Fabrication and refractive index control of transparent and luminescent HfO₂:Ln³⁺ (Ln³⁺ = Eu³⁺, Tb³⁺) thin films for enhanced surface emissions

Dense and porous thin films of luminescent Ln³⁺-doped HfO₂ materials (Ln³⁺ = Eu³⁺ and Tb³⁺) were fabricated on silica glass substrates by a sol-gel dip-coating method using different kinds of coating solutions. The refractive index of the HfO₂:Ln³⁺ thin films was controlled by changing their porosity based on the Lorentz-Lorenz relationship. Pores could be introduced systematically into the thin films with the addition of different amounts of a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer to the coating solutions and the subsequent heat-treatment at 700°C. While photoluminescence (PL) of the dense HfO₂:Ln³⁺ thin films with higher refractive index was inefficient mainly due to the reflection problem, the porous HfO₂:Ln³⁺ thin films with lower refractive index exhibited much improved PL intensity by suppressing total internal reflection of the emitted light. The existence of the pores was also beneficial to an increase in the absorption of the excitation light due to its multiple scattering inside the thin films. Our results then demonstrate that the porous luminescent thin films are suitable for applications where the transparency and the strong surface emission are both necessary.