N-Anchoring in Rare Earth-Doped Amorphous TiO₂ as a Route to Broadband Down-Conversion Phosphor

Developing light-harvesting materials with broadband spectral response has constantly been at the forefront of photonics and materials science. The variation in dopants and hosts allows for extension of spectral response, but construction of broadband down-conversion phosphor covering the entire ultra-violet region remains a daunting challenge. Here, we describe a material system in which amorphization and N-doping notably extend the spectral response. We demonstrate that the fabricated nitrided amorphous TiO₂ activated with high concentration of Eu³⁺ (∼15 mol %) can be excited by X-ray and ultraviolet light (200-400 nm) and present intense visible luminescence. We use hybrid density functional theory to perform structure simulation and clarify that N-anchoring is mediated by coordinately Ti-N bonding between the N lone pairs and the Ti^IV center. Accordingly, the simultaneous structure disordering and nitriding in semiconductors as demonstrated here in TiO₂:Eu³⁺ could be extended to other host and dopant systems for applications ranging from spectral modification to X-ray detection.