Strong red-emission of Eu³⁺:Li₄Ti₅O₁₂ powders for phosphor applications

Abstract

The synthesis and photoluminescence properties of trivalent europium doped lithium titanate (Eu³⁺:Li₄Ti₅O₁₂) with different Eu³⁺ concentrations (0.1 mol%, 0.3 mol%, 1.0 mol%, 3.0 mol%) are reported and analyzed as a phosphor. Europium (III) nitrate (Eu(NO₃)₃) was employed as Eu³⁺ source, while lithium acetate dihydrate (CH₃COOLi·2H₂O) and titanium n-butoxide (Ti(OC₄H₉)₄) were adopted as raw materials to synthesize the host lithium titanate with a Li:Ti stoichiometry of 4.5:1. Phase identification was performed using X-ray diffraction (XRD), and morphology was examined using scanning electron microscopy (SEM). Eu³⁺:Li₄Ti₅O₁₂ powders showed strong red emission at 612 nm, corresponding to the ⁵D₀–⁷F₂ transition, with the strongest excitation peak observed in the blue light region at 464 nm. Decay time analyses revealed relatively short lifetimes accompanying typical exponential decay rates. The effect of Eu³⁺ concentration (0.1 mol%, 0.3 mol%, 1.0 mol%, 3.0 mol%) on photoluminescence intensity and decay time was explored, and is reported here. It was determined that the CIE color coordinates (0.66, 0.34) of the doped Li₄Ti₅O₁₂ powders were independent of Eu³⁺ concentration, and that the coordinates are very similar to the ideal red chromaticity (0.67, 0.33) designated by the National Television Standard Committee (NTSC) system.

Introduction

In the development of white light emitting diodes (LED) as novel solid state light sources, rare earth (RE)-doped phosphor systems have attracted great interest due to their wide range of applications. RE ions perform well as luminescence centers, due to the f–f and f–d transitions generated by the site symmetry and ligand field surrounding RE ions. Ce³⁺ -doped materials are commonly used as yellow-light phosphors in combination with GaN-based blue LED to generate white light emission. However, the combination generates a color deficiency in the red region and thus has a poor color rendering index after conversion. As a result, the addition of a red-emitting component is an effective way to improve the properties of white light emitting devices. Herein, the f–f transition of Eu³⁺ is explored as a good candidate for red light emission.

Several host materials have been investigated in the past for their potential as host matrices for LED applications, including inorganic crystalline and non-crystalline matrices [1], [2]. Borates [3] and phosphates [4] have been demonstrated to be efficient host matrices. Moreover, titanates are promising matrices for RE ion activated luminescence materials on the basis of their facile preparation and high luminescence efficiency [5], [6], [7]. Accordingly, RE doped titanates can be readily applied to optoelectronics applications [7]. Multiple titanates have been explored as potential host materials, including BaTiO₃ [8], SrTiO₃ [9], and CaTiO₃ [10]. Meanwhile, many reports investigate the effect of lithium addition on the enhancement of the luminescence behavior and luminescence efficiency of RE ions [10], [11], [12], [13]. Lithium tantalate [14], lithium niobate [15], and lithium aluminate [16] have been extensively studied as lithium-based matrices. However, limited research has been done to investigate lithium titanate in the context of a phosphor host. Kumar and Buddhudu [17] reported the photoluminescence (PL) properties of monoclinic phase Li₂TiO₃; however there are no reports of using cubic Li₄Ti₅O₁₂ as a host material.

Li₄Ti₅O₁₂ has the spinel structure with space group Fd$\stackrel{3}{̅}$m. The unit cell contains eight (Li)^8a[Li_1/3Ti_5/3]^16dO₄ with all the tetrahedral 8a sites occupied by lithium, the octahedral 16d sites randomly occupied by lithium (1/6) and titanium (5/6) atoms, and the 32e sites occupied by oxygen atoms [18]. Interestingly, lithium diffusion can occur in Li₄Ti₅O₁₂ at room temperature in three ways [19]: 1) 8a→16c→8a; 2) 8a→16c→48f→16d; 3) direct lithium hopping between 16c and 16d or 8a and 48f. Lithium diffusion has the potential to enhance the photoluminescence processes of luminescent ions. Hence, in this work, Li₄Ti₅O₁₂ was adopted as a host material, with Eu³⁺ chosen as a luminescence center. Eu³⁺:Li₄Ti₅O₁₂ phosphors were synthesized by a sol–gel method with various concentrations of Eu³⁺ in order to analyze the effect of Eu³⁺ concentration (0.1 mol%, 0.3 mol%, 1.0 mol%, 3.0 mol%) on PL behavior at room temperature. The synthesized materials were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results were analyzed in the context of the material׳s potential as a red-emitting phosphor for optical applications.