The Enhancement And The Spectroscopic Investigation Of Upconversion Emissions In Rear Earth Doped Oxide And Fluoride Nanocrystals

Rare earth doped upconversion fluorescent materials have important applications in the fields of bio-medical, upconversion laser, three-demensional display and so on. The near infrared laser excited upconversion materials can be used as bio-fluorescent labels, which have the benefits of no interferences of autofluorescence, no harmful to organisms, deep penetration in tissue, and so on. This thesis carried on the enhancement of upconversion luminescence, the expandment of the upconversion spectrum and the in oxide and fluoride materials. The prepared nanocrystals have been used in vivo imaging and cell imaging, which demonstrated the important applications in the bio-fluorescence labels.The enhancement of upconversion emissions in Y₂O₃ nanocrystals has been investigated. In order to enhance the intensity of upconversion emissions in oxide nanocrystals Y₂O₃:Yb³⁺/Er³⁺, the ion Li+ has been codoped in the synthesize process, this method not only can enhance the visible upconversion emissions of Er³⁺, but also can enhance the ultraviolet upconversion luminescence significantly. The intensity of upconversion emissions can be further enhanced by codoping Zn²⁺ ions on the basis of Li+ ions. Steady-state rate equations display that the enhanced upconversion luminescence is mainly due to the prolonged lifetimes of the intermediate states, which are affected by the tailored local environment of the rare earth, the reduced dangling bonds in the surface of crystals and the decreased non-radiative relaxation.The synthesis of fluoride nanocrystals and the enhancement of the upconversion emissions in these nanocrystals have been investigated. By using the hydro-thermal, oil-thermal and other methods, the NaYF₄ nanocrystals with different morphologies and sizes can be synthesized. The increase of the nano-particle size can decrease the specific surface area, then reduced the non-radiative relaxation and enhance the upconversion efficiency. Increasing the Yb³⁺ concentration in NaYF₄:Yb³⁺/Tm³⁺ nanocrystals, can increase the absorption of 970 nm excitation light, and ultimately enhance the near infrared, visible and ultraviolet upconversion luminescence of Tm³⁺ ions. The steady-state rate equations and the measured time resolved spectrum can explain this phenomenon.The upconversion emissions of Tb³⁺ and Eu³⁺ ions have been investigated. Througn the cooperative upconversion process of Yb³⁺ ions, the upconversion emissions of Tb³⁺ ions in NaYF₄ nanocrystals can be firstly investigated. The upconversion emissions arising from the ⁵D₄ and ⁵D₃ energy levels of Tb³⁺ ions have been observed. The red, green and blue upconversion emissions of Tb³⁺ ions arising from the same long-lived energy levels ⁵D₃, which is important for the application of tissue optical. Under the 532 nm laser excitation, the upconversion emissions of Eu³⁺ ions in Y₂O₃å’ŒNaYF₄ host material have been firstly observed. In combination with the downconversion spectrum and the excitation spectrum, the upconversion mechanisms of Eu³⁺ ions can be given.The nanocrystals proper for bio-fluorescence have been synthesized and the in vivo imagine experiment have been carried out. By codoping Eu³⁺ or Ce³⁺ ions in NaYF₄ nanocrystals, using the energy transfer process can adjust the green to red ratio of Ho³⁺ ions, and thus make the nanocrystals be suitable for biological fluorescence labeling. This method provides a way to design the spectra of upconversion emissions. The in vivo imaging experiment have shown that the upconversion nanocrystals have the advantage of no auto-fluorescence interference, no background, and many other virtues, comparing to the conventional dye fluorescent labels and quantum dots; the surface-functionalized nanoparticles have been bonded with HeLa cancer cells, and cell imaging proved that these upconversion nanoparticles can be applied to multicolor cell image.