Insights into blue-light activated red-emitting persistent luminescence from Pr³⁺-doped phosphors

In recent years, a series of persistent luminescence materials excitable by blue light have been developed and widely used in many fields such as optical information storage, AC-LEDs, anti-counterfeiting and bio-imaging. However, it is still a long-standing challenge to develop a superior red-emitting persistent phosphor that can be efficiently excited by blue light. In this work, a novel blue-light excited red-emitting persistent phosphor CaCd₂Ga₂Ge₃O₁₂:Pr³⁺ was successfully synthesized by using a solid-state method, showing excellent luminescence properties. Moreover, the phase purity, crystal structure, photoluminescence spectra, afterglow emission spectra, and three-dimensional thermoluminescence spectrum were successfully investigated. Under 294 nm excitation, photoluminescence spectra show a single orange emission and a series of peaks centered at 492, 537, 568, 614 and 664 nm, which correspond to the ³P₀ → ³H₄, ³P₀ → ³H₅, ³P₂ → ³H₆, ¹D₂ → ³H₄, and ³P₀ → ³F₂ transitions of Pr³⁺, respectively. Interestingly, after blue light excitation, the afterglow luminescence exhibits red long emission, which is attributed to the ¹D₂ → ³H₄ transition of Pr³⁺. Through thermoluminescence spectra and three-dimensional thermoluminescence spectra, we analyze the reasons for the different colors of photoluminescence and afterglow luminescence. The results imply that there are two types of traps, and the depth of shallow traps and deep traps is calculated to be 0.684 and 0.776 eV, respectively. It is worth noting that the photoluminescence is attributed to the 4f² → 4f5d and f → f transitions of Pr³⁺, and the afterglow luminescence is ascribed to a tunneling-related process and the transition of electrons from the valence band to the conduction band. The obtained red-emitting persistent phosphors provide a promising pathway toward AC-LEDs, multi-cycle bio-imaging and other fields.