Strain-induced semi-metal to semiconductor transition and strong enhancement in thermopower of TiS$_{2}$

Electronic properties of transition-metal dichalcogenides (TMDs) (MX$_{2}$, where M = Mo, W and X = S, Se, Te) are very sensitive to the applied pressure/strain, causing a semiconductor to metal transition. Using first principles density functional theory calculations, we demonstrate that bulk TiS$_{2}$ changes from semi-metal to semi-conducting electronic phase upon application of uniform biaxial strain. This phase transition is responsible for the charge transfer from Ti to S and reduces the overlap between Ti-($d$) and S-($p$) orbitals. The transport calculations show a three-fold enhancement in thermopower for both $p$- and $n$-type TiS$_{2}$ due to opening of band gap along with changes in dispersion of bands. The electrical conductivity and thermopower shows a large anisotropy due to the difference in the effective masses along the in-plane and out-of-plane directions. We further demonstrate that the enhancement of thermoelectric performance, can also be achieved by doping TiS$_{2}$ with larger iso-electronic elements such as Zr or Hf at the Ti sites.