Charge Transfer State Transition Research Articles

Effect of Eu3+ Ion Concentration on Phase Transition, Site Symmetry and Quantum Efficiency of ZrO² Nanocrystal Rods.

This work report the influence of Eu3+ ion concentration on the photophysical properties of zirconia nanocrystal rods including its intrinsic quantum efficiency (IQE). A simple chemical route was employed in the synthesis procedure. X-ray diffraction results show mixed phases of monoclinic and tetragonal structures. Phase transition occurred at low (1 mol%) and high (7 and 8 mol%) Eu3+ concentrations. There are three forms of excitations for this phosphor; band edge excitation at 216 nm, charge transfer state transition at 247 and 263 nm, and direct excitation at 362, 395 and 535 nm. Photoluminescence emission for all the doped samples at room temperature appeared to be entirely from intraconfigurational Eu3+ emissions and depends both on the site symmetry as well as the Eu3+ concentration. Low temperature measurements indicate the presence of defect bands associated with oxygen vacancies. Defects were also shown to be temperature dependent. The Eu3+ ions were distributed in both phases especially at high ion concentrations (7 and 8 mol% Eu3+). Two multipolar processes where found to be responsible for the luminescence quenching process in the mixed-structure; the dipole-dipole and the dipole-quadrupole transitions. The intensity parameters (Ω2, Ω4), asymmetry ratio, R0 and the average decay lifetime of the nanocrystals show dependence on concentration and excitation wavelength. High IQE values were obtained at 1, 7 and 8 mol% Eu3+ where the monoclinic phase is dominant. The CIE coordinates values are comparable to existing red phosphors and in combination with high average IQE of 55% makes this phosphor a good candidate for red emitting phosphor application.

Preparation and Characteristics of Ca2NaMg2V3O12:Sm3+ Single-Phased White-Emitting Phosphors

Novel single-phased phosphors Ca2NaMg2-V3O12:xSm3+ that exhibit efficient white-light photoluminescence properties have been successfully synthesized by the solid-state reaction method. The formation of a single Ca2NaMg2V3O12 phase has been confirmed by X-ray powder diffraction (XRD). The wavelength-tunable white light is realized by coupling of the broad emission band at 500 nm due to a V5+−O2− charge-transfer state transition and the sharp emission peaks at 564, 616, 650 and 708 nm assigned to 4G5/2→6HJ (J = 5/2, 7/2, 9/2, 11/2) transitions of the Sm3+ ion. The luminescence color can be tuned from green to white by adjusting the doping concentration of Sm3+. When the doping concentration of Sm3+is increased from x = 0 to x = 0.20, the color rendering index of the Ca2NaMg2V3O12:xSm3+ phosphors increased from 55 to 83. The present white phosphors show a great potential for white LEDs.

Synthesis and fluorescence properties of Y 2O 3:Eu by molten salt synthesis method

Red phosphors Y 2O 3:Eu were synthesized via molten salt synthesis method with the eutectic mixtures of KNO 3-NaNO 3 or KCl-NaCl as the fluxes. The as-prepared samples were investigated by means of X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and fluorescence spectrophotometer. The results showed that as-synthesized phosphors had good cubic crystallinity and the effect of annealing temperature on crystallinity was significant. Furthermore, different special morphologies of products were obtained in terms of different fluxes. In the fluorescence spectra, not only did the products present common excitation band at 254 nm, which belongs to Eu 3+-O 2− charge-transfer state transition, two strong bands at 394 and 466 nm attributed to 7F 0, 1− 5L 6, 7F 0, 1− 5D 2 transitions of Eu 3+ ions, respectively, also appeared, both of which were in good agreement with the requirements of phosphors for white light-emitting diodes. Under different excitation wavelengths, the relative intensity of emission at 611 nm pumped by 466 nm was much stronger than that of emission excited by 254 nm and the effect of fluxes contents on the fluorescence property was also discussed.