Preparation And Luminescence Properties Of LuBO₃ And Lu₂o₃ Nanomaterials Activated By Rare Earth Ions

The reduction of particle size in a crystalline system can result in remarkable modifications of some of their bulk properties, hence, rare earth doped nanophosphors have attracted much attention due to their potential applications in high resolution display, fluorescence label, biological imaging as well as diagnosis and therapy of diseases and so on. LuBO3 and Lu2O3 are a kind of advanced hosts because of their high density, high atomic number of Lu, as well as high physical and chemical stability. This thesis mainly focuses on the rare earth ions activated LuBO3 and Lu2O3 nanophosphors. We have synthesized the LuBO3:Eu3+ nano/micro-crystals, Lu2O3:Eu3+ nanorods, nanosheets, and nanoparticles, as well as the Yb3+, Er3+ co-doped Lu2O3 nanocrystals and Yb3+, Er3+, Tm3+ tri-doped Lu2O3 nanoparticles by a hydrothermal approach. The dependences of their luminescence properties on the sizes and structures have been systematically investigated and compared. The major achievement obtained is as following:(1) The calcite and vaterite type LuBO3:Eu3+ nano/micro-crystals are synthesized by the hydrothermal approach for the first time. The pH value of the precursor solution has a crucial effect on the formation of phase type and morphology of LuBO3:Eu3+ sample. Comparing with the bulk materials prepared by traditional solid-state reaction method, the calcite type LuBO3:Eu3+ HT sample shows a symmetrical and narrower CTB, due to the absence of O-Lu excitonic excitation; the vaterite type LuBO3:Eu3+ HT sample with nanostructures, such as flower-like, demonstrates a red shift of the CTB and appearance of a long excitation tail at the long wavelength side of the CTB, the red shift is induced by an increase of crystal lattice parameters, and the long excitation tail is responsible to the Eu3+ ions located at outside with lower symmetric sites. As the outside Eu3+ are selectively excited, the 5D0-7F2 transition enhanced, thus, improving the color chromaticity in red.(2) The cubic Lu2O3:Eu3+ nanorods, nanosheets, and nanoparticles, are synthesized by a hydrothermal approach. The unique luminescence properties of the nanocrystals are presented by the appearances of a long tail at the long wavelength side of CTB, the enhanced 624 nm emission line and a novel 7F0-5D0 broad line. The observed phenomena are more obvious with decreasing the sample size in the order of nanorods>nanosheets>nanoparticles. Based on the experiment results of the thermal diffuse processes of Eu3+ from the surface to the inside of Lu2O3 particles, we conclude that the unique luminescence properties of the nanocrystals originate from the surface Eu3+. By the spectra decomposition, we obtain the excitation spectra of the interior Eu3+ and the surface Eu3+ of nanocrystals. In comparison with the bulk sample, the CTB of the interior Eu3+ presents an obvious blue-shift due to the size confinement effects, while that of the surface Eu3+ shows a prominent red-shift due to the distorted surface environments. The inbeing of the shifts and the broadening of the CTB in Eu3+ doped nanocrystals are clarified clearly.(3) The Yb3+, Er3+ co-doped Lu2O3 nanocrystals with different size are synthesized by the hydrothermal approach. The dependences of the upconversion luminescence of Er3+ ion on the size of nanocrystals are discussed in detail. It is observed that with decreasing the size of the nanocrystals, the relative intensity of the upconverted red emission to the green one is increased, and a three-photon process involved in the green upconversion is synchronously enhanced. An analysis based on steady-state rate equations indicates that the results can be induced by a large 4I11/2→ 4I13/2 nonradiative relaxation rate with a small 4F9/2→4I9/2 nonradiative relaxation rate.(4) The Yb3+, Er3+ and Tm3+ tri-doped Lu2O3 nanoparticles are synthesized by the hydrothermal approach. Under the excitation of a 980 nm single wavelength diode laser, bright white upconversion luminescence is observed by naked eye. The dependences of upconversion spectra upon impurities concentration and pump power are investigated systemically, and the best conditions to generate the white upconversion luminescence in Lu2O3 nanoparticles are obtained. The color coordinate of Lu1.906Yb0.08Er0.008Tm0.006O3 nanoparticles at laser pump power of about 160 mW is (0.327,0.339), which is very close to the standard equal energy white light illumination (0.33,0.33), indicating the potential application as the standard white light source. Additionally, the energy transfers from Er3+ to Tm3+ are proposed.