Study On Controllable Synthesis, Luminescence And Paramagnetism Of Ln³⁺(Yb³⁺/Er³⁺/Tm³⁺, Ce³⁺/Eu³⁺) Doped NaGdF₄and GdF₃Micro-/Nano-crystals

In recent years, with the development of chemistry and materials science,nano-and micro-materials doped with rare earth elements have been paid more andmore attention in many fields because of their unique physical and chemicalproperties. Especially the corresponding fluorides have become the research focussince these compounds possess several advantages, such as low phonon energy, hightransmittance and stability. With doping trivalent lanthanide ions (Ln³⁺), the obtainedmaterials might exhibit prominent luminescent and magnetic features based on the4felectrons in lanthanide ions. All these properties are highly depend on the size andshape of as-synthesized micro-/nano-scale particles. Therefore, to elucidate theirshape-and size-dependent properties, most efforts have been made to the controllablesynthesis of rare earth fluoride micro-/nano-materials in recent years. In particular,gadolinium fluorides such as GdF3and NaGdF₄have potential in multimodal imagingagents in biomedical fields. Then, it is significant to spend more efforts on theresearch of rare-earth doped gadolinium fluoride micro-/nano-materials. Based on theabove obtained conclusion, this thesis presents a systematic research focused on twotypical gadolinium fluorides, and the main contents are summarized as follows:（1） Hexagonal phase NaGdF₄microrods have been successfully synthesizedvia a facile hydrothermal method with Ethylene Diamine Tetraacetic Acid （EDTA） asthe structure-directing agent. The additive amounts of NaF and NaNO3as well as thereaction time have great effect on the morphology evolution of the products. Apossible growth mechanism was proposed for the formation of β-NaGdF₄microrods.Under the excitation of980nm near-infrared laser, bright green upconversion （UC）emissions can be readily achieved from NaGdF₄:10%Yb³⁺,1%Er³⁺microrods. Inaddition, the NaGdF₄microrods exhibit excellent paramagnetic features with themagnetization value of5.02emu/g （18kOe） at room temperature. Both the UCluminescent and magnetic properties are closely related to the size of as-prepared products.（2） Hexagonal phase NaGdF₄:Yb³⁺/Er³⁺/Li⁺microrods have been successfullysynthesized via a facile hydrothermal method with Ethylene Diamine Tetraacetic Acid（EDTA） as the structure-directing agent. The upconversion emission intensity wassignificantly enhanced by introducing different concentrations of Li⁺ions. Theinfluence of site occupancy of Li⁺on the upconversion emission ofβ-NaGdF₄:Yb³⁺/Er³⁺microrods was investigated in detail. In contrast to lithium-freeβ-NaGdF₄:Yb³⁺/Er³⁺, the green and red UC emission intensities of the microrodscodoped with5mol%Li⁺ions were enhanced by about8and7times, respectively.The luminescence enhancement should be attributed to the distortion of the localasymmetry around Er³⁺ions. In addition, it was found in our research work thatβ-NaGdF₄:Yb³⁺/Er³⁺microrods exhibited paramagnetic features at room temperatureand the magnetization was slightly increased by introducing Li⁺ions.（3） GdF3nano/submicroparticles have been successfully synthesized via afacile hydrothermal method by using Ethylene Diamine Tetraacetic Acid （EDTA） assurfactant. The shapes and sizes of the GdF3samples can be regulated precisely byintroducing different fluoride sources. Especially the substitution of NaBF₄for NaFand NH4F makes the size of products decrease from submicrometer to tens ofnanometer. The Ce³⁺/Eu³⁺and Yb³⁺/Tm³⁺codoped GdF3nanoparticles exhibitexcellent down-/up-conversion luminescent properties, respectively. TheGdF3:20%Yb³⁺/1%Tm³⁺nanoparticles also present paramagnetic properties with themeasured magnetization value of about2.1emu/g at18kOe at room temperature inair.