Synthesis and luminescent properties of rare earths doped Gd<sub>2</sub>Te<sub>4</sub>O<sub>11</sub> tellurite phosphors

Abstract

A series of rare earth Dy<sup>3+</sup>, Tb<sup>3+</sup>, Eu<sup>3+</sup> singly doped Gd<sub>2</sub>Te<sub>4</sub>O<sub>11</sub> (GTO) tellurite phosphors with intrinsic polarity are prepared by hydrothermal method. The phase structures, morphologies and thermal stabilities of these phosphors are characterized. Their luminescence properties are tested in detail. The results show that all those phosphors are crystalized into single phase of digadolinium tellurite with short rod-like shape. The maximum size in the axial direction is microns. The phosphor has good thermal stability. For the GTO:Dy<sup>3+</sup>, the fluorescence emission under UV excitation is mainly located in the yellow-green region. The optimal doping concentration corresponding to the strongest excitation and emission is 2.5%, and the CIE color coordinates are (0.39, 0.43). The fluorescence decay curve shows that the lifetime of the GTO:Dy<sup>3+</sup> on <sup>4</sup>F<sub>9/2</sub> energy level decreases gradually with doping concentration of Dy<sup>3+</sup> increasing, which may be related to the cross relaxation (CR) between Dy<sup>3+</sup> ions. For the GTO:Eu<sup>3+</sup>, the fluorescence emission under UV excitation is mainly located in the red region and orange-red region. The emission intensity is enhanced with the doping concentration of Eu<sup>3+</sup> increasing. When the doping concentration is 10%, the CIE color coordinates are (0.62, 0.38), which are located in the orange-red region with high color purity. The fluorescence lifetime of Eu<sup>3+</sup> on <sup>5</sup>D<sub>0</sub> energy level is hardly affected by the change of Eu<sup>3+</sup> doping concentration. For the GTO:Tb<sup>3+</sup>, with the increase of the Tb<sup>3+</sup> concentration, the fluorescence emission under UV excitation changes from blue-violet region to yellow-green region, which can be ascribed to the influence of CR between Tb<sup>3+</sup> ions. The fluorescence decay behavior reveals that the Tb<sup>3+</sup> ions on <sup>5</sup>D<sub>4</sub> excited state may undergo energy transfer and reabsorption, which can deviate the fluorescence decay from the single exponential model. When the concentration of Tb<sup>3+</sup> is 0.5%, the sample exhibits white light emission with the CIE color coordinates of (0.33, 0.35) and color rendering index of 86. The measurements of temperature-dependent emission spectra show that the above-mentioned phosphors have good luminescent thermal stability. The internal quantum efficiencies (IQEs) of those three types of phosphors are measured, and the IQE of GTO:Eu<sup>3+</sup> is better than those of GTO:Dy<sup>3+</sup> and GTO:Tb<sup>3+</sup>. There is still much room for improvement in the luminescent performance of all these phosphors. These phosphors have potential to be used in UV-excited white LEDs.