The optical transition in high-fraction polymorphs of titania (TiO2) were investigated to determine the band gap behavior of the most common polymorphs—brookite, rutile, and anatase—the values of which are varied in the literature. The direct optical band gaps of brookite, rutile, and anatase, were determined to be 3.37(7)eV, 3.41(11)eV and 3.59(2)eV respectively.
TiO2 was grown in-house via pulsed-laser deposition onto fused silica. The reflection and transmission spectra of high-phase fraction brookite, rutile and anatase, were measured with a grating spectrometer. Optical analysis software, SCOUT, was implemented with dielectric function models to perform a parameter fitting on transmission and reflection spectra, produced by the optical models, to the experimental data. From the parameter fitting, the optical absorption and index of refraction were extracted. The index of refraction was approximately 2.5 at 500 nm. A relationship in the high-phase brookite film showed a relationship between the thickness and band gap. Initially this appears to be a quantum well effect. However, the relationship is in face determined to be a consequence of our calculations favoring band gap fitting at low thicknesses–leading to an artificial trend.