FIGURE 3. Ultralong afterglow from CsCdCl3:4% Mn2+ single crystal. (a) Steady-state excitation spectra of PL and afterglow from crystal, showing a significant shift. (b) Afterglow spectra at different time delay after ceasing the excitation. (c) The afterglow decay curve showed an intense signal over background even after 12-hour delay. Note that a 30-min delay after ceasing excitation was used to avoid detector saturation. Insets were the afterglow photographs of a typical crystal. (d) A comparison of afterglow output power between two crystals.[17]
To shed more light on the afterglow of CsCdCl3:4% Mn2+ crystals, excitation spectra for both PL and afterglow were measured. Note that, the afterglow excitation was obtained by plotting afterglow intensity against varied excitation light source[31] (Figure 3a, Figure S9, Table S1). Intriguingly, afterglow excitation spectrum showed a red shift of 0.26 eV when compared with PL excitation. In addition, direct excitation of Mn2+ at 370, 420, and 515 nm failed to charge afterglow traps, indicating that the afterglow originated from the resonant energy transfer process from Cd2+ to Mn2+. To identify the emitting center of afterglow, afterglow PL spectra were recorded at varied time interval after ceasing the excitation (Figure 3b). The afterglow spectra featured a Gaussian profile centering at 598 nm which hardly changed with decay time, indicating that the afterglow emitter was Mn2+(4T16A1) in nature. The afterglow intensity remained two orders of magnitude higher than the background noise after 12-hour decay (Figure 3c), which was amongst the most durable afterglow phosphors to the best of our knowledge.[32-34] In contrast, the afterglow duration of the pristine crystal was quite limited (a few minutes) (Figure S10). Remarkedly, afterglow curve showed that the initial brightness of CsCdCl3:4% Mn2+ was about 40 times higher than that of the first afterglow perovskite,[17] namely, Cs2Ag0.8Na0.2InCl6:20% Mn2+ single crystals. (Figure 3d).