Pure and Dy-doped ZnNb2O6 nanoparticles have been prepared by a sol−gel combustion method using citric acid as a fuel and complexing agent and nitrates as oxidants at a relatively low temperature as compared to solid-state reaction methods. X-ray diffraction patterns of the pure ZnNb2O6 samples show that the ZnNb2O6 nanoparticles exhibit an orthorhombic phase. Photoluminescence (PL) properties of all the undoped and Dy-doped samples are studied in detail. For the pure ZnNb2O6 samples, a strong blue emission band centered at 445 nm and two weak emission bands centered at 525 and 625 nm, respectively, can be observed. For Dy-doped ZnNb2O6 samples, the luminescence of Dy3+ is observed with the host blue emission remaining and the host emission intensity varying with the Dy-doping concentrations. This novel PL characteristic of the doped samples may be attributed to the energy transfer between Dy3+ and niobate groups (NbO6).
YNbO4 and YNbO4:RE (RE = Er3+, Sm3+, Tb3+, and Tm3+) nanophosphors have been prepared using a citrate-gel combustion procedure. The results of X-ray diffraction reveal that the YNbO4 samples crystallized in both monoclinic and tetragonal phases in the investigated temperature range. The excitation spectra of the undoped YNbO4 samples exhibit a red-shift as annealing temperature or time increases, and the emission spectra show a broadband around 400 nm, corresponding to the self-activated luminescence center of YNbO4. Upon UV excitation of the niobate host, characteristic emissions from f−f transitions of the RE ions were observed. The luminescence covers the blue (Tm3+), green (Er3+, Tb3+), and orange−red (Sm3+). The mechanism of the energy transfer between the niobate host and the RE ions was deduced.
A novel class of orange-red phosphors based on Eu(3+)-activated LaNbTiO(6) was successfully fabricated by a wet chemical method, called a modified combustion approach. XRD, TG-DTA, SEM, and EDS results show that the heat-treatment of the powders above 1000 degrees C is enough to obtain highly crystallized and phase-pure LaNbTiO(6) and Eu(3+)-doped samples, which is of prime importance in investigating the optical properties of the novel phosphors using LaNbTiO(6) as the host material. UV-vis diffuse reflectance spectroscopy reveals that the direct band gap of LaNbTiO(6) with large grains (above 200 nm) is calculated to be 3.27 eV, while the absorption edge of the small particles shows an obvious blue-shift. Two blue emission bands centered at 440 and 470 nm ascribed to the self-trapped exciton emission of the distorted NbO(6) and TiO(6) groups for the pure LaNbTiO(6) can be obtained. Photoluminescence spectra of the Eu(3+)-doped phosphor particles illuminated the simultaneous occurrence of several intense orange-red band emissions due to the characteristic transitions of (5)D(0,1) --> (7)F(J) (J = 0, 1, 2, 3, 4) of Eu(3+) under 395 nm excitation. The mechanism of these multiplets possibly arising from the odd-parity distortions of the Eu(3+) ion environment and the effect of crystallanity of the compounds on luminescence were discussed, respectively. The highly bright and color-uniform fluorescence images of the doped samples with short luminescence decay times (nanosecond magnitude) confirmed the potential applications of the phosphors in luminescence and display devices.
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