| Rare earth (RE) doped upconversion materials capable of converting near-infrared (NIR) light to visible, are potential candidates for the upconversion laser, solar cell, anti-counterfeit labels, volumetric color imaging, white light display, temperature measurement and biological fluorescence labeling, ect. However, there are many problems limited their applications in photonics and biomedicine, such as the low upconversion efficiency, lack of multi-color emission, and so on. The purpose of this thesis is the enhancement of upconversion emission intensity. Firstly, mechanisms of Li+ions for upconversion emission enhancement were studied. Secondly, RE doped SrMoO4nanocrystals (NCs) by high excited state energy transfer were prepared, in order to decrease the nonradiative relaxation happened in the lower levels of the activators. Thirdly, RE ions high doped Gd2(MoO4)3NCs were prepared and the high excited state energy transfer was investigated. Finally, RE doped Gd2(MoO4)3NCs have been studied as a function of temperature. The main research contents and results are as follows:Effect of Li+ions on enhancement of NIR upconversion emission in Y2O3: Yb3+/Tm3+NCs were studied. Li+ions doped in Y2O3:Yb3+/Tm3+NCs greatly enhance the NIR upconversion emission intensity of Tm3+ions, up to14times, under976nm excitation. X-ray diffraction and Fourier transform infrared spectrometry were employed to study the enhancement mechanism. The cause of the enhancement is the modification of the local symmetry induced by the Li+ions, which increase the intra-4f transitions of Tm3+ions. Li+ions doped in Y2O3:Yb3+/Tm3+NCs also reduce the OH groups, lead to the reduction of the non-radiative relaxation. Moreover, the dependence of upconversion emission intensity on the pump power and the rate equation model were carried out.RE doped SrMoO4NCs by high excited state energy transfer were investigated. The upconversion emission properties of SrMoO4:Yb3+/Er3+NCs were studied in contrast to the corresponding dopants in Y2O3NCs under the same experimental conditions. And the relationship between the intensity ratio of green to red and the Yb3+concentrations in SrMoO4:Yb3+/Er3+NCs were studied too. The model on the high excited state energy transfer from the Yb3+(Er3+)-MoO42-dimer to the activator Er3+ion was proposed. The efficient energy transfer from the MoO42-ion to Er3+ion was confirmed under the266nm femtosecond laser excitation. It indicates that efficient upconversion and novel luminescence properties are attributed to the high excited state energy transfer from the sensitizer Yb3+-MoO42-dimer to the activator Er3+ion, which partly decreases the nonradiative relaxation happened in the lower levels of the activator. Then, the Yb3+/Tm3+, Yb3+/Ho3+and Yb3+/Tm3+/Ho3+doped SrMoO4NCs were prepared, which show the intense blue, yellow, and white upconversion emissions. The cross relaxation between the Tm3+ion and Ho3+ion was studied by the upconversion kinetic.The high excited state energy transfer was investigated in Er3+ions high doped Gd2(MoO4)3NCs. The luminescence intensity is the strongest, while the optimized Er3+ions concentration is20mol%. Based on the Dexter theory, the quenching mechanism of luminescence was investigated. Judd-Ofelt parameters of Gd2(MoO4)3:20Er3+NCs were calculated. Judd-Ofelt parameters of the sample were obtained by the measurement of the reflecting spectrum.Based on fluorescence intensity ratio technique, RE doped oxided NCs have been studied as a function of temperature. The thermally coupled2H11/2/4S3/2(Er3+) levels as a function of temperature were systematically analyzed in Gd2(MoO4)3: Yb3+/Er3+NCs. The different fluorescence peak intensity ratio, Yb3+concentration and the excitation power were investigated on the temperature sensitivity. The Stark sublevels of1G4level (Tm3+) can be regarded as the thermally coupled levels. Fluorescence intensity ratio variation of temperature sensitive blue upconversion emission from the Stark sublevels of1G4level was recorded. |
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