Trapping mechanism associated with electron delocalization and tunneling of CaAl2O4:Ce3+, a persistent phosphor

A long afterglow phosphor, CaAl2O4:Ce3+, was prepared and studied. It exhibits a broad-band emission in the deep blue with a persistence time longer than 10 h. The Ce3+ ions occupy three distinct Ca2+ sites in this material, and the emission and excitation spectra from Ce3+ ions in these sites have been identified. The thermoluminescence spectra were obtained following irradiation at different wavelengths. Five traps were found: two of the traps, with depths of 0.67 and 0.82 eV, are filled by electron transport though the conduction band; the other three, with depths of 0.4, 0.46, and 0.6 eV, are filled by electrons tunneling from the 5d levels of adjacent Ce3+ ions. Photoconductivity measurements were also made which indicated that the lowest 5d levels of Ce3+ lie below the conduction band and that the ground state of Ce3+ is about 1.1-1.4 eV above the valence band of the host. (C) 2003 The Electrochemical Society.