High resolution X-ray imaging via near unity emission organic–inorganic manganese bromide scintillator films using a suction filtration method

Abstract

In recent years, lead-free organic–inorganic hybrid scintillators have attracted much attention in the field of X-ray imaging because of progress in their solution synthesis, low toxicity, and high photoluminescence quantum yield (PLQY). However, the methodology for the facile and versatile fabrication of high-resolution scintillators grounded in manganese-based hybrid halide systems requires further exploration. In this paper, a novel zero-dimensional (0D) manganese-based organic–inorganic hybrid perovskite material (C₂₁H₂₂P)₂MnBr₄ is studied, which was synthesized via an anti-solvent method. The corresponding materials could emit strong green light under ultraviolet light or X-ray irradiation, and a photoluminescence quantum yield (PLQY) of 98.93% could be achieved. The near unity photoluminescence quantum yield leads to its excellent scintillator performance, with a high luminous yield of 46 786 photon MeV⁻¹ and a low detection limit of 217.7 nGy_air s⁻¹. Moreover, the flexible scintillator screen was prepared by using a suction filtration process, which achieved high-resolution and low-dose radiation imaging with a line pair (lp) of 20 lp mm⁻¹. This further proved that the suction filtration approach is feasible and meaningful for preparing high-resolution scintillation screens. The (C₂₁H₂₂P)₂MnBr₄ scintillator with high resolution and high light yield has great potential application in the field of X-ray imaging.

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1. Introduction

Scintillators can convert high-energy X-rays into low-energy visible light. X-ray scintillators are widely used in medical imaging, nondestructive testing, safety inspection, scientific research and geological surveys.^1–5 As science and technology continue to advance, it is very important to explore scintillator materials with high light yield and high resolution to promote the development of medical imaging and other fields. Scintillators with high light yield and high resolution guarantee the accuracy of diagnosis and detection. Recently, metal halides have been widely used in the field of X-ray imaging because of their excellent stability, high photoluminescence quantum yield (PLQY), high absorption coefficient, long carrier life and low-cost solution processability.^6–9 Among the many organic–inorganic hybrid metal halides, Mn²⁺-based compounds not only have the advantages of rich resources and environmental friendliness, but also have excellent physical and chemical characteristics.^10–13 In general, Mn²⁺-based mixed metal halides have broad application prospects in the field of X-ray scintillator imaging due to their excellent stability, high photoluminescence quantum yield (PLQY), long photoluminescence lifetime, high scintillation rate and low toxicity.^14–16

In the last several years, in order to obtain high light yield and high-resolution scintillators, flexible scintillation screens based on Mn²⁺-based hybrid halides have been continuously developed. For example, Xia et al. prepared a (ETP)₂MnB_r4 large transparent medium with a diameter of more than 10 cm using a fusible quenching strategy, with a light yield of 37 000 photons MeV⁻¹, a low detection limit of 103 nGy s⁻¹ and a competitive spatial resolution of 13.4 lp mm⁻¹.¹⁵ Xu et al. employed a melt-coating method to obtain a scintillator with a high light yield of 36 300 photons MeV⁻¹ and a resolution of up to 24.2 lp mm⁻¹.¹⁷ Our group reported (TEA)₂MnCl₄ and (TBA)₂MnCl₄ single crystals using an anti-solvent method, in which (TBA)₂MnCl₄ exhibited a light yield of 21 000 photon MeV⁻¹, a low detection limit of 381 nGy s⁻¹, and a resolution of 5.6 lp mm⁻¹.¹⁸ However, the preparation of glass scintillator screens by using a melting method under high temperature is complicated, the process cost is high and it is difficult to control the uniformity of glass thickness, which affects the performance of materials and leads to bad imaging quality. Moreover, the melting method imposes higher requirements on the material itself, so materials with lower melting points are more suitable for practical applications.¹⁷ Similarly, grinding the crystal and mixing it with polydimethylsiloxane (PDMS) to prepare scintillator screens will induce uneven coating of the scintillator and further affect the imaging resolution. Therefore, it is still a big challenge to realize scintillators with high light yield and high resolution.

This paper reports a novel zero-dimensional (0D) lead-free organic–inorganic hybrid perovskite crystal (C₂₁H₂₂P)₂MnBr₄. The crystal shows bright green emission under an ultraviolet lamp, and the PLQY reaches 98.93%, which is close to unity. Here, the [MnBr₄]²⁻ tetrahedron is separated by a huge organic cation [C₂₁H₂₂P]⁺, which forms a 0-dimensional (0D) crystal structure, which makes the crystal stable. In the zero-dimensional crystal structure, the isolated [MnBr₄]²⁻ tetrahedra undergo energy level transitions of Mn²⁺ ions. The d-orbitals of Mn²⁺ ions experience electron transitions from a low-energy level (ground state) to a high-energy level (excited state), and then return to the ground state through radiative transitions. This greatly reduces the energy loss caused by non-radiative transitions, which is beneficial for obtaining high light yields. A circular flexible and uniform (C₂₁H₂₂P)₂MnBr₄ scintillator film was conveniently fabricated using the filtration method. The (C₂₁H₂₂P)₂MnBr₄ flexible film has a high light yield of 46 786 photon MeV⁻¹ and a low detection limit of 217.7 nGy_air s⁻¹. By modulating the transfer function (MTF) curve, the resolution of the (C₂₁H₂₂P)₂MnBr₄ scintillation screen is 20 lp mm⁻¹, which is superior to that of most lead-free hybrid scintillators. Based on the performance of the (C₂₁H₂₂P)₂MnBr₄ flexible film with high light yield, low detection limit, and good resolution, the scintillator has become a potential scintillator material in X-ray applications.

2. Results and discussion

2.1. Structural characterization of the (C₂₁H₂₂P)₂MnBr₄ crystal

(C₂₁H₂₂P)₂MnBr₄ crystals were synthesized using the anti-solvent method (experimental part), and the experimental steps are shown in Fig. 1a. The (C₂₁H₂₂P)₂MnBr₄ crystal appears light green and emits bright green light under 254 nm ultraviolet light, illustrated in Fig. 1b. The crystal structure of (C₂₁H₂₂P)₂MnBr₄ is analyzed using single crystal ray diffraction at 273 K, in Fig. 1c and d. Clearly, the unique molecular architecture of (C₂₁H₂₂P)₂MnBr₄ is characterized by a central [MnBr₄]²⁻ tetrahedral anionic structural unit, which is surrounded by two peripheral [C₂₁H₂₂P]⁺ organic cations. Each Mn²⁺ ion is coordinated with four Br⁻ ions, resulting in the formation of a stable [MnBr₄]²⁻ tetrahedral configuration. These tetrahedral anions are effectively segregated via the large organic cations [C₂₁H₂₂P]⁺, thereby forming a zero-dimensional (0D) crystal structure at the molecular level. In the crystal structure, the tetrahedral molecular groups formed by [MnBr₄]²⁻ are isolated from each other by the spatial distribution of [C₂₁H₂₂P]⁺, a larger organic cation. The distribution of electrons within the molecule is altered by this unique spatial arrangement, thereby initiating the Jahn–Teller effect. The Jahn–Teller effect plays a critical role in comprehending and forecasting molecular geometrical configurations and electronic attributes.¹⁹ The (C₂₁H₂₂P)₂MnBr₄ crystal belongs to a typical monoclinic crystal structure with a P2₁/c space group, and its cell parameters are a = 17.316(4) Å, b = 14.982(3) Å, c = 18.264(3) Å, α = γ = 90°, β = 116.307(7)°, Z = 4. The detailed XRD parameters of the (C₂₁H₂₂P)₂MnBr₄ crystal are shown in Table S1 (ESI†). The bond lengths between Mn²⁺ and Br⁻ range from 2.4675 Å to 2.5369 Å, and the bond angles of Br⁻–Mn²⁺–Br⁻ range from 105.60° to 112.85° (Table S2, ESI†). The structural characteristics of this [MnBr₄]²⁻ tetrahedron are similar to those in known manganese-based halides.^20–22 The similarity of the [MnBr₄]²⁻ tetrahedra in manganese-based halides suggests that such structures are common in manganese-based halides, indicating that this type of structure is prevalent in manganese-based halides and exerts a significant influence on their electronic and optical properties. Because the larger organic cation [C₂₁H₂₂P] ⁺ surrounds the [MnBr₄]²⁻ anion, it provides a longer Mn²⁺–Mn²⁺ distance, with the shortest distance between adjacent Mn²⁺ being 11.1459 Å and the longest distance being 15.213 Å (Fig. S1, ESI†). This distance is larger than that reported previously for organic–inorganic hybrid metal halides (OIMHs). The larger distance may mean that the interaction between organic cations and inorganic metal halides is weakened, thus reducing the possibility of non-radiative transition, which is helpful to improve the efficiency of photoluminescence (PL).^21,23,24

Fig. 1 (a) The flow chart of (C₂₁H₂₂P)₂MnBr₄ crystal synthesized using an antisolvent method is shown. (b) Images of (C₂₁H₂₂P)₂MnBr₄ crystal under natural light and ultraviolet light. (c) and (d) Crystal structure of (C₂₁H₂₂P)₂MnBr₄. (e) Comparison between experimental XRD values and simulated XRD values of the (C₂₁H₂₂P)₂MnBr₄ crystal. (f) SEM image of (C₂₁H₂₂P)₂MnBr₄ and EDS mapping of C, P, Mn and Br in (C₂₁H₂₂P)₂MnBr₄, showing the homogeneous distribution of C, P, Mn, Br in the lattice.

By aligning the experimental XRD patterns of the single crystal powder samples with the theoretically simulated XRD values, a consistent diffraction peak was found (Fig. 1e). The comparison reveals the exceptional crystallinity and uniform phase composition of the crystal. To provide a more comprehensive analysis of the structural characteristics of the material, we further conducted Fourier transform infrared spectroscopy (FTIR) tests (Fig. S2, ESI†). From the FTIR spectrum, the peaks appearing in the region of 1650–1450 cm⁻¹ correspond to the skeletal vibrations of the aromatic ring. In the fingerprint region, the double peaks within the range of 770–730 cm⁻¹ indicate the presence of a monosubstituted benzene structure. Two weak peaks of equal intensity within the range of 3000–3100 cm⁻¹ correspond to the stretching vibrations of the C–H bonds on the benzene ring. The peak at 1100 cm⁻¹ represents the stretching motion between the phosphorus atom and the carbon atom. The peaks within the range of 2970–2880 cm⁻¹ correspond to the stretching vibrations of the C–H bonds in the methyl group. The bond between manganese and bromine generates absorption peaks in the lower wavenumber range, specifically within 400–500 cm⁻¹. The FTIR characteristic peaks further verify the specific bonding conditions within the crystal. To validate the consistency of these microscopic details on a macroscopic scale, the FTIR data were combined with XRD data. Comparative analysis through XRD confirmed that the crystal contains no impurity phases, and its crystal structure was further validated using FTIR. The long-range order information provided by XRD and the short-range molecular details revealed by FTIR jointly offer comprehensive structural insights into the (C₂₁H₂₂P)₂MnBr₄ crystal, spanning from macroscopic to microscopic perspectives. To further clarify the elemental composition of the material, scanning electron microscope (SEM) imaging and elemental analysis were conducted (Fig. 1f). It can be seen from the characterization diagram that the elements C, P, Mn, and Br in (C₂₁H₂₂P)₂MnBr₄ are evenly distributed. XPS (X-ray photoelectron spectroscopy) was employed for an in-depth analysis of the surface composition and chemical states of the elements present on the sample. The XPS spectrum of the obtained (C₂₁H₂₂P)₂MnBr₄ sample clearly shows the existence of Mn, Br, P and C elements (Fig. S3, ESI†). High-resolution X-ray photoelectron spectroscopy (XPS) analysis successfully reveals the intricate 2p energy level details of the Mn element in the (C₂₁H₂₂P)₂MnBr₄ compound, featuring two distinct peaks: 2p_3/2 and 2p_1/2, which are attributed to energy values of 640.43 eV and 651.86 eV, respectively. The existence of these energy level peaks directly evidences the occurrence of spin–orbit coupling splitting within the Mn²⁺-based material, where the interaction between the orbital and spin angular momenta of electrons leads to the splitting of the original single orbital energy level into two, clearly manifested as a double-peak characteristic in the high-resolution XPS spectrum.²⁵ The energy level peak of the sample material splits out the binding energy of 11.43 eV. In addition, the material's thermal stability was assessed through thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) (Fig. S4, ESI†). It can be seen that the material showed excellent stability before 292 °C, and began to decompose when the temperature reached 292 °C. After the material began to decompose in weightlessness, there was a significant endothermic phenomenon in the range of 318 °C to 367 °C. It can be seen that the (C₂₁H₂₂P)₂MnBr₄ material has high thermal stability.

2.2. Luminescence properties of (C₂₁H₂₂P)₂MnBr₄ crystal

To gain a deeper understanding of the optical characteristics of the crystal, its ultraviolet-visible absorption spectrum was analyzed (Fig. 2a). From the absorption spectrum, it can be observed that there is an absorption peak in the 260–300 nm ultraviolet region, along with a strong absorption band in the 330–350 nm ultraviolet range. Additionally, significant absorption bands are present in the 360–400 nm and 420–480 nm spectral regions. The peaks observed in the spectral regions of 360–400 nm and 420–480 nm stem from the interaction between the ground and the excited states of Mn²⁺ ions within the crystal field.²⁶ These absorption peaks correspond to ⁶A₁(S)-⁴A₂(F), ⁶A₁(S)-⁴T₁(P), ⁶A₁(S)-⁴E(D), ⁶A₁(S)-⁴T₂(D), ⁶A₁(S)-⁴A₁(G), 4E(G) and ⁶A₁(S)-⁴T₂(G) d–d radiation transitions in Mn²⁺ respectively. Their band gaps can be calculated using the Tauc plot rule:

αhv = A(hv − E_g)ⁿ (1)

where α represents the absorption coefficient, h denotes the Planck's constant, v stands for the frequency, A is the proportionality constant, and E_g signifies the semiconductor band gap. The value of the exponent n is contingent upon the type of semiconductor: for direct band gap semiconductors, n equals 1/2, whereas for indirect band gap semiconductors, n is 2. The energy band structure of the crystal was calculated using density functional theory (DFT), as shown in Fig. 2b. The figure illustrates that the conduction band minimum (CBM) is located at 2.131 eV, while the valence band maximum (VBM) reaches −0.02 eV, resulting in an indirect band gap of E_g = 2.15 eV. Additionally, Fig. 2c visually illustrates the energy band information, including allowed and forbidden bands, through the density of states (DOS) diagram. The conduction band (CBM) minimum in the (C₂₁H₂₂P)₂MnBr₄ crystal is primarily influenced by Br⁻¹-4p and Mn²⁺-3d orbitals, while the valence band (VBM) maximum is predominantly determined by Br⁻¹-4p orbitals, as depicted in Fig. S5 (ESI†). The minimal contribution of organic cations to the valence and conduction bands highlights that the luminescence properties of manganese-based hybrid metal halides are primarily determined by metal and halide ions. Gaussian simulations reveal that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are localized within the [MnBr₄]²⁻ tetrahedron (Fig. S5, ESI†). Theoretical calculations correspond closely to the results obtained for the HOMO and the LUMO. Therefore, it can be further explained that metals and halogen elements mainly determine the luminescence properties of crystals.

Fig. 2 (a) Optical absorption spectrum of (C₂₁H₂₂P)₂MnBr₄ crystals. (b) Electronic band structure of (C₂₁H₂₂P)₂MnBr₄. (c) Projected density of states (PDOS) of (C₂₁H₂₂P)₂MnBr₄. (d) Photoluminescence excitation (PLE) and photoluminescence (PL) emission spectra of (C₂₁H₂₂P)₂MnBr₄ crystal. (e) Illustration of Mn²⁺ transitions.

We also systematically studied the photophysical properties of (C₂₁H₂₂P)₂MnBr₄ single crystal. Fig. 2d shows photoluminescence's excitation spectrum (PLE) and emission fluorescence spectrum (PL) at ambient temperature. The excitation spectrum measured at an emission wavelength of 511 nm shows peaks at around 284 nm, 359 nm, and 441 nm. (C₂₁H₂₂P)₂MnBr₄ crystal shows a strong bright green emission at 511 nm under the excitation of 284 nm, 359 nm, and 441 nm, and its full width at half maximum (FWHM) is 185.5 meV (39.4 nm), which can be attributed to the d–d transition of Mn²⁺ in the d⁵ structure in tetrahedral [MnBr4]²⁻,²⁷ illustrated in Fig. 2e. Meanwhile, we have calculated through PL and PLE that the (C₂₁H₂₂P)₂MnBr₄ crystal exhibits a large Stokes shift of 1.94 eV. This substantial Stokes shift can effectively reduce self-absorption, rendering the crystal's self-absorption negligible. Consequently, this property is conducive to achieving a high light yield in scintillators. Furthermore, the emission spectra under different excitations (Fig. S6, ESI†) reveal that the emission center remains unshifted. This stability is attributed to the low concentration of defects within the crystal, which reduces the likelihood of non-radiative recombination and thus maintains the stability of the emission center. Illustrated in Fig. 3a, the PL decay of the crystal shows a single exponential decay and can be well fitted. The fitted PL lifetime is 379.5 μs, indicating that there is a uniform crystal field environment around Mn²⁺ ions.^28–30Fig. 3b demonstrates that the PLQY of the (C₂₁H₂₂P)₂MnBr₄ crystal under 284 nm excitation reaches an impressive 98.93%. This value, representing the ratio of emitted photons to absorbed photons, is higher than that of most previously reported organic–inorganic hybrid metal halides (OIMHs) (Table S3, ESI†). The (C₂₁H₂₂P)₂MnBr₄ crystal exhibits an intermittent bandgap, yet its photoluminescence quantum yield (PLQY) is significantly higher than that of materials typically characterized by such bandgap properties. Through photoluminescence (PL) spectral analysis, we found that the emission peak energy is located at approximately 2.43 eV, which is independent of the material's bandgap (2.15 eV). This emission peak corresponds to the typical d-d transition (⁴T₁(G) → ⁶A₁(S)) of Mn²⁺. Furthermore, the excitation peaks correspond to the transitions of Mn²⁺ from ⁶A₁(S) to ⁴A₂(F), 4E(D), and ⁴T₂(G), further confirming that the luminescence originates from localized d–d transitions. Simultaneously, it has been found that the photoluminescence quantum yield (PLQY) is affected by the distance between manganese ions (Mn²⁺). According to the research results of Ram Seshadri²¹ and Xia,²³ the longer Mn²⁺–Mn²⁺ spacing is helpful in improving the photoluminescence quantum yield (PLQY). Due to the large Mn²⁺–Mn²⁺ distance (∼15.2 Å) in the (C₂₁H₂₂P)₂MnBr₄ crystal material, the photoluminescence quantum yield (PLQY) approaches unity. Consequently, this material can suppress non-radiative energy loss processes in Mn²⁺ through dipole and spin interactions between adjacent Mn²⁺ ions.^21,23,24

Fig. 3 (a) Decay curve of (C₂₁H₂₂P)₂MnBr₄ under excitation at 284 nm. (b) The PLQY spectrum of (C₂₁H₂₂P)₂MnBr₄ crystal. The reference curve is the whiteboard in the integrating sphere (λ_ex = 284 nm). The calculation formula of crystal PLQY is as follows . Here N_e and N_a are emitted photons and absorbed photons respectively; L_s is the emission intensity of the sample; E_r and E_s are the photoluminescence intensities in the presence of the reference blank and sample (C₂₁H₂₂P)₂MnBr₄, respectively. (c) Temperature-variable spectra within the range of 80–300 K. (d) The reciprocal function of PL intensity of (C₂₁H₂₂P)₂MnBr₄ with temperature. (e) Dependence of FWHM and PL intensity of the (C₂₁H₂₂P)₂MnBr₄ crystal on temperature T (T = 80–300 K). (f) Temperature-dependent PL decay curve (λ_em = 511 nm) of the (C₂₁H₂₂P)₂MnBr₄ crystal at room temperature (RT) and in the temperature range of 80–280 K.

Then we characterized the optical properties related to temperature and tested the luminescence characteristics of (C₂₁H₂₂P)₂MnBr₄ material at different temperatures. The temperature-dependent photoluminescence spectrum of the crystal is shown in Fig. 3c, and the emission intensity gradually increases with the temperature dropping from 300 K to 80 K. When the temperature rises, the interaction between electrons and lattice vibrations (phonons) in the crystal increases, which leads to more excited electrons releasing energy through a non-radiation process, rather than returning to the ground state through luminescence, which will reduce the luminous efficiency of the crystal. However, when the temperature is reduced to 80 K, the coupling between the electrons and the vibration modes is weakened, which reduces the occurrence of a non-radiative process, so that more electrons can return to the ground state through radiation recombination and release energy in the form of luminescence, so the emission intensity gradually increases. Additionally, as the temperature decreases, the crystal lattice may contract, leading to an increase in PL intensity. This lattice contraction may also contribute to the changes in the luminescence properties of the crystal. Furthermore, it is evident that the FWHM of the PL spectrum gradually increases with temperature. The exciton activation energy of the (C₂₁H₂₂P)₂MnBr₄ crystal was calculated using the Arrhenius equation (Fig. 3d):³¹

Here, I(T) and I₀ represent the emission intensities at temperature T and 0 K, respectively, A is a constant, and k_B is the Boltzmann constant. The data fitted from 80 to 300 K show that the exciton activation energy is 80 meV, significantly higher than the thermal ionization energy (25 meV) at room temperature.³² It shows that under room temperature conditions, excitons can remain stable and are less likely to dissociate due to thermal energy, which helps enhance the photoelectric conversion efficiency. Huang–Rhys factor (S) and optical phonon energy (ħω_phonon) can be obtained from the following formula (Fig. 3e):³³where h is Planck's constant, ω_phonon is the frequency of optical phonon, k_B is the Boltzmann constant, and S is the Huang–Rhys factor of electron–phonon coupling strength. The calculated S and ħω_phonon are 4.93 and 25.7 meV respectively. The S of (C₂₁H₂₂P)₂MnBr₄ is smaller than that of typical hybrid materials such as (TBA)CuBr₂³⁴ and (C₁₃H₂₂N)₂MnCl₄.²⁵ According to G. Blasse et al., the electron–phonon coupling of Mn²⁺ in (C₂₁H₂₂P)₂MnBr₄ belongs to medium intensity (1 < S < 5).³⁵ The temperature-dependent spectrum reveals a slight red shift in the emission peak, transitioning from 510 nm to 512 nm as the temperature decreases from 300 K to 80 K. This is because the lattice contraction increases the energy difference (d–d splitting) between the d-orbital electrons of Mn²⁺, which leads to the decrease of the energy required for the electron transition, so the wavelength of the emitted light becomes longer, showing a redshift (supporting the temperature-dependent PL isoline diagram of the information Fig. S7, ESI†).³⁶ When the temperature is in the range of 80 K to 280 K and at room temperature (RT), the PL lifetime of the crystal shows the same single exponential decay, and the fitted PL lifetime is 0.37 ms (Fig. 3f). We can see that the PL lifetime of (C₂₁H₂₂P)₂MnBr₄ remains unchanged at different temperatures, which is different from the performance of traditional Mn²⁺ doped inorganic fluorescent materials. In some inorganic fluorescent materials, when the temperature decreases, the process of non-radiation energy loss is usually inhibited, which leads to the increase of luminescence lifetime with the decrease of temperature.¹⁴ This phenomenon shows that the nonradiative relaxation of Mn²⁺ ions in (C₂₁H₂₂P)₂MnBr₄ is very weak in the temperature range of 80–280 K and RT. Therefore, it can well explain why the PLQY of the crystal is close to unity at RT. This also demonstrates that the material can maintain stable luminescence properties under temperature fluctuations. Therefore, with the temperature decreasing from 300 K to 80 K, the emission spectral intensity of the crystal gradually increases, which is not due to the influence of the non-radiation relaxation of Mn²⁺. Instead, the lattice contraction deforms the [MnBr₄]²⁻ tetrahedron, enhancing the absorption of excitation light by Mn²⁺ ions and increasing the emission intensity.³⁷

2.3. X-ray properties of scintillator based on (C₂₁H₂₂P)₂MnBr₄@PVDF

Because of its high PLQY, minimal self-absorption, and excellent thermal stability, (C₂₁H₂₂P)₂MnBr₄ is a good X-ray scintillator material. The crystal (C₂₁H₂₂P)₂MnBr₄ exhibits excellent radiative luminescence (RL) characteristics under X-ray irradiation.

Under the irradiation of 80 kV/200 μm X-rays, the imaging ability of the (C₂₁H₂₂P)₂MnBr₄ crystal was studied. As shown in Fig. 4a, the RL spectrum of the (C₂₁H₂₂P)₂MnBr₄ crystal is very similar to that of the PL spectrum, indicating that the RL of the crystal is generated by the transition of Mn²⁺ from the excited state ⁴T₁ to the ground state ⁶A₁.³⁷ The RL emission peak of the (C₂₁H₂₂P)₂MnBr₄ crystal is centered at 510 nm, and the FWHM is 223.1 meV (Fig. 4b). The light yield (LY) of the (C₂₁H₂₂P)₂MnBr₄ crystal was evaluated by comparing it with the commercial scintillator LuAG(Ce) as a benchmark. The LY of LuAG(Ce) is reported to be 25 000 photons MeV⁻¹.³⁸ The RL spectrum of (C₂₁H₂₂P)₂MnBr₄@PVDF scintillator thin film was compared with that of the reference material LuAG(Ce). We employed a self-built RL spectroscopy probe system, which includes an X-ray source, a hollow black box, a circular film clamp, optical fibers, and a spectrometer (Fig. S8, ESI†). The scintillator powder was then pressed into circular film clamps of the same size and placed at the same position within the black box, aligned with the spectrometer's fiberoptic probe. The RL spectra were measured under identical conditions of an X-ray tube voltage of 80 kV and a tube current of 200 μA. Illustrated in Fig. 4c, it can be determined that the light yield of (C₂₁H₂₂P)₂MnBr₄ is about 46 786 photons MeV⁻¹. Its performance significantly surpasses that of LuAG(Ce) and the previously highly regarded lead halide perovskite CsPbBr₃ (21 000 photons MeV⁻¹).^39,40 The light yield of (C₂₁H₂₂P)₂MnBr₄ is slightly lower than that of the commercial scintillator CsI: TI (54 000 photons MeV⁻¹).⁴¹ In addition, as illustrated in Fig. 4d, the comparison also reveals the photoluminescent yield of (C₂₁H₂₂P)₂MnBr₄ against commercially available BGO,⁴² (C₈H₂₀N)₂MnBr₄ crystals,⁴³ and (C₂₂H₂₂O₂P)₂MnBr₄ crystals.⁴⁴ The attenuation characteristics of (C₂₁H₂₂P)₂MnBr₄, GAGG(Ce), LuAG(Ce), and carbon were theoretically calculated using the NIST database.⁴⁵ As illustrated in Fig. 4e, the absorption coefficient of (C₂₁H₂₂P)₂MnBr₄ was compared with that of commercial GAGG(Ce), LuAG(Ce) and carbon. The absorption coefficient of (C₂₁H₂₂P)₂MnBr₄ is comparable to that of commercial GAGG(Ce) and LuAG(Ce), but is stronger than that of carbon. In Fig. 4f, it is evident that the absorption coefficient of (C₂₁H₂₂P)₂MnBr₄ is stronger than that of GAGG(Ce) and LuAG(Ce) in the range of 15–40 KeV. At the same time, the absorption efficiencies of (C₂₁H₂₂P)₂MnBr₄, GAGG(Ce), LuAG(Ce) and C are calculated. As illustrated in Fig. 4g, the X-ray absorption efficiency at a thickness of 200 μm is indicated by the dotted line, showing that the absorption efficiency of (C₂₁H₂₂P)₂MnBr₄ is 40%. Furthermore, the RL intensity of the scintillator exhibits an excellent linear correlation with the X-ray dose rate. In Fig. 4h, the detection limit of (C₂₁H₂₂P)₂MnBr₄ is 217.7 nGy_air s⁻¹, which is significantly lower than the dose rate required for X-ray medical diagnostics (5.5 μGy_air s⁻¹).⁴ We also tested the radiation stability of (C₂₁H₂₂P)₂MnBr₄ under the radiation conditions of 80 KV and 200 μA. Illustrated in Fig. 4i, the radiation intensity of (C₂₁H₂₂P)₂MnBr₄ has no obvious change under 230 s X-ray irradiation, showing excellent irradiation stability.

Fig. 4 (a) The radiative luminescence (RL) spectrum of the (C₂₁H₂₂P)₂MnBr₄ crystal at 80 KV/200 μm X-ray. (b) X-ray-induced RL spectra of the (C₂₁H₂₂P)₂MnBr₄ crystal in the dose rate range of 329.6–1098.7 nGy_air s⁻¹. (c) The RL spectra of the (C₂₁H₂₂P)₂MnBr₄ crystal and LuAG (Ce) crystal under the same measurement conditions. (d) Comparison chart of light yield. (e) Comparison of absorption coefficients of (C₂₁H₂₂P)₂MnBr₄ and commercial GAGG(Ce), LuAG (Ce) and carbon. (f) Comparison of absorption coefficients of (C₂₁H₂₂P)₂MnBr₄ and commercial GAGG(Ce), LuAG(Ce) and carbon in the range of 0–50 KeV. (g) Absorption efficiency of (C₂₁H₂₂P)₂MnBr₄ at the thickness of 200 μm. (h) The linear relationship between the dose rate and RL intensity for (C₂₁H₂₂P)₂MnBr₄ transparent medium scintillation. (i) RL intensity spectra of (C₂₁H₂₂P)₂MnBr₄ continuously excited by X-rays.

Illustrated in Fig. 5a, is the flow chart for preparing (C₂₁H₂₂P)₂MnBr₄@PVDF flexible films using a suction filtration method. The scintillator (C₂₁H₂₂P)₂MnBr₄@PVDF was prepared by the method described in the experimental procedure. The film showed bright green emission (Fig. 5a). By conducting SEM tests on the (C₂₁H₂₂P)₂MnBr₄@PVDF film, it was found that the thickness of the film is approximately 80 μm. The surface of the PVDF film is uniformly filled with (C₂₁H₂₂P)₂MnBr₄ microcrystals, with the microcrystal particle size ranging from about 2 to 10 μm (Fig. S9, ESI†). The flexibility and fluorescence of the film were tested under indoor lighting and ultraviolet lamp irradiation (Fig. S10, ESI†). The (C₂₁H₂₂P)₂MnBr₄@PVDF film exhibited excellent flexibility, allowing it to be bent to any angle and then easily recovered. Additionally, the film displayed bright green luminescence under ultraviolet lamp irradiation. To evaluate the changes in luminescence performance after the crystal transformed into thin films, we conducted additional RL (radioluminescence) tests of the scintillator films. The overlapped RL spectra confirmed the same X-ray conversion performance of both (C₂₁H₂₂P)₂MnBr₄ crystal and fabricated scintillator thin films (Fig. S11, ESI†). Additionally, to evaluate the X-ray imaging applications of the (C₂₁H₂₂P)₂MnBr₄ scintillator, a system schematic diagram was constructed (Fig. 5b). The X-ray source, imaging table (imaging object), reflector and special digital camera are placed in sequence to form a special system. We use an ISO19232 standard line pair card to test the resolution of the scintillator. From Fig. S12 (ESI†), we can see that D16 line pair with resolution of 20 lp mm⁻¹ can be distinguished. The parameter of image indexing represented by an ISO 19232 standard line pair card can be obtained from Table S5 (ESI†). In order to better verify the ability of the (C₂₁H₂₂P)₂MnBr₄ scintillation screen to form high spatial resolution images, the modulation transfer function (MTF) curve was obtained using the hypotenuse method, and the spatial resolution was calculated.⁴⁶ As illustrated in Fig. 5c, the resolution of the (C₂₁H₂₂P)₂MnBr₄ scintillation screen is 20 lp mm⁻¹. This value stands out as highly competitive within the realm of most lead-free hybrid metal halide scintillators, shown in Table S4 (ESI†). As shown in Table 1, we compared the (C₂₁H₂₂P)₂MnBr₄ scintillation screen with some commercial scintillators. The light yield and resolution of the (C₂₁H₂₂P)₂MnBr₄ scintillation screen are significantly superior to those of these commercial scintillators. Fig. 5d presents an image of the chip under normal light (left) and an X-ray image with a scintillator screen of (C₂₁H₂₂P)₂MnBr₄ (middle). From the enlarged view of the chip in the X-ray image, the iron piece inside the chip can be clearly observed (top right), and even the fine wires within the chip are detectable (bottom right), measuring approximately 60 μm. Fig. S13 (ESI†) demonstrates that the spring inside a capsule can also be distinctly visualized when the scintillator is exposed to X-rays. Fig. 5e shows an X-ray image of a dried fish, where it is evident that under X-ray exposure, the internal structure of the dried fish is completely revealed. Additionally, an enlarged view of the dried fish allows for a clear observation of the shape and size of the fish's eyeballs, the structure of the tail fins, and the shape and size of the heart. These results indicate that a (C₂₁H₂₂P)₂MnBr₄@PVDF flexible screen exhibits excellent ray imaging performance and holds significant promise for applications in medical and industrial ray imaging.

Fig. 5 (a) The schematic diagram of a scintillation screen made using a suction filtration method is given. (b) Schematic diagram of a scintillator image detection system. (c) Modulation transfer function (MTF) curve of the (C₂₁H₂₂P)₂MnBr₄@PVDF scintillator. (d) Digital photos of the target object (chip) under natural light (left) and X-ray irradiation (middle), as well as the partial enlarged view of the X-ray image of the chip (right). (e) Physical image (left) and imaging picture (right) of dried small fish, as well as local enlarged images of dried fish.

Table 1 The comparison of parameters between (C₂₁H₂₂P)₂MnBr₄ and some reported commercial detectors

Compound	LY [photons MeV^−1]	δ [lp mm^−1]	Ref.
BGO	10 000	9.1	47
LuAG(Ce)	25 000	—	38
CsPbBr3	21 000	0.210 mm	40
CsI: Tl	35 000	8.5	47
Gd2O2S: Pr, Ce, F	22 000	6.2	47
Bi4Ge3O12	10 000	9.1	47
(C21H22P)2MnBr4	46 786	20	This work

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3. Conclusions

We have successfully synthesized a novel zero-dimensional lead-free organic–inorganic hybrid single crystal material, (C₂₁H₂₂P)₂MnBr₄. Under ultraviolet excitation, this material exhibits a bright green emission with a PLQY of up to 98.93%, along with excellent thermal stability and negligible self-absorption. Compared to most reported manganese-based halides, our material demonstrates significant advantages in optical performance and X-ray scintillation properties. Through an innovative filtration preparation technique, we have combined (C₂₁H₂₂P)₂MnBr₄ with PVDF to develop a flexible scintillator screen with uniform thickness and emission. This screen achieves a light yield of 46 786 photons MeV⁻¹, spatial resolution up to 20 lp mm⁻¹, and detection limit as low as 217.7 nGy_air s⁻¹, outperforming most reported materials of the same type. In contrast to the complex preparation processes found in the existing literature, our method is simple, low-cost, and highly versatile, offering a new pathway for the large-scale production of high-performance scintillators. This research represents a significant breakthrough in the synthesis of novel manganese-based hybrid perovskite materials, the innovation of preparation techniques, and the development of high-performance X-ray scintillators, providing new possibilities for application in medical imaging, security inspection, and industrial detection.

4. Experimental section

4.1. Chemicals and materials

Isopropyl triphenyl phosphorus bromide (C₂₁H₂₂PBr, 98%), manganese bromide (MnBr₂, 99.9%), and N,N-dimethylformamide (DMF, 99.8%) were purchased from Adamas. Ether (≥99.5%) and acetone (≥99.5%) were purchased from KL. Fluororubber. Polyvinylidene fluoride microporous membrane (PVDF) was purchased from Delvstlab. All materials were not further purified.

4.2. Synthesis of the (C₂₁H₂₂P)₂MnBr₄ single crystal

The compound (C₂₁H₂₂P)₂MnBr₄ is prepared using an antisolvent method. Generally, 2 mmol of C₂₁H₂₂BrP (770.58 mg) and 1 mmol of MnBr₂ (214.75 mg) are placed in a wide-mouth glass bottle, and 2 mL of N,N-dimethylformamide (DMF) solution is added. The sealed bottle containing the mixture is placed on a heating table at 60 °C and stirred continuously until a clear and transparent solution is formed, which takes about 30 minutes. After unsealing the bottle, it is put into a jar with an ether anti-solvent to seal it. The jar is allowed to stand for a day or two, during which crystals precipitate in the vial. The crystals are then washed with ether three times repeatedly to obtain a single crystal.

4.3. Synthesis of (C₂₁H₂₂P)₂MnBr₄@PVDF thin films

A 500 mg sample, derived from the synthesized (C₂₁H₂₂P)₂MnBr₄ crystalline material, is subjected to fine grinding in an agate mortar. After the crystal particle size is reduced, a solvent, prepared by combining 15 mL of acetone solution with 150 mg of fluororubber, is added to continue the grinding, ensuring that the crystal size in the solution reaches a state of uniform and fine particles. The solution is transferred to a glass bottle, where it undergoes ultrasonic vibration and thorough mixing. The mixed solution is then stirred overnight. Subsequently, the completely mixed solution is pumped through a polyvinylidene fluoride microporous membrane (PVDF) with a diameter of 47 mm and a pore size of 0.45 micrometers using a suction filter for 10 minutes, thereby obtaining a (C₂₁H₂₂P)₂MnBr₄@PVDF film.

4.4. Measurement and calculation of light output

The scintillator was placed at the same position of a carefully calibrated spectrometer (Maya 2000 Pro, Ocean Optics), and the RL (radioluminescence) spectrum was measured under the conditions of an X-ray tube voltage of 80 kV and a tube current of 200 μA.

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

XAE(ε,d) = 1 − e^−c(ε)ρd × 100% (4)

where c(ε) is the photon cross-section function obtained from the XCOM database of the National Institute of Standards and Technology, ε is the photon energy, ρ is the density of the scintillator, and d is the thickness. Then, the X-ray attenuation coefficient (α) is defined as follows:

α = c(ε) × ρ (5)

The attenuation efficiency of X-rays varies with the thickness of the scintillator across the entire X-ray photon energy range (0–50 keV). The calculation formula is as follows:where r(ε) is the X-ray output spectrum of our X-ray tube and the unit of ε is keV.

The steady-state RL spectrum was integrated to obtain the corresponding photon count results (measured RL). The photon count of the scintillator's emission was normalized to the same X-ray attenuation using the following formula:

where XAE(d) is the X-ray attenuation efficiency (%) of scintillators for the entire X-ray energy range (from 0 to 80 keV) at a certain thickness.

The steady-state internal X-ray to light conversion efficiency of organic scintillators (LY_sample) can be calculated using the following equation:

where LY is the light yield of the sample or reference scintillator and RL_normalized is the photon count of the sample and reference scintillator normalized to the respective X-ray attenuation efficiencies.

4.5. Physical characterization

Powder X-ray diffraction measurements (PXRD) were performed on a PANalytical X’Pert powder diffractometer. During the measurement, the Bragg's diffraction angle (2°) range was set to 10°–80° and scanning time was 20 minutes. Scanning electron microscopy (SEM) images were taken using Quattro S. Cold FE-SEM, Hitach High-Technologies with an acceleration voltage of 10 KV, and EDX was used to characterize the elements of doping samples. Pure (C₂₁H₂₂P)₂MnBr₄ was scanned using a ZEISS field emission scanning electron microscope (GeminiSEM 300), and all elements were characterized by energy dispersive spectroscopy (EDS). The UV-Vis transmission spectra and absorption spectra were recorded using a UV-3600 spectrophotometer (Shimadzu, Japan) in the wavelength range of 200 to 800 nm. The decay curves were recorded using a time-resolved fluorescence spectrophotometer (FLS1000, Edinburgh Instrument Ltd, Edinburgh, UK). The photoluminescence (PL) spectra were measured by using a 450 W Xe lamp for ozone removal as the excitation source and a FLS1000 spectrophotometer (Edinburgh Instrument Ltd, Edinburgh, UK). The thermogravimetric analysis of crystals was performed by TGA/DSC1/1600LF (Mettler Toledo, Switzerland) at a heating rate of 20 °C min⁻¹ up to 800 °C. The DFT calculation was performed on the Cambridge Sequential Total Energy Packge (CASTEP) module.⁴⁸ The generalized gradient approximation with the Burke–Ernzerhof potential was used for periodic solids. The cutoff energy was set as 400 eV and Brillouin zone was using 3 × 3 × 3 Monkhorst–Pack grid. Spin-polarized calculations within the DFT+U framework have been applied for Mn²⁺ to correct the well-known DFT self-interaction errors.^49,50 For the structural optimization, the maximum force limit, maximum energy variation tolerance and maximum displacement were set as 0.03 eV Å⁻¹, 10⁻⁵ eV per atom and 0.002 Å. The self-consistent field (SCF) tolerance was set as 2 × 10⁻⁶ eV per atom. Excitation energy diagrams were performed using the B3LYP hybrid function implemented in the Gaussian-16 suite of programs.⁵¹ The time-dependent density functional theory (TDDFT) calculations were performed using the PBE0-D3(BJ)/def2-SVP level of theory⁵² on the geometries obtained from XRD experiments. The radioluminescence (RL) spectrum was characterized on a Maya2000 Pro spectrometer. The X-ray radiation for detection and imaging was produced using an X-ray source (HAMAMATSU, L10321). The type of Charge-Coupled Device (CCD) chip used for detection and imaging was KAF-16803.

Author contributions

Mengyue Wu: methodology, investigation, formal analysis, writing – original draft. Jun’an Lai: conceptualization, methodology, investigation, visualization, writing – review & editing, Zixian Wang: methodology, investigation. Faguang Kuang: methodology, investigation. Yongqiang Zhou: methodology, investigation. Kang An: methodology, investigation.

Sijun Cao: methodology, investigation. Yayun Pu: methodology, investigation. Baofei Sun: methodology, investigation. Zhengzheng Liu: methodology, investigation. Juan Du: methodology, investigation. Heng Luo: methodology, investigation. Peng He: methodology, investigation. Xiaosheng Tang: conceptualization, writing – review & editing, supervision.

Data availability

The data supporting this article have been included as part of the ESI.† CCDC 2414560 contain data for (C₂₁H₂₂P)₂MnBr₄.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 62375032, 62104023, 52302059), Natural Science Foundation of Chongqing (No. CSTB2023TIAD-KPX0017, CSTB2022NSCQ-MSX0360), The Open Fund of the State Key Laboratory of High Field Laser Physics (Shanghai Institute of Optics and Fine Mechanics). China Postdoctoral Science Foundation (Grant Number: BX20230355). Sponsored by Natural Science Foundation of Chongqing, China, cstc2019jcyj-msxmX0737.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2414560. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5tc00183h

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