Nanosized rutile (TiO₂) thin film upon ion irradiation and thermal annealing

Among three TiO₂ polymorphs, rutile is thermodynamically unstable as compared with anatase and brookite when the crystal size is ∼10 nm. In this study, nanocrystalline rutile with an average grain size of ∼6 nm was synthesized in a thin film geometry by ion beam-assisted deposition (IBAD) with an amorphous TiO₂ interlayer between the rutile film and Si-substrate. Nonstoichiometry produced by high-intensity ion bombardment during deposition may have stabilized the metastable rutile phase on the nanoscale. The phase stability of nanosized rutile was investigated by irradiation using 1 MeV Kr²⁺ combined with thermal annealing. In situ transmission electron microscopy (TEM) results indicate that partial amorphization occurred in nanocrystalline rutile when irradiated at room temperature, whereas the nanocrystals remained stable upon irradiation at 573 K. Ion beam-induced recrystallization occurred in the amorphous TiO₂ at a 573 K, significantly lower than the temperature for thermally induced recrystallization, which occurs at 673 K. This is an example of radiation-enhanced kinetics of a phase transformation. With a further increase in the irradiation temperature, to 1073 K, the tetragonal rutile transformed to triclinic Ti₅O₉.