A narrowband ultraviolet-B-emitting LiCaPO4:Gd3+ phosphor with super-long persistent luminescence for over 100 h

The past decades have witnessed a significant increase in interest in inorganic luminescent materials that emit in the narrowband ultraviolet-B (NB-UVB; 310-313 nm) spectral region due to the growing need for applications in photochemistry and photomedicine. However, the majority of existing NB-UVB phosphors rely on photoluminescence, which requires constant external excitation. This common but inconvenient photoluminescence style significantly slows down the progress of NB-UVB luminescence technology. Herein, we report the design and synthesis of a new Gd3+-doped NB-UVB-emitting persistent phosphor, LiCaPO4:Gd3+, which shows strong NB-UVB persistent luminescence peaking at 312 nm and a super-long persistence time of >100 h after ceasing X-ray excitation. Owing to the zero-background noise from the ambient light, a UVB camera can detect the NB-UVB light emission originating from the charged LiCaPO4:Gd3+ phosphor in a bright indoor environment. Through spectroscopic investigations and first-principles calculations, the nature of energy traps and the persistent luminescence mechanism of Gd3+ in the LiCaPO4 host have been thoroughly studied. Besides, remarkable photochromic behavior when irradiating the phosphor with X-rays is also observed, and the possible intrinsic point defects that contribute to the colorization are proposed. This NB-UVB persistent phosphor shows great potential in indoor optical tagging, optical information storage, and dermatological therapy.