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.