Composition and Antithermal Quenching of Noninteger Stoichiometric Eu²⁺-Doped Na-β-Alumina with Cyan Emission for Near-UV WLEDs

Abstract

Phosphors with high quantum efficiency and thermal stability play a key role in improving the performance of phosphor-converted white light-emitting diodes (pc-WLEDs). A near-UV-pumped LED shows a great advantage due to its reduction of the negative effect of blue light on human health. In this work, we propose a series of near-UV excitable cyan-emitting Eu²⁺-activated phosphors with a nominal composition of Na_2-2<i>x</i>Al₁₁O_17+<i>a</i>:<i>x</i>Eu²⁺ (<i>x</i> = 0.01-0.40), which crystallize in a sodium β-alumina phase with a composition close to Na_1.22Al₁₁O_17.11. An excess amount of the sodium carbonate raw material makes up the volatile Na during the high-temperature process. The noninteger stoichiometric composition promotes the rigidity of the crystal structure with a slight excess of Na insertion into layers between spinel blocks of the NaAl₁₁O₁₇ matrix. The nonequivalent substitution of Na⁺ by Eu²⁺ generates intrinsic defects acting as carrier traps. As a result, the phosphor with an optimal nominal composition Na_1.6Al₁₁O_17+<i>a</i>:0.20Eu²⁺, under the excitation at 365 nm, shows an asymmetric cyan emission band at 468 nm with internal and external quantum efficiencies of 81.3 and 56.9%, respectively. Remarkably, the phosphor exhibits antithermal quenching within 200 °C. A pc-WLED with a high color rendering index (87.2) suggests great potential of the phosphor in pc-WLEDs. Therefore, a combination of a rigid structure and deep trap level is an effective way in exploring new phosphors with high quantum efficiency and thermal stability.