Review-Photoluminescence Spectroscopy of Eu2+-Activated Phosphors: From Near-UV to Deep Red Luminescence


 In this review article, the characteristic luminescence behaviors of the Eu2+-activated phosphors are discussed from a spectroscopic point of view. This spectroscopic point of view makes possible to classify the Eu2+-activated phosphors into two groups: types Eu2-A and Eu2-B, where an energy inequality relation of the lowest excited state of type Eu2-A to be given by 4f
 7 (6
 P
 7/2)→4f
 65d, whereas that of type Eu2-B is given by 4f
 7 (6
 P
 7/2)→4f
 65d. Simply saying, the 6
 P
 7/2 excited manifold of the 4f
 7 configuration is located just below and well above the lowest-energy 5d level of the Eu2+ ion in types Eu2-A and Eu2-B phosphors, respectively. While generally uncommon, in some host materials the 4f
 7 (6
 P
 7/2)→ 4f
 7 (8
 S
 7/2) intra-configurational transitions of Eu2+ occur with exhibiting the characteristic narrow-line emissions. An analysis method based on the Franck–Condon principle within the configurational-coordinate motel is proposed to exactly determine the lowest excited-state 4f
 65d energy in both types of phosphors Eu2-A and Eu2-B. Thermal stability of the phosphor properties is of crucial importance and scientific interest. Therefore, key understandings of the temperature dependences of luminescence intensity and decay time in the Eu2+-activated phosphors are also presented as the additional and important concern in the present review article.