Synthesis And Luminescence Properties Of A₃PO₇:Eu³⁺ (A=La,Gd) And MMoO₄:Eu³⁺,Sm³⁺ (M=Ca,Sr,Ba) Phosphors

In order to improve the screen resolution, rare-earth phosphors in nanoscale have been investigated in recently years due to their promising technological applications in flat panel displays. Among red phosphors in nanometer, Y2O3:Eu3+ and YBO3:Eu3+ have attracted much attention for the applications under ultraviolet excitation. Y2O3:Eu3+ in nanometer is an efficient red phosphor, but it is not as bright under vacuum ultraviolet excitation. YBO3:Eu3+ nanopowder presents strong absorption and high fluorescence efficiency under vacuum ultraviolet excitation. However, the low color purity hampers its applications in displays. Therefore, it is urgent to develop novel red-emitting phosphors with improved color purity for the application such as plasma display panels (PDP) and field emission displays (FED). Eu3+-doped oxyphosphate A3PO7 (A=La, Gd) have been studied。For its promising applications on solid state lighting, the white light emitting diodes (LEDs) have been the hottest topic recently. The near-ultraviolet (UV) based white LEDs is the most important component as its high color rendering index and stability. For near-UV chip excitation, yellow, green and blue emitting phosphors as candidates have been reported, but there are few red phosphors with environmental stability and nontoxicity. The main purpose of one part of this work is to obtained red phosphors for the near-ultraviolet based white LEDs. MMoO4:Eu3+, Sm3+ (M = Ca, Ba, Sr) have been synthesised and their luminescent properties, energy transfer and mechanism have been studied. The main results obtained are listed as follows:1. Eu3+-doped oxyphosphate (La3PO7) nanoparticles were synthesized by a straightforward combustion method. Under 254-nm excitation, La3PO7:Eu3+ nanoparticles emitted mainly red fluorescence, assigned to the 5D0→7F2 transition of Eu3+ ions and revealed the Eu3+ ions occupied asymmetric sites in the oxyphosphate host. After annealed at different temperatures from 400°C to 1200°C, we found that the higher the annealing temperature, the better the samples crystallized and the bigger the intensity ratio of I(5D0-7F2)/I(5D0-7F1). In comparison with LaPO4: Eu3+, an orange phosphor with stronger 5D0→7F1 emission, La3PO7:Eu3+ has prominent red luminescence under the same excitation and maybe becomes a promising phosphor for red PDPs and Hg-free fluorescent lamp in the future.2. Eu3+-doped oxyphosphate (La3PO7:Eu3+) nanoparticles were synthesized by a straightforward combustion method. Under 254-nm excitation, the La3PO7:Eu3+ nanoparticles emitted intense red fluorescence from the 5D0→7F2 transition of Eu3+ ions. When the ratio of glycine to nitrate was about 2.7:1, the optimal intensity of the 5D0→7F2 transition was obtained. The Commission Internationale de l'Eclairage chromaticity coordinates of La3PO7:Eu3+ nanoparticles are in the red area of CIE1931 XY chromaticity coordinate graph and close to that of Y2O3:Eu3+ than YBO3:Eu3+.3. Eu3+-doped Gd3PO7 nanospheres with an average diameter of ~300 nm and a narrow size distribution have been prepared by a facile combustion method and structurally characterized by X-ray diffraction and field emission scanning electron microscopy. The luminescent properties were systemically studied by the measurement of excitation/emission spectra, and emission spectra under different temperatures, as well as photostability. The strong red emission intensity peaking at 614 nm originates the 5D0→7F2 transition is observed under 254-nm irradiation, indicating that Eu3+ ions in Gd3PO7 mainly occupied non-centrosymmetry sites. The CIE1931 XY chromaticity coordinates of Gd3PO7:Eu3+ nanospheres are (x = 0.654, y = 0.345) in the red area, which is near the National Television Standard Committee standard chromaticity coordinates for red. Thus, Gd3PO7:Eu3+ nanospheres may be potential red-emitting phosphor for PDP and Xe-based mercury-free lamps.4. Eu3+ and Sm3+codoped micro/nano sized MMoO4 (M = Ca, Ba, Sr) phosphors with various shapes have been synthesized via a facile hydrothermal method with surfactant-free environment. The morphology of the materials was found to be manipulated by the pH value of the precursor solution. MMoO4:Eu3+, Sm3+ (M = Ca, Ba, Sr) nanoflakes, microflowers, cubes, and spheres were obtained and characterized by X-ray diffraction and field emission scanning electron microscopy. The luminescent properties of the molybdate phosphors were systematically studied. The introduction of Sm3+ into each of the red emitting phosphors MMoO4:Eu3+ (M = Ca, Ba, Sr) can generate a strong excitation line at 405 nm, originating from 6H5/2→6P5/2 transition of Sm3+, significantly extending the excitation region for matching the near-ultraviolet light-emitting diodes (~400 nm). Energy transfer from Sm3+ to Eu3+ was observed in the whole MMoO4 systems and investigated in detail in CaMoO4 as a function of Sm3+ concentrations. A back energy transfer was also evidenced by the shortening of the 5D0 lifetimes of Eu3+ with increasing Sm3+ concentrations.5. The occurrence of energy transfer from Sm3+ to Eu3+ in the red emitting phosphor CaMoO4:Eu3+, Sm3+ significantly extends the excitation region for better matching the near UV LED. Photoluminescence spectra indicate that the energy transfer pathway concerns the relaxation from 4K11/2 to 4G5/2 of Sm3+ and subsequent transfer to 5D0 of Eu3+ rather than Eu3+ 5D1 after the 4K11/2 state of Sm3+ is excited by 405 nm. The transfer pathway is valid for both nano and bulk samples. The fluorescent decay patterns of Sm3+ 4G5/2 level as a function of Eu3+ concentration are studied based on the Inokuti and Hirayama model for energy transfer in bulk samples at 77K, revealing an electronic dipole-dipole interaction for Sm3+→Eu3+ transfer. The fluorescent rise and decay patterns of Eu3+ 5D0 level at 77K is well described by the dynamical processes of Sm3+→Eu3+ energy transfer.