Synthesis And Optical Properties Of Rare Earth Doped Nanofluorides

Rare-earth doped luminescent materials have wide applications, including phosphors, display minitor, X-ray imaging, scintillators, lasers, biology, and amplifies for fiber-optic communications. Recently, low-dimensional nanostructured materials have sparked a worldwide interest due to their unique electronic, optical, and mechanical properties and their potential applications in nanodevices and functional materials. It is well known that strcture, dimentsion, shape, and size have great effect on the properties of low-dmensional nanomaterials. Therefore, the main aim in my thesis'job is to synthesize some rare-earth ions doped nanostructures with speccail morphologies and to investigate their physical properties. In this paper, we synthesized different luminescence nanomaterials by different methods, and characterized their properties. The main contents in this thesis are as follows.1. YF₃:Yb³⁺/Tm³⁺ nanobundles and nanoellipses were synthesized by a microemulsion method. Results of X-ray diffraction and transmission electron microscopy reveal that each nanobundle consists of numerous nanowhiskers with a mean length of ⁷00 nm and a mean diameter of ² nm. The growth mechanism of YF₃ nanocrystals forming in water/cetyltrimethylammonium bromide (CTAB)/cyclohexane/1-pentanol was discussed in detail. In the proposed mechanism, primary particles are formed by constant collision, fusion, and fission of micelles. These primary particles can further self-organize to one dimensional nanostructure at room temperature or aggregate disorderly to form nanoellipses at high temperature. Under 980-nm excitation, blue (¹G₄→³H₆ and ¹D₂→³F₄) and ultraviolet (¹D₂→³H₆ and ¹I₆→³F₄/³H₆) upconversion fluorescence emitted from the YF₃:Yb³⁺/Tm³⁺ nanobundles. The relative intensity of the ultraviolet to the blue emissions increases with decreasing the size of nanowhiskers. Three mechanisms might cause the enhancement of UV emissions. (1) The excitation light is trapped inside nanowhiskers due to their small sizes and large surface/volume ratio, so the actual excitation power density inside increases with decreasing the size of nanowhiskers. (2) We attributed the UV enhancement to the decrease of Judd-Ofelt (J-O) parameter ?2, which reflects the asymmetry of the crystal field.2. YF₃:Yb³⁺(20%)/Tm³⁺(2%) octahedral nanocrystals were synthesized by a microemulsion method. After annealing in an argon atmosphere, the nanocrystals emitted weak blue and intense ultraviolet light under 980-nm excitation. Especially, unusual ³P₂→³H₆ (²65 nm) and ³P₂→³F₄ (³09 nm) emissions were observed for the first time. The emissions from ¹D₂ and ¹I₆ were much stronger than those from ¹G₄ and ³H₄. In our previous observations, the ³P₂ level usually relaxed to ¹I₆ level and emitted 291- and 347-nm UV UC fluorescence. Radiative transitions from the ³P₂ level are difficult to occur due to the nearby ³P_0,1 and ¹I₆ levels offer the routes for rapidly nonradiative relaxation. For a relative high doping concentration, the microcrystals therefore emitted intense 347- and 291-nm fluorescence under the 980-nm excitation. The upconversion mechanism was discussed in detail.3. YF₃:Er³⁺/Yb³⁺ nanocrystals were synthesized by a microwave-assisted microemulsion method. Pumped with a 980-nm diode laser, violet/ultraviolet upconversion fluorescence was presented in YF₃:Er³⁺/Yb³⁺ nanocrystals. 318-nm emission, coming from a four-photon excitation process, was observed for the first time. In comparison with a bulk sample having the same chemical compositions, the nanocrystals had a markedly enhanced ability of emitting violet/ultraviolet upconversion fluorescence. Presumably, three mechanisms might cause the enhancement of violet/UV UC in the nanocrystals. (1) The excitation light was trapped inside nanoparticles due to their small sizes and relative large surface/volume ratio, so the actual excitation power density inside was much higher than that in a bulk sample. (2) The actual concentrations of dopants in nanocrystals were different from that in bulk sample. (3) The surrounding of Er³⁺ ions in the nanocrystals was more benefit for violet/UV UC emission. By employing Tm³⁺ ions as structural probes in the samples, we found that the enhancement could be attributed to the decrease of Judd-Ofelt parameterΩ₂. A model for revealing the four-photon excitation process was proposed based on spectral analysis.4. Hexagonal NaYF₄: Yb³⁺/Tm³⁺ microcrystals were synthesized by a microemulsion method. Under 980-nm excitation, novel UC luminescent properties were presented in hexagonal NaYF₄:Tm³⁺(1.5%)/Yb³⁺(20%) microcrystals. The ³P₂→³H₆ (²64 nm) and ³P₂→³F₄ (³09 nm) emissions were observed. In comparison with the strong ¹D₂ and ¹I₆ emissions, the ¹G₄ and ³H₄ emissions were almost vanished. The dependence of the Tm³⁺ concentration on the UC luminescence indicates that the unusual phenomenon was caused by the efficient cross-relaxation of ¹G₄ + ³H₄→³F₄ + ¹D₂ (Tm³⁺) under high Tm³⁺ concentration. In addition, the number of laser photons absorbed in one UC excitation process, n, marvelously changed in different excitation power range, which was theoretically explained considering different UC excitation mechanism together with thermal effect under higher excitation power.5. Cubic and hexagonal NaYF₄:Ln³⁺ (Ln = Eu and Yb/Tm) microcrystals were separately synthesized by an EDTA-assisted solvothermal method. Under 393-nm excitation, the emission spectra of NaYF₄:Eu³⁺ microcrystals show the characteristic Eu³⁺ emissions. In the excitation spectra of the 610-nm emission, the ⁷F₀→⁵D₃ is dominant for the cubic microcrystals, while the ⁷F₀→⁵L₆ is dominant for the hexagonal sample. Presumably, two mechanisms might cause the spectral difference between hexagonal and cubic samples. (1) The 393- and 409-nm absorption transitions in the two samples may be different because the crystal structure is changed from cubic to hexagonal. (2) The nonradiative relaxation of ⁵L₆→⁵D₃ for the hexagonal is more efficient than that for the cubic. Since the nonradiative relaxation is closely related to the phonon energies of the hosts, the Raman spectra of the samples were studied. For the hexagonal microcrystals, two Raman bands centered at 230 and 520 cm^-1 are observed. However, for the cubic sample, only one broad Raman band centered at 260 cm^-1 can be observed. The energy separation between ⁵L₆ and ⁵D₃ is about 1000 cm^-1. Two phonons (900 - 1200 cm^-1) in the hexagonal sample exactly match the energy separation between ⁵L₆ and ⁵D₃, resulting in the effective nonradiative relaxation of ⁵L₆→⁵D₃. Consequently, the excitation band of ⁷F₀→⁵L₆ is dominant for the hexagonal, while the ⁷F₀→⁵D₃ is dominant for the cubic. In addition, unusually strong ultraviolet emissions (¹I₆→³H₆, ¹I₆→³F₄, and ¹D₂→³H₆) were observed in the hexagonal NaYF₄:Yb³⁺/Tm³⁺ microcrystals under 980-nm excitation. In comparison with a cubic sample having the same chemical compositions, the hexagonal microcrystals had a markedly enhanced ability of emitting ultraviolet upconversion luminescence.6. Cubic NaYF₄:Yb³⁺/Er³⁺ microspheres were synthesized by EDTA-assisted hydrothermal method. Under 980-nm excitation, blue (⁴G_11/2→⁴I_15/2), violet (²H_9/2→⁴I_15/2), green (⁴F_7/2→⁴I_15/2, ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2), and red (⁴F_9/2→⁴I_15/2) upconversion fluorescence were observed. The number of laser photons absorbed in one upconversion excitation process, n, was determined to be 3.89, 1.61, 2.55, and 1.09 for the blue, violet, green, and red emissions, respectively. Obviously, n = 3.89 indicates that a four-photon process has been involved in populating the ⁴G_11/2 state, and n = 2.55 indicates that a three-photon process has been involved in populating the ⁴F_7/2/²H_11/2/⁴S_3/2 levels. For the violet and red emissions, the population of the states ²H_9/2 and ⁴F_9/2 separately come from three-photon and two-photon processes. The decrease of n was well explained by the mechanism of competition between linear decay and upconversion processes for the depletion of the intermediate excited states.7. One-dimensional BaSiF₆:Yb³⁺/Tm³⁺ nanorods were synthesized by a facile microemulsion method for the first time. Results of X-ray diffraction reveal that the nanorods have a pure rhombohedral structure. Under 980-nm excitation, the ¹D₂→³H₆, ¹D₂→³F₄, ¹G₄→³H₆, ¹G₄→³F₄, ³F₂→³H₆, ³F₃→³H₆, and ³H₄→³H₆ emissions was observed, indicating that BaSiF6 is a new host material for producing desirable upconversion luminescence.8. Water soluble YVO₄:Ln³⁺ and YVO₄:Ln³⁺/Ba²⁺ (Ln = Ce, Dy, Eu, and Sm) nanocrystals were synthesized by a polyvinylpyrrolidone-assisted hydrothermal method. Under the excitation of the host absorption, phosphors can emit blue light for YVO4:Ce³⁺/Ba²⁺, yellow light for YVO4:Dy³⁺/Ba²⁺, red light for YVO4:Eu³⁺/Ba²⁺, and reddish orange light for YVO4:Sm³⁺/Ba²⁺. In comparison with those of YVO4:Ln³⁺ nanocrystals, the emissions of YVO4:Ln³⁺/Ba²⁺ are greatly enhanced. Furthermore, the excitation spectra of YVO4:Eu³⁺/Ba²⁺ and YVO4:Dy³⁺/Ba²⁺ show the similar features, which are different from those of YVO4:Sm³⁺/Ba²⁺ and YVO4:Ce³⁺/Ba²⁺.