High-performance organic–inorganic hybrid manganese halide scintillator array for superior-resolution X-ray imaging

Abstract

Pb-free organic–inorganic metal halides play an increasingly important role in X-ray scintillator imaging applications because of their high luminescent quantum yield, excellent stability, and environmentally friendly characteristics. However, traditional scintillator materials suffer from high toxicity and pose environmental risks, while current lead-free materials generally face challenges of low luminescence efficiency, poor stability, severe optical crosstalk and complicated preparation processes, which severely limit the development of high-resolution imaging. To overcome these industrial challenges, we developed a high-performance manganese-based (C₁₉H₁₇ClP)₂MnCl₄ single crystal that achieves an extremely high PLQY of 96.63%, along with an excellent light yield of 53 159.6 photons per MeV and a low detection limit of 182.5 nGy_air per s. Furthermore, the pixelated scintillator fabricated via a solution process features a 25 μm aperture in the array, significantly reducing optical crosstalk while achieving an outstanding spatial resolution of 24.6 lp mm⁻¹. This demonstrates its strong potential for medical and security imaging applications.

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1. Introduction

Scintillators, which can convert high-energy radiation into ultraviolet-visible light, have been widely used in medical diagnosis, nondestructive testing, space exploration, and safety inspection.^1–6 NaI:Tl,⁷ CsI:TI crystals,⁸ Gd₂O₂S:Tb ceramics,⁹ and PbWO₄¹⁰ are commonly used materials for X-ray scintillators. All of them exhibit high scintillation efficiency but suffer from unwanted afterglow, high toxicity, and high manufacturing costs.^11–13 In recent years, organic–inorganic hybrid metal halides have been extensively explored because of their prominent PLQY, high absorption coefficient and low cost.^14–16 However, most materials reported so far contain toxic lead, which severely limits their practical application.^17–23 To solve this problem, lead-free metal halides, for example, Mn-based organic–inorganic hybrids, were rationally designed and synthesized. Because of the similar radii of Mn²⁺ and Pb²⁺, significant lattice distortion can be avoided.²⁴ In addition, Mn itself bears unique characteristics. Xia's group found that a sufficiently long Mn–Mn distance in the crystal improved the photoluminescence yield.²⁵ This suggests that adjusting the distance by selecting appropriate organic cations is an effective method to improve the PL. Tetramethylammonium manganese chloride (C₄H₁₂NMnCl₃), with a relatively small organic size, offered a PLQY of 91.8% and a light yield of 50 500 photons per MeV.²⁶ The larger [C₃₈H₃₄P₂]⁺ cation in ethyne bis-triphenylphosphine manganese(II) bromide ((C₃₈H₃₄P₂)MnBr₄) gave an improved PLQY of 95% and a light yield of 80 000 photons per MeV.²⁷ Mn-based hybrid metal halides exhibit excellent characteristics, including high stability, excellent PLQY, high scintillation rates, negligible self-absorption effects, extremely low detection limits, and low toxicity, making these materials potential candidates for X-ray scintillators.

In addition to material engineering, great efforts have been made to develop methodologies for constructing high-quality scintillator films. For example, Chen et al. prepared a (MTP)₂MnBr₄ transparent wafer by the melt-quenching method, which provided a high light yield of 67 000 photons per MeV, a detection limit of 82.4 nGy_air per s, and an X-ray imaging spatial resolution of 6.2 lp mm⁻¹.²⁸ Liu et al. prepared cesium iodide scintillation thin films with a special microcolumn structure on Shi Ying glass substrates by a vacuum thermal evaporation method.²⁹ A (BTPP)₂MnCl₄-polydimethylsiloxane (PDMS) flexible scintillation screen achieved a high spatial resolution of 14.1 lp mm⁻¹.³⁰ Han et al. combined (C₇H₂₀N₂)₃Mn₄Cl₁₄, (C₇H₂₀N₂)MnBr₄, and (C₇H₂₀N₂)MnI₄ with PDMS to prepare scintillator films, achieving resolutions of 2.7 lp mm⁻¹, 4.7 lp mm⁻¹, and 7.3 lp mm⁻¹, respectively.³¹ Despite the inimitable advantages of the high-temperature melting method in preparing high-performance scintillators, the process is relatively complex. Although array scintillation screens with thermal evaporation structure have good transmission performance, they still face the problem of complicated processes and mismatch of adhesion between crystal columns, which leads to transverse optical crosstalk and reduces resolution.³² Scintillators prepared by grinding materials with PDMS offer a simple and low-cost preparation process. However, thin films made from powder materials face challenges in optical applications because it is not possible to ensure the material powder is uniformly distributed within the PDMS matrix, leading to internal structure defects and light scattering problems.³³ Solving optical crosstalk remains a great challenge for improving imaging resolution.

In this work, we developed a new zero-dimensional (0D) organic–inorganic hybrid halide (C₁₉H₁₇ClP)₂MnCl₄ crystal that exhibits a high PLQY (96.63%). The quadruple coordination of Mn²⁺ tetrahedron ions [MnCl₄]²⁻ resulted in green fluorescence emission. Under X-ray radiation (RL), the obtained light yield was as high as 53 159.6 photons per MeV. A (C₁₉H₁₇ClP)₂MnCl₄ array scintillator screen was fabricated through a simple, low-cost, and universal deposition process by combining the sample with a silicon wafer. The tiny apertures separate the particles, thus effectively reducing the optical crosstalk, showing a highly competitive resolution of 24.6 lp mm⁻¹ and a low detection limit of 182.5 nGy_air per s in X-ray imaging. Benefiting from its straightforward fabrication process and outstanding scintillation performance, the (C₁₉H₁₇ClP)₂MnCl₄ array scintillator shows great potential in the fields of medical diagnosis, public safety and industrial testing.

2. Results and discussion

2.1. Synthesis and characterization of the (C₁₉H₁₇ClP)₂MnCl₄ crystal

The (C₁₉H₁₇ClP)₂MnCl₄ crystal was synthesized by a simple anti-solvent method, as shown in Fig. 1a and the Experimental section. The crystal structure was solved by single-crystal X-ray diffraction (SCXRD) at 193 K. From Fig. 1b and c, it can be seen that one [MnCl₄]²⁻ tetrahedron is surrounded by two larger [C₁₉H₁₇ClP]⁺ organic cations, forming a periodic zero-dimensional (0D) structure. The (C₁₉H₁₇ClP)₂MnCl₄ crystal presented a typical monoclinic structure with space group P2₁/C. The cell parameters are a = 12.3203(5) Å, b = 15.5683(6) Å, c = 20.210(1) Å, α = γ = 90°, β = 96.814(2)° and Z = 4. Detailed XRD data are summarized in Table S1. The average Mn–Cl bond length was 2.37 Å and the Cl–Mn–Cl bond angle was 109.44° in the [MnCl₄]²⁻ tetrahedron (Table S2). With a larger organic cation [C₁₉H₁₇ClP]⁺ separating the [MnCl₄]²⁻ tetrahedra, a longer Mn–Mn distance was obtained. The shortest distance between two adjacent Mn²⁺ is 11.2026 Å and the longest is 11.4812 Å, as shown in Fig. S1. The luminescence properties of crystals are closely related to the crystal structure, especially to the Mn–Mn distance.^25,26,34,35 The longer Mn–Mn distance effectively reduces the spin–spin coupling between Mn ions, reducing the possibility of non-radiative transition, which is helpful for improving the PL efficiency.²⁸ As shown in Fig. 1d, the (C₁₉H₁₇ClP)₂MnCl₄ crystal appears pale green under natural light and brilliant green under 254 nm ultraviolet light. According to the literature, such green emission luminescence should be ascribed to the d–d transition of Mn ions.³⁴ Consistent diffraction peaks of the experimental XRD and the simulated result shown in Fig. 1e confirmed that (C₁₉H₁₇ClP)₂MnCl₄ was successfully synthesized with high crystallinity. In this study, (chloromethyl)triphenylphosphonium (C₁₉H₁₇Cl₂P) was selected as the organic cation due to its significantly superior performance over structurally simpler analogues such as methyltriphenylphosphonium (MTP) and ethyltriphenylphosphonium (ETP). The larger molecular volume of the [C₁₉H₁₇ClP]⁺ cation enables more effective spatial separation of the [MnCl₄]²⁻ tetrahedra, achieving a shortest Mn–Mn atomic distance of 11.2026 Å. This structural feature plays a critical role in enhancing the PLQY by suppressing concentration quenching and non-radiative energy transfer. Beyond spatial effects, the introduction of the chlorine atom in the chloromethyl group enhances molecular polarity, thereby strengthening the organic–inorganic interactions within the crystal lattice. This enhanced interaction significantly improves the structural rigidity, which more effectively suppresses non-radiative transitions and further contributes to achieving a high PLQY. These combined effects ultimately enable the (C₁₉H₁₇ClP)₂MnCl₄ crystal to achieve a PLQY of 96.63%, significantly surpassing the reported values for (ETP)₂MnBr₄ (84%)³⁶ and (MTP)₂MnCl₄ (74.61%).³⁷ Benefiting from the enhanced structural rigidity, the material also exhibits outstanding performance in both thermal stability and moisture resistance. The material maintains structural stability up to 310 °C, demonstrating superior thermal stability compared to (ETP)₂MnBr₄, which decomposes at approximately 205 °C.³⁶ Additionally, the material retains its structural and luminescent integrity even after direct water immersion, highlighting its excellent moisture resistance. These combined advantages underscore its potential as a candidate for high-performance scintillator materials.

Fig. 1 (a) Synthesis of (C₁₉H₁₇ClP)₂MnCl₄ crystals. (b and c) Structure of the (C₁₉H₁₇ClP)₂MnCl₄ crystal. (d) Images of (C₁₉H₁₇ClP)₂MnCl₄ crystals under natural and ultraviolet light. (e) Comparison between the XRD diffraction peaks of the (C₁₉H₁₇ClP)₂MnCl₄ single-crystal powder sample and the simulated XRD diffraction peaks. (f) Energy-dispersive X-ray energy spectrum (EDS) of the (C₁₉H₁₇ClP)₂MnCl₄ crystal. (g) Thermal gravimetric analysis of the (C₁₉H₁₇ClP)₂MnCl₄ crystal.

To verify the chemical composition of the (C₁₉H₁₇ClP)₂MnCl₄ crystal, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were performed (Fig. 1f and Fig. S2). All the elements C, P, Mn and Cl are uniformly distributed in the (C₁₉H₁₇ClP)₂MnCl₄ crystal. The atomic ratio of Mn/Cl is 1 : 5.75, which is relatively consistent with the stoichiometry of (C₁₉H₁₇ClP)₂MnCl₄. In addition, X-ray photoelectron spectroscopy (XPS) was used to analyze the surface elemental composition. Fig. S3 clearly shows the presence of C, P, Mn, and Cl elements. The peaks at 640.13 eV and 652.09 eV correspond to Mn 2p_3/2 and 2p_1/2, which are attributed to the spin–orbit splitting of Mn in the tetrahedral environment.³⁸ As shown in Fig. 1g, the thermal stability of the crystal was evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). No obvious weight loss was found below 310 °C, indicating the excellent thermal stability without any material decomposition. The sharp endothermic peak at 203 °C in the DSC plot implies that the material began to melt. At this stage, the organic cations can still maintain an intact structure due to the strong chemical bond, while the material loses its ordered periodic structure due to the weak interaction between organic cations and MnCl₄ tetrahedra. Such glass-like features make the material easier to process.

2.2. Photophysical properties of the (C₁₉H₁₇ClP)₂MnCl₄ crystals

Ultraviolet-visible absorption was used to study the optical properties of the crystals. From Fig. 2a, it can be seen that the crystal exhibits a strong and broad absorption band in the deep-ultraviolet region, with a peak located at 280 nm. Two weak absorption bands appear in the regions of 344–396 nm and 420–500 nm, which originate from the electronic interaction of Mn²⁺ ions from the ground state to the excited state.³⁹ The band gap was calculated from the Tauc plot with a value of 2.64 eV, as shown in Fig. 2b. Furthermore, the energy band structure of the crystal was calculated and analyzed by DFT calculations. Fig. 2c shows that the material exhibits a direct band gap of 2.61 eV, which agrees well with our experimental results. The band gap of this material is fairly close to those of lead-containing hybrid perovskites such as (BA)₂MAPb₂Br₇ (2.55 eV)⁴⁰ and (BA)₂CsPb₂Br₇ (2.68 eV),⁴¹ both of which have been widely studied because of their excellent photoelectric properties and emission capabilities in the UV-Vis region. The density of states (DOS) diagram in Fig. 2d shows that the minimum conduction band of (C₁₉H₁₇ClP)₂MnCl₄ is primarily composed of Mn 4p orbitals, and the maximum valence band is mainly composed of Cl 3p (Fig. S4). A clear ligand to metal (LM) transition path was confirmed in our material, indicating that the luminescence properties are fundamentally dominated by the metal–halide tetrahedra rather than the organic counterpart. The degree of distortion of the MnCl₄ unit and the distance between any two Mn sites are commonly used to tailor the optical properties.²⁵

Fig. 2 (a) The ultraviolet-visible absorption spectrum of (C₁₉H₁₇ClP)₂MnCl₄ crystals. The bandgap value calculated by the Tauc method is shown in (b). (c) and (d) are the energy band structure and density of states diagram of (C₁₉H₁₇ClP)₂MnCl₄ crystals, respectively. (e) Excitation and emission spectra of (C₁₉H₁₇ClP)₂MnCl₄ crystals. (f) PL and PLE isoline plots of the crystals.

To explore the photoluminescence properties of (C₁₉H₁₇ClP)₂MnCl₄, the excitation (PLE) and emission (PL) spectra of the crystal at room temperature were measured. As shown in Fig. 2e, the excitation spectrum of (C₁₉H₁₇ClP)₂MnCl₄ includes peaks at approximately 286, 357, 432, and 447 nm, which correspond to the electronic transitions of Mn²⁺ from the ground state ⁶A₁ to ⁴A₂(F), ⁴E(D), ⁴A₁(G) + ⁴E(G), and ⁴T₂(G), respectively. A strong emission peak corresponding to green light appeared at 517 nm under different excitations, and the full width at half maximum (FWHM) was 46.9 nm. The peak corresponds to the transition of the Mn²⁺ ion in the [MnCl₄]²⁻ tetrahedron from the excited state ⁴T₁(G) to the ground state ⁶A₁.^42,43 The Stokes shift of the crystal can be calculated to be 231 nm by using the PL and PLE plots. The significant energy difference can effectively reduce the reabsorption of emitted light by the luminescent centers, thereby enhancing the light output efficiency of the crystal,⁴⁴ which guarantees our material excellent light yield below. The PL and PLE isolines shown in Fig. 2f clearly show that the emission wavelength peak position remains unchanged under different excitation, which further proves that the green emission of the (C₁₉H₁₇ClP)₂MnCl₄ crystal comes from the d–d transition of Mn²⁺.⁴⁵ Fig. S5 also confirms that the position of the emission peaks was not affected by varying the excitation wavelengths.

Transient fluorescence spectrometry was applied to measure the PLQY, which is determined by the number of emitted photons and the number of absorbed photons. As shown in Fig. 3a, the PLQY of the (C₁₉H₁₇ClP)₂MnCl₄ crystal is 96.63%. This value is superior to most reported organic–inorganic hybrid manganese halides (Table S3). The photoluminescence characteristics of Mn²⁺ in Mn-based halides are closely related to the distance between two adjacent Mn sites, and increased Mn–Mn distance leads to increased PLQY.^35,46,47 In this work, the [C₁₉H₁₇ClP]⁺ cation, with its significant steric hindrance, effectively increases the Mn–Mn distance by acting as a molecular spacer. More importantly, the asymmetric geometry and restricted rotational freedom of this cation lock the [MnCl₄]²⁻ tetrahedra into a specific arrangement, with interatomic distances ranging from 11.2026 to 11.4812 Å (Fig. 1b and Fig. S1). This precise molecular-scale regulation effectively suppresses Mn–Mn coupling and non-radiative energy transfer, serving as a key factor in achieving near-unity photoluminescence quantum yield. The PL lifetime was measured at room temperature and fitted with a single-exponential function (Fig. 3b). Under 286 nm excitation, the well-fitted (R² = 0.997) decay curve shows that the lifetime is 3.02 ms, which indicates that the excited state particles (excitons) return to the ground state as the main way and are not affected by significant non-radiation defects.⁴⁸ As shown in Fig. S6, we tested the afterglow lifetime of the crystal. We can see that the crystal continuously emits under ultraviolet excitation, the emission intensity instantly becomes zero after the ultraviolet excitation is turned off, indicating that the crystal has no afterglow phenomenon. In order to further examine whether the crystal can cause afterglow or not, the crystal was tested for two-dimensional X-ray thermoluminescence. However, the crystal did not exhibit any thermoluminescence signal (Fig. S7), confirming its lack of persistent afterglow. Afterglow is often caused by defect traps in crystals and is displayed by the thermoluminescence phenomenon.⁴⁹ In our case, the high-quality (C₁₉H₁₇ClP)₂MnCl₄ single crystal did not exhibit these unwanted defects.

Fig. 3 (a) PLQY curve of (C₁₉H₁₇ClP)₂MnCl₄ crystal excited with ultraviolet light at 286 nm. (b) PL attenuation curve of (C₁₉H₁₇ClP)₂MnCl₄ crystal under excitation at 286 nm and emission at 517 nm. (c) The PL emission spectra of the (C₁₉H₁₇ClP)₂MnCl₄ crystal in the temperature range of 80 K–450 K. (d) The reciprocal function of PL intensity of (C₁₉H₁₇ClP)₂MnCl₄ with temperature. (e) Emission intensity and FWHM diagram at different temperatures. (f) PL attenuation curves at different temperatures. (g) Daylight and ultraviolet light images of a (C₁₉H₁₇ClP)₂MnCl₄ crystal immersed in water for 30 minutes. (h) Comparison of PL spectra of the (C₁₉H₁₇ClP)₂MnCl₄-PDMS thin film kept at 60% ambient humidity for one month.

The temperature-dependent photoluminescence behavior of (C₁₉H₁₇ClP)₂MnCl₄ was further studied, as shown in Fig. 3c. In the range of 80 K −260 K, the luminous intensity gradually decreases with temperature increases, which can be attributed to the thermal quenching effect. However, from 260 K to 400 K, the emission intensity increased slightly. This can be attributed to the negative thermal quenching, which is contrary to the normal PL thermal quenching behavior.⁵⁰ As the temperature is elevated further from 400 K to 470 K, the emission intensity drops. The phenomenon can be explained by considering that, at low temperatures, direct recombination of initial electrons and final holes gives adequate PL intensity. Increased temperature causes vigorous phonon vibration and activates the traps, which jointly result in weakened radioluminescence. The integral intensity of the PL correspondingly shows a monotonous downward trend. However, when the temperature rises to a certain level, thermal excitation effects become significant, so the remaining electrons in the final state can be transferred to the non-radiation state. This increases the hole concentration and promotes the recombination of electrons with these extra holes, thus enhancing the efficiency of the PL transition at higher temperatures,⁵¹ for example, 260 K–400 K in this work.

In addition to the PL intensity evolution, a blue shift of the emission peak position (14 nm) was detected. This can be attributed to the thermal expansion of the crystal lattice with the temperature increase (Fig. S8). Thermal expansion of the lattice leads to an increase in the Mn–Mn distance, reducing energy quenching and thus the spin–spin interaction (E_sc) between them.³⁸ Shortened emission wavelengths correspond to the blueshift. As shown in Fig. 3d, the exciton activation energy of (C₁₉H₁₇ClP)₂MnCl₄ crystals can be calculated by using the Arrhenius formula:⁵²

where I(T) and I₀ represent the emission intensities at various temperatures (T) and 0 K, respectively, A is a constant, and k_B represents the Boltzmann constant. The data fitted from 80 °C–470 °C show that the activation energy of excitons is 79.9 meV, which is much higher than the thermal ionization energy (25.0 meV) at room temperature.⁵³ This shows that excitons can exist stably at room temperature and are not easily destroyed by thermal energy, thus improving the photoelectric conversion efficiency. As the criteria for judging the electron–phonon coupling strength in crystals, the Huang–Rhys factor (S) and the optical phonon energy (hω_phonon) can be calculated by the following formula:where hω_phonon is the maximum phonon energy, k_B is the Boltzmann constant, and S is the Huang–Phys factor of electron–phonon coupling strength. The S and ħω_phonon are calculated to be 4.49 and 30.52 meV, respectively (Fig. 3e). The obtained S value is smaller than that of reported organic–inorganic hybrid materials, such as (BTPP)₂MnCl₄, (BTPP)₂MnBr₄³⁰ and (MTP)₂MnBr₄.²⁸ G. Blasse et al. reported that the electron–phonon coupling of (C₁₉H₁₇ClP)₂MnCl₄ is at a moderate intensity (1 < S < 5).⁵⁴

Temperature-dependent attenuation curves in the range of 80 K–290 K are plotted in Fig. 3f. The lifetime decay curves of the crystal remained unchanged at the varying temperatures, exhibiting the same single exponential decay. This can be attributed to the internal electronic transition mechanism. Furthermore, the humidity stability of the (C₁₉H₁₇ClP)₂MnCl₄ crystal was monitored by immersing the crystal in aqueous solution. As shown in Fig. 3g, the luminous intensity of the crystal remains almost unchanged after continuous immersion in water for 30 minutes. As shown in Fig. S9, the XRD pattern of the soaked crystals retains all the diffraction peak positions, consistent with the original pattern, with no emergence of new peaks or shifts in peak positions. Therefore, the water immersion treatment did not induce phase transitions, structural degradation, or other alterations in the crystals, further confirming the excellent water stability of the (C₁₉H₁₇ClP)₂MnCl₄ crystals. At the same time, we ground the crystal with polydimethylsiloxane (PDMS), smeared the mixture on a glass sheet to obtain a scintillator film (Fig. S10), and monitored and tested its luminous intensity under 60% environmental humidity. After continuous monitoring for one month, the luminous intensity of the film decreased only slightly (Fig. 3h). These results confirm that our crystal has good humidity stability and can be stored in high-humidity environments for a long time.

2.3. (C₁₉H₁₇ClP)₂MnCl₄ array performance as an X-ray scintillator

The attenuation characteristics of (C₁₉H₁₇ClP)₂MnCl₄, and commercial materials C, Si, GAGG (Ce), and LuAG (Ce) were calculated and compared (using the NIST database⁵⁵). The (C₁₉H₁₇ClP)₂MnCl₄ crystal showed excellent scintillator characteristics. As shown in Fig. 4a and Fig. S11, the attenuation coefficient of (C₁₉H₁₇ClP)₂MnCl₄ is larger than those of silicon and carbon in the overall range, while it is superior to those of GAGG (Ce) and LuAG (Ce) at energies higher than 205 keV. Fig. 4b shows the relationship between absorption efficiency and thickness at an X-ray energy of 50 keV. The absorption efficiency of (C₁₉H₁₇ClP)₂MnCl₄ is remarkably higher than those of Si and C, while it is slightly lower than those of GAGG (Ce) and LuAG (Ce). The absorption efficiency of (C₁₉H₁₇ClP)₂MnCl₄ was calculated to be 65% at a thickness of 0.2 mm.

Fig. 4 (a) Attenuation curves of (C₁₉H₁₇ClP)₂MnCl₄, C, Si, GAGG (Ce) and LuAG (Ce). (b) The absorption efficiency of (C₁₉H₁₇ClP)₂MnCl₄, C, Si, GAGG (Ce), and LuAG (Ce) versus thickness. (c) RL radiation spectrum of the (C₁₉H₁₇ClP)₂MnCl₄ crystal. (d) Comparison of the RL and PL spectra of the (C₁₉H₁₇ClP)₂MnCl₄ crystal. (e) Comparison of the scintillation spectra of the (C₁₉H₁₇ClP)₂MnCl₄ and LuAG (Ce) scintillators under the same dose rate of X-ray irradiation. (f) Comparison of the energy and light yields of (C₁₉H₁₇ClP)₂MnCl₄ with commercial scintillators and some reported manganese halide hybrid scintillators. (g) RL spectra of (C₁₉H₁₇ClP)₂MnCl₄ at various doses. (h) Fitting curve showing the minimum detection limit of the (C₁₉H₁₇ClP)₂MnCl₄ crystal. (i) Variation curve of (C₁₉H₁₇ClP)₂MnCl₄ crystal irradiated at 50 kV and 500 μA for 1 h.

The radiation emission of (C₁₉H₁₇ClP)₂MnCl₄ at 50 kV/200 μA was further recorded. Fig. 4c and d shows the same profile as the PL spectrum, with a half-peak width of 49.1 nm, indicating that the radiation emission follows the mode of Mn²⁺ d–d transition.^26,56 The light yield was estimated using the following formula:²⁷

where I(λ) is the radiation intensity of the RL, ε is the photon energy, R(ε) is the X-ray output spectrum of the tube, and AE(ε) is the radiation attenuation efficiency. A commercial LuAG (Ce) single crystal was used for comparison to evaluate the light yield of the (C₁₉H₁₇ClP)₂MnCl₄ crystal further. We employed a self-built radioluminescence spectroscopy system consisting of an X-ray source, a dark box, a circular film holder, optical fibers, and a spectrometer (Fig. S12). The scintillators were secured in the holder and positioned at the same location inside the dark box, ensuring precise alignment with the optical fiber probe of the spectrometer. The radioluminescence spectra of both materials were acquired under identical experimental conditions with an X-ray tube voltage of 50 kV and a tube current of 200 μA. From Fig. 4e, the radiation photon number of (C₁₉H₁₇ClP)₂MnCl₄ is about 2.13 times larger than that of LuAG (Ce) (Fig. S13),⁵⁷ with a light yield value of 53 159.6 photons per MeV. The light yields of (C₁₉H₁₇ClP)₂MnCl₄, (BTPP)₂MnBr₄,³⁰ LuAG (Ce)⁵⁷ and commercial BGO⁵⁸ are compared in Fig. 4f. (C₁₉H₁₇ClP)₂MnCl₄ outperformed most of the reported Mn-based organic–inorganic hybrid materials (Table S4).

The RL spectra of (C₁₉H₁₇ClP)₂MnCl₄ at different doses were measured and summarized in Fig. 4g. The radiation intensity of (C₁₉H₁₇ClP)₂MnCl₄ increased as the dose increased from 1098.7 nGy_air per s to 2117.1 nGy_air per s. In addition, a fairly good linear relationship between the radiation intensity and X-ray dose was found, as shown in Fig. 4h. The lowest detection limit of the (C₁₉H₁₇ClP)₂MnCl₄ crystal is 182.5 nGy_air per s with a signal-to-noise ratio of 3, which is far lower than the medical diagnostic requirement for X-ray (5.5 μGy_air per s).⁵⁹Fig. 4i shows the radiation stability of (C₁₉H₁₇ClP)₂MnCl₄ measured at 50 kV and 500 μA. After 1 hour of irradiation, the intensity remains unchanged, demonstrating the excellent working stability. When the RL spectrum of (C₁₉H₁₇ClP)₂MnCl₄ was tested after one month under an environmental humidity of 60%, the radiation intensity remained almost unchanged (Fig. S14), which once again proved that the crystal has good humidity stability.

2.4. X-ray imaging based on the (C₁₉H₁₇ClP)₂MnCl₄ array scintillator

An imaging system was constructed to study the X-ray imaging performance of the (C₁₉H₁₇ClP)₂MnCl₄ array scintillator, consisting of a ray source, an object table, a reflector, and an ultraviolet-visible camera in sequence, as shown in Fig. 5b. Fig. 5a shows the flow chart of the (C₁₉H₁₇ClP)₂MnCl₄ array fabrication process to make scintillators with high resolution and low light scattering. The inset photograph shows the uniform scintillator film with bright green light emitted under ultraviolet irradiation. The wafer array is cut into 3 × 3 cm squares. The diameter of the individual pores on the wafer was 25 μm, and the hole spacing was about 10 μm, which effectively reduced the optical crosstalk. Optical microscopy analysis was performed on the (C₁₉H₁₇ClP)₂MnCl₄ array to observe the filling of the (C₁₉H₁₇ClP)₂MnCl₄ particles (Fig. S15), which showed that the pixel holes in the array were all filled with materials.

Fig. 5 (a) Manufacturing schematic of the array scintillator and an image of the (C₁₉H₁₇ClP)₂MnCl₄ array under natural light and ultraviolet light. (b) Schematic of the self-built X-ray imaging system. (c) Modulation transfer function curve of the (C₁₉H₁₇ClP)₂MnCl₄ array scintillator. (d) A visual comparison of light yield and resolution of (C₁₉H₁₇ClP)₂MnCl₄ with some published manganese-based scintillators and commercial scintillators. (e) Bluetooth headset image and partial enlargement under natural light and X-ray. The internal structure of the high-quality earphone can be seen by local enlargement, and the solder joints and wiring of the internal circuit board can be clearly seen. (f) Photographs of the chip under light and radiographic images of the (C₁₉H₁₇ClP)₂MnCl₄ array, as well as enlarged views of regions of the chip.

The image resolution of the (C₁₉H₁₇ClP)₂MnCl₄ array was tested by using the ISO 19232 standard dual-line card. Pair calipers consist of 14 pairs of platinum wires (D4–D17). The parameters of the image index represented by the ISO 19232 standard line pair card are given in the SI, Table S5. The D16 line pair with a resolution of 20 lp mm⁻¹ on the ISO 19232 standard card under the (C₁₉H₁₇ClP)₂MnCl₄ array can be identified (Fig. S16), indicating that the resolution of the scintillator of the (C₁₉H₁₇ClP)₂MnCl₄ array is within 25 lp mm⁻¹.

In order to obtain the imaging resolution of the (C₁₉H₁₇ClP)₂MnCl₄ array more accurately, the edge of the blade image is oversampled by the hypotenuse method, and a black-and-white straight line is obtained from which the edge spread (ESF) function is obtained (Fig. S17a). Then, the derivative of ESF is used to calculate the rate of change of the straight line to obtain the line spread (LSF) function (Fig. S17b). Finally, the modulation transfer (MTF) function curve is obtained by a one-dimensional Fourier transform. The final resolution reaches 24.6 lp mm⁻¹ (Fig. 5c). This resolution is superior to those of most reported Mn-based metal halides and commercial scintillators (Table S4 and Fig. 5d). A comprehensive analysis of scintillator performance based on Table S4 reveals that (C₁₉H₁₇ClP)₂MnCl₄ exhibits a resolution that is approximately 2.7 times higher than that of conventional BGO scintillators, while maintaining a high standard among manganese-based hybrid materials. Additionally, its light yield surpasses that of most reported materials, and its detection limit reaches industry-leading levels. This exceptional overall performance directly translates into outstanding imaging capabilities. The internal wiring and solder joints of the earphone and chip (Fig. 5e and f), the USB interface, as well as the spring in the capsule (Fig. S18) can be clearly seen. Especially, for the ultra-high-definition image of the 5 × 10 mm chip in Fig. 5e, the width of the thinnest wire was calculated to be ∼16.9 μm, which can be clearly distinguished. Additionally, a (C₁₉H₁₇ClP)₂MnCl₄-PDMS composite scintillator film was fabricated for comparison (Fig. S19b). Resolution test pattern results demonstrate that the film achieves a spatial resolution exceeding 12.5 lp mm⁻¹ (Fig. S19a). X-ray imaging of an integrated circuit chip using (C₁₉H₁₇ClP)₂MnCl₄-PDMS (Fig. S19c) reveals its internal structural outlines. Compared with the PDMS-based composite film, the (C₁₉H₁₇ClP)₂MnCl₄-silicone array exhibits superior spatial resolution.

3. Conclusion

In summary, single crystals of (C₁₉H₁₇ClP)₂MnCl₄ with high light yield, negligible self-absorption, good thermal stability, and low toxicity were successfully prepared. An impressive photoluminescence yield of 96.63% was obtained. Under X-ray irradiation, the light yield of the (C₁₉H₁₇ClP)₂MnCl₄ crystal achieved an exceptional 53 159.6 photons per MeV, which greatly contributes to its excellent scintillator performance. By sinking the solution into a pixel-structured silicon wafer array, the scintillator screen of a (C₁₉H₁₇ClP)₂MnCl₄ array was successfully fabricated. The array scintillator screen features tiny apertures that separate the scintillator material, effectively reducing optical crosstalk. As a result, the scintillator screen exhibits a competitive resolution of 24.6 lp mm⁻¹ and a low detection limit of 182.5 nGy_air per s under X-ray imaging. The (C₁₉H₁₇ClP)₂MnCl₄ array scintillator screen demonstrates significant potential for wide applications in X-ray imaging, particularly in medical diagnosis, public safety, and industrial detection. Its simple preparation process, outstanding resolution characteristics, and low detection limit make it highly promising for these important fields.

Author contributions

Mengyue Wu: methodology, investigation, formal analysis, writing – original draft. Jun'an Lai: conceptualization, methodology, investigation, visualization, writing – review & editing. Yongqiang Zhou: methodology, investigation. Faguang Kuang: methodology, investigation. Kang An: methodology, investigation. Sijun Cao: methodology, investigation. Yayun Pu: methodology, investigation. Heng Luo: methodology, investigation. Peng He: methodology, investigation. Baofei Sun: methodology, investigation. Xiaosheng Tang: conceptualization, writing – review & editing, supervision.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: (C₁₉H₁₇ClP)₂MnCl₄ crystal detailed experimental steps, specific parameters of crystal structure, luminous performance of light, radiant luminous performance, ISO 19232 double-line image quality indicator and so on. Crystal detailed experimental steps, specific parameters of crystal structure, luminescent properties of light, luminescent properties of radiation and other specific parameters. SEM, EDS, XPS images of (C₁₉H₁₇ClP)₂MnCl₄ crystal, DFT calculation of the crystal, emission spectra under different excitation, PL isoline diagram at variable temperature, absorption efficiency analysis diagram and array optical microscope diagram. See DOI: https://doi.org/10.1039/d5qi01886b.

CCDC 2487361 ((C₁₉H₁₇ClP)₂MnCl₄) contains the supplementary crystallographic data for this paper.⁶⁰

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 62375032, 62104023 and 52302059), the Natural Science Foundation of Chongqing (No. CSTB2023TIAD-KPX0017 and CSTB2022NSCQ-MSX0360), the Fundamental Research Funds for the Central Universities (No. 2023CDJKYJH086), the Open Fund of the State Key Laboratory of High Field Laser Physics (Shanghai Institute of Optics and Fine Mechanics) and the China Postdoctoral Science Foundation (Grant Number: BX20230355). Additionally, this work was sponsored by Natural Science Foundation of Chongqing, China (cstc2019jcyj-msxmX0737) and supported by the Science and Technology Research Program of Chongqing Municipal Education Commission (No. KJQN202400619).

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