Improved upconversion efficiency and thermal stability of NaYF₄@SiO₂ photonic crystal film

Fluorescent photonic crystal films (PCFs), which consisted of monodisperse NaYF₄:10Yb, 0.5Tm@SiO₂ or NaYF₄:10Yb, 0.5Er, 0.2Tm@SiO₂ core-shell spheres (10%, 0.5% and 0.2% represent the reactant mole percentage), were successfully fabricated. The morphologies and structures of PCFs were measured, while the fluorescent properties of NaYF₄ in PCFs with different photonic band gap (PBG) positions were investigated. Besides, the energy transfer process in thermal coupled energy levels of NaYF₄:10Yb, 0.5Tm@SiO₂ under different temperatures was discussed. The results indicated that the short wave emission of NaYF₄ can be enhanced by the regulation of photon mode density (PMD) in photonic crystals. Particularly, when the infrared 800 nm emission peak was in the range of PBG position of the PCF sample, the energy transfer process of ³H₄-³H₆ was suppressed and the short wave emission was enhanced by 1.64 times. Due to the well thermal radiation ability of silica shell, the thermal coupled energy levels (³F₃ and ³H₄) of NaYF₄:10Yb, 0.5Tm@SiO₂ core-shell spheres exhibited more stable emission intensities than NaYF₄:10Yb, 0.5Tm nanoparticles, when the experimental temperatures varied from 30 to 300 °C.