Strain tuned structural phase transition and optical switching in 1T-ZrS₂ and 1T-ZrSe₂

Strain is a versatile and powerful tool to manipulate electronic properties of 2D-materials. In addition to continuous tuning of the electronic band-structure, applied strain can drive a structural phase transition, generating new states with drastically different optical and electronic properties. While this has not been observed in the most actively investigated semiconducting transition metal dichalcogenides, 2H-MoS₂ and 2H-WSe₂, we found strain-induced structural phase transitions to take place in 1T-ZrS₂ and 1T-ZrSe₂, which possess optical and electronic properties similar to those of Mo- and W- compounds, but have different crystalline structure and orbital character. Our high-pressure Raman scattering, X-ray diffraction and optical absorption experiments revealed a reversible metallization of 1T-ZrSe₂ at 8 GPa and an irreversible transformation of 1T-ZrS₂ to a new semiconducting phase at 3 GPa, with smaller band gap by 1 eV (from 1.7 eV to 0.7 eV), a change that is optically evident. The study is complemented by structural investigation under pressure and ab initio band structure calculations.