A brief review on red to near-infrared persistent luminescence in transition-metal-activated phosphors

• Red-NIR persistent phosphors activated by Cr3+ and Mn2+ are reviewed.
• Radiance is found better than luminance for the persistent luminescence evaluation.
• Construction of energy level diagram in Cr-doped persistent phosphor is discussed.

In the past few years, red to near-infrared persistent phosphors activated by transition metals (Cr3+ or Mn2+) received much attention. The research was triggered by a recently proposed in vivo imaging application and it may show more significance in the future. In this review, we try to make a full list of known Cr3+ or Mn2+-activated compounds showing red to near-infrared persistent luminescence, which may be useful for further exploration of new compounds. We also notice that radiance (in units of W sr−1 m−2) is even more suitable than luminance (in units of cd m−2) to quantify the brightness of red to near-infrared persistent phosphors for the in vivo imaging application. Finally, we discuss the mechanism of the persistent luminescence by taking an example of Cr3+-activated Zn0.98(Ga1−xAlx)2O3.98 persistent phosphors.