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+ (3Eg1A1g) and Mn2+(4T16A1), 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.