Effect Of Yb³⁺ Concentration On Upconversion Luminescence Of Rare-Earth Doped Nano-Oxide

Upconversion (UC) luminescence of nano-oxide doped with rare-earth (RE) ions has attracted much attention due to a wide range of applications, such as fluorescent labels for biomolecules, optical data storage, upconversion lasers, color display, etc. The properties of nanocrystals are strongly influenced by the efficiency of upconversion. Codoped with sensitizer in materials can increase the upconversion efficiency. Yb³⁺ ion is just a good sensitizer. Moreover, the two energy level structure of Yb³⁺ ion is splendid, and Yb³⁺ ions can mighty increase the efficiency of upconversion through sensitizing Er³⁺ and Tm³⁺ ions. Thus, in the thesis, the samples of RE ions doped in nano-oxide are synthesized by a complex precursor method and excitated by 980nm diode lasers. Furthermore, the effect of Yb³⁺ concentration on upconversion luminescence is studied.In the system of Y₂O₃: Tm³⁺, Yb³⁺, the intensity of blue (¹G₄→³H₆), red (¹G₄→³H₄), and infrared (³F₄→³H₆) fluorescence increases at first and then decreases with the increasing of Yb³⁺ concentration. The effect of Yb³⁺ concentration on the UC luminescence in Y2O3: Tm³⁺, Yb³⁺ nanocrystals is investigated on the basis of spectra and power dependence. When the concentration of Yb³⁺ ions is high, the cooperative sensitization and the back energy transfer from Tm³⁺ to Yb³⁺ are presented and discussed. The phenomenon observed in the experiment is well explained.It is first discussed that the intensity ratio of blue band (476/488nm) of Tm³⁺ ion changed with the concentration of Yb³⁺ and also changed with pump power. It is believed that the change of temperature of laser fleck induced the change of the intensity ratio. The upper separated level of ¹G₄ state is thermal populated owing to Boltzmann's distribution. So the intensity ratio of 476/488nm increases with the temperature increasing of the laser fleck.Furthermore, the UV spectra of 298nm, 364nm and 391nm are observed in Y₂O3:0.2 mol%Tm³⁺, 3mol%Yb³⁺ sample. The power dependence indicates that they are six- and five-photon processes respectively.In the system of ZrO₂: Er³⁺, Yb³⁺, with the increasing of Yb³⁺ concentration, the intensity of green fluorescence relative to red fluorescence decreases. When the concentration of Yb³⁺ is 10mol%, a single red UC spectrum is observed. Based on the analyses of spectra and power dependence, it is suggested that the process of energy transfer changed at high Yb³⁺ concentration, and the back energy transfer (EBT) process is dominating. The particles number populated on the green and red emissions changes with Yb³⁺ concentration, which causes that the green and red UC fluorescence is tuned by Yb³⁺ concentration.