The cyan-green luminescent behaviour of nitrided Ba9Y2Si6O24: Eu2+ phosphors for W-LED
Introduction
The luminescence properties of phosphors directly affect the luminous efficiency, color rendering index (CRI), and color temperature (CT) of W-LED. The color of phosphor can be adjusted to meet the needs of different lighting fields due to the fixed-range emission band of LED chip [1], [2]. At present, partially converting YAG:Ce3+ yellow phosphor is a common way to creat a W-LED [3]. But YAG:Ce3+ phosphors have defects of low color rendering index and high color temperature, which attributes to the lack of red component in the emission spectrum [4]. Therefore, many researchers have made many attempts to improve the luminescence of YAG phosphors by changing the synthesis method [5]. Setlur et al. [6] studied the luminescent properties of Si3N4-doped RE3Al5-xO12:Ce3+ (RE = Lu3+ or Y3+) phosphors, effectively improved the decay life and thermal stability and realized red-shift of spectrum. Similarly, Sopicka-Lizer et al. [7] used a substituting technique to replace the Al-O bond with a Si-N bond for forming Y2.94Ce0.06Al5-xSixO12-xNx solid solution, which showed that the emission spectrum had a significant red shift. Therefore, Si3N4 doping oxide-matrix phosphors have become an important and effective method to improve phosphors’ luminescence.
Moreover, nitride phosphors and oxynitride phosphors have received widespread attention because of their high luminous efficiency and thermal stability [8]. However, the nitride compounds are often to be sintered at above 1700 °C. On the other side, rare-earth doped silicate phosphors have been favored due to their wide optical spectrum range, good stability, and low cost and suitable synthesis temperature [9], [10], [11]. However, most silicate phosphors have still some shortcomings compared with YAG:Ce3+ phosphors in the aspect of luminous efficiencies and thermal stability. In order to further improve the luminescent properties of silicate phosphors, more and more researchers are beginning to use nitriding technology in the study of silicate phosphors. Furthermore, the novel silicate Ba9M2Si6O24 (M=Sc, Y, Lu) phosphor has been reported in recent years. T Nakano et al. has prepared Ba9Sc2Si6O24:Eu2+ phosphor, in which luminescent properties was analyzed, and the effects of Si3N4 on the luminescence properties are discussed [12]. Hyun Cho et al. [13] prepared Ba9Y2Si6O24: Eu2+ phosphor and analyzed the luminescence properties, shown superior luminescence properties relative to Ba9Sc2Si6O24: Eu2+. Liu et al. systematically studied the luminescence properties of Ba9M2Si6O24: N (N = earth element, M= Sc, Y, Lu), and proved the application prospect of phosphor in white LEDs [14], [15]. But some unclear luminescent mechanism and shortcoming still exist in the aspect of luminous efficiencies and thermal stability compared to commercial YAG: Ce3+. Thus, for the improvement on luminescent performance, we doped Si3N4 to actualizing the nitriding process.
In this paper, we reported the Si3N4 doped Ba9Y2Si6O24: 0.6Eu2+ phosphors. The effect of nitriding on Eu2+ luminescent behaviors of the phosphor was studied. The luminescence properties and thermal characteristics were investigated to different activating center in crystal structures. The overall luminescent performances of nitrided phosphors are better than that of the pristine, which has more possibility to acting as a potential phosphor for W-LED.
Section snippets
Experimental section
The samples of Ba9Y1.94Si6O24–3×/2Nx: 0.6Eu2+ (x = 0–0.4) were synthesized via conventional solid-state reaction. The raw materials are BaCO3 (SP), SiO2 (99.99%), Eu2O3 (99.99%), Y2O3 (99.99%), and Si3N4 (99.99%). Every raw material was weighed out according the stoichiometric and thoroughly mixed in the agate mortar by enough grinding. Afterward, the obtained powder was fired at 1400 °C in 5%H2/95%N2 atmosphere for 4 h. Finally, the as-synthesized samples were slowly cooled to room temperature
Results and discussions
Fig. 1(a) shows all XRD profiles of Ba9Y2Si6O24–1.5xNx: Eu2+ (x = 0, 0.1, 0.2, 0.3 and 0.4) phosphors were indexed by Ba9Sc2Si6O24 (PDF#82–1119) apart from a little amount of Ba2SiO4 phase. The XRD pattern is shifted to small angle compared to PDF when x = 0, because the ionic radius of Y3+ ions (RY = 0.9 Å) is greater than that of Sc3+ ions (RSc = 0.745 Å). The results indicate that the Eu2+ and N3- can completely accommodate in the Ba9Y2Si6O24 crystal lattices. The lattice parameters were
Conclusions
Ba9Y2Si6O24–3×/2Nx: 0.6Eu2+ phosphors were prepared by high temperature solid-state reaction. N3- is penetrated into Ba9Y2Si6O24 crystal lattice in replacement of O2- sites, and Eu2+ ions occupy the lattice sites of Ba2+ ions. Researches show that Nitriding does not change the original crystal structure. To Eu-Ba(1)-center corresponds to short-wave cyan emission peaking at ~490 nm and Eu-Ba(2, 3)-center corresponds to long-wave green emission peaking at ~525 nm, there is an effective energy
Acknowledgements
This work was supported by the National Natural Science Foundation of China under Grant Number: 51672063, 51202051, and Science and Technology Program of Zhejiang Province under Grant Number: 2016C31110. Open Projects of Key Laboratory of Inorganic Functional Materials and Devices, Shanghai Silicate Institutes, Chinese Academy of Sciences under Grant Number: KLIFMD201708
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