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Characterizing Optical Signatures of TiO₂ Amorphous Precursors

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  • TiO₂ typically crystallizes into three polymorphs: brookite, rutile, and anatase. The phase selection problem in TiO₂ is the lack of understanding what conditions cause amorphous TiO₂ to crystallize into one of its three polymorphs during an annealing process. It has been recently shown that phase selection in TiO₂ is highly dependent on thin-film, amorphous precursor thickness and oxygen content. This is contrary to the previous thought that amorphous precursors are indistinguishable, as indicated by common identification techniques like X-ray diffraction and Raman spectroscopy. By way of simple transmission and reflection spectroscopy, this paper expands on this idea by searching for optical signatures present in amorphous TiO₂ that indicate what phase will be “selected” during an annealing process. Absorption spectra in range of 300-1000 nm distinguishes the anatase precursor from the other precursors, with the anatase precursor having an absorption coefficient of approximately 6.2 x 10³ cm⁻¹, while the brookite and rutile precursors show approximately 3.3 x 10⁴ cm⁻¹. Furthermore, the behavior of the transmission spectra in the long wavelength range can be linked to oxygen content, confirming the hypothesis that oxygen content plays a major role in phase selection. Tauc analysis performed on absorption spectra in range on 200-350 nm did not sufficiently differentiate the precursors from one another. However, Tauc analysis performed on crystalline samples showed consistent differences, indicating that transmission and reflection spectroscopy may be a viable method of polymorph identification. This paper further explores the phase selection problem by investigating optical properties that can be used to predict what polymorph amorphous TiO₂ will crystalize into during an annealing process.
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