Multicolor persistent luminescence realized by persistent color conversion
Graphical abstract
Introduction
The emission of persistent phosphors can persist for a long time after the cessation of excitation. Persistent luminescence and persistent phosphors are arousing increasing interests and have found abundant applications in night vision materials, security signals, art painting, mechanical stress detection, high energy particle detection, optical information storage, catalysis, analysis, time-resolved bioimaging, etc [1], [2], [3]. Apparently, these applications rely on the synthesis of persistent phosphors with abundant spectral properties. Many new phosphors have been reported, usually by developing new matrix and/or new dopants. For example, Ca0.2Mg0.9Zn0.9Si2O6:Eu,Dy,Mn is the first developed persistent phosphor in persistent luminescence imaging [4], [5]. Cr-doped gallium oxide related phosphors represents the current dominating ones [6], [7], [8], [9], [10], [11], [12]. Novel Cr/Ga-free persistent phosphors are being explored such as Zn3Ga2Ge2O10:Ni2+ [13], LaAlO3:Mn4+,Ge4+ [14], BaZrSi3O9:Eu2+, Pr3+ [15], KGaGeO4:Bi3+ [16], semiconducting polymer [17], metal organic framework [18], etc. Although a number of new persistent phosphors have been explored, to date, CaAl2O4:Eu,Nd (blue, CAO) and SrAl2O4:Eu,Dy (green) still represents the most efficient commercial ones [19], [20].
Regarding spectral tuning, in addition to explore new matrix or dopants, a color conversion strategy has been widely applied in the field of LED lighting to obtain white light [21]. Generally, a blue semiconductor lighting chip and color conversion phosphors are mixed together. Then, conversion phosphors absorb part of the blue light and emit green, yellow and/or red light to generate abundant visible colors based on the trichromatic theory [22], [23], [24], [25], [26]. Inspired by these works in LED color tuning, we propose that multicolor persistent spectral tuning can also be realized by color conversion. Here, CAO is chosen as a persistent donor as it is currently the most efficient blue persistent phosphor [19]. Ce3+-doped lutetium aluminum garnet (Lu3Al5O12:Ce3+, LuAG), which is one of the most efficient green color conversion phosphors in LED industry, is used as a color conversion phosphor [27], [28], [29]. We try to tune the blue persistent luminescence by changing the mass ratio of LuAG in CAO/LuAG composite. This color conversion strategy was further explored by using four other conversion phosphors to generate abundant persistent color.
Section snippets
Materials
Ca(NO3)2·4H2O, Al(NO3)3·9H2O, HBO3, urea and nitric acid are AR reagents and used as received. Europium oxide, neodymium oxide, and cerium oxide are 4N reagents and their nitrate solutions are prepared by dissolving oxides in nitric acid. Granular activated charcoal are used to generate weak reductive atmosphere to reduce Eu3+ and Ce4+ during annealing. Four commercial phosphors (LMY-4453-C, LMY-5049-C, LAM-R-6534, LAM-R-6831), were purchased from Luming Technology Group Co., LTD (China).
Synthesis of CAO
The
Results and discussion
As indicated in Fig. 1a, the diffraction pattern of the as-synthesized CAO corresponds well with that of monoclinic CaAl2O4 (JCPDS card no. 23-1036). No apparent impurity diffraction peaks can be observed, indicating the successful doping of Eu and Nd into CaAl2O4 lattice. The size of CAO particles range from ~100 to ~10 µm, as shown in Fig. 1c. The as-synthesized LuAG possesses a garnet crystal structure with diffraction pattern corresponding with Lu3Al5O12 garnet (JCPDS card no. 731368) (Fig.
Conclusion
CAO, an efficient blue persistent phosphor can serve as an efficient persistent energy donor to excite phosphors without persistent properties. Continuous color tuning persistent luminescence composite material or device can be obtained by simply changing the mass ratio of the donor persistent phosphor and conversion phosphor. Multicolor spectral tuning of persistent luminescence can be realized by choosing matched persistent/conversion phosphor pairs using this color conversion strategy.
Acknowledgments
This research was supported by the Talent Introduction Special Fund (88016676), Scientific Cultivation and Innovation Fund (11617323) of Jinan University, and the National Natural Science Foundation of China (21701052).
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