Preparation And Luminescent Properties Of Rare Earth Doped Lu₂O₃Nanomaterials

Rare earth doped nanometer luminescent materials have drawn much attention due to the potential applications in phosphors, biological fluorescence labels,3D display, solid lasers, transparent ceramics, temperature sensors and solar cells, and so on. Lutetium oxide (Lu2O3) is an excellent host matrix due to its stable physical and chemical properties, lower phonon energy, facilitating the substitution of rare earth. The rare earth doped Lu2O3nanometer luminescent materials were mainly investigated in this thesis. The research content of this thesis consists of three parts. In the first part, the spectroscopic properties of rare earth ions were firstly summarized and the research hotspots of rare earth luminescent materials were given, then several synthesis methods of rare earth luminescent materials were introduced. At last, the research status and its application of upconversion and downconversion luminescent materials were stated.In the second part, the rare earth doped Lu2O3upconversion luminescent materials were studied, which is discussed from Chapter2to Chapter4. In the third part, the rare earth doped Lu2O3downconversion luminescent materials were investigated, which is discussed in Chapter5.The major research contents and its results are listed below:1. Lutetium oxide nanocrystals codoped with Tm3+and Yb3+were synthesized by the co-precipitation method. Effect of the Tm3+, Yb3+molar concentration and precipitant solution pH on the structural and upconversion luminescent properties of the Lu2O3nanocrystals have been investigated. The experimental results show that all the prepared nanocrystals can be readily indexed to pure cubic phase of Lu2O3with good crystallinity. The strong blue (490nm), the weak red (653nm) and the near infrared (811nm) emissions from the prepared nanocrystals were observed under980nm excitation, and attributed to the1G4â†'3H6,1G4â†'3F4and3H4â†'3H6transitions of Tm3+ion, respectively. Power-dependent investigation reveals that the blue (490nm) and red (653nm) upconversion fluorescence is a three-photon upconversion process. The near infrared (811nm) upconversion fluorescence is a two-photon upconversion process. The experimental results show that concentration quenching occurs when the Tm3+concentration is larger than0.2%. The luminescent intensities of blue, red, and near infrared increase at first and then decrease with the increase of Yb3+concentration. When the Yb3+concentration is4%, the upconversion intensity is strongest. Thus, the optimal Tm3+and Yb3+doped concentration is0.2%and2%, respectively. It can be seen that the lifetimes of1G4and3H4of Tm3+decrease with increasing the Yb3+concentration, which is ascribed to the back energy transfer from Tm3+to Yb3+and introducing more defects into the host matrix.The upconversion emission intensity of Lu2O3:2%Yb3+,0.2%Tm3+nanocrystals obtained from the precipitant solution with pH=11is the strongest. The enhancement of the upconversion luminescence is suggested to be the consequence of reducing the number of OH-groups and the enlarged nanocrystals size. The photoluminescent intensity of Lu2O3:0.2%Tm3+nanocrystals is stronger than Lu2O3:2%Yb3+,0.2%Tm3+nanocrystals under266nm excitation. On one hand, it possibly exists the energy transfer from Tm3+to Yb3+. On the other hand, the larger particle size of Lu2O3:2%Yb3+,0.2%Tm3+nanocrystals can also contribute to the emission enhancement, since the larger nanocrystal size has lower surface-to-volume ratio and less defects.2. Lu2O3nanocrystals doped with2%Yb3+,0.5%Tm3+and various doping concentrations of Li+were synthesized by the sol-gel method. The upconversion luminescence enhancement effect of Li+ions doping was investigated. The experimental results show that Li+doping in Lu2O3:2%Yb3+,0.5%Tm3+nanocrystals can greatly enhance the upconversion emission intensity. When the Li+doped concentration is7%, the upconversion intensity is strongest. The mechanism of upconversion enhancement effect was proposed based on the experimental and theoretical analysis. Li+can not change the Lu2O3crystal structure, but it destroys the inverse symmetry of Lu3+in Lu2O3crystal. Meanwhile, it lowers the local crystal field symmetry around the Tm3+and Yb3+ions in favor of the transition of the RE ions. Moreover, it is worthwhile to point out that the grain size of Li+-doped Lu2O3:2%Yb3+,0.5%Tm3+nanocrystals is larger than Lu2O3:2%Yb3+,0.5%Tm3+nanocrystals. The increased nanocrystal size induced by Li+ions can also contribute to the upconversion emission enhancement, since the larger nanocrystal size have less defects.Effect of the temperature on the upconversion luminescent intensity was studied. It was found that the upconversion intensity decreases at low temperature.The phonon-assisted energy transfer from Yb3+to Tm3+takes place owing to the energy mismatch, whereas the phonon density decreases at the lower temperature.Therefore, the upcoversion intensity becomes weak.3. Lutetium oxide nanocrystals codoped with0.2%Tm3+and2%Yb3+have been successfully synthesized by the hydrothermal method. Effect of the precursor solution pH on the structure and morphology of precursors and naocrystals were investigated. The samples were systematically characterized by X-ray diffraction, Field emission-scanning microscopy, Thermogravimetric analyzer and Fourier transform infrared transmittance. The results show that the precursors did not yield Lu2O3directly, which can be assumed to be a new phase-Lu4O(OH)9(NO3). The as-formed precursors could transform to cubic Lu2O3with the same morphology and a slight shrinkage in size. All the obtained Lu2O3:2%Yb3+,0.2%Tm3+nanocrystals after calcinating the precursors can be readily indexed to pure cubic phase of Lu2O3with good crystallinity. The upconversion emission intensity of Lu2O3:2%Yb3+,0.2%Tm3+nanocrystals obtained from the precursor solution with pH=9is the strongest. The enhancement of the upconversion luminescence is suggested to be the consequence of reducing the number of OH-groups and the enlarged nanocrystals size.4. Tb3+and Yb3+codoped Lu2O3nanophosphors were synthesized by the co-precipitation method. The structure and photoluminescence (PL) spectra of nanophosphors were investigated. The XRD results show that all the obtained nanophosphors can be readily indexed to pure cubic phase of Lu2O3. The experimental results show that the strong visible emission around543nm from Tb3+(5D4â†'7F5) and near-infrared (NIR) emission around973nm from Yb3+(2F5/2â†'2F7/2) of Lu2O3:Tb3+,Yb3+nanophosphors were observed under ultraviolet light excitation, respectively. Excited Tb3+can cooperatively transfer energy to two neighboring Yb3+ions, which is followed by the emission of two photons. The Yb3+concentration dependence on luminescent properties and lifetimes of both the visible and NIR emissions have also been studied. Cooperative energy transfer (CET) from Tb3+to Yb3+is discussed as a possible mechanism for the near-infrared emission. When doped concentrations are1mol%Tb3+and2mol%Yb3+, the intensity of NIR emission is the strongest. Lu2O3:Tb3+,Yb3+nanophosphors that can cut one photon into two NIR photons of around973nm may be used to reduce thermalization loss of the solar cell.