VUV photoluminescence characteristics of (Y,Gd)VO4:Eu,Zn phosphors produced by ultrasonic spray pyrolysis
Introduction
Vacuum ultraviolet (VUV) phosphors have received considerable attention over the past few years due to a wide range of applications in plasma display panels (PDPs) and a new generation of Hg-free fluorescent lamps [1], [2], [3], [4], [5]. In recent years, PDPs are widely used as flat-panel information-display devices. In PDPs, mixed Ne–Xe gas is mainly subjected to discharge between two glass panels in order to generate VUV light. The VUV photon energy is absorbed by the host lattice and then the absorbed energy is transferred to the activator.
YBO3:Eu3+ has been used as an important material in PDPs due to its high quantum yield under VUV excitation and high ultraviolet (UV) transparency as well as exceptional optical damage threshold [5]. However, in spite of all these advantages, it is considered not to be a desired VUV phosphor because of its poor chromaticity being orange-red rather than red. Therefore, (Y,Gd)BO3:Eu3+ phosphor has been conventionally used as a red-emitting material in PDPs due to its enhanced chromaticity under VUV excitation, compared to YBO3:Eu3+ [6], [7]. The improvement of color purity in (Y,Gd)BO3:Eu3+ originates from lowering the local symmetry of Eu3+, resulting in an increase in the electric dipole 5D0 → 7F2 transition of Eu3+ [5]. However, for high-performance PDPs, its chromaticity is still lacking [8], [9], [10], [11], [12]. In order to further improve the performance of PDPs, it is necessary to explore a novel efficient VUV-excited phosphor with high red purity.
Eu3+-activated vanadate-based phosphates have been of considerable interest due to a strong absorption of UV or VUV light by the host lattice (VO4)3− as well as efficient energy transfer from the host lattice (VO4)3− to the activator Eu3+ [13], [14], [15], [16], [17]. Among the phosphates, both YVO4:Eu3+ and GdVO4:Eu3+ are strong candidates for red phosphors in displays [13], [14], [15], [16], [17]. In our previous study, the photoluminescence (PL) properties of (Y0.5Gd0.5)0.94Eu0.06VO4 under an excitation of 147 nm were much better than those of Y0.94Eu0.06VO4 and Gd0.94Eu0.06VO4 [18]. In addition, it is well known that the incorporation of metal ions into the host lattice distorts the lattice to modify the energy absorption and the transfer behaviors, thereby enhancing the PL characteristics [19], [20], [21]. In the present work, Zn2+ was selected as a doped metal ion in (Y0.5Gd0.5)0.94Eu0.06VO4. We investigated the microstructure and PL characteristics of (Y0.5Gd0.5)0.94−xZnxEu0.06VO4 (0 ≤ x ≤ 0.04) phosphors, depending on the Zn content and annealing temperature.
Section snippets
Experimental
The ultrasonic spray pyrolysis method was used to prepare red-emitting (Y0.5Gd0.5)0.94−xZnxEu0.06VO4 (x = 0, 0.01, 0.02, 0.03, and 0.04) phosphor powders. Y2O3 (99.99%, High Purity Chemicals), Gd2O3 (99.99%, High Purity Chemicals), and NH4VO3 (99%, High Purity Chemicals) powders were used as host materials. Also, Eu2O3 (99.99%, High Purity Chemicals) powder and Zn(NO3)2·6H2O (99.9%, High Purity Chemicals) were used as activator and sensitizer, respectively. First, Y2O3, Gd2O3, and Eu2O3 were
Results and discussion
In order to find the appropriate content of ethylene glycol and citric acid, we investigated the (Y0.5Gd0.5)0.94−xZnxEu0.06VO4 (0 ≤ x ≤ 0.04) powders synthesized by the ultrasonic spray pyrolysis method using various organic additives with different ratios of ethylene glycol and citric acid. Fig. 1(a)–(c) shows the typical SEM images for one of the powders, Zn-free (Y0.5Gd0.5)0.94Eu0.06VO4, synthesized with various organic additives of ethylene glycol:citric acid = 0.25:0.75, 0.5:0.5, and 0.75:0.25,
Conclusions
The (Y0.5Gd0.5)0.94−xZnxEu0.06VO4 (0 ≤ x ≤ 0.04) synthesized by the ultrasonic spray pyrolysis method with an organic additive of ethylene glycol:citric acid = 0.5:0.5 showed high-quality powder characteristics, i.e., spherical and regular morphologies as well as smooth surfaces. The doped Zn had no significant influence on the morphological characteristics of synthesized powders. In addition, the annealing gave rise to an increase in the powder size. Higher Zn content yielded a higher degree of
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