  The electronic, phonon, and thermoelectric properties of a two-dimensional
HfS2 monolayer are investigated by using the first-principles calculations
combined with the Boltzmann transport theory. The band valleys of the HfS2
monolayer can be effectively tuned by the applied biaxial strain. The Seebeck
coefficient and therefore the peak value of the power factor (with the
relaxation time inserted) increase when the degeneracy of the band valleys is
increased by the strain. When no strain is applied, the HfS2 monolayer is an
excellent n-type thermoelectric material, while the thermoelectric performance
of the p-type doped one is poor. The applied tensile strain of 6% can increase
the room-temperature ZT value of the p-type doped system to 3.67, which is five
times larger than that of the unstrained one. The much more balanced ZT values
of the p- and n-type doping are favorable for fabrication of both p- and n-legs
of thermoelectric modules. Our results indicate that the thermoelectric
performance of the HfS2 monolayer can be greatly improved by the valley
engineering through the method of strain.
