Abstract
Persistent luminescent materials are able to emit light for hours after being excited. The majority of persistent phosphors emit in the blue or green region of the visible spectrum. Orange- or red-emitting phosphors, strongly desired for emergency signage and medical imaging, are scarce. We prepared the nitrido-silicates Ca2Si5N8:Eu (orange), Sr2Si5N8:Eu (reddish), Ba2Si5N8:Eu (yellowish orange), and their rare-earth codoped variants (R = Nd, Dy, Sm, Tm) through a solid state reaction, and investigated their luminescence and afterglow properties. In this paper, we describe how the persistent luminescence is affected by the type of codopant and the choice and ratio of the starting products. All the materials exhibit some form of persistent luminescence, but for Sr2Si5N8:Eu,R this is very weak. In Ba2Si5N8:Eu the afterglow remains visible for about 400 s, and Ca2Si5N8:Eu,Tm shows the brightest and longest afterglow, lasting about 2,500 s. For optimal persistent luminescence, the dopant and codopant should be added in their fluoride form, in concentrations below 1 mol%. A Ca3N2 deficiency of about 5% triples the afterglow intensity. Our results show that Ba2Si5N8:Eu(,R) and Ca2Si5N8:Eu(,R) are promising persistent phosphors for applications requiring orange or red light.
Highlights
Some luminescent materials continue emitting light for minutes or hours after removal of the excitation. This phenomenon, known as persistent luminescence, is sometimes undesired, as it limits the use of these materials in applications that require high switching speeds, such as LEDs or information displays
The very long lifetime of the emission without the need of a constant energy input opens up possibilities in many other fields such as, but not limited to, emergency signage [1], traffic safety, medical imaging [2], dials and displays, and decoration
The lack of potent red-emitting afterglow phosphors, which are desired for emergency signage and medical imaging [2], is caused by two reasons:
Summary
Some luminescent materials continue emitting light for minutes or hours after removal of the excitation This phenomenon, known as persistent luminescence, is sometimes undesired, as it limits the use of these materials in applications that require high switching speeds, such as LEDs or information displays. The lack of potent red-emitting afterglow phosphors, which are desired for emergency signage and medical imaging [2], is caused by two reasons: Firstly, the sensitivity of the human eye is low in the orange-to-red region of the visible spectrum. This effect is even more dramatic at low light conditions, which are typical for persistent luminescent applications. As we describe in this paper, the afterglow is influenced by codoping, and depends on the starting materials and the dopant and codopant concentrations
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