2.1.

Synthesis and Structural Characterization of Cs₃MnBr₅ NCs in the Glass

A precursor glass (PG) containing cesium, manganese, and bromine elements is designed and fabricated by the melt-quenching method, and ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5\mathrm{NCs}$ 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 ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ crystal consists of $[{\mathrm{MnBr}}_4{]}^{2-}$ tetrahedrons filled in the (3D) ${\mathrm{BrCs}}_6$ octahedral skeleton [Fig. 1(a)]. From the density functional theory (DFT) calculations of band structures and corresponding density of states [Figs. 1(b) and 1(c)], the direct bandgap of ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ crystal is $\sim 3.367\mathrm{eV}$, contributing to the absorption band in the ultraviolet (UV) region and high optical transmittance in the visible region.³⁵ The diffraction peaks at 22.783 deg, 26.914 deg, and 29.454 deg corresponding to (004), (213), and (310) crystal facets of ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ (PDF#27-0117) are observed in the X-ray diffraction (XRD) pattern after the glass was annealed at 570°C for 5 h [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 $2{\mathrm{p}}_{1/2}$ (653.4 eV) and $2{\mathrm{p}}_{3/2}$ (642.1 eV) are observed, which are higher than that in manganese halide ($<640.7\mathrm{eV}$), indicating that the electron density and covalent bond ratio of ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5\mathrm{NC}$-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 ${\mathrm{Mn}}^{2+}$ ions after the ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NCs crystallized in the glass.³⁸ A transmission electron microscope (TEM) image shows dispersed NCs with an average size of 18.3 nm, indicating the formation of NCs with good crystal quality in the glass matrix [Figs. 1(e) and 1(f)]. The crystal lattice fringes with a spacing of 0.305 nm can be seen in the high-resolution transmission electron microscope (HRTEM) image, which corresponds to the (310) crystal facet of ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ [Fig. 1(g)]. The above results confirm the successful precipitation of ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NCs in the glass after annealing.

Fig. 1

Structural properties of ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NCs crystallized in the glass. (a) Schematic diagram of glass network structure before (left) and after (middle) annealing and the anti-perovskite structure of the ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ crystal (right). (b) DFT-calculated band structures and (c) electronic density of states of the ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ crystal. (d) XRD patterns of the PG and the glass sample after annealing at 570°C for 5 h. (e) TEM image, (f) corresponding size distribution, and (g) HRTEM image of the glass annealed at 530°C for 5 h. The inset in (g) is the fast Fourier transform pattern corresponding to the (310), (213), and (004) crystal facet. The scale bars in (e) and (g) are 100 and 10 nm, respectively.

2.2.

Photoluminescence Properties of Cs₃MnBr₅ NCs in the Glass

As a typical transition metal ion, the optical properties of ${\mathrm{Mn}}^{2+}$ 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–Sugano diagram in Fig. 2(a) reveals the change in the energy level of ${\mathrm{Mn}}^{2+}$ as the electronic configuration of $3{\mathrm{d}}_5$ is sensitive to local perturbations. The 10Dq and $B$ for Mn (IV) in ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NCs are calculated to be 8192 and $811{\mathrm{cm}}^{-1}$, and for Mn (VI) in glass are 11,306 and $732{\mathrm{cm}}^{-1}$, respectively (see note 1 in the Supplemental Material). These calculations confirm that the $3{\mathrm{d}}_5$ electronic configuration of ${\mathrm{Mn}}^{2+}$ doped in the glass was significantly changed after the crystallization of the glass.

Fig. 2

PL properties of ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NCs in the glass. (a) Tanabe–Sugano diagram of $3{\mathrm{d}}_5$ electronic configuration of ${\mathrm{Mn}}^{2+}$ ions. (b) Absorption and (c) PL spectra of the PG and samples annealed at different temperatures for 5 h. The inset in (b) shows the photographs of the glass samples taken under daylight (top) and 365 nm UV light (bottom). (d) PLE mapping spectra of the glass sample annealed at 570°C for 5 h. (e) Fluorescence decay curves of ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NCs and (f) ${\mathrm{Mn}}^{2+}$ in glass annealed at different temperatures. The fitting curves are fitted with (e) single-exponential and (f) double-exponential, respectively. The excitation wavelength used in (c), (e), (f) is 365 nm.

The absorption spectra of the PG and the glass samples annealing at different temperatures are recorded in the wavelength region of 300 to 800 nm [Fig. 2(b)]. The narrow peak at 413 nm is attributed to the electronic transition of ${{}^6\mathrm{A}}_1(\mathrm{S})\to {{}^4\mathrm{A}}_1$, ${}^4\mathrm{E}(\mathrm{G})$ of ${\mathrm{Mn}}^{2+}$ ions. Under the excitation of 365 nm UV light, a green emission band peaking at 523 nm emerges after crystallization, which originates from ${\mathrm{Mn}}^{2+}$ in the tetrahedron of ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NCs [Fig. 2(c)]. It is worth noting that there are two luminescence centers in the glass after annealing, attributed to ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NCs and the remaining ${\mathrm{Mn}}^{2+}$ ions in the glass matrix. Thus, the luminescence color can also be adjusted by controlling the excitation wavelength due to the two different luminescence centers, as shown in Fig. 2(d) and Figs. S4(a, b) in the Supplemental Material. The variation of two different emission bands in the time-resolved emission spectrum (1 to 10 ms) shows two different coordination environments around ${\mathrm{Mn}}^{2+}$ in the annealed glass sample (Fig. S5 in the Supplemental Material). With the increase of annealing temperature, the fluorescence lifetimes of the green and red emission change a little [Figs. 2(e) and 2(f)]. This is because ${\mathrm{Mn}}^{2+}$ is in a stable tetragonal and hexagonal field environment, respectively. Due to the crystal coordination environment of ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NCs, the green emission lifetime reaches the microsecond range, which is often required for dynamical X-ray imaging.

To further increase the concentration of ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ 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 ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NCs. However, the scattering caused by the increase in crystal size also leads to a large optical loss of the glass samples. Thus, the sample treated at 570°C for 10 h has the highest PL quantum yield of 35.5% (Fig. S7 in the Supplemental Material). Depending on the annealing temperature and duration, the emission color of the glass samples can be precisely adjusted from red to green (Fig. S8 in the Supplemental Material). In contrast to the typical ${\mathrm{CsPbX}}_3$ ($\mathrm{X}$ = Cl, Br, I) materials, which have poor thermal and optical stability, the stability of ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ 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 $6.4\mathrm{W}/{\mathrm{cm}}^2$ for 60 min, the PL intensity of ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ 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 ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ anti-perovskite configuration and their larger binding energies.

2.3.

Radioluminescence Properties of Cs₃MnBr₅ NCs in the Glass

The tunable PL, excellent stability, and large Stokes shift (negligible self-absorption) demonstrate the large potential of ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ 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 ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ 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 ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NC-embedded glass from 1 to 1000 keV. The absorption coefficient of ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NC-embedded glass is comparable with the typical scintillators, such as ${\mathrm{Bi}}_4{\mathrm{Ge}}_3{\mathrm{O}}_{12}$ (BGO) and ${\mathrm{CsPbBr}}_3$ [Fig. 3(b)].⁴¹ The light yield of ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NC-embedded glass is $\sim 5200\text{photons}{\mathrm{MeV}}^{-1}$ 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 X-ray irradiation. The ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ 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 ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NCs have a better linear response to the dose rate of X-rays than that of ${\mathrm{Mn}}^{2+}$ 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 $787{\mathrm{nGy}}_{\text{air}}{\mathrm{s}}^{-1}$ for ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ 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 ($5.5\mu {\mathrm{Gy}}_{\text{air}}{\mathrm{s}}^{-1}$ dose rate), as shown in Fig. 3(e).⁴³ Therefore, these merits of ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NC-embedded glass make it possible to achieve high-performance X-ray detection and real-time X-ray imaging.

Fig. 3

RL properties of ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NCs in the glass. (a) Schematic diagram of X-ray-induced luminescence mechanism of ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NCs and ${\mathrm{Mn}}^{2+}$ ions in the glass. (b) X-ray attenuation efficiency of ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NC-embedded glass, BGO, and ${\mathrm{CsPbBr}}_3$ crystal. (c) RL spectra of ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NC-embedded glass annealed at 570°C for different durations recorded under X-ray excitation with a dose rate of $4.814{\mathrm{mGy}}_{\text{air}}{\mathrm{s}}^{-1}$. The inset shows the photographs of the corresponding samples taken under X-ray irradiation. (d) RL spectra of the sample annealed at 570°C for 40 h recorded under different low X-ray dose rates. (e) Linear relationship between the low dose rate and RL intensity of the sample annealed at 570°C for 40 h.

2.4.

X-Ray Imaging Performance of Cs₃MnBr₅ NCs in the Glass

Due to the high optical transparency of glass (Fig. S14 in the Supplemental Material) and the excellent RL properties, this ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ 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 ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ 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 ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NC-embedded glass is promising for electronics inspection and damage imaging. To further demonstrate the X-ray imaging capability of ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ 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 $20\mathrm{lp}{\mathrm{mm}}^{-1}$, 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 ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NCs, the spatial resolution of our glass sample reaches $19.1\mathrm{lp}{\mathrm{mm}}^{-1}$ at $\mathrm{MTF}=0.2$, 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 $\pi /12\mathrm{rad}{\mathrm{s}}^{-1}$. 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 ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NC-embedded glass.

Fig. 4

Demonstrations for real-time radiography. (a) The schematic of the X-ray imaging system. (b) Photographs of an AI chip (left), charging cable (middle), and circuit board (right) under daylight and X-ray irradiation. Scale bars, 1 cm. (c) Bright-field and X-ray images of the standard X-ray resolution pattern plate with the ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NC-embedded glass. (d) MTF of X-ray images obtained from the ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NC-embedded glass (the thickness is 0.6 mm). (e) Comparisons of spatial resolutions in representative scintillators.^{9,16–18,44–46} (f) Real-time dynamic X-ray images recording the procedure of two-dimensional rotation of an iron spring; the speed of angular velocity is $\pi /12\mathrm{rad}{\mathrm{s}}^{-1}$ (Video 1, MP4, 14 MB [URL: https://doi.org/10.1117/1.AP.5.4.046002.s1]). Scale bar, 5 mm.

2.5.

Ultrastable X-Ray Imaging Application

Owing to the encapsulation by the glass medium and the stability of the anti-perovskite structures, the ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ 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 ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ 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 ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ 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 ${\mathrm{Mn}}^{2+}$ in the glass drops sharply, while the green emission from ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ 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 ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ 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 ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NC-embedded glass possesses strong thermal stability.

Fig. 5

Ultrastable X-ray imaging. (a) Temperature-dependent RL spectra and (b) emission mapping of the glass sample under X-ray irradiation with a dose rate of $4.814{\mathrm{mGy}}_{\text{air}}{\mathrm{s}}^{-1}$. (c) RL intensity of the ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NCs in the glass sample upon six heating/cooling cycling processes over the temperature ranging from 303 to 563 K. (d) Photographs of a cylindrical ABS resin embedded with an iron spring in the air (top) and dimethyl silicone oil (bottom). (e) Thermal imaging photographs (top) and X-ray images (bottom) of the cylindrical ABS resin embedded with an iron spring immersed in dimethyl silicone oil at different temperatures. Scale bar, 1 cm. (f) RL intensity of ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NCs in the glass recorded over continuous 120 on/off cycles during 60 min. (g) Photograph (left) and X-ray images (right) of the chip taken under continuous irradiation for 2 h. Scale bar, 2 mm.

We continue with a series of demonstrations of the imaging performance of the ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NC-embedded glass in different environments. First, an iron spring is encapsulated in an ABS cylindrical resin and placed together with the ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NC-embedded glass in a $1\mathrm{cm}\times 1\mathrm{cm}\times 10\mathrm{cm}$ colorimetric dish filled with dimethyl silicone oil [Fig. 5(d)]. The spring inside is not visible under daylight but can be imaged by ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ 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 ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NC-embedded glass [Fig. 5(e)]. In this experiment, the temperature of the ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ 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 ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NC-embedded glass can be used for high-temperature X-ray imaging. In another experiment, we place an integrated circuit chip and the ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ 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 ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ 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 ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NC-embedded glass recorded under repeated high-dose X-ray ($5.775{\mathrm{mGy}}_{\text{air}}{\mathrm{s}}^{-1}$) 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 $9.66{\mathrm{mG}}_{\text{air}}{\mathrm{s}}^{-1}$, its luminescence was only slightly affected for a short time (Fig. S20 in the Supplemental Material). In addition, the X-ray images from ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NC-embedded glass can still be observed after continuous X-ray irradiation for 2 h at a high dose rate up to $5.775{\mathrm{mGy}}_{\text{air}}{\mathrm{s}}^{-1}$ [Fig. 5(g)]. Considering the above X-ray imaging demonstrations in different environments, the ${\mathrm{Cs}}_3{\mathrm{MnBr}}_5$ NC-embedded glass is expected to be applied for the next generation of scintillation materials.