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+(4T1→6A1)
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).