Modulation of halogens in organic manganese halides for high-resolution and large-area flexible X-ray imaging

Abstract

Manganese-based organic metal halides, due to their excellent optoelectronic properties and stable chemical nature, have been widely used in optoelectronic devices and sensors. In this study, through rational modulation of halogens, three novel zero-dimensional organic manganese halides [(C₆H₈N)₂MnX₄ (X = Cl, Br, and I)] were obtained by changing the halogen atom via the solvent evaporation method. Due to the d–d electronic transition of Mn²⁺ in the tetrahedrally coordinated [MnX₄]²⁻ polyhedron, all samples exhibit strong green emission. Notably,(C₆H₈N)₂MnBr₄ exhibited a remarkable light yield of 18 224 photons per MeV and a low detection limit of 1.9 μGy s⁻¹, which is below the X-ray diagnostic limit and even superior to that of commercial BGO scintillators. Moreover, a 9 cm × 9 cm flexible scintillator film was successfully fabricated by mixing (C₆H₈N)₂MnBr₄ crystalline powder with polymethyl methacrylate, and this film manifested superior radiation stability and an X-ray imaging resolution of 12.1 lp mm⁻¹. Notably, the X-ray images captured by the flexible film demonstrate distinguished clarity when adhered perfectly to curved metal sheets. The combination of excellent performance and facile solution processing opens new opportunities for low-cost, high-performance organic metal halide-based large-area flexible scintillators for X-ray detection and imaging.

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Introduction

As crucial elements in indirect X-ray imaging, X-ray scintillators have garnered significant attention in the realms of medical imaging, non-destructive testing, industrial flaw detection, and scientific research.^1–4 Scintillators can absorb X-rays and convert them into visible light, which can then be transformed into electrical signals using photoelectric sensors like CCD cameras or photomultiplier tubes. These signals are ultimately transmitted to an intelligent display terminal for image processing, resulting in the final imaged picture.^5,6 A series of representative traditional inorganic scintillators, such as Bi₄Ge₃O₁₂ (BGO), NaI : Tl, and CsI : Tl, have been successfully applied to X-ray imaging due to their high X-ray absorption capacity.⁷ However, the high-temperature conditions required for growing these crystals make the production process complex and expensive.^8,9 Therefore, research is imperative for achieving low-cost and high-performance scintillators.

In recent years, organic metal halide materials have become a focal point in the research of new-generation scintillator materials. These materials are favored for their cost-effective production, adjustable band gaps, and excellent luminescence properties, presenting vast opportunities in light-emitting diodes and detectors.^10–13 Among them, organic lead halide perovskite materials demonstrate suitability for X-ray imaging, benefiting from their high X-ray attenuation ability, and have achieved notable advancements.^14,15 However, the high toxicity and poor stability of lead-based perovskites limit their further practical application. With reports of scintillator materials based on the metals of tin (Sn), copper (Cu), antimony (Sb) and manganese (Mn), such as crystals of (C₈H₁₇NH₃)₂SnBr₄,¹⁶ (4-bzpy)₄Cu₄I₄,¹⁷ C₅₀H₄₄P₂SbCl₅ ^18,19 and (C₃₈H₃₄P₂)MnBr₄,²⁰ new environmentally friendly lead-free counterparts have emerged.²¹ It is noteworthy that organic manganese halides stand out among other materials due to their unique luminous mechanism, high luminous efficiency, facile synthesis, and simple structure design, sparking waves of research enthusiasm.²²

Compared with other lead-free halide materials, the luminescence of organic manganese halide scintillators originates from the energy transition emission of the Mn²⁺ luminescence center.²³ It can be excited in the UV or blue region, exhibiting a larger Stokes shift and a high PLQY.²⁴ The luminescence intensity is influenced by the crystal field intensity. Depending on the coordination environment of Mn²⁺ ions, octahedral Mn²⁺ halides exhibit red emission, while tetrahedral Mn²⁺ halides exhibit green emission.²⁵ In addition, their low-dimensional crystal structure endows them with enhanced electron localization, which promotes radiative recombination and results in higher PLQY.²² For example, Jiang et al.²⁴ reported that the C₄H₁₂NMnCl₃ crystal with Mn²⁺ octahedral coordination exhibits red emission at 635 nm, with a PLQY as high as 91.8%. This scintillator demonstrates good radiation hardness and can achieve X-ray imaging resolutions of up to 5 lp mm⁻¹. In addition, previously reported crystals such as TEA₂MnI₄,²⁶ TPP₂MnBr₄ ²⁷ and (C₃₈H₃₄P₂)MnBr₄,²³ which all have tetrahedrally coordinated Mn²⁺, exhibit green light emission, and possess excellent photophysical properties. Their light yield (LY) can even rival that of commercial scintillators, showcasing immense potential in the field of X-ray imaging.

The photophysical properties of organic metal halides are significantly influenced by various halogen elements.²⁸ Each halogen not only affects the chemical stability of these materials, but also alters their optical characteristics. For instance, the previous literature indicates that owing to the smaller radius and higher electronegativity of chlorine, MAPbCl₃ exhibits stronger ionic bond stability in thermal environments compared to MAPbBr₃ and MAPbI₃.^29,30 In contrast, the incorporation of bromine often yields a favorable balance between light absorption and luminescence efficiency,³¹ demonstrating considerable potential for applications. Additionally, iodine, with its larger atomic radius and atomic number, enhances the adaptability of the crystal structure and increases X-ray attenuation.²⁶ Thus, the careful selection and manipulation of different halogens are crucial for optimizing the optoelectronic properties of organic metal halides, providing valuable insights for the development of new and efficient optoelectronic devices.

For X-ray imaging equipment, such as commercial computed X-ray tomography scanners, the internal X-ray detection component is typically designed in a ring shape to achieve comprehensive scanning of the target object. Unfortunately, most available scintillators are rigid single crystals (e.g., NaI : Tl and CsI : Tl) that are assembled into rings using multiple scintillators,^32,33 resulting in loss of image information at the joints. Therefore, there arises a demand for a large-area flexible film that can adapt to detectors of various shapes and configurations. Additionally, for irregularly shaped imaging objects, a flexible scintillation film can conform to the object's surface, reducing scattering lines and optical crosstalk, consequently improving image quality and spatial resolution.³⁴

Herein, three types of 0D manganese-based organic metal halides [(C₆H₈N)₂MnX₄ (X = Cl, Br, and I)] were rationally designed and prepared via solvent evaporation. 2-Methylpyridine was selected as the organic ligand for its ability to effectively integrate into [MnX₄]²⁻, consequently increasing the Mn–Mn distance. Furthermore, its strong interactions with halides can enhance structural rigidity and reduce non-radiative transitions from thermal vibrations.^24,25,35 Based on the d–d transition of the [MnX₄]²⁻ tetrahedron, the three compounds exhibit efficient green luminescence, with (C₆H₈N)₂MnBr₄ achieving a high PLQY of 64.7% that significantly surpasses the 17.18% of (C₆H₈N)₂MnCl₄ and the 15.31% of (C₆H₈N)₂MnI₄. Furthermore, the single crystals of (C₆H₈N)₂MnBr₄ display extraordinary scintillation properties, exhibiting an LY of 18 224 photons per MeV and a low detection limit of 1.9 μGy s⁻¹. By mixing the crystal powder with PMMA, a large-area flexible scintillation film was successfully fabricated with size of 9 cm × 9 cm. This film was utilized for imaging objects, achieving a spatial resolution of 12.1 lp mm⁻¹. Additionally, the images of bent metal sheets further validated the superior performance of the flexible film in X-ray imaging. The film demonstrates its capability to conform to various complex and irregular surfaces, highlighting its vast range of application prospects in medical, industrial, security, and other fields.

Results and discussion

We selected 2-methylpyridine as the organic molecule and prepared single crystals (SCs) of (C₆H₈N)₂MnX₄ (X = Cl, Br, and I) using the solvent evaporation method (Fig. S1†). As shown in Fig. 1(a), the three different halide crystals vary in appearance from green to yellow under natural light. However, they all exhibit green luminescence when irradiated with a 365 nm UV lamp. The single-crystal X-ray diffraction (SCXRD) results indicate that the crystals of (C₆H₈N)₂MnCl₄ and (C₆H₈N)₂MnBr₄ crystallize in monoclinic space groups of I2/a and C2/c, respectively, while the single crystal of (C₆H₈N)₂MnI₄ belongs to the orthorhombic space group of Pbca (Table S1†). As illustrated in Fig. 1(b–d), within these organic manganese halides, each Mn²⁺ ion forms a distorted [MnX₄]²⁻ (X = Cl, Br, and I) tetrahedron by coordinating with four halogen ions, which is surrounded by C₆H₈N⁺ cations, creating a 0D structure at the molecular level. These tetrahedra exhibit varying degrees of distortion, with their Mn–X bond lengths ranging from 2.3421 to 2.3739 Å, 2.4807 to 2.5193 Å, and 2.6839 to 2.7199 Å, respectively. The X–Mn–X bond angles vary from 106.06° to 112.86°, 105.96° to 112.69°, and 105.50° to 112.24° (Table S2†). Referring to the previous literature,³⁶ we calculated the bond length distortions and bond angle variances for the three crystals. The distortions in the bond lengths of [MnX₄]²⁻ (X = Cl, Br, and I) were found to be 3.37 × 10⁻⁵, 3.30 × 10⁻⁵, and 3.83 × 10⁻⁵, while the corresponding bond angle variances were 5.98, 5.51, and 6.69. Among these, [MnBr₄]²⁻ showed the least distortion and highest symmetry, making it the closest to an ideal tetrahedron, which can enhance the photoluminescence performance of the crystal.³⁷ Additionally, the shortest Mn–Mn distances in (C₆H₈N)₂MnX₄ (X = Cl, Br, and I) SCs are 7.62 Å, 7.81 Å, and 8.27 Å, respectively (Fig. S2†). This change results from the differing electronegativities of Cl⁻, Br⁻, and I⁻, leading to variations in hydrogen bond strength between halides and organic molecules, which in turn affects the separation of Mn tetrahedra.³⁶ Detailed crystal parameters and refinement data for each SC are provided in the ESI.†

Fig. 1 (a) Photographs of the (C₆H₈N)₂MnX₄ (X = Cl, Br, and I) crystals under visible light (top) and 365 nm UV light (bottom). Crystal structures of (b) (C₆H₈N)₂MnCl₄, (c) (C₆H₈N)₂MnBr₄ and (d) (C₆H₈N)₂MnI₄ single crystals (hydrogen atoms have been omitted for clarity).

The powder X-ray diffraction (PXRD) results of the three crystals align with the SCXRD simulation results, confirming that the synthesized SCs possess a high-purity phase (Fig. 2(a)). The Fourier transform infrared (FTIR) spectra show notable peaks, including those for C–N stretching at 1099 cm⁻¹ and N–H vibration at 1565 cm⁻¹. These observations determine the presence of organic molecules (Fig. 2(b)). Additionally, the high-resolution X-ray photoelectron spectroscopy (XPS) spectra also demonstrate the presence of C, N, Mn, and X (X = Cl, Br, and I) elements in the three crystals (Fig. 2(c)). As shown in Fig. S3–S5,† there are two distinct Mn 2p characteristic peaks in the (C₆H₈N)₂MnX₄ (X = Cl, Br, and I) crystals, corresponding to Mn 2p_3/2 and Mn 2p_1/2 respectively. These peaks are observed at 641.7 and 653.5 eV for (C₆H₈N)₂MnCl₄, 641.8 and 653.8 eV for (C₆H₈N)₂MnBr₄, and 640.6 and 652.6 eV for (C₆H₈N)₂MnI₄. Additionally, the two peaks at 198.1 and 199.7 eV in Fig. S3(b)† can be attributed to Cl 2p_3/2 and Cl 2p_1/2 in the (C₆H₈N)₂MnCl₄ crystals. The peaks at 68.4 and 69.4 eV correspond to Br 3d_5/2 and Br 3d_3/2 in the (C₆H₈N)₂MnBr₄ crystals (Fig. S4(b)†), while the peaks at 617.6 and 629 eV are assigned to I 3d_2/5 and I 3d_3/2 in the (C₆H₈N)₂MnI₄ crystals (Fig. S5(b)†), respectively. The results of scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) of the (C₆H₈N)₂MnX₄ (X = Cl, Br, and I) SCs in Fig. S6–S8† further verify that elements such as C, N, Mn and X (X = Cl, Br, and I) are present and evenly distributed in the crystals. Furthermore, the thermal stability of the three halide crystals was evaluated through thermogravimetric analysis (TGA), showing that the (C₆H₈N)₂MnX₄ (X = Cl, Br, and I) crystals began to decompose at 156 °C, 185 °C, and 247 °C, respectively (Fig. 2(d)).

Fig. 2 (a) Experimental PXRD and simulated PXRD patterns, (b) FTIR spectra, (c) survey XPS spectra and (d) TG curves of (C₆H₈N)₂MnX₄ (X = Cl, Br, and I).

The steady-state photoluminescence (PL) and excitation (PLE) spectra reveal that all three organic manganese halide crystals exhibit narrow-band green emission in the visible light range (Fig. 3(a–c)). Notably, the PL and PLE spectra are red-shifted with increasing halogen atomic number. Specifically, the peak emission for the crystals based on Cl⁻, Br⁻, and I⁻ are centered at 520, 525, and 548 nm, respectively. The emission wavelength is influenced by several factors, particularly the strength of the ligand field and the splitting of the d-orbitals in a tetrahedral environment. For Mn(II) ions, a weaker ligand field results in smaller d-orbital splitting.³⁸ Consequently, samples incorporating Br⁻ exhibit a slight red shift compared to those with Cl⁻. Additionally, the covalent bond strength of the Mn–I interaction affects the electronic environment of the Mn ions and their d-orbital splitting. Strong covalent bonds can lower the energy required for electronic transitions, leading to further red shift in the emission wavelength.³⁹ UV-vis absorption spectra are used to study the absorption characteristics of the three manganese halide crystals (Fig. 3(d–f)). The three samples exhibit multiple absorption peaks at 263, 298, 364, 377, 434, 455, and 476 nm, corresponding to the ⁶A₁ → ⁴A₂(F), ⁶A₁ → ⁴T₁(F), ⁶A₁ → ⁴Tg(⁴p), ⁶A₁ → ⁴Eg(⁴D), ⁶A₁ → ⁴Ag, ⁴Eg(⁴G), ⁶A₁ → ⁴T₂(⁴G) and ⁶A₁ → ⁴T₁(⁴G) transitions, respectively. After excitation, the excited electrons relax to the lowest excited state ⁴T₁ through several non-radiative processes. Subsequently, they emit bright light via the ⁴T₁ → ⁶A₁ radiative transition (Fig. S9†). Additionally, the direct band gaps of (C₆H₈N)₂MnX₄ (X = Cl, Br, and I) were calculated using the Tauc method, yielding values of 2.50, 2.45, and 2.33 eV, respectively (inset of Fig. 3(d–f)). In the tetrahedral coordinated environment of manganese halides, the weak crystal field strength results in a high separation level of ⁴T₁ → ⁶A₁, contributing to the narrow-band green emission of the crystals.²⁴ Afterwards, we recorded the PL and PLE spectra of the three samples at various excitation and emission wavelengths. The contour plots clearly show that the emission centers of the three samples do not shift (Fig. S10†). This consistency further indicates that the green light emission characteristics of (C₆H₈N)₂MnX₄ (X = Cl, Br, and I) arise solely from the tetrahedrally coordinated manganese center and are the result of relaxation from the same excited state. Additionally, the decay curves of time-resolved photoluminescence (TRPL) spectra for the (C₆H₈N)₂MnX₄ (X = Cl, Br, and I) crystals were fitted using a single exponential function. The calculated lifetimes are 509.4, 262.6 and 15.8 μs, respectively, showcasing a discernible trend of faster attenuation from Cl to Br to I (Fig. 3(g–i)). This trend is mainly due to the heavy-atom-enhanced reverse intersystem crossing (RISC) process.^40,41

Fig. 3 (a–c) PLE and PL spectra, (d–f) UV-vis absorption spectra, and (g–i) PL decay curves of (C₆H₈N)₂MnX₄ (X = Cl, Br, and I).

To evaluate the photophysical properties of the title samples, their PLQYs were measured as 17.18%, 64.7%, and 15.31%, respectively (Fig. S11†). Studies have shown that the Mn–Mn distance significantly influences the luminescence properties of the crystals. Generally, a larger Mn–Mn distance can reduce non-radiative transitions between manganese ions, thereby improving the excitation and recombination efficiency of the photo-generated carriers, which can effectively enhance the PLQY.^42,43 Moreover, lower symmetry can accelerate PL decay, which is unfavorable for enhancing the PLQY.⁴³ Consequently, (C₆H₈N)₂MnBr₄ exhibits a higher PLQY compared to that of (C₆H₈N)₂MnCl₄, which may arise from a longer Mn–Mn distance and a higher symmetry as suggested from the single-crystal structure data. It is also noteworthy that the lower PLQY of (C₆H₈N)₂MnI₄ may be ascribed to the significant structural distortion of the inorganic unit, as well as the enhanced spin–orbit coupling effects and strong self-absorption.^28,42,43 Specifically, the larger atomic radius and lower electronegativity may lead to a stronger spin–orbit coupling effect, increasing the probability of non-radiative recombination. On the other hand, the substantial overlap between the absorption and emission peaks can enhance the self-absorption of the crystal.^28,42 This results in some of the excitation light being absorbed by the material rather than being effectively converted to light emission, ultimately leading to a lower PLQY.

Using density functional theory (DFT), we performed the calculation and investigation of the electronic properties of (C₆H₈N)₂MnBr₄, as illustrated in Fig. 4(a). Our analysis reveals that the valence band maximum (VBM) and the conduction band minimum (CBM) are situated at the same high-symmetry point, resulting in a calculated direct bandgap of 2.4 eV, which closely aligns with the experimental values. Further examination indicates that both the VBM and CBM exhibit a significant degree of flatness, suggesting that electrons are in a highly localized state.⁴⁴ Through partial density of states (PDOS) analysis, we observed that the VBM is primarily composed of Br-p and Mn-d orbitals, while the CBM is predominantly contributed to by the organic components (Fig. 4(b)). According to previous reports, this result indicates that organic molecules not only play an isolating role within the structure, but also transfer energy to the luminescent [MnX₄]²⁻ units during the luminescence process.^28,45,46

Fig. 4 (a) Electronic energy band structures of (C₆H₈N)₂MnBr₄. (b) Orbital projection of the partial density of states of (C₆H₈N)₂MnBr₄.

To further assess the scintillation performance of the (C₆H₈N)₂MnX₄ (X = Cl, Br, and I) crystals, we first calculated the relationship between the X-ray absorption coefficient and the photon energy for the three different samples based on the XCOM database. The results are shown in Fig. 5(a). It is evident that the X-ray absorption coefficients of (C₆H₈N)₂MnX₄ (X = Cl, Br, and I) exhibit a progressive increase with the halogen atomic number. This is attributed to their high atomic number, which can effectively interact with X-rays, leading to significant increases in their absorption cross-section for X-rays.⁴⁷ Subsequently, we plotted the thickness-dependent attenuation efficiency at a photon energy of 22 keV (the average photon energy under 50 kV X-ray irradiation). It is noteworthy that despite the absorption coefficient and attenuation efficiency of these three samples being lower than those of commercial BGO scintillators, the absorption coefficient of (C₆H₈N)₂MnI₄ and (C₆H₈N)₂MnBr₄ is significantly superior to those of previously reported scintillators such as (C₃₈H₃₄P₂)MnBr₄,²³ (HTPP₂)MnBr₄,⁴⁸ C₄H₁₂NMnCl₃,²⁴ and (ETP)₂MnBr₄ ⁷ in the range of 40–120 keV. As shown in Fig. 5(b), the attenuation efficiencies of the 1 mm-thick (C₆H₈N)₂MnX₄ (X = Cl, Br, and I) scintillator materials for 22 keV X-ray photons are 51.5%, 98.8% and 96.2%, respectively. Notably, at this incident photon energy, the attenuation efficiency of the Br⁻-based sample is significantly higher than that of the I⁻-based sample. This phenomenon can be attributed to the fact that the incident photon energy is located near the K absorption edge of (C₆H₈N)₂MnBr₄. In this energy range, the photoelectric effect is significantly enhanced, resulting in a notable increase in the absorption capacity of X-rays as they pass through the material.⁴⁹

Fig. 5 (a) The functional relationship between the absorption coefficients and the photon energies of (C₆H₈N)₂MnX₄ (X = Cl, Br, and I), BGO scintillators and previously reported Mn-based scintillators. (b) Comparison of the attenuation efficiencies under 22 keV X-ray irradiation for the (C₆H₈N)₂MnX₄ (X = Cl, Br, and I) and standard BGO scintillators. (c) RL spectra of (C₆H₈N)₂MnX₄ (X = Cl, Br, and I) and standard BGO scintillators (tube voltage: 50 kV). (d) RL intensities of the (C₆H₈N)₂MnX₄ (X = Cl, Br, and I) and standard BGO scintillators as a function of different X-ray dose rates.

The light yield (LY) is another critical parameter for evaluating scintillator performance, as it affects the sensitivity and detection limits of the X-ray detector.¹ For comparison, a commercial BGO scintillator serves as a standard reference, along with a mini X-ray generator equipped with a tungsten target and a spectrometer to obtain radioluminescence (RL) spectra. When the scintillator material interacts with incident high-energy X-rays, numerous electron–hole pairs are generated. These electron–hole pairs are then transferred to the tetrahedrally coordinated manganese luminescent centers. Ultimately, the excited Mn²⁺ ions relax from the ⁴T₁ state to the ⁶A₁ state, emitting scintillation light.²⁶ As depicted in Fig. 5(c), the RL spectra of (C₆H₈N)₂MnX₄ (X = Cl, Br, and I) share similar characteristics with their PL spectra, indicating identical radiative recombination pathways for both forms of excitation. At equivalent dose rates, the RL intensity of (C₆H₈N)₂MnBr₄ dramatically surpasses that of (C₆H₈N)₂MnCl₄, (C₆H₈N)₂MnI₄ and the commercial BGO scintillator. The BGO reference sample has an LY of 10 600 photons per MeV.⁵⁰ In comparison, the calculated LY values for (C₆H₈N)₂MnX₄ (X = Cl, Br, and I) are 3009, 18 224 and 3770 photons per MeV, respectively. Additionally, we collected RL spectra of the samples at different dose rates (Fig. 5(d) and Fig. S12†). The (C₆H₈N)₂MnX₄ (X = Cl, Br, and I) crystals exhibit perfectly linear responses in the dose rate range of 0.96–5.92 mGy s⁻¹. By calculation, the detection limit for (C₆H₈N)₂MnX₄ (X = Cl, Br, and I) is 23.2, 1.9, and 6.1 μGy s⁻¹, respectively. Notably, under the same measurement conditions, the detection limit of (C₆H₈N)₂MnBr₄ is only a seventh of that of the BGO scintillator and is under the critical threshold for the X-ray diagnostic limit (5.5 μGy s⁻¹).^51–53

To further investigate the temperature-dependent radiation stability of the three samples, their RL spectra were recorded over the temperature range of 300–380 K (Fig. S13†). The results indicate that under the same conditions, (C₆H₈N)₂MnBr₄ exhibits lower attenuation and higher stability of RL intensity. At 380 K, its strength reduces to 59% of the initial value, which is significantly better than the 12% recorded for (C₆H₈N)₂MnCl₄ and the 24% for (C₆H₈N)₂MnI₄. This phenomenon underscores the fact that (C₆H₈N)₂MnBr₄ not only performs excellently at ambient temperature, but also maintains excellent stability and effectiveness in high-temperature environments. The above-mentioned exceptional performance of (C₆H₈N)₂MnBr₄ establishes it as a strong contender for use in future efficient and reliable X-ray imaging scintillators.

Considering the high X-ray attenuation efficiency and LY of (C₆H₈N)₂MnBr₄ crystals, we further investigated their potential as high-resolution X-ray imaging scintillators. Powdered (C₆H₈N)₂MnBr₄ crystals were mixed with PMMA to prepare large-area flexible scintillation screens, as shown in Fig. 6(a) and Fig. S14.† The (C₆H₈N)₂MnBr₄@PMMA film was approximately 9 cm × 9 cm with a thickness of 79 μm, emitting uniform green light under 365 nm UV irradiation and exhibiting high transparency under daylight, with clearly discernible letters beneath the film (Fig. 6(a) and Fig. S15†). Additionally, the film demonstrated remarkable flexibility, capable of withstanding multiple folds (Fig. 6(a)). Moreover, the SEM-EDS images confirm the uniform distribution of elements within the film (Fig. S16†). Furthermore, the X-ray-induced RL characteristics of the film and crystal remained consistent, indicating no alteration in the luminescent centers (Fig. S17†). To further assess the stability of the scintillating film, we performed continuous irradiation at a dose rate of 8.99 mGy s⁻¹. As illustrated in Fig. 6(b), when the cumulative dose reached 9 Gy, the RL intensity showed no significant changes, indicating that the film possesses excellent radiation stability.

Fig. 6 (a) Photographs of the (C₆H₈N)₂MnBr₄@PMMA film under daylight (top) and UV light (bottom). (b) Radiation stability of the (C₆H₈N)₂MnBr₄@PMMA film under continuous X-ray irradiation at a dose rate of 8.99 mGy s⁻¹. The inset shows the RL intensities at the first and 100th irradiation cycles. (c) Bright-field images and X-ray images of a microchip, a spring-loaded pen, headphones, chicken feet, a shrimp and a small crab based on the (C₆H₈N)₂MnBr₄@PMMA film.

The X-ray imaging quality of (C₆H₈N)₂MnBr₄@PMMA flexible films has been systematically studied using a self-constructed imaging system, as depicted in Fig. S18.† Due to the varying X-ray absorption capacities of materials with different densities, the differential RL intensity of the scintillator films captures these variations, allowing optical cameras to obtain internal images of objects.⁵⁴ At a tube voltage of 50 kV, the internal information of a microchip, a spring-loaded pen, and an earphone was clearly visible, which was imperceptible to the naked eye in daylight (Fig. 6c). To delve deeper into the potential of (C₆H₈N)₂MnBr₄ in medical imaging, we selected a shrimp, chicken feet, and a small crab as subjects for experimentation. The X-ray images revealed the internal bones and other tissue structures of these organisms, highlighting the promising application of (C₆H₈N)₂MnBr₄@PMMA scintillator films in medical diagnostics.

As shown in Fig. 7(a), the spatial resolution of a standard X-ray image is approximately 12 lp mm⁻¹. By calculating the modulation transfer function (MTF) of the tilting edge X-ray image of the scintillator film (Fig. 7(b)), the spatial resolution was quantified as being 12.1 lp mm⁻¹. This demonstrates its excellent performance in high-resolution imaging applications.

Fig. 7 (a) X-ray image of the standard line pair card. (b) Modulation transfer functions (MTFs) of the X-ray images obtained from the (C₆H₈N)₂MnBr₄@PMMA flexible film scintillators. X-ray images of a bent metal sheet taken using the (C₆H₈N)₂MnBr₄@PMMA film in its (c) plane and (d) curved states.

The flexible film can adjust to different shapes and sizes of objects, offering superior fit and coverage. This adaptability makes X-ray imaging of complex structures or irregular surfaces more efficient and precise.⁵⁵ Moreover, the flexible film can minimize artifacts and distortions that occur when a rigid detector cannot fully conform to the object's surface.⁵⁶ This enhancement boosts the resolution and clarity of X-ray imaging, resulting in more accurate diagnoses and analytical outcomes. As displayed in Fig. 7(c and d), (C₆H₈N)₂MnBr₄@PMMA exhibits a significant difference in X-ray imaging quality between images of a metal sheet in its curved and flat states. When the scintillation film is pressed against the irregular metal sheet, the image becomes clearer and the overall image quality is significantly improved. Therefore, the scintillation film not only enhances the resolution and clarity of the image, but also demonstrates its substantial application potential in flexible X-ray imaging.

Conclusions

In summary, by selecting 2-methylpyridine as the organic cation, we obtained three non-toxic 0D manganese-based organic metal halides (C₆H₈N)₂MnX₄ (X = Cl, Br, and I). Under UV and blue light excitation, these halides displayed efficient green emission stemming from the d–d transitions of Mn²⁺. With its advantages of a high PLQY value and robust stability, (C₆H₈N)₂MnBr₄ demonstrated potential for X-ray detection with excellent scintillation performance, featuring a high LY of 18 224 photons per MeV and a low detection limit of 1.9 μGy s⁻¹, outperforming commercial BGO scintillators and showcasing extensive application prospects. More importantly, the flexible film presented clearer imaging on bent metal sheets in contrast to planar imaging, making it more effective for detecting irregularly shaped objects or covering large areas. This work enriches the excellence of organic manganese halides and demonstrates their latent application prospects in medical imaging, industrial inspection, and security screening.

Experimental section

Materials and crystal growth

2-Methylpyridine (C₆H₇N, 99%), manganese(II) bromide tetrahydrate (MnBr₂·4H₂O, 98%), manganese(II) chloride tetrahydrate (MnCl₂·4H₂O, 99.99%), manganese (Mn, 99.95%), hydrobromic acid (HBr, 40%), hydroiodic acid (HI, 55.0%–58.0%), methanol (CH₃OH, 99.5%), hypophosphorous acid (H₃PO₂, 50 wt% in water), and polymethyl methacrylate ((C₅H₈O₂)_n) were all purchased from Aladdin. Hydrochloric acid (HCl, 36.0%–38.0%) and toluene (C₇H₈, 99.5%) were obtained from Tieta Reagent, and dioctyl phthalate (C₂₄H₃₈O₄, AR) was obtained from Macklin. All reagents were used as received without further purification.

Growth of (C₆H₈N)₂MnCl₄ single crystals. In a typical synthesis of (C₆H₈N)₂MnCl₄, C₆H₇N and MnCl₂·4H₂O with a molar ratio of 2 : 1 were first dissolved in a mixed solution of HCl and CH₃OH (1/3, v/v) and the mixture was then heated and stirred at 90 °C to form a transparent precursor solution. Subsequently, the solution was then slowly evaporated on a heating plate at 60 °C in an atmospheric environment. After a few days, light green crystals of (C₆H₈N)₂MnCl₄ can be obtained.

Growth of (C₆H₈N)₂MnBr₄ single crystals. For the synthesis of (C₆H₈N)₂MnBr₄, C₆H₇N and MnBr₂·4H₂O with a molar ratio of 2 : 1 were dissolved in a mixed solution of HBr and CH₃OH (1/3, v/v) and the mixture was then heated and stirred at 90 °C to form a transparent precursor solution. Subsequently, the solution was then slowly evaporated on a heating plate at 60 °C in an atmospheric environment. After a few days, green crystals of (C₆H₈N)₂MnBr₄ can be obtained.

Growth of (C₆H₈N)₂MnI₄ single crystals. For the synthesis of (C₆H₈N)₂MnI₄, C₆H₇N and Mn powder were dissolved in a mixed solution of HI and H₃PO₂ (4/1, v/v). The mixture was heated and stirred at 90 °C to form a transparent precursor solution. Subsequently, the solution was then slowly evaporated on a heating plate at 60 °C under atmospheric conditions. After a few days, yellow single crystals of (C₆H₈N)₂MnI₄ can be obtained.

Preparation of (C₆H₈N)₂MnBr₄@PMMA flexible thin films

First, PMMA particles were dissolved in toluene at a concentration of 0.25 g mL⁻¹ by stirring at 45 °C. Then, 0.9 g of finely ground (C₆H₈N)₂MnBr₄ crystal powder was added to 2.5 g of the mixed solution, followed by the addition of 0.2 g of the plasticizer dioctyl phthalate. The mixture was stirred continuously for several hours until it reached a uniform consistency. A scraper was then employed to apply the mixture onto a glass substrate, with adjustments made to control the film thickness. After the solvent had completely evaporated and the film had dried, it was carefully peeled off the glass substrate, resulting in a large-area flexible film.

Measurements and characterization

Single-crystal X-ray diffraction (SCXRD) data were obtained using a Bruker SMART APEX-II diffractometer, which features a CCD detector (Mo Kα, λ = 0.71073 Å), and processed using APEX2 software. Structural analysis and refinement were conducted using direct methods with the SHELXTL97 software package. Powder X-ray diffraction (PXRD) was conducted using Cu Kα1 radiation (λ = 1.54186 Å) on a Bruker AXS D8 Advance diffractometer, with diffraction patterns collected over a 2θ range of 10–70° at a step size of 0.02°. Simulated PXRD data were derived from the single-crystal structure. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Thermo Fisher Scientific ESCALAB 250 instrument. Thermogravimetric analysis (TG) was conducted using a Netzsch STA 449C analyzer, covering a temperature range from 35 to 800 °C at a heating rate of 5 °C min⁻¹ under a nitrogen atmosphere. The UV-vis absorption spectral data were collected at room temperature using a Shimadzu UV2550 spectrophotometer equipped with an integrating sphere (200–1500 nm). Using an Edinburgh FS5 full-function fluorescence spectrometer equipped with an integrating sphere, steady-state photoluminescence excitation/emission (PLE/PL) spectra, time-resolved photoluminescence (TRPL) spectra, and PLQY values were recorded. The RL spectrum and detection limit were recorded using the FS5 fluorescence spectrometer, employing a mini X-ray tube (Moxtek Inc.) with a tungsten target as the X-ray source. X-ray imaging was carried out with a Nikon D7200 camera and an AF-S VR 105 mm f/2.8G lens.

Calculation of X-ray attenuation efficiency

The X-ray attenuation efficiency (AE) was calculated using the following formula:

AE = [1 − exp(−tρd)] × 100%

where t is the X-ray absorption coefficient, sourced from the XCOM database by the National Institute of Standards and Technology (NIST), d denotes the scintillator thickness, and ρ is the crystal density. The theoretical densities of the (C₆H₈N)₂MnX₄ (X = Cl, Br, and I) single crystals are 1.469, 1.984 and 2.375 g cm⁻³, respectively. The correlation between attenuation efficiency and scintillator thickness was evaluated at a photon energy of 22 keV.

Calculation of light yield

Using a commercial BGO scintillator (LY = 10 600 photons per MeV) as a reference, we calculated the LYs of (C₆H₈N)₂MnX₄ (X = Cl, Br, and I). The experiments were conducted at a tube voltage of 50 kV, with the sample powder compressed to a uniform thickness of approximately 1 mm. The attenuation efficiencies (AE(d)) for (C₆H₈N)₂MnX₄ (X = Cl, Br, and I) and BGO can be extracted from Fig. 5(b). The photon counting results (PC_measured) were obtained by integrating the RL spectrum. Finally, the relative LY was calculated using the following eqn (1) and (2):where PC_measured represents the measured photon count, PC_normalized denotes the normalized photon count, AE(d) indicates the attenuation efficiency, LY_sample refers to the LY of the measured sample, and LY_BGO signifies the LY of the BGO commercial scintillator.

Calculation of detection limit

Within the dose rate range of 0.96–5.92 mGy s⁻¹, we measured the RL spectra of (C₆H₈N)₂MnX₄ (X = Cl, Br, and I) and BGO scintillators. By employing linear fitting, the relationship was established between the RL intensity and the dose rate for each sample, resulting in the slope (k), which represents the sensitivity of the X-ray detector. Additionally, noise measurements were conducted in the absence of samples, and Gaussian statistical analysis of the noise intensity data provided the FWHM as the average noise level. Finally, the detection limit (DL) was calculated based on the detector sensitivity (k) and the average noise FWHM using the formula DL = 3 × FWHM/k.

Calculation of modulation transfer function (MTF)

The modulation transfer function (MTF) curve of the scintillator film was determined using the slanted edge method. Initially, a piece of lead with a sharp edge, approximately 1 mm thick, was positioned on the scintillator for X-ray imaging, and the edge image was obtained. Subsequently, the edge spread function (ESF) was extracted from the image, followed by acquisition of the line spread function (LSF). Finally, the MTF value was computed using the Fourier transform of the LSF, as follows:
where ν represents the spatial frequency and x denotes the pixel position. The spatial resolution is obtained at MTF = 0.2.

Author contributions

Xiaomei Jiang conceived the idea, designed the experiments, revised the manuscript and obtained the research grants; Ying Sun conducted the materials synthesis, characterization, and wrote the manuscript; Pan Gao, Qi Wang and Zeyu Guo conceived the discussion and conducted the device performance measurements. Qian Ma and Dongheng Zhao conducted the DFT calculations. All authors contributed to the general discussion.

Data availability

The data supporting this article have been included as part of the ESI.† Crystallographic data for 2381212, 2270667 and 2381207 have been deposited at the CCDC.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We sincerely acknowledge financial support from the National Natural Science Foundation of China (Grant No. 62305195), the projects ZR2024ME223 and ZR2022QF036 supported by the Shandong Provincial Natural Science Foundation, and the Opening Project of State Key Laboratory of Crystal Materials (Shandong University, KF2105).

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Footnote

† Electronic supplementary information (ESI) available: Single-crystal X-ray diffraction data, XPS spectra, excitation wavelength-dependent PL and PLE spectra, X-ray dose rate-dependent RL spectra, RL spectrum of the scintillation film and X-ray images. CCDC 2270667, 2381207 and 2381212. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi02522a

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