Effect of Yb~(3+) Concentration on Upconversion Luminescence of Rare-Earth Doped Nano-Oxide

Abstract: 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. Yb~(3+) ion is just a good sensitizer. Moreover, the two energy level structure of Yb~(3+) ion is splendid, and Yb~(3+) ions can mighty increase the efficiency of upconversion through sensitizing Er~(3+) and Tm~(3+) 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 Yb~(3+) concentration on upconversion luminescence is studied.In the system of Y_2O_3: Tm~(3+), Yb~(3+), the intensity of blue (~1G_4→~3H_6), red (~1G_4→~3H_4), and infrared (~3F_4→~3H_6) fluorescence increases at first and then decreases with the increasing of Yb~(3+) concentration. The effect of Yb~(3+) concentration on the UC luminescence in Y2O3: Tm~(3+), Yb~(3+) nanocrystals is investigated on the basis of spectra and power dependence. When the concentration of Yb~(3+) ions is high, the cooperative sensitization and the back energy transfer from Tm~(3+) to Yb~(3+) 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 Tm~(3+) ion changed with the concentration of Yb~(3+) 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 ~1G_4 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 Y_2O3:0.2 mol%Tm~(3+), 3mol%Yb~(3+) sample. The power dependence indicates that they are six- and five-photon processes respectively.In the system of ZrO_2: Er~(3+), Yb~(3+), with the increasing of Yb~(3+) concentration, the intensity of green fluorescence relative to red fluorescence decreases. When the concentration of Yb~(3+) 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 Yb~(3+) concentration, and the back energy transfer (EBT) process is dominating. The particles number populated on the green and red emissions changes with Yb~(3+) concentration, which causes that the green and red UC fluorescence is tuned by Yb~(3+) concentration…
Key words: upconversion; energy transfer; sensitization

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