The Effect Of Li~+ On Enhancing Upconversion Photoluminescence Of Rare Earth Ions And Its Mechanism

Rare-earth ions are suitable to be upconversion luminescent materials, for that they have stable chemical property, rich energy levels in the ultraviolet-visible- near infrared spectral region, and long lifetime of excited state. These years there have been many reports of rare-earth ion doped materials on the upconversion phenomenon, mechanism of fluorescence generation and potential application. The upconversion luminescence of rare-earth doped materials can be applied in the fields such as compact solid-state laser, information processing, optical storage, color display, temperature measurement and biological fluorescence labeling ect. In this paper, we study the preparation of Li⁺ and rare-earth ions codoped oxidation nanocrystals and its upconversion photoluminescence at room temperature. It is found that the upconversion light intensity of rare-earth ions has been increased greatly by adding Li⁺, further we research on the mechanism of upconversion intensity increased. The main contents and results are as follows:Er³⁺ doped ZnO, Li⁺/Er³⁺ codoped ZnO, and Li⁺/Er³⁺/Yb³⁺ codoped ZnO nanocrystals are prepared by sol-gel method. The upconversion luminescence of Er³⁺ is observed by 976 nm laser excitation, and its intensity has been greatly increased (at most 50 times) by adding Li⁺, so it is indicated that Li⁺ is a wonderful photoluminescence intensifier for Er³⁺. Then we also find the upconversion luminescence intensity of other rare-earth ions can be greatly increased by adding Li⁺. That means Li⁺ ion is a wonderful photoluminescence intensifier for rare-earth ions.Visible light emission of rare-earth ions generate from the transitions between intra-4f shell electrons. The distortion of the local symmetry around rare-earth ions increases the transition probabilities governing various intra-4f shell transitions of rare-earth ions. Extended X-ray absorption spectroscopy is utilized to study the local structure of Er³⁺ ions in ZnO codoped with Li⁺/Er³⁺ nanocrystals, and it is found that the bond length and coordination number of Er-O and Er-Er have been altered by adding Li⁺, which indicated the local crystal field around Er³⁺ ions had been changed. The absorption spectrum is measured, and we analyze the effect of Li⁺ on the absorption coefficient and radiation coefficient of Er³⁺ with J-O theory, which indicates that both the absorption coefficient and radiation coefficient of Er³⁺ have been increased by adding Li⁺.It is shown that the OH clusters reduced greatly by adding Li⁺ by fourier transform infrared spectrometry, while there is no change for phonon energy. OH clusters with a larger phonon energy can increase the multiphonon relaxation between two neighboring levels. This is one of the reasons for upconversion photoluminescence enhanced.Fluorescence intensity of Er³⁺ doped Y₂O₃ and Li⁺/Er³⁺ codoped Y₂O₃ nanocrystals have been studied as a function of temperature. Multi-phonon relaxation model and rate equation are used to fit the results of fluorescence intensity versus temperature, which shows that multi-phonon relaxation is the main procession between neighboring Er³⁺ energy levels, and the radiative transition can be neglected. In this research, it is found that the multi-phonon relaxation probability between neighboring energy levels can be reduced by adding Li⁺, so the radiative transition increased.White fluorescent powder of Er³⁺/Tm³⁺/Yb³⁺ codoped Y₂O₃ nanocrystals is prepared, which can emit bright white light emission under 976 nm laser excitation. The white light emission intensity increased by adding Li⁺; The optical temperature sensors based on the fluorescence intensity ratio of Er³⁺ have been study in Li⁺/Er³⁺ codoped Y₂O₃ nanocrystals, and this material is suitable for optical temperature sensor.