FIGURE 2. Temperature-dependent and time-resolved photoluminescence ofCsCd1-xCl3:x% Mn crystals. Temperature-dependent PL (a) and absorption (b) spectra. Insets showed the luminescent photographs at varied temperatures. A 275-nm LED was used as the excitation source. (c) PL QY was plotted against Mn2+-doping content, suggesting an optimal doping at 4 mol%. (d) The PL decay curves at varied doping contents indicated a concentration quenching at heavy doping level. (e) Time-resolved PL spectra within a time window of 200 ms, showing a clear trace of afterglow signal.
As mentioned above, the PL behavior between Cd2+ and Mn2+ were almost identical regarding to band width, peak position, and even lifetime. However, they showed a large difference in temperature-dependent luminescence. As shown in Figure 2a, the pristine crystal exhibited a substantial enhancement in PL intensity at high temperature while the doped crystal remained almost identical. To shed more light on their difference, the temperature-dependent PL spectra were collected in a temperature range from 25 to 200oC (Figure S7). Indeed, the PL intensity of pristine crystals increased strikingly by 4 folds while that of the doped one decreased by 20%, which was in good agreement with the photographs (Figure 2a, Figure S8). The PL decrease was readily ascribed to thermal quenching, yet the 4-fold enhancement remained to be explained.
To understand such an unusual enhancement, temperature-dependent absorption at 275 nm was conducted in a homemade temperature module (Figure 2b). The incident light intensity of a 275-nm LED before and after the doped crystal was recorded with varied temperature, and the difference was considered as the absorbed portion. Surprisingly, the absorption of the pristine crystals increased by 2.3 folds from 25 oC to 200 oC while that of the doped one remained almost constant. In this sense, the additional thermal-induced absorption was the major factor for the 4-fold enhancement of PL. Note that the such temperature dependence of absorption has been well documented in many semiconductors, such as crystalline silicon[25]and CH3NH3PbI3.[26]Another possible factor could be the phonon-assisted processes, which stimulated the trapped excitons to radiative channel.[27, 28] The boosted PL QY eventually enhanced the PL intensity by the residual 1.7-fold. Based on this assumption, the PL QY of pristine crystals were 73% at 200oC.
Apart from phonon-assisted process, Mn2+-doping content was found a facile tool to boost the PL QY of crystals. To this end, a series of Mn2+-doped crystals were grown for an optimal doping concentration (Figure 2c). The PL QY increased from 42% to 100% when doping content increased from 0 to 4 mol%, followed by dropping to 80% at heavy doping of 15 mol%. To investigate the PL QY dropping, lifetime measurement at ~ 600 nm was conducted for varied doping content (Figure 2d). The decay curves showed that with the Mn2+ content increasing from 0% to 15%, the lifetime decreased from 14.5 to 5.7 ms. It was attributed to the concentration quenching effect, whereby excitation energy was migrated to quenching sites via multiple cross-relaxation processes.[29, 30] Surprisingly, time-resolved PL mapping in the range from 480 nm to 760 nm demonstrated a clear afterglow feature of CsCdCl3:4% Mn2+crystals. Two kinds of decay centers were clearly distinguished in time domain, including a Gaussian profile at 598 nm of PL decay and an ultralong afterglow trace extending to 0.2 s (Figure 2e).