Influence of TiO₂ Space layer on plasmon enhanced Luminescence of Ag/TiO₂/NaGdF₄:Er³⁺/Yb³⁺/Al³⁺ composite Films

Lanthanide-doped upconversion nanoparticles (UCNPs)have potential applications in optical and optoelectronic devices due to their novel optical properties. The surface plasmon is an effective method in improving the upconversion luminescence (UCL) properties, and it is essential to tunee the distance between the metal NPs and UCNPs for improving UCL. In this work, the Ag/TiO2/NaGdF4:Er3+/Yb3+/Al3+ composite films were fabricated, and the effect of the TiO2 layer thickness on spectral characteristics in the composite films was investigated. It is shown that the thickness of TiO2 film has a significant effect on the Ag NPs plasma resonance and fluorescence intensity. The maximum upconversion emission enhancement factors of the red and green emission were obtained for the sample with 15-nm-thickness TiO2 under the 980 nm excitation. That is mainly originated from the enhancement of the resonate energy transfer (Yb3+→Er3+) rate under the competition between non-radiative transition and local field effects.