Red persistent luminescence excited by visible light in CaS:Eu2+,Tm3+

Excitation wavelength dependence of red persistent luminescence in CaS:Eu2+,Tm3+ phosphor is reported. Persistent luminescence appears under visible light excitation in the wavelength region of 400–600nm. Photon energy from white light-emitting diode lamps is possibly stored in this material. This sulfide phosphor is synthesized using iodine vapor. Under iodine vapor, Eu2+ and Tm3+ are found to be efficiently included in CaS. The concentration dependence of Eu2+ is studied, and the optimum concentration is 0.05%. Trap depth of 0.27–0.33eV contributing to persistent luminescence is evaluated by using thermoluminescence

Red afterglow and luminescence arising from defects in CaS:Eu2+, Tm3+

CaS:Eu^2+, Tm^3+ is a phosphor known to emit a long afterglow of red emission (650 nm) when excited by blue light (450 nm). It shows a long afterglow time of 700 s for Eu = 0.05% and Tm = 2%. The mechanism of this afterglow is investigated using time-resolved fluorescence (TR-F) spectroscopy from the nanosecond to millisecond region. At room temperature, it is not possible to investigate shallow levels because of the effects of thermal vibrations. The mechanism of the emission characteristics at room temperature would be affected by these levels that can be observed only at low temperatures. Therefore, the samples are cooled to 15 K for the TR-F measurements. The host material CaS emits blue light (420 nm) arising from sulfur defects, and the typical decay time is measured to be 6 ms. This blue emission becomes stronger when Tm^3+ is doped. Furthermore, the doped Eu ions emit a broad red spectrum at 650 nm originating from the Eu^2+ -specific 4f^6 5d^1 –4f^7 transition. When the excitation is ceased, the red emission decays with a fast time constant of 0.6 μs. This value is a typical decay time for Eu^2+. This red emission has multiple decay time constants, and a component with a decay time of 6 ms appears. This 6 ms decay time is the same as that of the blue emission from the sulfur defects, which have an important role on the red afterglow