Study On Synthesis And Upconversion Luminescence Of The Ion-codoping NaYF₄

As an important optoelectronic materials, rare earth doped NaYF4 shows a wide range of potential applications in biological marking, near-infrared detection, solid-state lasers and other fields. According to Judd-Ofelt theory, the upconversion performance of the rare earth ions is closely related their local crystal field environment. Moreover, the upconversion performance is also related with the crystal phase. For example, the upconversion luminescence of hexagonal NaYF4 is stronger than that of cubic NaYF4, because the former has lower phonon energy. Therefore, in this paper, we codoped NaYF4 with rare earth ions and non-rare-earth ions, to improve the upconversion performance of the ion codoped NaYF4, by modulating crystal field around the activator ions and improving the crystal phase transition.In this work NaYF4:Yb3+, Er3+ was codoped with different non-rare-earth ions by hydrothermal method, respectively. The samples were characterized by X-ray powder diffraction, field emission scanning electron microscopy, high resolution transition electron microscopy, fluorescence spectroscopy and other methods. On the basis of these methods, a systematic study was performed on the crystal phase transition and structural changes of NaYF4, which influenced the upconversion mechanism.The main research works are listed as follows:1) Hexagonal Sn co-doped NaYF4:Yb3+, Er3+ was synthesized by hydrothermal method, to study the influence of co-doped Sn on the decay time constants and the luminescence performance. The results indicated that Sn showed variable-valence in the samples and that we could adjust the concentration of Sn to modulate the local symmetry of the crystal field. We also found when the molar concentration of co-doped Sn ions was 3%, the Er3+ crystal field asymmetry reached the maximum, resulting that the upconversion luminescence intensity became the highest, while the decay time constant was reduced.2) In3+ co-doped NaYF4:Yb3+, Er3+ was synthesized by hydrothermal method, to study the influence of co-doped In3+ on the crystal phase transition and the luminescence performance. The results showed that with rising of the In3+ concentration, the lattice strain of cubic NaYF4 would be raised, improving its phase transition from cubic to hexagonal under a low temperature. The with rising of the In3+ concentration, the upconversion luminescence intensity of hexagonal NaYF4 became higher and reached the maximum when the concentration of co-doped In3+ was 3%.3) Zr4+/Hf4+ co-doped NaYF4:Yb3+, Er3+ was synthesized by the same method, respectively. We also studied the influence of co-doped Zr4+/Hf4+ on the luminescence performance.Moreover, we tried to synthesize mesoporous NaYF4 with hard template. Ordered mesoporous SiO2 was used as template to synthesized ordered mesoporous carbon. Then we tried to synthesize mesoporous NaYF4 with ordered mesoporous carbon as template.In this paper we explored the influence of codoped ions on the structure of NaYF4 and the relationships between the upconversion performance and the structural changes. This work may be helpful for design and synthesis of high-performance upconversion materials.