FIGURE 4. Afterglow mechanism ofCsCdCl3:4%
Mn2+ crystal. (a) Thermoluminescence curves from 25oC to 300 oC. at a temperature
ramping rate of 4 oC/s. (b) Thermoluminescence curves
measured at different charging temperatures from 20 oC
to 100 oC. Inset was the calculated trap distribution
by two distinct methods, including
thermal cleaning method and
initial rise method. (c) Afterglow decay trace after ceasing excitation,
where the spikes were the stimulated signal with a 980-nm laser. (d)
Afterglow mechanism involving two traps was proposed.
To understand the ultralong afterglow mechanism of crystals, the
thermoluminescence (TL) curves of
CsCd1-xCl3:x% Mn2+crystals were measured (Figure 4a). The pristine CsCdCl3crystal exhibited an ultra-deep trap at 280 oC.
Interestingly, the trap disappeared after 4-mol% Mn2+doping. Meanwhile two shallow traps at 140 oC and 180oC emerged significantly, which shifted slightly
towards low-temperature end after heavy doping at 10 mol% and 15 mol%.
To investigate the distribution of traps, TL curves of a typical crystal
were measured at different charging temperature (Figure 4b, Figure S11).
The initial rise method was then used to estimate the top edge of two
kinds of traps, suggesting a shallowest level at 0.05 eV and 0.26 eV,
respectively. Based on the thermal-cleaning method, the average depths
of traps were calculated to be 0.72 eV to 0.92 eV,
respectively.[35] As shown in Figure 4b inset, the
two traps were discretely distributed with an energy difference about
0.2 eV (Supporting Video), which echoed well with difference between PL
and afterglow excitation (Figure 3a). In this sense, we speculated that
the afterglow could be charged more effectively by direct filling of
deep traps which has a shallow edge exactly below conduction band by
0.26 eV. In addition to thermal excitation, trapped electrons in crystal
could also be stimulated and released by photoexcitation at 980 nm which
has a sufficient energy (~ 1 eV) to the trap depth
(Figure 4c). After multiple cycles of photon stimulation (4-min for each
cycle), the afterglow intensity was still about an order of magnitude
higher than the background noise. Both the multiple-cycle photon
stimulation and the long-term thermoluminescence (Supporting Video)
suggested a large density of traps.
Based on the above discussion, both the luminescence and afterglow
mechanism were proposed as shown in Figure 4d. Before
Mn2+ doping, electrons in the conduction band mainly
have two transition routes: one part of electrons radiatively
transitioned to 3Eg energy level of
Cd2+ while the other part were non-radiatively
quenched by defects, leading to a low PL QY about 42%. After
Mn2+ doping, the latter route involving quenching was
outperformed by the rapid resonant energy transfer from
Cd2+ to Mn2+ due to their almost
identical bandgap. In this sense, the PL QY was significantly boosted as
a result of the high efficiency of radiative d-d transition from
Mn2+. As to the afterglow, two kinds of traps below
bandgap were illustrated in Figure 4d. Upon thermal perturbance, the
trapped electrons were elevated to conduction band, followed by
relaxation to nearby Cd2+ due to its rich content
(97.5-mol%). Thereafter, the Mn2+ acceptor was
excited by resonant energy transfer from Cd2+ donor,
whereby a strong afterglow was generated.
In conclusion, a new optical host for afterglow, namely
CsCdCl3, was grown in a hydrothermal reactor. The
pristine crystal showed a relatively PL QY at room temperature, which
was further boosted up to 73% at 200 oC. After
Mn2+ doping, the PL QY was further elevated to 100%
due to the efficient resonant energy transfer from
Cd2+ to Mn2+. Importantly, an
ultralong afterglow up to 12 hours were observed for the doped crystals,
which was attributed to the interplay between two kinds of traps based
on TL measurement. This work
brought a new member to the
library of transparent afterglow crystal, opening many possibilities to
advanced applications such as
volumetric display and
three-dimensional information
encryption.