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/mK(2) 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/mK(2) 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/mK(2) and 2.27 W/mK, respectively.