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.