Luminescence enhancement of Al³⁺ co-doped Ca₃Sr₃(VO₄)₄:Eu³⁺ red-emitting phosphors for white LEDs

Abstract

A series of red-emitting Al³⁺ co-doped Ca₃Sr₃(VO₄)₄:Eu³⁺ and Ca₃Sr₃(VO₄)₄:Eu³⁺ phosphors with standard rhombohedral structures synthesized through the combustion method fueled by citric acid. By using X-ray diffraction, scanning electron microscopy, and photoluminescence spectroscopy, the crystal structure and photoluminescence properties of the prepared powder samples are investigated. The results indicate that the phosphors can be efficiently excited by blue light to achieve intense red luminescence (618 nm), which is associated with the hypersensitive ⁵D₀→⁷F₂ transition of Eu³⁺. Furthermore, significant enhancement in the photoluminescence intensity from Al³⁺ co-doped Ca₃Sr₃(VO₄)₄:Eu³⁺ phosphors is observed for an Al³⁺ concentration of 6 mol%. The influence of Al³⁺ on the crystal structure of Ca₃Sr₃(VO₄)₄:Eu³⁺ and the mechanism of the photoluminescence enhancement are discussed.

Introduction

White light-emitting diodes (WLEDs) are ideal eco-friendly light sources. They hold high potential for applications owing to their solidification properties, long lifetimes, nonpolluting nature, the absence of flash frequency, and high efficiency allowing energy saving [1], [2], [3]. The fluorescent transformation method has been applied in various implementations of white LEDs. Under excitation with a low direct current voltage, an InGaN chip can emit light in the spectral range from the near-ultraviolet (NUV) to the blue wavelengths, which could excite the three-primary-color phosphors and produce white light [4]. This method for white light emission has the advantages of low cost and easy production; however, it is impeded by the lack of red-light-generating components. Furthermore, this method has the drawbacks of low luminous efficiency, poor color rendering of the light source, and non-uniform luminance as a result of the high color temperature [5], [6]. Hence, new inorganic red-emitting phosphors are being actively searched for, with the goal to combine them with NUV or blue LEDs.

Alkaline earth metal vanadates possess excellent properties such as high thermal stability, good crystallinity, and visible-light transparency [7], [8], [9]. Therefore, they can serve as an excellent luminescent matrix. As reported in the literature, there is energy transfer from VO₄³⁻ to Eu³⁺ [10], [11]. There have been several studies on red-emitting phosphors using Ca₃Sr₃(VO₄)₄ as the host lattice material. Choi et al. [12] investigated the effects of charge compensators on the photoluminescence properties of the Ca₃Sr₃(VO₄)₄:Eu³⁺ red-emitting phosphor with the conventional high-temperature solid-state routine. Tang et al. [13] studied the photoluminescent properties of the Ca₃Sr₃(VO₄)₄:Eu³⁺ red-emitting phosphor; they found that the intensity of the Ca₃Sr₃(VO₄)₄:Eu³⁺ red emission could be considerably enhanced by co-doping with Sm³⁺ ions. Wang et al. [14] used a conventional solid-state reaction to synthesize the LaBSiO₅:Eu³⁺ red-emitting phosphor by Al³⁺ co-doping to enhance its photoluminescence intensity. As we can see from these reports, most red-emitting phosphors were synthesized using the solid-state routine. However, this method usually requires high temperatures and long reaction times, which results in high energy consumption [15], [16]. In addition, the products need to be ground, and the process of grinding can damage the crystal structure of phosphors and result in a decreased emission intensity. In comparison to the solid-state method, the combustion method has advantages such as a low reaction temperature, low energy consumption, and homogeneous morphology of the product [17]. Therefore, the combustion method is chosen in the present work to synthesize the samples. Furthermore, the ⁵D₀→⁷F₂ transition of Eu³⁺, which dominates the emission intensity, is hypersensitive to the crystal symmetry; thus, to enhance the emission intensity and simultaneously reduce the cost of the Ca₃Sr₃(VO₄)₄:Eu³⁺ phosphor, we choose Al³⁺ ions with small radii over the expensive rare earth ions for doping the Ca₃Sr₃(VO₄)₄ host lattice.

In this study, the Eu³⁺ and Al³⁺ co-doped Ca₃Sr₃(VO₄)₄ red phosphors are synthesized using the combustion method. The influence of Al³⁺ on the photoluminescence properties of the Ca₃Sr₃(VO₄)₄:Eu³⁺ phosphor is discussed, and fluorescence lifetimes are investigated.