Synthesis and characterization
Similar to other low-dimensional organic metal halide hybrids reported by our group28,29,30, 0D (C38H34P2)MnBr4 single crystals were obtained by diffusing diethyl ether into a dichloromethane (DCM) precursor solution containing ethylenebis(triphenylphosphonium bromide) (C38H34P2Br2) and MnBr2 in a ratio of 1:1. The details of synthesis and purification could be found in Supplementary Scheme 1 and Fig. 1. The crystal structure of (C38H34P2)MnBr4 single crystals was determined by single-crystal X-ray diffraction (SCXRD). As shown in Fig. 1a and Supplementary Fig. 2, (C38H34P2)MnBr4 crystallizes at a monoclinic space group of C2/c, possessing 0D structure at the molecular level with MnBr4 tetrahedrons isolated and surrounded by C38H34P22+ cations. The manganese center adopts a typical tetra-coordinated geometry bonded to bromide ions, with an average Mn–Br bond length of 2.51 Å and bond angle of 108.48° (Supplementary Tables 1 and 2), similar to those of previously reported MnBr4 complexes35. The powder XRD pattern of (C38H34P2)MnBr4 powder is identical to the simulated result from SCXRD data (Fig. 1b), suggesting the high phase purity of the as-prepared single crystals. No weight loss was observed before 310 °C in thermogravimetric analysis (TGA) as shown in Supplementary Fig. 3, suggesting a high thermal stability. The differential scanning calorimetry (DSC) results with an endothermic peak at 295 °C (Supplementary Fig. 3), which could be the melting point of (C38H34P2)MnBr4, suggest its high phase stability at elevated temperatures below 295 °C.
The (C38H34P2)MnBr4 single crystals are pale green under ambient light and become highly emissive upon irradiating with ultraviolet (UV) light as shown in Fig. 1c, d. The photophysical properties were further investigated using UV–vis absorption and steady-state PL spectroscopies. As shown in Fig. 1e, (C38H34P2)MnBr4 exhibits an intense absorption band around 285 nm along with two absorption peaks at 360 and 450 nm. The excitation spectrum has the same features as the absorption spectrum in a low-energy band, which are corresponding to two groups of transitions: 6A1 → 4G and 6A1 → 4D. (See the optical transitions in tetrahedrally coordinated Mn2+ ion in Supplementary Scheme 2.) Upon irradiation in the range of 300–400 nm, bright green emission peaked at 517 nm was observed with a full-width at half-maximum of 51 nm, a high PLQE of ~95%, and a long single-exponential decay lifetime of 318 µs (R2 = 0.999) (Supplementary Fig. 4). The strong green emission is well known to be from d–d 4T1 → 6A1 transition of Mn2+ ion with a tetrahedral coordination geometry. Moreover, (C38H34P2)MnBr4 demonstrated great moisture stability with PL intensity unchanged after exposure in an ambient atmosphere for 1 month (Supplementary Fig. 5). The high emission efficiency together with good quality of facilely prepared single crystals suggest the suitability of (C38H34P2)MnBr4 for luminescent devices.
X-ray scintillation properties
To explore the scintillation performance of (C38H34P2)MnBr4, a commercially available scintillation material, cerium-doped lutetium aluminum garnet (Ce:LuAG), was used as a standard reference as it exhibits a similar PL emission peaked at ~520 nm that could minimize the influence of response difference by detectors. The X-ray radioluminescence (RL) spectra of (C38H34P2)MnBr4 and Ce:LuAG were obtained by using Edinburgh FS5 fluorescence spectrophotometer equipped with a X-ray generator (Amptek Mini-X tube, Au target, 4 W). As shown in Supplementary Fig. 6, both RL emissions are identical to their PL emissions. Interestingly, the RL intensity of (C38H34P2)MnBr4 is >3 times higher than that of Ce:LuAG under the same X-ray dose rate irradiation. Moreover, the X-ray image of (C38H34P2)MnBr4 single crystals is much brighter than that of Ce:LuAG, as shown in Fig. 2a, suggesting that (C38H34P2)MnBr4 is more sensitive to X-ray irradiation than Ce:LuAG. To evaluate the scintillator response to X-ray dose rate, the RL intensities were measured under various X-ray dose rates for (C38H34P2)MnBr4 and Ce:LuAG. Figure 2b and Supplementary Fig. 7 show that both scintillators exhibit excellent linearities to the X-ray dose rates in a large range from 36.7 nGy s−1 to 89.4 μGy s−1. Moreover, (C38H34P2)MnBr4 exhibits a higher response to X-ray dose than Ce:LuAG with a larger slope. The reproducibility of the responses to X-ray for (C38H34P2)MnBr4 was validated by using single crystals with different sizes and shapes. Almost the same sensitivity was recorded for all the samples, as shown in Supplementary Fig. 8. The detection limit of X-ray dose rate was derived to be 72.8 nGy s−1 for (C38H34P2)MnBr4 when the signal-to-noise ratio (SNR) is 3, which is ~75 times lower than the dose rate required for X-ray diagnostics (5.5 μGy s−1)12. Light yield is another important parameter to evaluate the performance of scintillators, which is dependent on the amplitude of X-ray response and the RL spectra. Since the X-ray dose response of (C38H34P2)MnBr4 is 3.2 times higher than that of Ce:LuAG (with a light yield of 25,000 photon MeV−1) and they have a similar RL spectrum, the light yield of (C38H34P2)MnBr4 could be derived to be ~79,800 photon MeV−1. As shown in Fig. 2c, the light yield of (C38H34P2)MnBr4 is comparable to those of recently reported lead-free metal halides, such as Cs3Cu2I5 (79,279 photon MeV−1)43 and Rb2CuBr3 (91,056 photon MeV−1)33, and much better than those of Rb2CuCl3 (16,600 photon MeV−1)34, widely investigated CsPbBr3 nanocrystals (21,000 photon MeV−1)22, and many commercially available scintillators, such as CsI:Tl (54,000 photon MeV−1) and CdWO4 (28,000 photon MeV−1). Moreover, based on the toxicity classification (health and environment) information of metal halides from material safety data sheet, (C38H34P2)MnBr4 is believed to be significantly less toxic than existing scintillators mentioned above. As shown in Supplementary Table 3, Pb(II), Cu(I), CsI, and GdWO4 possess the most severe toxicity in the environment, and Tl(I) and CsI are moderately toxic to health. Also, 87Rb isotope is radioactive34. Mn(II) is considered to be less toxic for health and friendly to the environment. The stability of (C38H34P2)MnBr4 single crystals against X-ray irradiation was evaluated by monitoring the changes of RL intensity under continuous X-ray irradiation with a dose rate of 89.4 μGy s−1. Figure 2d shows that little-to-no radio-degradation was observed after 4 h exposure to X-ray irradiation, suggesting high stability for scintillator applications.
To further validate the potential of (C38H34P2)MnBr4 as scintillation material for practical X-ray imaging, a home-built X-ray imaging system was constructed, as shown in Supplementary Fig. 9. The scintillator screen was prepared by refilling the glass holder with (C38H34P2)MnBr4 fine powders with the particle size <3 µm (see scanning electron microscope (SEM) images in Supplementary Fig. 10). A speaker chip with a size of 9 mm × 6 mm, as shown in Fig. 2e, was used as a target placed between the X-ray source and the scintillator screen for X-ray image. The configuration inside of the chip cannot be seen directly with our eyes, which however could be revealed clearly by X-ray imaging using a (C38H34P2)MnBr4-based scintillator (Fig. 2f). The large difference in X-ray absorption for different materials in the chip resulted in spatial intensity contrast displayed in the scintillator screen. The spatial resolution was calculated as 0.322 mm by fitting the point spread function of the intensity profile (Fig. 2g). Image contrast is another important parameter for practical imaging applications; image lag or ghosting would happen if the emission with a long lifetime has a strong afterglow after X-ray being turned off. To exclude the effect of afterglow, we measured the afterglow intensities of (C38H34P2)MnBr4, as shown in Supplementary Fig. 11. The intensity decreased to the background level in 10 ms after the cease of the excitation source, indicating the suitability for high contrast imaging. The excellent performance of X-ray imaging could be attributed to the negligible self-absorption, high PLQE, light yield, and low detection limit of (C38H34P2)MnBr433,34,43,44.
Flexible devices have received tremendous attention nowadays for their good foldability, high crack resistance, favorable compatibility, and potential application in portable and wearable devices. Here, flexible scintillators with large size (4.5 × 5.8 cm2) were demonstrated by blending (C38H34P2)MnBr4 fine powders with polydimethylsiloxane (PDMS). As shown in Fig. 3a–c, the resulting films show excellent flexibility, which can be easily bent and stretched. Moreover, the film shows high uniformity and strong emission under UV irradiation (Fig. 3d–f). The scintillation performance of flexible scintillation screens was characterized as shown in Supplementary Fig. 12, which exhibit excellent linearities to the X-ray dose rates in a large range from 36.7 nGy s−1 to 89.4 μGy s−1, with a slightly lower light yield (66,256 photon MeV−1) and detection limit (461.1 nGy s−1), as compared to those of single crystals. This is not surprising, considering that the content of (C38H34P2)MnBr4 is reduced in the blends, the distribution of (C38H34P2)MnBr4 might not be perfectly uniform in the blends, and PDMS could also affect the X-ray absorption. To demonstrate the capability of the X-ray imaging, a wrench and a speaker chip were scanned as the targets (Fig. 3g, h). Distinct color contrast and detail inside of the chip can be displayed in the flexible film with good resolution.