Theoretical analysis of optical and thermoelectric characteristics of TinO2n−1†

This study investigates the optical and thermoelectric properties of metallic (cubic TiO, monoclinic TiO, and γ-Ti₃O₅) and semiconducting (Ti₂O₃, α-Ti₃O₅, β-Ti₃O₅, Ti₄O₇, and Ti₅O₉) phases of Ti_nO_2n−1, using various functionals including PBE, mBJ, PBE+U, and YS-PBE0. The YS-PBE0 and random phase approximation approaches accurately predict the electronic and optical bandgaps for semiconducting phases, which align well with the experimental data. For the case of semiconducting phases, two significant optical absorption peaks are identified: one in the infrared due to (t_2g–e_g) interband transitions and another in the ultraviolet due to O-2p to Ti-3d intraband transitions. The semiconducting phases demonstrate impressive Seebeck coefficients (800–1200 μV K⁻¹), driven by large effective mass and flat bands near the Fermi level. The electronic relaxation times are estimated to be 10⁻¹⁴ to 10⁻¹⁶ seconds for Ti_nO_2n−1 structures. Although the thermoelectric figure of merit of Ti_nO_2n−1 phases is low (<1), there is possibility for improvement through optimizing carrier concentration. These findings suggest that Ti_nO_2n−1 can be considered as potential materials for thermoelectric applications.