FIGURE 1. Crystal structure and luminescence property ofCsCd1-xCl3:x%
Mn crystals. (a) Schematic structure of CsCdCl3. (b)
XRD pattern suggested a pure phase
after doping content up to 15%. (c)
Steady-state photoluminescence
excitation and emission spectrum before (bottom) and after (top) 4%
Mn2+ doping. Insets are photographs of a crystal under
day light and UV light, respectively.
(d) Photoluminescence decay curves of
CsCdCl3 and CsCdCl3:4% Mn crystals.
CsCdCl3 single crystal was grown by a hydrothermal
method.[17] Typically, powders of CsCl and
CdCl2 (or a mixture with MnCl2) were
stoichiometrically weighted and loaded in a vessel with Teflon lining.
The vessel was then sealed in a steel cup and kept at 180 °C for 12 h in
a muffle furnace, whereby the reagents were allowed to dissolve
completely. The crystal growth was initiated by a slow cooling procedure
from 180 °C to 30 °C at a rate of 3 oC/h. Transparent
crystals at the bottom of vessel were then collected and rinsed with
isopropanol before drying at room temperature (Figure S1). To identify
the thermal stability of CsCdCl3:4% Mn,
thermogravimetric analysis (TGA) test was performed in a temperature
range from 30 to 1000 oC (Figure S2). There was no
significant weight loss under 550 oC, suggesting a
robust thermal stability.
CsCdCl3 single crystal adapted a hexagonal structure
(ICSD:16575) in a space group of
P63/mmc,[21] where a pair of
face-sharing
[Cd2Cl9]5-octahedra were corn-connected with the other six
[CdCl6]4- octahedra (Figure 1a),
as further evidenced by the powder X-ray diffraction (XRD) measurement.
Importantly, the crystal structure retained after heavy doping of
Mn2+ up to 15 mol%, largely due to the similarity of
ionic valence between Cd2+ and Mn2+.
To probe the actual concentration of Mn ions in lattice, energy
dispersive spectroscopy (EDS) measurement was performed (Figure S3). The
actual concentration showed a linear increase with increasing nominal
doping content, although a substantial deviation was also observed.
Owing to their identical valence, the Mn2+ likely
substituted the lattice site of Cd2+. Since
Mn2+ (0.67 Å) ion was significantly smaller than
Cd2+ (0.96 Å), the substitution was further confirmed
by the shrinkage of unit cell, as shown in XRD where the (204) peak
shifted towards high angle (Figure 1b). Before doping, the pristine
crystals were transparent in visible region as a result of the wide
bandgap (4.80 eV) (Figure S4). After doping of Mn2+,
however, the crystal showed a pink tint which further deepened as dopant
content increased (Figure S5).
Unlike their changed color, the photoluminescence of
CsCdCl3 and CsCdCl3:4%
Mn2+ single crystals were essentially identical under
UV lamp excitation (Figure 1c). The pristine crystal showed an orange
photoluminescence (PL) at 589 nm with a full-width-at-half-maximum
(FWHM) of 78 nm and a sharp photoluminescence excitation (PLE) peak at
258 nm, leading to an extremely large Stokes shift up to 2.70 eV. After
doping, the major excitation peak shifted from 258 nm to 292 nm.
Besides, a series of weak excitation peaks emerged, which was ascribed
to the d-d transition of
Mn2+.[18, 22] The emission peak
slightly shifted to 598 nm with a Stokes shift up to 2.09 eV.
Importantly, the PL QY was improved from 42% to ~ 100%
after 4-mol% Mn2+ doping. To understand such PL
improvement, emission lifetime measurement was conducted (Figure 1d).
Interestingly, the lifetime slightly decreased from 14.5 ms to 13.0 ms,
suggesting no significant change. We further collected the excitation
maps of both samples, and found a large overlap between two excitation
centers (Figure S6). Such resonant emission centers could be ascribed to
Cd2+ (3Eg →1A1g) and Mn2+(4T1 →6A1), respectively, featuring a common
forbidden d-d transition.[22] Compared to
self-trapped exciton,[23]Cd2+ions were more likely responsible for the yellow emission of millisecond
lifetime.[24] Therefore, the doping of
Mn2+ passivated the non-radiative channel of
Cd2+ by a fast resonant energy transfer, resulting in
a boosted PL QY.