He⁺ ion irradiation response of Fe-TiO₂ multilayers

The accumulation of radiation-induced defect clusters and He bubble formation in He⁺ ion irradiated nanocrystalline TiO₂ and Fe-TiO₂ multilayer thin films were investigated using transmission electron microscopy (TEM). Prior to ion irradiation it was found that the crystallinity of TiO₂ layers depends on the individual layer thickness: While all TiO₂ layers are amorphous at 5 nm individual layer thickness, at 100 nm they are crystalline with a rutile polymorph. After He⁺ irradiation up to ∼6 dpa at room temperature, amorphization of TiO₂ layers was not observed in both nanocrystalline TiO ₂ single layers and Fe-TiO₂ multilayers. The suppression of radiation-induced amorphization in TiO₂ is interpreted in terms of a high density of defect sinks in these nano-composites in the form of Fe-TiO₂ interphase boundaries and columnar grains within each layer with nano-scale intercolumnar porosity. In addition, a high concentration of He is believed to be trapped at these interfaces in the form of sub-nanometer-scale clusters retarding the formation of relatively larger He bubbles that can be resolved in TEM.