Upconversion Emissions And Kinetic Processes Of Eu³⁺, Tm³⁺, Er³⁺ And Yb³⁺ Codoped Fluoride Nanocrystals

Upconversion refers to nonlinear optical processes characterized by the successive absorption of two or more pump photons via intermediate long-lived energy states followed by the emission of the output radiation at a shorter wavelength than the pump wavelength. Principles and applications of such upconversion phosphors were presented in several reviews up to the 1970s by Auzel, Garlick, and Wright. Since then, upconversion has become a pervading effect in all RE-doped materials under high-density infrared excitation. Due to their unique and excellent optical properties, rare-earth fluoride upconversion nano-materials have important applications in high-resolution displays, solid lasers, integrated optical systems, infrared imaging, biological analysis and medical diagnosis and have attracted much attention. Establishing well-controlled synthesis strategies to these materials, understanding their growth and the relationships of their structures, morphologies and optical behaviors are crucial to the production of nanophosphors with designated optical properties. In this thesis, we reported some distinct upconversion emissions of several kinds of fluoride nanocrystals doped with Eu³⁺, Tm³⁺, Er³⁺ and Yb³⁺ ions, and characterized their properties. (1) YF3:Yb/Tm/Eu nanobundles with a mean diameter of ² nm were synthesized by microemulsion method. Besides the UC emissions from Tm³⁺ ions, not only the unusual ⁵D₂→⁷F₃, ⁵D₃→⁷F_J emissions of Eu³⁺ were observed, but also the ultraviolet (UV) UC fluorescence from ⁵H_3-7, ⁵G_2-6, and ⁵L₆ levels were recorded under room temperature. To the best of our knowledge, it is the first time to report the ⁵H_3-7, ⁵G_2-6, ⁵L₆→⁷F₀ transitions, which expand the vast space for the research, development and application of Europium. Furthermore, we investigated the excitation power dependence of upconversion luminescence intensities. The upconversion emission peaked at 326.7 nm came from the transitions of ⁵H_J→⁷F₀. Its n value was 4.8±0.3, indicating that populating the 5HJ levels needed five 980 nm photons. Also, we found that populating the ⁵D₀ level was the result of three photons and four photons processes. These unique optical properties were attributed to the bridging function of Tm³⁺ ions in populating high-energy states of Eu³⁺ ions. The population of the Eu³⁺ ions occured through the energy transfer from Tm ³⁺ ions and resulted in the reduction of upconversion intensity of the Tm³⁺ ions. Through adjusting the doping proportion of Tm ³⁺ to Eu³⁺ ions, strong emissions of both Tm³⁺ and Eu³⁺ ions can be obtained. In addition, we investigated the temporal evolution of upconversion luminescence in the nanocrystals. Temporal evolutions of ⁵H_J, ⁵G_J, and ⁵L₆ levels of Eu³⁺ ions were reported for the first time. The analyses of lifetimes indicated that the lifetimes were increasing along the ladder of the energy levels ⁵H_J, ⁵L₆, ⁵D₃, ⁵D₂, ⁵D₁, and ⁵D₀ except for ⁵G_J. The ⁵G_J levels presented a short lifetime due to the narrow energy gap to ⁵L_J, which increased the nonradiative relaxation rate from ⁵G_J to ⁵L_J and destroyed the monotony along the energy ladder. This phenomenon indicates that in the trivalent europium ions, the higher the energy level locates, the more unstable the level is. These high-energy states prefer to de-excite to low-energy levels radiatively or nonradiatively.(2) NaYF₄: Yb/Er/Eu NCs with an average size about 6.7 nm were synthesized using oleic acid as capping ligand. Under 980 nm excitation, besides the UC emissions from Tm³⁺ ions, ⁵D₃→⁷F₃, ⁵D₂→⁷F₀,1,2 and more abundant ⁵H_3-7→⁷F_0-3 transitions were reported for the first time under room temperature. Furthermore, we study the integrated emission intensity ratios of ⁴G_11/2, ²H_11/2, ⁴F_9/2 levels of Er³⁺ ions between NaYF₄: 20%Yb³⁺, 1.5%Er³⁺, 1% (or 4%)Eu³⁺ and NaYF₄: 20%Yb³⁺, 1.5%Er³⁺ NCs under 980 nm excitation, and conclude the energy transfer law of ⁴G₍11/2_, ²H_11/2, ⁴F_9/2 levels of Er³⁺ ions. When the excited Er³⁺ ions serve as activators, it is easier to transfer their energy to acceptors after they are excited to the longer lifetime level.(3) Water-soluble PVP-stabilized NaYF₄: Yb/Er/Eu and NaYF₄: Yb/Tm/Eu NCs were synthesized by hydrothermal method. The NCs were coated with a very thin silica shell. And amino groups were introduced to the surface of silica shells by copolymerization of 3-aminopropyl (triethoxy) silane. The core/shell NCs can be dispersed in ethanol and water to form stable colloidal solution. This kind of distinct tri-doped NCs can emit dual-color characteristic luminescence under 980 nm and 394 nm excitation.(4) Three kinds of Tm³⁺ / Yb³⁺ codoped fluoride microphosphors with similar sizes were synthesized by hydrothermal method. The results indicate 20% Yb³⁺ doping is sufficient for hexagonal LaF₃ microparticles to crystallize completely in the orthorhombic phase. And crystal symmetry has obvious effect on the luminescence spectra of Tm³⁺ ions and lifetimes of the radiative levels of Tm³⁺ ions.(5) Hexagonal-phase NaYF₄: Yb/Er microcrystals with an average size about 200 nm were synthesized using oleic acid as capping ligand. The microcrystals are capable of being dispersed in nonpolar organic solvents to form fully transparent and stable colloidal solutions. Under 980 nm excitation, the Yb³⁺/Er³⁺ codoped microcrystals colloidal solution presents bright and almost-monochromatic green upconversion fluorescence. Temperature-change spectra indicate: thermalization effects between the ²H_11/2 and ⁴S_3/2 levels separated by 765 cm^-1 in NaYF₄: Yb/Er microcrystals were the reason of the 520 nm emissions. And the green emission intensity of Er³⁺ ions had a maximum at 160K under 980 nm excitation, which was attributed to the thermal activated distribution of electrons and the thermal quenching effect.