Highly efficient eco-friendly X-ray scintillators based on an organic manganese halide

Scintillators, materials converting ionizing radiation into visible light, are crucial in diverse applications, including medical imaging, security, and space exploration. However, existing organic and inorganic scintillators face challenges such as demanding synthesis conditions, hygroscopicity, anisotropic scintillation, and low light yields. Lead halide perovskites have shown promise due to their strong X-ray absorption and efficient charge carrier conversion, but their toxicity limits widespread use. Lead-free alternatives, including low-dimensional metal hybrids, are attracting increasing attention. Organic manganese(II) halide hybrids offer strong photoluminescence with tunable colors, but their application as scintillators remained unexplored until this study. This research investigates the potential of a zero-dimensional (0D) organic manganese(II) halide hybrid, (C38H34P2)MnBr4, as a high-performance, eco-friendly X-ray scintillator.

The literature review highlights the ongoing search for efficient and environmentally friendly X-ray scintillators. It discusses the limitations of existing materials, including inorganic scintillators with stringent synthesis requirements and hygroscopicity, organic crystals with anisotropic scintillation and low light yields, and plastics with low light yields. The review then focuses on the recent emergence of lead halide perovskites as promising scintillators due to their strong X-ray absorption and efficient charge carrier conversion. However, concerns regarding the toxicity of lead necessitate the exploration of lead-free alternatives. The potential of lead-free metal halide perovskites and low-dimensional metal hybrids, such as Cs2AgBiBr6, Bmpip2SnBr4, and Rb2CuX3 (X = Cl, Br), are mentioned as examples of ongoing research in this area. Finally, the study notes the established strong photoluminescence properties of eco-friendly organic manganese(II) halide hybrids, showcasing their potential for various applications including organic light-emitting diodes and luminescent vapochromism, while emphasizing that their application in X-ray scintillation was previously unreported.

The 0D (C38H34P2)MnBr4 single crystals were synthesized using a facile room-temperature solvent diffusion method. Diethyl ether was diffused into a dichloromethane solution containing ethylenebis(triphenylphosphonium bromide) and MnBr2 (1:1 ratio). Single-crystal X-ray diffraction (SCXRD) determined the crystal structure, revealing a monoclinic space group (C2/o) with isolated MnBr4 tetrahedrons. Powder X-ray diffraction (PXRD) confirmed the high phase purity. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed to assess thermal stability. UV-Vis absorption and steady-state photoluminescence (PL) spectroscopy were employed to characterize the optical properties. X-ray radioluminescence (RL) spectra were measured using an Edinburgh FS5 fluorescence spectrophotometer equipped with an X-ray generator (Amptek Mini-X tube). The scintillator response to various X-ray dose rates was evaluated, and the detection limit was determined using the signal-to-noise ratio (SNR). The light yield was estimated by comparison with cerium-doped lutetium aluminum garnet (Ce:LuAG) as a reference standard. X-ray imaging tests were conducted using a home-built system and (C38H34P2)MnBr4 powders as the scintillator screen. Flexible scintillators were prepared by blending (C38H34P2)MnBr4 with polydimethylsiloxane (PDMS). Scanning electron microscopy (SEM) was used to determine the particle size of the (C38H34P2)MnBr4 powders, and additional characterization techniques were used to assess afterglow.

The synthesized (C38H34P2)MnBr4 single crystals exhibited high thermal stability (no weight loss before 310°C). The crystals showed bright green emission peaked at 517 nm with a full width at half maximum (FWHM) of 51 nm, a high photoluminescence quantum efficiency (PLQE) of ~95%, and a long decay lifetime of 318 μs. The material showed excellent moisture stability, with no change in PL intensity after one month in ambient conditions. In X-ray scintillation studies, (C38H34P2)MnBr4 demonstrated a linear response to X-ray dose rates from 36.7 nGy s⁻¹ to 89.4 μGy s⁻¹, exceeding the response of Ce:LuAG by a factor of 3.2. The detection limit was determined to be 72.8 nGy s⁻¹, significantly lower than that required for X-ray diagnostics. A light yield of ~80,000 photon MeV⁻¹ was calculated, comparable to the best-performing lead-free metal halides and significantly exceeding that of many commercially available scintillators. The material also showed high stability under continuous X-ray irradiation. X-ray imaging tests using (C38H34P2)MnBr4 powders produced high-resolution images (spatial resolution of 322 µm) with negligible afterglow. Furthermore, flexible scintillators were successfully fabricated by blending (C38H34P2)MnBr4 with PDMS, maintaining excellent linearity and providing high-quality X-ray images.

The findings demonstrate that (C38H34P2)MnBr4 is a highly promising eco-friendly X-ray scintillator. Its superior performance compared to many lead-free metal halide perovskites and commercial scintillators highlights its potential for practical applications. The facile synthesis method, combined with the material's high light yield, low detection limit, and excellent linearity, makes it a strong candidate for cost-effective and environmentally responsible X-ray imaging technology. The successful fabrication of flexible scintillators further expands its application potential in portable and wearable devices. This research opens up new avenues for exploring organic manganese(II) halide hybrids for advanced radiation detection technologies.

This study successfully developed a new 0D organic manganese(II) halide hybrid, (C38H34P2)MnBr4, showing highly efficient green emission under photo and X-ray excitation. Its facile room-temperature synthesis, remarkable scintillation properties (high light yield, low detection limit, excellent linearity), and successful demonstration in high-resolution X-ray imaging, along with the creation of flexible scintillators, make it a highly promising material for commercial applications. Future research could focus on further optimization of the material properties and exploration of its use in various X-ray imaging applications.

While the study demonstrates excellent performance, there are limitations. The light yield and detection limit of the flexible scintillators were slightly lower than those of the single crystals. This could be attributed to the lower concentration of (C38H34P2)MnBr4 in the blend and potential non-uniform distribution. The impact of PDMS on X-ray absorption should be further investigated. Long-term stability under harsh conditions and scalability for mass production require further evaluation.