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).