Improved Thermoelectric Performance of Monolayer HfS₂ by Strain Engineering

Strain engineering can effectively improve the energy band degeneracy of two-dimensional transition metal dichalcogenides so that they exhibit good thermoelectric properties under strain. In this work, we have studied the phonon, electronic, thermal, and thermoelectric properties of 1T-phase monolayer HfS₂ with biaxial strain based on first-principles calculations combined with Boltzmann equations. At 0% strain, the results show that the lattice thermal conductivity of monolayer HfS₂ is 5.01 W m^-1 K^-1 and the electronic thermal conductivities of n-type and p-type doped monolayer HfS₂ are 2.94 and 0.39 W m^-1 K^-1, respectively, when the doping concentration is around 5 × 10¹² cm^-2. The power factors of the n-type and p-type doped monolayer HfS₂ are different, 29.4 and 1.6 mW mK^-2, respectively. Finally, the maximum ZT value of the n-type monolayer HfS₂ is 1.09, which is higher than 0.09 of the p-type monolayer HfS₂. Under biaxial strain, for n-type HfS₂, the lattice thermal conductivity, the electronic thermal conductivity, and the power factor are 1.55 W m^-1 K^-1, 1.44 W m^-1 K^-1, and 22.9 mW mK^-2 at 6% strain, respectively. Based on the above factor, the ZT value reaches its maximum of 2.29 at 6% strain. For p-type HfS₂, the lattice thermal conductivity and the electronic thermal conductivity are 1.12 and 1.53 W m^-1 K^-1 at 7% strain, respectively. Moreover, the power factor is greatly improved to 29.5 mW mK^-2. Finally, the maximum ZT value of the p-type monolayer HfS₂ is 3.35 at 7% strain. It is obvious that strain can greatly improve the thermoelectric performance of monolayer HfS₂, especially for p-type HfS₂. We hope that the research results can provide data references for future experimental exploration.