Controllable Synthesis And Upconversion Luminescence Properties Of Lanthanide-doped Fluoride Nano And Micro Crystals

In recent years, with the popularity of commercial infrared laser diode, the upconversion (UC) materials gradually become a hot research field. Due to their excellent optical properties, Rare Earth (RE)-doped UC materials have been applied extensively in many fields, such as solid-state lasers, optical communication, biolabels and photodynamic therapy. Under the same doping concentration of RE ions, the UC luminescence has close relationships with the crystal structure, sizes and morphologies of UC materials. Therefore, controllable synthesizing UC materials that plays a more crucial role. The main contents in this thesis are as follows:(1)Synthesizing Tm³⁺/Yb³⁺ co-dopedβ-NaYF₄ micro-prisms by hydrothermal method successfully, and measuring their UC spectrum under the excitation of 980 nm. Although the sample's size is large, with average length of 13μm and average diameter of 6μm, its ultraviolet (UV) UC efficiency is quite high in that the 291 nm and 346 nm emissions which come from five-photon process and four-photon process respectively are strong. This phenomenon shows the potential applications in short wavelength solid-state laser. By adjusting various reaction parameters such as reaction time, feeding amount and the molar ratio of NaF/Ln(NO3)3 ,et al, the influence of these parameters on crystal structures, sizes and morphologies of products has been studied. As the feeding amount increasing, the phenomenon of crystal structure transformation has been observed that YF3 would transform toα-NaYF₄ andα-NaYF₄ toβ-NaYF₄. The reaction time would make product's size larger and reduce the slenderness. As time increasing, the crystal structure would transform fromβ-NaYF₄ toα-NaYF₄ andα-NaYF₄ to YF3, which is the opposite phenomenon observed in changing feeding concentration. The increasing of NaF/Ln(NO3)3 molar ratio would not only help the crystal structure transforming toβ-NaYF₄, but also reduce product size and increase slenderness.(2) The role Na+ ions play has been studied in by introducing NaNO3 into system. The results indicate that Na+ ions could regulate the morphology of YF3 and also enter the crystal lattice to formα-NaYF₄. As the amount of Na+ ions increasing, the product crystal structure would transform toβ-NaYF₄ fromα-NaYF₄. And these added sodium salts could also control the diameters ofα-NaYF₄ in a wide concentration range that diameters would shrink as the salt amount increasing. Toβ-NaYF₄, once NaNO3 introducing, the slenderness would increase, and would keep almost unchanged. Introduce other cations like H⁺, Li⁺ and K⁺ ions to study the cation effect. Compared to Na⁺ ions, H⁺ and Li⁺ have advantages in competition, due to the smaller ion radius, and would be more capable of entering lattice; while the larger radius K+ ions would barely change the crystal structure fromα-NaYF₄ to KYF₄.(3) NaYF₄:Yb³⁺,Tm³⁺ nanocrystals have been successfully synthesized with the diameters of about 4 nm. Since the surface modification of oleic acid, the obtained NaYF₄:Yb³⁺,Tm³⁺ nanocrystals could well solute in the nonpolar solvent, forming a transparent solution. Under the excitation of 980 nm, record the UC spectrum of as-prepared solution and analyze the corresponding luminescent mechanisms. By the hydrolysis of TBOT, the NaYF₄: Yb³⁺, Tm³⁺/TiO₂ nano core-shell structure has been successfully synthesized, with the diameter of about 40 nm. After annealing, the TiO₂ outer layer would have crystal structure. Under the same testing conditions, compared the UC optical property of NaYF₄:Yb³⁺,Tm³⁺ nanocrystals and NaYF₄: Yb³⁺, Tm³⁺/TiO₂ nanostructure, the result indicates that the amorphous TiO₂ shell could enhance but the annealed TiO₂ shell could effectively reduce the UV UC emissions.