Photoluminescence Properties Of (Er, Y)₂Ti₂O₇ Nanocrystalline Powders Prepared By A Citric Acid Chelated Sol-gel Method

The (Er,Y)₂Ti₂O₇ (EYTO) nanocrystalline powders with Er³⁺ doping concentrations of 0.1-5.0 mol% at the sintering temperature of 700-1000℃have been successfully prepared by citric acid chelated sol-gel method, using tetrabutyltitanate as precursor, citric acid as chelator, absolute ethanol solvent. The Y³⁺, Er³⁺ were introduced in the form of Y(NO₃)·5H₂O and Er(NO₃)₃·5H₂O respectively, the molar ratio of Y³⁺ and Er³⁺ to Ti⁴⁺ was 1:1, the molar ratio range of Er³⁺ to Ti⁴⁺ was (0.001-0.05): 1. The effect of Er³⁺ doping concentration and sintering temperature on the structure of EYTO nanocrystalline powders, and the photoluminescence (PL) properties in near-infrared and visible region have been systematically studied, and the underlying mechanisms discussed.The crystallization of EYTO phase was poor at the sintering temperature lower than 800℃, and greatly improved with increasing the sintering temperature up to 1000℃. The content of hydroxyl group in the EYTO nanocrystalline powders gradually decreased with increasing the sintering temperature. The near-infrared PL spectra in wavelength range 1.40-1.70μm corresponding to the ⁴I_13/2→⁴I_15/2 transition of Er³⁺, with the green and red upconversion luminescence spectra in visible wavelength range 400-750 nm corresponding to the (²H_11/2,⁴S_3/2)→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions respectively, were obtained for the EYTO nanocrystalline powders by using semiconductor laser diodes pumping at 980nm.The broadband near-infrared PL spectrum with the full width at half maximum of a single peak about 112 nm for the EYTO nanocrystalline powders sintered at 700℃, was replaced by the multi-peak PL spectra due to Stark splitting sintered at 800-1000℃. Both the PL intensity and fluorescence lifetime at the 1.53μm wavelength monotonically increased with increasing the sintering temperature, due to the gradual removal of hydroxyl group in powders and the improving crystallization of EYTO phase. Under the condition of 1000℃, both the PL intensity and fluorescence lifetime at 1.53μm wavelength, which presented increased trend attributed to the homodispersion provided by EYTO phase with Er³⁺ doping concentrations of 0.1-1.5 mol%, simultaneously reached the maximums with the maximal lifetime of 20.9 ms at Er³⁺ doping concentration of 1.5 mol%, and then presented reductive trend caused by the enhanced interaction between Er³⁺-Er³⁺ as Er³⁺ doping concentration exceeded 1.5 mol%.Both the red and green upconversion luminescence of EYTO nanocrystalline powders enhanced with the increasing sintering temperature, owing to the gradual removal of hydroxyl group and the improving crystallization of EYTO phase as well. With increasing Er³⁺ doping concentrations, the red emission monotonically enhanced, while the green emission first increased and then decreased, the ratio of I_red to I_green continuously increased. At Er³⁺ doping concentration range 0.1-1.5 mol%, Er³⁺ first arrived at ⁴F_7/2 energy level by excited state absorption, then rapidly relaxed to ²H_11/2 and ⁴S_3/2 energy levels, the population of which increased, the remarkable upconversion green emission was obtained when Er³⁺ directly went back to ground state by radiative transition. As Er³⁺ doping concentration exceeded 1.5 mol%, energy transfer acted as the main upconversion pathway due tO the shortened distance of Er³⁺-Er³⁺. Er³⁺ readily arrived at ⁴F_9/2 energy level by the cooperative upconversion between ⁴F_7/2 and ⁴I_11/2 energy levels, resulted in the increasing magnitude of Er³⁺ populated at ⁴F_9/2 energy level, but decreasing at ²H_11/2 and ⁴S_3/2 energy levels. Therefore the red emission continuously enhanced, with the green emission gradually decreasing.