficient knowledge on their ML phase. In this study, density functional theory based full-potential linearised augmented plane wave method embedded in WIEN2k code is used to determine the structural and electronic properties of palladium dichalcogenide (PdX2; X=S, Se, Te). Different exchange correlation (xc) energy functionals are considered. From the data of band energies obtained from WIEN2k calculations, BoltzTraP code is used to calculate the TE properties. All calculated lattice constants on average are less than 5%of the experimental values. Optimised structures of PdX2 calculated with Perdew-Burke-Ernzerhof generalised gradient approximation give better values of band gap energy. Bulk phase PdS2 and PdSe2 have ZT of 0.99, while in ML phase the achieved value of ZT is 1.01. The largest improvement on ZT is on PdTe2 where the obtained ZT is 0.48 for bulk phase and ZT is 1.00 for ML phase. This study has successfully demonstrated the enhancement of the TE properties for PdX2 by reducing their dimensionality.