Blue afterglow of undoped CaAl

This paper reports on the intense blue afterglow from undoped CaAl2O4 nanocrystals in an attempt to achieve a clear understanding of the afterglow mechanism of Eu2+-doped CaAl2O4. Undoped CaAl2O4 nanocrystals were obtained via solution combustion technique. Intense blue afterglow, which is quite similar to that of Eu2+-doped CaAl2O4, is observed with naked eyes for undoped CaAl2O4 regardless of no Eu2+ species in the phosphors. Based on density functional calculations, the electronic structures of oxygen-deficient CaAl2O4 are given, and the mechanism of the blue afterglow from undoped CaAl2O4 is discussed in terms of the oxygen vacancy introduced midgaps. The recorded blue afterglow from undoped CaAl2O4 nanocrystals leads to significant progress in our fundamental understanding of the afterglow mechanisms of Eu2+-doped CaAl2O4