Design, synthesis and characterization of near-infrared long persistent phosphors Ca4(PO4)2O:Eu2+,R3+ (R = Lu, La, Gd, Ce, Tm, Y)

Abstract

Near-infrared (NIR) persistent phosphors have many remarkable optical advantages in bioimaging applications. However, only a very few activators and hosts have been found to exhibit NIR persistent luminescence. Here, we report Eu²⁺-activated phosphate NIR long persistent phosphors, Ca₄(PO₄)₂O:Eu²⁺,R³⁺ (R = Lu, La, Gd, Ce, Tm, Y). All the codoped samples showed persistent luminescence peaking at a long wavelength of ca. 690 nm due to the 5d–4f transitions of Eu²⁺. The persistent luminescence of Ca_3.97(PO₄)₂O:Eu_0.01²⁺,Y_0.02³⁺ can be detected for 2 h after the removal of excitation. As a key parameter of persistent phosphors, the trap depths can be tuned from 0.62 to 0.77 eV by codoping with trivalent rare earth ions. The I⁻¹–t curves reveal that two effective traps contribute to long persistent luminescence of Y³⁺, Tm³⁺ and Ce³⁺ codoped samples, whereas there is only one effective trap for La³⁺, Gd³⁺ and Lu³⁺ codoped samples, which was confirmed by the thermoluminescence curves of the codoped samples. This investigation provided a new long persistent phosphor which enriches the activator and hosts of existing NIR long persistent phosphors for potential application in vivo bioimaging.