Strain-induced enhancement of thermoelectric performance of TiS monolayer based on first-principles phonon and electron band structures.

Using first-principle calculations combined with Boltzmann transport theory, we investigate the biaxial strain effect on the electronic and phonon thermal transport properties of a 1 T (CdI-type) structural TiS monolayer, a recent experimental two-dimensional (2D) material. It is found that the electronic band structure can be effectively modulated and that the band gap experiences an indirect-direct-indirect transition with increasing tensile strain. The band convergence induced by the tensile strain increases the Seebeck coefficient and the power factor, while the lattice thermal conductivity is decreased under the tensile strain due to the decreasing group velocity and the increasing scattering chances between the acoustic phonon modes and the optical phonon modes, which together greatly increase the thermoelectric performance. The figure of merit can reach 0.95 (0.82) at an 8 percent tensile strain for the p-type (n-type) doping, which is much larger than that without strain. The present work suggests that the TiS monolayer is a good candidate for 2D thermoelectric materials, and that biaxial strain is a powerful tool with which to enhance thermoelectric performance.