Synthesis And Luminescent Properties Of Nanosized Rare Earth Phosphate Luminescent Materials

With the technical development of FED and PDP, the requirement including crystal size distribution, stability, luminescence efficiency, brightness, and conductivity of the luminescent powder also has been improved. Meanwhile, with the development of nanotechnology, nanoscale luminescent materials are likely to be the next generation novel luminescent materials due to their numerous unique properties. Among many nano-materials, rare-earth phosphates are ideal to be the fluorescence host material for their good chemical and thermal stability. However, up to now, the investigation of the fluorescence properties of these materials is limited. In this paper, rare-earth phosphate fluorescence nano-materials were prepared through many different ways, and the fluorescence properties were also systematically investigated. These results are helpful for us to find a new type of high-powered fluorescence materials.1. In the first chapter, we briefly expound the research condition of fluorescence nano-materials; systematically introduce the spectrum theory of rare earth ions and the research background of rare earth ions doped fluorescence nano-materials; besides that, we generalize and comment the improvement of the synthesis and surface modification of fluorescence nano-materials, as well as the problem which confronts; moreover, the research direction also be prospected and summarized.2. In order to improve the properties for application of YPO₄:Re³⁺, We have synthesized a series of nano-scale YPO₄: Re³⁺ (Re = Eu, Dy)through the improved co-precipitation method with Li₂CO₃ as a flux. At the same time, we have investigated the best synthesis conditions: such as pH, the concentration of Re³⁺, the additional concentration of Li₂CO₃ and the anneal temperature. The optimum luminescent intensity could be achieved when the concentration of Eu³⁺ was 6%, the concentration of Li₂CO₃ is 10%, the annealed temperature is 950℃and the value of pH is 2. The nanoparticles we synthesized have a well crystalline morphology with nanometer dimension and the particle sizes of obtained nanocrystals were distributed in the range of 20-100 nm without Li₂CO₃ flux through XRD. Under the irradiation of UV, we studied the fluorescence properties of the powder, YPO₄: Eu,YPO₄: Dy nanocrystals imply orangish and white-emitting, which is corresponding to transitions of ⁵D₀ - ⁷F₁ and ⁴F_9/2→⁶H_15/2. Furthermore, a small shift of the excitation bands could be observed for the YPO₄: Eu nanoparticles, duing to the small particle size of the sample. When the flux Li₂CO₃ is used, the crystallization of samples is improved, and the particle size is increased. Furthermore, the energy loss of non-radiative is descreased, and the activate ions are easy to enter the lattice to form the luminescence centers, which result in the strong luminescent intensity.3. At the same time, we also prepared Eu³⁺ iron doped LaPO₄ nanocrystalline by co-precipitation method at 950℃, 3 h. The obtained LaPO₄ have nano-crystalline, particle size of about 70 nm, and the improvement of the calcination temperature can be increased the degree of crystallization of the samples. X-ray powder diffraction confirmed the structure of LaPO₄ monoclinic, cell parameters a = 6.84, b = 7.08, c = 6.46,β= 103.85°, is P2₁ / n (No·14) space group. The PL properties of the Eu³⁺-doped LaPO₄ nanocrystals were discussed. Fluorescence spectra indicate that: the charge transfer excited states of LaPO₄ can effectively transfer energy to the Eu³⁺ ions. Four emission peaks, which are at 592, 612 nm emission peak near the two split both phenomena, description of the Eu³⁺ ion doped in the crystal LaPO₄ have not the same coordination environment, that Eu³⁺ ions exsite sites D_2d or C₃ symmetry. The optimum quantity of concentration Eu³⁺ is 5%. The concentration quenching is due to Eu³⁺ ions exchange interaction.4. Ortherwise, the luminescence of Eu³⁺ in different host was deduced. Eu³⁺ -doped (Y_xLa_1-x)_0.95Eu_0.05PO₄ and (La_xGd_1-x)₍0.95_Eu_0.05PO₄ phosphors were prepared by a facile co-precipitation method. It is found that (La, Gd) PO₄:Eu³⁺ phosphors have the same crystal structure as LaPO₄:Eu³⁺, which is monoclinic with a little different lattice parameters. In the case of (La, Y) PO₄:Eu³⁺ phosphors, however, the gradual change from monoclinic to tetragonal structure of host lattice was observed as the amount of Y ion increased. Finally, we investigate (Y_xLa_1-x)_0.95Eu_0.05PO₄ and (La_xGd_1-x) _0.95Eu_0.05PO₄ phosphor samples ⁵D₀→⁷F₁ under the source of excitation at different excitation and the emission intensity changes of X values, results showed that (Y_xLa_1-x)_0.95Eu_0.05PO₄, with the addition of Y³⁺, the first increase in emission intensity in the X = 0.5 at the maximum, and then decrease. Then (La_xGd_1-x) 0.95Eu0.05PO₄ first emission intensity of samples with increasing concentrations of Gd3 + and enhanced in the X = 0.5 at maximum, and then with the Gd³⁺ concentration further increased, emission intensity started to decline, that is, quenching concentration of X = 0.5, show that the Gd³⁺ to Eu³⁺ has a significant energy transfer. Consequently, it was found that Gd or Y ions in orange-emitting LaPO₄:Eu³⁺ phosphor affected the site symmetry around Eu³⁺ ion, which influenced the PL intensity and PL spectra of Eu³⁺-activated orthophosphate phosphors. The 5D0→7F1 emission intensity of (Y_xLa_1-x) _0.95Eu_0.05PO₄ phosphor samples is increased by 70% than LaPO₄: Eu intensity, while emission intensity of (LaxGd1-x) _0.95Eu_0.05PO₄ phosphor samples is increased 30% than LaPO₄: Eu.5. In order to study the energy of Ce³⁺ to Tb³⁺ in YPO₄, co- precipitation was applied as the method to attain the nanosized luminescent grains YPO₄: Ce³⁺, YPO₄: Tb³⁺ and YPO₄: Ce / Tb, the doped Re³⁺ and the calcination temperature on the structure to determine the phase transition temperature was studied . The results show that with the increasing of heat treatment temperature, the crystallization of the product increased, but the particle size does not grow significantly, and the particles,non-aggregation and 20 -100 nm size. The luminescence properties of Ce³⁺-doped YPO₄ samples with ⁵d→²F_5/2 and ⁵d→²F_7/2 transitions have broad band, its excitation and emission spectra are broad band spectrum; Tb³⁺-doped YPO₄ samples under the UV excitation, the emissioon spectra of samples is is divided into two parts, represents the 4f–5d transition of Tb³⁺ and the 4f– 4f transition of Tb³⁺. The emissions from Tb doped YPO₄ mainly result from the transitions of ⁵D₃ and ⁵D₄ to ⁷F_J (where J = 1- 6). With an increase of Tb concentration, the emissions from ⁵D₃ to ⁷F_J level are quenched gradually by the cross relaxation process; For Ce³⁺ and Tb³⁺ co-doped YPO₄ sample, as monitoring the emission of Tb³⁺ at 542 nm, the strong allowed f– d transitions in the 250 nm 290 nm of Ce³⁺ and the weak forbidden f–f transitions of Tb³⁺ ions were observed, maximum excitation peak cemtered at 272 nm, demonstrated the phenomenon of blue shift of spectral peaks, which is more conducive to Ce³⁺→Tb³⁺ energy transfer. Under 272 nm excitation, the emission spectrum has four emission peaks for 490 nm, 542 nm, 589 nm, 626 nm, attributed to Tb³⁺ the ⁵D₄→⁷F_J (J = 6, 5, 4, 3) transitions launch, which launched the strongest transition for the ⁵D₄→⁷F₅, there is no characteristic of Ce³⁺ emission peaks appeared, show that the Ce³⁺ ion effectively sensitized luminescence of Tb³⁺. With the increase in concentration, the ⁵D₄→⁷F₅ increase in luminous intensity, Ce³⁺ doping concentration of 4 percent of the time in the emission intensity of the strongest, Ce³⁺ to increase the amount of doping, the luminescence intensity decreased. The energy transfer process, energy transfer efficiency, and the dynamic process of Ce³⁺ and Tb³⁺. The results showed that the effective, energy transfer efficiency of codoping YPO₄ material exists in the Ce³⁺ to Tb³⁺ energy transfer is 70%.6. Lanthanide orthophosphate 1D nanostructures with different crystalline phases and morphologies have been successfully synthesized using a hydrothermal method under mild conditions. It has been shown that the obtained LaPO₄ and GdPO₄ have a monoclinic and hexagonal structure, while YPO₄ exists in the tetragonal structure. When CTAB after accession, GdPO₄ the structure into a monoclinic structure, while YPO₄ the structure into a hexagonal structure。Irregularly LnPO₄ (Ln = La, Gd) nanorods with diameters of 50– 100 nm and lengths ranging from several hundreds of nanometres to several micrometres were obtained. The FE-SEM also indicates that the presence of CTAB can also promote oriented growth of the hexagonal and tetragonal structure. The possible growth mechanism of LnPO₄ (Ln = La, Gd, Y) with CTAB nanomaterial was explored. A study of the photoluminescence in Eu³⁺ and Tb³⁺-doped lanthanide phosphates has shown that the optical properties of these nanophosphors are strongly dependent on their crystal structures and morphologies. It was indicating the possibility of modiyfing their optical properties by structural design. These Re³⁺-doped rare-earth phosphate can also change the optical properties of other rare earth elements, and is expected to fluorescent light, UV excitation of the new plasma display devices, as well as other important biological fluorescent labeling technology has been applied.