Toward tunable and bright deep‐red persistent luminescence of Cr ³⁺ in garnets

Garnet structure ( A 3 B 2 C 3 O 12 ) with three different cation sites is a very flexible host material widely used for w ‐ LED s, solid‐state lasers, scintillators, and so on. In this work, we have successfully developed six different Cr 3+ ‐doped garnets: Y 3 Ga 4.99 Cr 0.01 O 12 ( YG G :Cr), Gd 3 Ga 4.99 Cr 0.01 O 12 ( GG G :Cr), Lu 3 Ga 4.99 Cr 0.01 O 12 (Lu G G :Cr), Y 3 Sc 1.99 Cr 0.01 Ga 3 O 12 ( YSG G :Cr), Gd 3 Sc 1.99 Cr 0.01 Ga 3 O 12 ( GSG G :Cr), and Lu 3 Sc 1.99 Cr 0.01 Ga 3 O 12 (Lu SG G :Cr), which exhibit persistent luminescence (PersL) due to Cr 3+ emission matching well with both the response curve of the Si detector and the wavelength region of the first biological window ( NIR ‐I, 650‐950 nm). The main emission band of Cr 3+ in these garnets can be easily tunable from the sharp R ‐line emission due to the 2 E ( 2 G)→ 4 A 2 ( 4 F) transition in the strong crystal field to the broad band emission due to the 4 T 2 ( 4 F)→ 4 A 2 ( 4 F) transition in the weak one when Lu 3+ in the A site and Ga 3+ in the B site are, respectively, replaced by larger cations, Y 3+ /Gd 3+ and Sc 3+ . Furthermore, based on the knowledge of 4 f energy levels of 15 lanthanide ions in the host‐referred binding energy ( HRBE ) diagram, we took the GSG G host as a typical example to discuss the feasibility of four trivalent lanthanides (Sm 3+ , Eu 3+ , Tm 3+ , Yb 3+ ) as potential sensitizers for enhancing Cr 3+ PersL.