Mechanisms of persistent luminescence in Eu 2+, RE 3+ doped alkaline earth aluminates

Abstract

Persistent luminescence of Eu 2+ doped calcium aluminate, CaAl 2O 4 : Eu 2+, was studied under UV- and visible illumination. Broad band luminescence was observed in the blue region ( λ max=440 nm) due to the 4f 65d 1→4f 7 transition of Eu 2+. The threshold for the visible excitation was found at about 530 nm, which is not high enough in energy to create a monovalent Eu + ion required by the mechanisms presented previously. The new mechanism proposed involves the excited state absorption of two 530 nm photons via deep traps followed by trapping of electrons in shallow traps. The persistent luminescence is observed after a temperature controlled electron–hole recombination and subsequent energy transfer to the Eu 2+ luminescent center.