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.