Preparation And Luminescent Properties Of Rare Earth Ions Doped Gadolinium-based Up-conversion Phosphors

Up-conversion luminescence materials are one kind of light-emitting materials with the feature that emitting short wavelength photons after excited with long wavelength photons. Most of them are rare earth doped materials. These up-conversion luminescence materials have been intensively studied due to their potential applications in many fields, such as three-dimensional displays, infrared radiation detectors, and laser anti-counterfeiting brands. This kind of materials has become a research focus at home and abroad in recent years.Er³⁺ ion has not only a rich energy-level structure but also can easily realize up-conversion efficiency when sensitized by other rare earth ions. In particular, this ion is a good activator with Yb³⁺ as a sensitizer in up-conversion luminescence materials when pumped by 980 nm LD. Yb³⁺ ion has a unique two-level structure, and Er³⁺ ion is in accordance with the energy matching of the gap (between 2F11/2 and 2F5/2) of Yb³⁺, so it can improve the up-conversion efficiency.Both Gd₂O₃ with cubic phase and garnet-structural Gd₃Al₅O₁₂ have favorable physical and chemical properties, so they are promising luminescent hosts in many optical applications. Thus this research chose these two Gadolinium-based oxides (Gd₂O₃, Gd₃Al₅O₁₂) as the research object, and the preparation and luminescence properties of the Er³⁺/Yb³⁺ co-doped Gd₂O₃ and Gd₃Al₅O₁₂ were systematically investigated by Fourier transform infrared spectroscopy (FTIR), Thermogravimetry-differential scanning calorimetry (TG-DSC), X-ray diffraction (XRD), Scanning electron microscope (SEM) and up-conversion emission measurement.Er³⁺ doped and Er³⁺/Yb³⁺ co-doped Gd₂O₃ powders have been prepared by the co-precipitation method. It's mainly discussed the effects of sintering temperatures on the size, structure, morphology and up-conversion luminescence properties of the powders. It was shown that the structures of the powders were all pure cubic phase. It's also found that the crystallinity increased as the heat-treat temperature rising. Moreover, both red and green up-conversion emissions were enhanced. It's proved that the up-conversion luminescence efficiency can be improved greatly by doping Yb³⁺ as the sensitizer. It was also explored the up-conversion emission mechanism of Er³⁺/Yb³⁺ co-doped Gd₂O₃ powders were attributed to two-photon absorption up-conversion process.Er³⁺/Yb³⁺ co-doped Gd₂O₃ powders had been also prepared by the modified co-precipitation method. This method was modified by adding quantities of ethanol into the solution, and the effects of sintering temperatures and solvent (ethanol) on phase-structure, size, morphology and up-conversion emission properties were systematically investigated. It's shown that the powders synthesized by this method were all pure cubic phase, and the powders had good diffusion with the size about 2μm and the morphology was regular zigzag when the heat-treatment temperature was 900oC. As the sintering temperature rising, the morphology of the particles seemed like loss slices. It can be seen from the up-conversion luminescence spectra that the main emission of the samples was red up-conversion, and the luminescent intensity of the powders prepared by the modified co-precipitation method was much stronger than that of the powders prepared by traditional co-precipitation method at the same sintering temperature. On the contrary, when ethanol was not added into the solvent, it can be found that the main emission of the samples was green up-conversion.Er³⁺/Yb³⁺ co-doped Gd₃Al₅O₁₂ powders had been also prepared by the co-precipitation method, and the effects of sintering temperatures and the concentration of precipitant on the phase structure and up-conversion emission properties were systematically investigated. It's found that the pure garnet structure GAG phase was obtained at lower sintering temperature and shorter heat-treatment time when the concentration ratio of NH4HCO3 to all the cations was 7. The photoluminescence spectra showed that the luminescent intensity of the powders was the strongest under the excitation of 980 nm LD.