Optical absorption and small-polaron hopping in oxygen deficient and lithium-ion-intercalated amorphous titanium oxide films.

Optical absorption in oxygen-deficient and Li⁺-ion inserted titanium oxide films was studied in the framework of small-polaron hopping. Non-stoichiometric TiO_y films with 1.68 ≤ y ≤ 2.00 were deposited by reactive DC magnetron sputtering and were subjected to electrochemical intercalation of Li⁺-ions and charge-balancing electrons to obtain LixTiOy films with 0.12 ≤ x ≤ 0.34. Dispersion analysis was applied to calculate the complex dielectric function ε(ℏω) ≡ ε₁(ℏω) + iε₂ (ℏω) from numerical inversion of optical transmittance and reflectance spectra; a superposition of Tauc-Lorentz and Lorentz oscillator models was used for this purpose. Data on ε₂ (ℏω) were employed to calculate the optical conductivity and fit this property to a small-polaron model for disordered systems with strong electron-phonon interaction and involving transitions near the Fermi level. The introduction of oxygen vacancies and/or Li⁺ insertion yielded band gap widening by ∼0.20-0.35 eV, and both processes induced similar low-energy optical absorption. The small-polaron-based analysis indicated increases in the Fermi level by ∼0.15-0.3 eV for sub-stoichiometric and/or Li⁺-inserted films. This suggests the existence of polaronic Ti³⁺ states in the lower part of the conduction band arising from transfer of electrons from oxygen vacancies and/or inserted Li⁺ species. The present article is a sequel to an earlier paper on oxygen-deficient and/or Li⁺-inserted amorphous WO_y thin films and forms part of a comprehensive investigation of optical absorption in amorphous transition metal oxides with different valence states of the metallic ions. [ABSTRACT FROM AUTHOR]