Energy transfer induced color-tunable emissions from Ba2Gd5B5O17:Ce3+/Tb3+ borate phosphors for white LEDs
Graphical abstract
Introduction
For the past few decades, inorganic luminescent materials have been widely investigated by many researchers due to their potential applications in field emission displays, white light emitting diodes (w-LEDs), sensors, photoelectric devices, etc., [[1], [2], [3], [4], [5], [6], [7], [8], [9]]. Recently, w-LEDs have been extensively studied for applications in various fields, such as automobile headlamps, backlights, flash lights, high definition televisions, device indicators and general illumination, because of their superior characteristics of high brightness, low energy consumption, long operating time and environmental friendliness [10,11]. The most widely used w-LEDs are based on the combination of InGaN based blue LED chips with YAG:Ce yellow phosphor. However, the white light obtained from such devices always exhibit low color rendering index (CRI) and high correlated color temperature (CCT > 4500 K) due to the lack of red emission [[11], [12], [13], [14], [15]]. In order to conquer this difficulty, the combination of near-UV LED chip with blue, green and red phosphors can be proposed as an alternative to offer w-LEDs with high color uniformity and high CRI [[16], [17], [18]]. Among all the trivalent rare earth ions, Tb3+ ion is the most significant activator ion of green emitting phosphors because of its dominant green emission around 543 nm corresponding to the 5D4→7F5 transition [19,20]. Still, it has a drawback of weak excitation band due to parity-forbidden intra-configurational 4f→4f transition, which could restrict its application in w-LEDs. Hence, Ce3+ ion can be used as an effective sensitizer to enhance Tb3+ excitation, because the 4f→5d parity-allowed transition of Ce3+ ions gives broad excitation in the near-ultraviolet (near-UV) region from 250 to 450 nm [[21], [22], [23]].
The Tb3+ and Ce3+ ions can be successfully doped into various host materials like silicate, borate, tungstate, molybdate and phosphate systems. Among all the host materials, borates have been found to be suitable for co-doping of Ce3+/Tb3+ ions due to their advantages including low synthesis temperature, wide UV transparency, high optical damage threshold, broad band gap and high chemical stability. Tb3+ and Ce3+ ions co-doped borate phosphors are unique luminescent materials and have been studied by many researchers for w-LED application, such as Ce3+/Tb3+ co-doped Na2Y2B2O7, GdAl3(BO3)4, Sr3La(BO3)3, Ca3Gd(AlO)3(BO3)4, K3YB6O12, and Sr3Y(BO3)3 phosphors [[24], [25], [26], [27], [28], [29]]. In our previous reports, we have successfully synthesized yellow- and red-emitting Ba2Gd5B5O17 phosphors doped with Dy3+ and Eu3+ ions [30,31]. However, to the best of our knowledge, there is no report on the luminescence properties of Ce3+ and Tb3+ ions doped Ba2Gd5B5O17 phosphors.
Herein, the aim of the present work was to study the luminescence properties of Ce3+ singly doped and Ce3+/Tb3+ co-doped Ba2Gd5B5O17 phosphors for w-LED applications. The Ce3+/Tb3+ co-doped Ba2Gd5B5O17 phosphors exhibited two intense emission bands in blue and green regions under near-UV excitation, in which the blue emission was originating from Ce3+ ions and the green emission was related to the Tb3+ ions. Furthermore, energy transfer from Ce3+ to Tb3+ ions was systematically analyzed. The variation in the emission color induced by the energy transfer from Ce3+ to Tb3+ ions was discussed in detail by tuning the Tb3+ ion concentration. The obtained results clearly show that the Ce3+/Tb3+ co-doped Ba2Gd5B5O17 can be used as a potential dual-color emitting phosphor in the fabrication of near-UV-pumped phosphor-converted w-LEDs.
Section snippets
Experimental section
Power samples of Ce3+ singly doped Ba2Gd5(1-x)B5O17:xCe3+ (named as BGBO:xCe3+; x = 0.004, 0.008, 0.012, 0.016 and 0.02) and Ce3+/Tb3+ co-doped Ba2Gd5(0.992-y)B5O17:0.008Ce3+,yTb3+ (named as BGBO:0.008Ce3+,yTb3+; y = 0.01, 0.03, 0.05, 0.1 and 0.2) were synthesized by high-temperature solid-state reaction technique. Herein, some chemicals including BaCO3 (analytical reagent; Aladdin Industrial Corporation, Shanghai, China), Gd2O3 (99.99%; Jining Tianyi New Materials Co., Ltd), H3BO3 (analytical
Phase purity and structural analysis
The XRD patterns of pure BGBO host, BGBO:0.008Ce3+, BGBO:0.008Ce3+,0.1Tb3+ and BGBO:0.008Ce3+,0.2Tb3+ phosphors were presented in Fig. 1. All the diffraction patterns were coincided well with the Ba2Y5B5O17 compound (JCPDS card No. 00-056-0113), which was reported to crystallize in an orthorhombic crystal system with space group of Pbcn [[32], [33], [34]]. Moreover, no impurity peaks were observed in XRD patterns of the as-synthesized phosphors, which illustrated the fact that the doping of Ce3+
Conclusion
In summary, novel BGBO:xCe3+ and BGBO:0.008Ce3+,yTb3+ powder phosphors with different Ce3+ and Tb3+ ion concentrations were prepared by the solid-state reaction technique. The XRD results revealed that the as-synthesized phosphors were in the orthorhombic crystal system with Pbcn space group. The optimal doping concentrations of Ce3+ and Tb3+ ions were 0.8 mol% and 5 mol%, respectively. The Ce3+→Tb3+ energy transfer efficiency was found to be 52% for BGBO:0.008Ce3+,0.05Tb3+ sample and it
Author contribution statement
R. Vijayakumar: Investigation, Writing-Original draft preparation. Balaji Devakumar: Software, Investigation. Xiaoyong Huang: Conceptualization, Investigation, Supervision, Writing-Reviewing and Editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 51502190), Young Sanjin Scholars Distinguished Professor Program of Shanxi Province, and the Open Fund of the State Key Laboratory of Luminescent Materials and Devices (South China University of Technology, No. 2020-skllmd-01).
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