Transparent glassy composites incorporating lead-free anti-perovskite halide nanocrystals enable tunable emission and ultrastable X-ray imaging

Abstract. Lead halide perovskite materials exhibit excellent scintillation performance, which, however, suffer from serious stability and toxicity problems. In contrast, the heavy metal-free anti-perovskite materials   [  MX4  ]  XA3 (A = alkali metal; M = transition metal; X = Cl, Br, I), a class of electron-inverted perovskite derivatives, exhibit robust structural and photophysical stability. Here, we design and prepare a lead-free   [  MnBr4  ]  BrCs3 anti-perovskite nanocrystal (NC)-embedded glass for efficient X-ray-excited luminescence with high-resolution X-ray imaging with a spatial resolution of 19.1  lp mm  −  1. Due to the unique crystal structure and the protection of the glass matrix, the Cs3MnBr5 NC-embedded glass exhibits excellent X-ray irradiation stability, thermal stability, and water resistance. These merits enable the demonstration of real-time and durable X-ray radiography based on the developed glassy composite. This work could stimulate the research and development of novel metal halide anti-perovskite materials and open a new path for future development in the field of high-resolution and ultrastable X-ray imaging.


Introduction
High-resolution and ultrastable X-ray imaging methods required in material inspection, medical diagnostics, astronomical discovery, and scientific research have stimulated extensive research on X-ray-responsive materials with high X-ray attenuation, efficient scintillation, fast light decay, and robust durability. [1][2][3][4] Recently, lead halide-based perovskites have attracted growing attention in the field of X-ray imaging, due to their excellent high luminescence efficiency, high X-ray attenuation ability, and short fluorescence lifetime. [5][6][7][8] However, they are restricted in the scintillation field due to the toxicity of heavy metal Pb, low photon yield caused by severe self-absorption effect, and poor X-ray irradiation stability. 9,10 In order to solve the above problems, many types of lead-free zero-dimensional (0D) metal halides, 11 such as Cu-based halides, [12][13][14] Ag-based halides, 15 Zr-based halides, 16 and Mn-based halides, 17,18 have been developed as effective scintillators for X-ray detection and imaging on account of their high photon yield, diversity of composition and structure, and unique self-trapped excitons luminescence mechanism. Nevertheless, most of them are fabricated in thin-film form or wafers for X-ray imaging, 11 which usually demonstrate low imaging resolution due to the light scattering by the large particles and crystal boundary. 9 In addition, lead-free 0D metal halides suffer from poor stability, especially in a hot and humid environment. 19 To improve the imaging resolution and durability, a more practical solution would be the encapsulation of the lead-free 0D metal halides with controllable size into a stable matrix. Recently, lead-free halide NCs have been crystalized in glass and exhibit potential in light-emitting devices, 20 but the dynamic X-ray imaging in a high-temperature and humid environment is hard to realize.
Compared with lead halide perovskite materials, the configuration of anti-perovskite materials can be represented as ABX 3 , but electronically inverted (X is a cation, and A and B are anions or anionic groups). 21,22 From the point of view of structural chemistry, anti-perovskite can accommodate a variety of elements, forming a large family of functional materials. Anti-perovskite materials have shown various interesting properties, such as magnetism, 23 ionic conductivity, 24 superconductivity, 25 and negative thermal expansibility, 26 but have rarely been reported as photoluminescent materials. One type of anti-perovskite is ½MX 4 XA 3 (A = alkali metal; M = transition metal; X = Cl, Br, I), in which the luminescence center is the ½MX 4 2− tetrahedron filled in the three-dimensional (3D) XA 6 octahedral anti-perovskite skeleton. This unique structure can effectively reduce the interaction of the luminescence center and increase the spatial confinement effect so that anti-perovskite materials generally have high quantum efficiency and luminescence stability. 27 Recently, the specific requirements for performing X-ray imaging at high temperatures have increased dramatically. Nondestructive inspection of some high temperature industrial equipment can be done easily by X-ray imaging applications. 28,29 In addition, some investigation of the effects of high temperature on a polymer electrolyte fuel cell also needs high temperature X-ray imaging technology. 30,31 What is more, high-temperature X-ray microtomographic imaging can supply a new technique for studying mechanical behavior of multiphase composites. 32 Most importantly, the specific requirements for detectors used in space aimed at achieving stunning elevated temperature stability. 33,34 Therefore, it would be highly attractive to explore and design an anti-perovskite material for X-ray imaging applications in a high-temperature environment.
Here, we demonstrate state-of-the-art high-resolution and ultrastable X-ray imaging in a high-temperature and humid environment using the designed lead-free Cs 3 MnBr 5 antiperovskite nanocrystal (NC)-embedded glass. Mn 2þ ions are transferred into Cs 3 MnBr 5 NCs through in situ crystallization in the glass matrix during annealing, therefore displaying tunable luminescence color from red to green controlled by the annealing schedule. With the protection of the transparent glass matrix, Cs 3 MnBr 5 NCs show excellent optical properties, good machinability, and high stability. As expected, the Cs 3 MnBr 5 NC-embedded glass exhibits X-ray detection limit of 767 nGy air s −1 , a high X-ray imaging spatial resolution of 19.1 lp mm −1 and a high X-ray dose irradiation stability at 5.775 mGy air s −1 . More importantly, this transparent NC-glass composite enables high-resolution X-ray imaging even in a high-temperature and humid environment. Our results would have strong implications for the development of next-generation X-ray imaging devices.

Synthesis and Structural Characterization of Cs 3 MnBr 5 NCs in the Glass
A precursor glass (PG) containing cesium, manganese, and bromine elements is designed and fabricated by the melt-quenching method, and Cs 3 MnBr 5 NCs are crystallized in the glass matrix by annealing above the glass transition (T g ) temperature ( Fig. S1 in the Supplemental Material), as shown in Fig. 1(a). As a lead-free anti-perovskite material, the Cs 3 MnBr 5 crystal consists of ½MnBr 4 2− tetrahedrons filled in the (3D) BrCs 6 octahedral skeleton [ Fig. 1 Fig. 1(d)]. In addition, X-ray photoelectron spectroscopy (XPS) was performed on the glass sample before and after annealing ( Fig. S2 in the Supplemental Material). The characteristic peaks of Mn 2p 1∕2 (653.4 eV) and 2p 3∕2 (642.1 eV) are observed, which are higher than that in manganese halide (<640.7 eV), indicating that the electron density and covalent bond ratio of Cs 3 MnBr 5 NC-embedded glass are increased. 36 This may result in stronger ionic bonding between manganese ions and bromine ions, which could enhance the stability under high-energy-ray irradiation. 17 The electron paramagnetic resonance (EPR) spectra show a wide EPR signal with a line width of 609.4 G instead of six fine structures because of the high concentration in the glass (Fig. S3 in the Supplemental Material). 37 The signal is more obvious after annealing, which is due to the stronger magnetic coupling caused by the decrease in the spacing of Mn 2þ ions after the Cs 3 MnBr 5 NCs crystallized in the glass. 38  NCs in the glass after annealing.

Photoluminescence Properties of Cs 3 MnBr 5 NCs in the Glass
As a typical transition metal ion, the optical properties of Mn 2þ ions are influenced by the strong interactions between electrons in their outermost d orbitals and their ligands, which can be described by the crystal field strength (10Dq) and the Racah parameter (B), respectively. 39,40 The Tanabe . This is because Mn 2þ is in a stable tetragonal and hexagonal field environment, respectively. Due to the crystal coordination environment of Cs 3 MnBr 5 NCs, the green emission lifetime reaches the microsecond range, which is often required for dynamical X-ray imaging.
To further increase the concentration of Cs 3 MnBr 5 NCs, we also investigated the effect of annealing duration on the luminescence properties of the samples. The glass samples were treated at 570°C for 5 to 40 h, respectively, and a series of photoluminescence (PL) spectra and lifetime decay curves were recorded [Figs. S6(a-d) in the Supplemental Material]. It can be found that with the increase in annealing duration, the green emission of the samples is gradually enhanced due to the increase in the concentration of Cs 3 MnBr 5 NCs. However, the scattering caused by the increase in crystal size also leads to . In contrast to the typical CsPbX 3 (X = Cl, Br, I) materials, which have poor thermal and optical stability, the stability of Cs 3 MnBr 5 NC-embedded glass has been greatly improved. At 503 K, the green emission of the sample retains 66% of the intensity recorded at room temperature [ Fig. S9(a, b) in the Supplemental Material]. In addition, under the continuous irradiation of a 375 nm laser with 6.4 W∕cm 2 for 60 min, the PL intensity of Cs 3 MnBr 5 remained unchanged ( Fig. S10(a, b) in the Supplemental Material). These characteristics can be attributed to the unique restriction of the high-density luminescent center in the Cs 3 MnBr 5 antiperovskite configuration and their larger binding energies.

Radioluminescence Properties of Cs 3 MnBr 5 NCs in the Glass
The tunable PL, excellent stability, and large Stokes shift (negligible self-absorption) demonstrate the large potential of Cs 3 MnBr 5 NC-embedded glass for applications in the field of X-ray detection applications. In contrast to the PL mechanism, the radioluminescence (RL) mechanism involves an additional photoelectron conversion process that converts high-energy electrons to low-energy ones for radiative recombination [ Fig. 3(a)]. 15 The X-ray is absorbed by heavy atoms, such as Cs, Mn, Ge from Cs 3 MnBr 5 NC-embedded glass through photoelectric effect and Compton scattering. Then, a large number of hot electrons are released and captured by the luminescence center after being thermalized. We investigate the X-ray absorption coefficient of the Cs 3 MnBr 5 NC-embedded glass from 1 to 1000 keV. The absorption coefficient of Cs 3 MnBr 5 NCembedded glass is comparable with the typical scintillators, such as Bi 4 Ge 3 O 12 (BGO) and CsPbBr 3 [ Fig. 3(b)]. 41 The light yield of Cs 3 MnBr 5 NC-embedded glass is ∼5200 photons MeV −1 estimated by using commercial scintillator BGO as a standard sample (Fig. S11 in the Supplemental Material). 42 Due to the dual-emission centers, the glass samples demonstrate RL with an adjustable red-green ratio under X-ray excitation by adjusting the annealing temperature and duration [ Fig. 3(c) and Fig. S12 in the Supplemental Material]. It is shown in Fig. 3(c) that with the increase of annealing duration, the emission color of glass samples changes from light yellow to yellow-green under Xray irradiation. The Cs 3 MnBr 5 NC-embedded glass shows the similar PL and RL spectra, indicating that they originate from the same radiative recombination channel upon X-ray and UV excitation. Interestingly, from the RL spectra recorded under X-ray excitation with different dose rates, we observe that Cs 3 MnBr 5 NCs have a better linear response to the dose rate of X-rays than that of Mn 2þ ions in the glass sample (Fig. S13 in the Supplemental Material). In Fig. 3(d), we show the RL spectra of a series of low-dose X-ray excitations. For scintillation materials, the detection limit is one of the most important parameters for medical examination. Here, the detection limit of the X-ray dose rate is 787 nGy air s −1 for Cs 3 MnBr 5 NC-embedded glass when the signal-to-noise ratio is 3. This value is significantly lower than the dose rate used in X-ray medical diagnosis (5.5 μGy air s −1 dose rate), as shown in Fig. 3(e). 43 Therefore, these merits of Cs 3 MnBr 5 NC-embedded glass make it possible to achieve high-performance X-ray detection and real-time X-ray imaging.

X-Ray Imaging Performance of Cs 3 MnBr 5 NCs in the Glass
Due to the high optical transparency of glass (Fig. S14 in the Supplemental Material) and the excellent RL properties, this Cs 3 MnBr 5 NC-embedded glass is used for high-resolution X-ray imaging. Here, we construct a self-made X-ray imaging system [ Fig. 4(a)], and a series of objects, such as an AI chip, a charging cable, and a circuit board, were imaged by Cs 3 MnBr 5 NC-embedded glass under X-ray and captured by a commercial digital camera. As shown in Fig. 4(b), the internal structures are directly observed, indicating that Cs 3 MnBr 5 NC-embedded glass is promising for electronics inspection and damage imaging. To further demonstrate the X-ray imaging capability of Cs 3 MnBr 5 NC-embedded glass, images of the standard X-ray resolution pattern plate with different thicknesses of glasses were used (Fig. S15 in the Supplemental Material). The highest resolution of the X-ray image was achieved while the glass thickness is 0.6 mm (Fig. S16 in the Supplemental Material). The observation down-limit is between 18 and 20 lp mm −1 , which is consistent with the calculated results of modulation transfer functions (MTFs) according to the slanted-edge method [ Fig. 4(d) and Fig. S17 in the Supplemental Material]. Due to the high transparency of the glass, the high X-ray luminescence efficiency and the negligible self-absorption effect of Cs 3 MnBr 5 NCs, the spatial resolution of our glass sample reaches 19.1 lp mm −1 at MTF ¼ 0.2, which exceeds most recently reported materials for X-ray imaging [ Fig. 4(e)]. In addition, real-time radiography was successfully performed by recording the rotation procedure of an iron spring with an angular velocity of π∕12 rad s −1 . As shown in Fig. 4(f) and Video 1, we can see the image of the rotary spring, further confirming the high-quality and rapid X-ray imaging based on the Cs 3 MnBr 5 NC-embedded glass.

Ultrastable X-Ray Imaging Application
Owing to the encapsulation by the glass medium and the stability of the anti-perovskite structures, the Cs 3 MnBr 5 NC- embedded glass is expected to be used for X-ray imaging applications in medical fields and space stations. Moreover, we are surprised to find that the Cs 3 MnBr 5 NC-embedded glass has very stable X-ray-excited optical properties when tested in a variety of harsh environments. The temperature-dependent RL spectra of Cs 3 MnBr 5 NC-embedded glass are shown in Figs. 5(a) and 5(b). It can be found that at temperature up to 563 K, the RL intensity of red emission from Mn 2þ in the glass drops sharply, while the green emission from Cs 3 MnBr 5 NCs retains 73% of the room temperature intensity. In addition, the high thermal stability is further supported by the periodic change in emission intensity under repeated heating/cooling from 303 to 563 K for six cycles, as shown in Fig. 5(c). The excellent RL stability of Cs 3 MnBr 5 NC-embedded glass in high-temperature environment exceeds most recently reported materials for X-ray imaging (Table S1 in the Supplemental Material). These results indicate that Cs 3 MnBr 5 NC-embedded glass possesses strong thermal stability. We continue with a series of demonstrations of the imaging performance of the Cs 3 MnBr 5 NC-embedded glass in different environments. First, an iron spring is encapsulated in an ABS cylindrical resin and placed together with the Cs 3 MnBr 5 NCembedded glass in a 1 cm × 1 cm × 10 cm colorimetric dish filled with dimethyl silicone oil [ Fig. 5(d)]. The spring inside is not visible under daylight but can be imaged by Cs 3 MnBr 5 NC-embedded glass under X-ray irradiation. In addition, the X-ray images of the spring can be detected at different temperatures, benefited by the stable thermal RL property of Cs 3 MnBr 5 NC-embedded glass [ Fig. 5(e)]. In this experiment, the temperature of the Cs 3 MnBr 5 NC-embedded glass is detected by using an infrared camera. The X-ray image at the temperature up to 121.6°C is still clear, indicating that the Cs 3 MnBr 5 NC-embedded glass can be used for high-temperature X-ray imaging. In another experiment, we place an integrated circuit chip and the Cs 3 MnBr 5 NC-embedded glass in deionized water and recorded the underwater X-ray imaging for 24 h [ Fig. S18(a, b) in the Supplemental Material]. After the Cs 3 MnBr 5 NC-embedded glass was immersed in deionized water for 0, 5, 15, and 24 h, the X-ray images of the internal structure of the chip are still clear. Moreover, the underwater X-ray images remain unchanged at different temperatures (Fig. S19 in the Supplemental Material). Figure 5(f) presents the RL intensity of Cs 3 MnBr 5 NC-embedded glass recorded under repeated high-dose X-ray (5.775 mGy air s −1 ) excitation. It can be seen that after 120 on-off cycles, the RL intensity remains unchanged, showing good long-term X-ray irradiation stability. Moreover, even under a higher dose rate up to 9.66 mG air s −1 , its luminescence was only slightly affected for a short time (Fig. S20 in the Supplemental Material). In addition, the X-ray images from Cs 3 MnBr 5 NC-embedded glass can still be observed after continuous X-ray irradiation for 2 h at a high dose rate up to 5.775 mGy air s −1 [ Fig. 5(g)].
Considering the above X-ray imaging demonstrations in different environments, the Cs 3 MnBr 5 NC-embedded glass is expected to be applied for the next generation of scintillation materials.

Conclusion
In summary, we have successfully prepared an ultrastable monolithic scintillator based on a lead-free Cs 3 MnBr 5 antiperovskite NC-embedded glass. The experimental results show that the Cs 3 MnBr 5 NC-embedded glass has high optical transmittance, excellent tunable optical properties, and durable stability. Therefore, the Cs 3 MnBr 5 NC-embedded glass can achieve an X-ray detection limit of 767 nGy air s −1 , high Xray imaging spatial resolution of 19.1 lp mm −1 , and excellent stability under high-dose X-ray irradiation. The Cs 3 MnBr 5 NC-embedded glass can be made into optical fibers, so the X-ray imaging performance and resolution may be further improved by using the pixelated dual tapered fiber arrays method. 47 More importantly, with the protection of the glass matrix and the stability of the anti-perovskite structure, we demonstrate highresolution X-ray imaging under a high-temperature and humidity environment by using Cs 3 MnBr 5 NC-embedded glass. Our findings not only provide an effective strategy for achieving ultrastable X-ray-excited luminescence in a harsh environment but also broaden the applications of lead-free anti-perovskite materials in the applications of advanced X-ray radiography.

Appendix A: Sample Preparation
The glass samples were fabricated by using the melting-quenching method. Reagent-grade raw materials including GeO 2 , B 2 O 3 , ZnO, CaCO 3 , NaBr, Cs 2 CO 3 , and MnO (glass composition: 37GeO 2 -31B 2 O 3 -3CaO-2ZnO-3MnO-6Cs 2 O-18NaBr) were mixed and then melted at 1100°C. The PG was made by pouring the melt into a mold. Then the glass was annealed at 350°C for 5 h and cooled for 10 h to room temperature to release the thermal stress. The PG samples were cut after removal from the mold and well-polished for crystallization of Cs 3 MnBr 5 anti-perovskite NCs by annealing at 490°C, 530°C, and 570°C.

Appendix B: Sample Characterization
Differential scanning calorimetry was measured by an STA449C Jupiter (Netzsch, Bavaria, Germany) in an air atmosphere with a heating rate of 10°C min −1 . The XRD patterns were recorded by a D8 ADVANCE X-ray diffractometer (Bruker, Faellanden, Switzerland) with Cu∕K α (λ ¼ 0.1541 nm) radiation. Absorption spectra were measured on a Lambda 900 (Perkin Elmer, Waltham, MA) spectrophotometer. An Edinburgh FLS 920 instrument (Edinburgh Instruments Ltd., Livingston, United Kingdom) equipped with a photomultiplier tube for light detection (Hamamatsu, Japan) was used to measure the PL spectra, excitation spectra, and lifetime decay of bulk glass sample. A 450 W ozone-free xenon lamp and a microsecond-pulsed xenon flash lamp were used as excitation sources during PL and lifetime decay measurement, respectively. For thermal and optical stability tests, PL spectra were excited by a 375 nm laser and recorded by an Ocean Optics HR4000 spectrometer. XPS measurements were performed on an Axis Ultra DLD XPS instrument (Kratos, England) with a monochromatic Al K α source (1486.6 eV). The morphology and size distribution of Cs 3 MnBr 5 NCs were measured by transmission electron microscopy (TEM, JEM-2100F, JEOL, Japan). The glass sample after crystallization was put into an agate mortar and ground to powder in ethanol for 20 min, and then dispersed in ethanol with ultrasonic treatment for 10 min. Finally, three to five drops of supernatant were dropped on the copper net for the TEM image test. The model of the electric rotary displacement table used for real-time imaging was Thorlabs PRM1Z8. FLS1000 spectrofluorometer (Edinburgh Instruments Ltd., United Kingdom) equipped with an X-ray tube (MAGPRO, Moxtek, W target and tube voltage 50 kV) is used to measure the RL spectra. The digital camera used for X-ray imaging is a Nikon d610.

Appendix C: Calculation of the X-Ray Attenuation Coefficients
The detail of calculation is fully described in the subsection entitled "Supplementary Note 2" in the Supplemental Material.

Code, Data, and Material Availability
Data underlying the results presented in this paper may be obtained from the corresponding author upon reasonable request. All software is also available from the corresponding author upon reasonable request.