Structure and optical properties of (Gd,Eu)F₃-nanocrystallized sol-gel silica films

Transparent, luminescent sol-gel silica films were fabricated on the basis of a SiO₂-(Gd,Eu)F₃ nanocomposite structure, in which (Gd,Eu)F₃ nanocrystals were dispersed in a silica matrix. The films were coated on silica glass substrates at low temperatures of 300-500°C using trifluoroacetic acid as a fluorine source. Doping Eu³⁺ ions into the films led to narrow emission lines upon ultraviolet (UV) excitation, indicating that Eu³⁺ was selectively incorporated into the GdF ₃ or GdOF nanocrystals. While magnetic dipole ⁵D ₀ → ⁷F₁ transitions were dominant in the SiO₂-(Gd,Eu)F₃ films that were heat-treated at 300 or 400°C, forced electric dipole ⁵D₀ → ⁷F₂ transitions had a higher probability in the SiO ₂-(Gd,Eu)OF film that was heat-treated at 500°C. The efficiency of the UV light excitation at 274 and 256 nm was promoted by the energy transfer from Gd³⁺ to Eu³⁺ and the charge transfer from O ^2- to Eu³⁺ in photoluminescence, respectively.