Dynamics of Charges in Superlong Blacklight-Emitting CaB2O4:Ce3+ Persistent Phosphor

The optical and persistent luminescence properties of CaB2O4:Ce3+ phosphor are presented. The optical emission for excitation in the 250–340 nm wavelength region is dominated by two bands at 365 and 460 nm. Lifetime measurements suggested that the 365 nm emission band is due to interconfigurational Ce3+ 5d → 4f transitions. Upon excitation with a 254 nm UV lamp, a superlong persistent luminescence in the UVA1 region (340–400 nm, blacklight) was observed, lasting for at least 15 h, and with excellent reproducibility, which is perfectly suitable for phototherapy application. The initial-rise method was applied on the thermoluminescence glow curves to determine the trap distribution and trap depth. The results suggest that one distinct trap, with an activation energy of ∼0.52 eV, was solely responsible for the persistent luminescence in the CaB2O4:Ce3+ phosphor. The other traps had a quasi-continuous distribution, with activation energies between 0.56 and 1.15 eV. The proposed persistent luminescence and the thermoluminescence mechanisms are elucidated using experimental parameters obtained from the optical and thermoluminescence results and the theoretically calculated electronic structure of the Ce3+ ion in CaB2O4. The lowest Ce3+ 5d1 level was found to be ∼0.97 eV below the conduction band, and the persistent luminescence/thermoluminescence emission was dominated by the radiative transitions between Ce3+ energy levels, 5d → 2F5/2,7/2.