[The Different Phase, Morphology Controllable Synthesis and Luminescent Properties Investigation of NaYF⁴: Yb, Er]

Using sodium fluoride and rare earth nitrate as raw materials and sodium citrate as surfactant, micron grade NaYF4 upconversion luminescent materials were prepared by hydrothermal method. By X-ray diffraction(XRD), scanning electron microscope(SEM) and fluorescence spectrometer, the crystal phase, morphology and luminescent characteristics of the prepared samples were investigated. The results showed that the phase of the samples could generate a transition from cubic phase to hexagonal phase by adjusting the proportion (5, 6, 7, 8, 9, 11) of NaF/RE , and the X ray diffraction peaks for the cubic and hexagonal phase of samples exactly matched with those of the standard card of PDF# 77-2042 and PDF# 16-0334, respectively. The SEM photographs showed that the crystallinity of samples was high and the dispersibility was favourable, the morphology were translated from microrods to hexagonal microplates. The samples upconversion luminescent spectra showed the intensity enhancement of red and green light emission peaks with increasement of the ratio of NaF/RE3+. The green emission peaks of samples at 520 and 539 nm corresponded to the ²H¹¹/²-⁴-->I₁₅/₂ and ⁴S₃/₂-->⁴I₁₅/₂ level transition of Er³⁺ ion, and the red light emission peaks of samples at 653 nm corresponded to the ⁴F₉/₂-->⁴I₁₅/₂ levelt ransition of Er+ ion. The chromaticity coordinate diagram exhibited that the change of the luminescent color of samples could be achieved by adjusting the ratio of NaF/RE³⁺. With the increasing of NaF/RE³⁺ ratio, for the whole light-emitting colors of samples, the shift from yellow region to near red region could be realized. It can be concluded that through the relatively simple experimental procedure and lower cost materials, the change of phase and morphology, the moving of light-emitting color for sample NaYF4:Yb³⁺, Er³⁺ could be well controlled only by changing the single component (NaF) molar ratio in the raw materials. The effect of phase and morphology of fluorescent materials on their upconversion luminescence has great potential applications in photonic devices and bioanalysis research.