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This work reports the realization of Gd 3+ persistent luminescence in the narrowband ultraviolet-B (NB-UVB; 310–313 nm) through persistent energy transfer from a sensitizer of Pr 3+ , Pb 2+ or Bi 3+ . We propose a general design concept to develop Gd 3+ -activated NB-UVB persistent phosphors from Pr 3+ -, Pb 2+ - or Bi 3+ -activated ultraviolet-C (200–280 nm) or ultraviolet-B (280–315 nm) persistent phosphors, leading to the discovery of ten Gd 3+ NB-UVB persistent phosphors such as Sr 3 Gd 2 Si 6 O 18 :Pr 3+ , Sr 3 Gd 2 Si 6 O 18 :Pb 2+ and Y 2 GdAl 2 Ga 3 O 12 :Bi 3+ as well as five ultraviolet-B persistent phosphors such as Y 3 Al 2 Ga 3 O 12 :Pr 3+ , Sr 3 Y 2 Si 6 O 18 :Pb 2+ and Y 3 Al 2 Ga 3 O 12 :Bi 3+ . The persistent energy transfer from the sensitizers to Gd 3+ is very efficient and the Gd 3+ NB-UVB afterglow can last for more than 10 hours. This study expands the persistent luminescence research to the NB-UVB as well as the broader ultraviolet-B spectral regions. The NB-UVB persistent phosphors may act as self-sustained glowing NB-UVB radiation sources for dermatological therapy. 

Visible-light and infrared-light persistent phosphors are extensively studied and are being used as self-sustained glowing tags in darkness. In contrast, persistent phosphors for higher-energy, solar-blind ultraviolet-C wavelengths (200–280 nm) are lacking. Also, persistent tags working in bright environments are not available. Here we report five types of Pr³⁺-doped silicates (melilite, cyclosilicate, silicate garnet, oxyorthosilicate, and orthosilicate) ultraviolet-C persistent phosphors that can act as self-sustained glowing tags in bright environments. These ultraviolet-C persistent phosphors can be effectively charged by a standard 254 nm lamp and emit intense, long-lasting afterglow at 265–270 nm, which can be clearly monitored and imaged by a corona camera in daylight and room light. Besides thermal-stimulation, in bright environments, photo-stimulation also contributes to the afterglow emission and its contribution can be dominant when ambient light is strong. This study expands persistent luminescence research to the ultraviolet-C wavelengths and brings persistent luminescence applications to light.

 

It is general knowledge in persistent luminescence that high-energy illumination, mostly ultraviolet light, is usually necessary in order to effectively charge persistent phosphors. However, the need for high-energy ultraviolet light excitation compromises some applications. In his pioneering work on ruby (Al 2 O 3 :Cr 3+ ) laser materials in 1960, Theodore Maiman observed an excited-state absorption phenomenon under the excitation of a high-intensity green-light flash tube. Inspired by Maiman's observation, here we propose a new two-photon up-conversion charging (UCC) concept to effectively charge Cr 3+ -activated near-infrared persistent phosphors using low-energy, high-intensity visible-light laser diodes. As an example, we demonstrate that a low-energy 635 nm laser diode can produce persistent luminescence in the LiGa 5 O 8 :Cr 3+ persistent phosphor at the same magnitude as that produced by high-energy 335 nm ultraviolet light from a xenon arc lamp. Moreover, the UCC appears to be a common phenomenon in persistent phosphors containing other UCC-enabling activators such as rare-earth Pr 3+ ions and transition metal Mn 2+ ions. The UCC technique offers a new way to study persistent luminescence and utilize persistent phosphors; for instance, in bioimaging it makes effective in vivo charging persistent optical probes using tissue-friendly visible light possible.