Enhanced Thermoelectric Performance of a HfS2 Bilayer by Strain Engineering.

For two-dimensional transition metal dichalcogenides, the thermoelectric properties of the material are affected by layer thickness and lattice strain. In this paper, we investigate the thermoelectric properties of a HfS₂ bilayer under different biaxial tensile strains by first-principles calculations combined with Boltzmann equations. The presence of degenerate bands in the HfS₂ bilayer and the absence of its monolayer results in the better thermoelectric performance of the HfS₂ bilayer than its monolayer. Moreover, this strain increases the band degeneracy of the HfS₂ bilayer even more, and the degenerate bands and stepped 2D density of states lead to a high power factor. In addition, the lattice strain increases the phonon scattering rate and reduces the phonon lifetime of the HfS₂ bilayer, resulting in a decrease in the lattice thermal conductivity. Ultimately, we obtained a maximum ZT value of 1.76 for the unstrained HfS₂ bilayer at the optimal doping concentration. At this time, its power factor and thermal conductivity are 53.01 mW/mK² and 9.06 W/mK, respectively. When the strain reaches 3%, for the n-type doped HfS₂ bilayer, the power factor and thermal conductivity are 69.87 mW/mK² and 6.36 W/mK, respectively, and the maximum ZT value is 3.29. For the p-type doped HfS₂ bilayer, the maximum ZT value appears at 6% strain, which is 1.83, at which the power factor and thermal conductivity are 13.81 mW/mK² and 2.27 W/mK, respectively. [ABSTRACT FROM AUTHOR]