Strain-induced electronic phase transition and strong enhancement of thermopower of TiS2

TL;DR

This study uses first principles calculations to show how biaxial tensile strain induces a phase transition in TiS2, significantly enhancing its thermopower and thermoelectric performance through electronic structure modifications.

Contribution

It reveals a strain-induced semimetal to semiconductor transition in TiS2 and proposes doping with Zr or Hf to achieve similar thermoelectric enhancements.

Findings

Strain causes a semimetal to semiconductor transition in TiS2.

Thermopower is enhanced threefold under tensile strain.

Doping with Zr or Hf can mimic strain effects to improve thermoelectric performance.

Abstract

Using first principles density functional theory calculations, we show a semimetal to semiconducting electronic phase transition for bulk TiS 2 by applying uniform biaxial tensile strain. This electronic phase transition is triggered by charge transfer from Ti to S, which eventually reduces the overlap between Ti-(d) and S-(p) orbitals. The electronic transport calculations show a large anisotropy in electrical conductivity and thermopower, which is due to the difference in the effective masses along the in-plane and out of plane directions. Strain induced opening of band gap together with changes in dispersion of bands lead to three-fold enhancement in thermopower for both p- and n-type TiS2 . We further demonstrate that the uniform tensile strain, which enhances the thermoelectric performance, can be achieved by doping TiS2 with larger iso-electronic elements such as Zr or Hf at Ti sites.