Capillary Manganese Halide Needle‐Like Array Scintillator with Isolated Light Crosstalk for Micro‐X‐Ray Imaging

The exacerbation of inherent light scattering with increasing scintillator thickness poses a major challenge for balancing the thickness‐dependent spatial resolution and scintillation brightness in X‐ray imaging scintillators. Herein, a thick pixelated needle‐like array scintillator capable of micrometer resolution is fabricated via waveguide structure engineering. Specifically, this involves integrating a straightforward low‐temperature melting process of manganese halide with an aluminum‐clad capillary template. In this waveguide structure, the oriented scintillation photons propagate along the well‐aligned scintillator and are confined within individual pixels by the aluminum reflective cladding, as substantiated from the comprehensive analysis including laser diffraction experiments. Consequently, thanks to isolated light‐crosstalk channels and robust light output due to increased thickness, ultrahigh spatial resolutions of 60.8 and 51.7 lp mm−1 at a modulation transfer function (MTF) of 0.2 are achieved on 0.5 mm and even 1 mm thick scintillators, respectively, which both exceed the pore diameter of the capillary arrays’ template (Φ = 10 µm). As far as it is known, these micrometer resolutions are among the highest reported metal halide scintillators and are never demonstrated on such thick scintillators. Here an avenue is presented to the demand for thick scintillators in high‐resolution X‐ray imaging across diverse scientific and practical fields.


Introduction
[3][4][5][6][7] One pertinent example of the X-ray imaging scintillator is the nondestructive testing within the vast and rapidly evolving microelectronic components' market.In this case, the diameter of bonding wires in electronic chips typically falls below 30 μm, which necessitates a scintillator with sufficiently high spatial resolution to discern such fine structural features.In addition, these electronic products, which consist of intricate packaging casings, metal assembly, and diverse silicon-based chips, demand higher energy X-rays to achieve adequate penetration for clear visualization of the internal structure.Thus, a scintillator with a thickness of the order of several hundred micrometers or even millimeters was required to ensure enough deposition of X-ray energy, leading to a high signal-tonoise ratio (SNR).Generally, to balance the X-ray imaging spatial resolution and scintillation brightness, it is desirable to maintain low lateral light crosstalk on a sufficiently thick scintillation screen.However, conventional scintillation screens consisting of polycrystals with nonoriented crystal structures inevitably suffer from inherent light scattering. [1,8,9]15][16][17][18][19][20][21][22][23][24][25][26][27][28] Figure 1.a) Statistical data of scintillator thickness versus X-ray imaging resolution in previous publications and comparison with this work.The numbers in this panel represent references.b) Statistics of linear attenuation coefficients (40 keV X-ray as an example) for commercial and previously reported novel metal halide scintillators.The inset shows a schematic of the photoelectric effect in the interaction of X-rays with matter.c) Light propagation mechanisms and results of conventional nonstructured scintillators and structured capillary needle-like array scintillators in X-ray imaging.
[15][16][17][18][19] On the other hand, thicker scintillators normally yield an unsatisfactory resolution.Therefore, the conflict between scintillator thickness and high resolution remains a formidable challenge in X-ray imaging-related applications.
][17]20,24] In the case of well-established CsI:Tl scintillators, vertical vapor deposition growth of columnar scintillators and melt filling into silica-coated silicon templates are the two most common pixelation schemes. [16,17,29]Despite the effectiveness of photon management engineering for improving the X-ray imaging resolution, high-temperature, time-consuming, and complicated vacuum deposition or high-cost deep silicon etching, and oxidation processes still need to be simplified and optimized.36] However, this strategy comes with several limitations: 1) the thin thickness (typically below 50 μm) and low porosity of AAO greatly limit the X-ray absorption and light output of the array scintillators; 2) the fragility of AAO templates reduces the qualification rate during array scintillator processing, which is very costly for large-scale production; 3) the unavoidable self-absorption behavior caused by the intrinsic small Stokes shift in MHPNCs, which greatly affects the light output efficiency during longdistance propagation; and 4) the poor stability and toxicity associated with lead also hinder their practical applications.Furthermore, some recent works have reported the fabrication of selectively oriented 1D perovskite-like metal halide scintillators by the close-space sublimation.Although such an oriented optical propagation strategy has been successfully demonstrated to suppress lateral light crosstalk to improve the imaging spatial resolution, the seed screening process and the absence of reflective layers between the columns remain challenging for large-scale production and for obtaining high resolution on thick scintillators. [27,37,38]][41][42] More notably, the ultralow melting points of some OMH materials enable them to be readily converted from the crystalline state to the glassy state, endowing them with unparalleled low-temperature processing capabilities. [41,42] tricky issue, however, is the insufficiency of X-ray absorption arising from their low linear attenuation coefficient (Figure 1b).The photoelectric effect, which is the dominant process in the interaction of X-rays with matter, occurs mainly on the K-shell electrons with the highest binding energy in the electron orbitals (Figure 1b, inset).The X-ray absorption increases dramatically (i.e., K-edge) when the incident X-ray energy is higher than the binding energy of the K-shell orbital electrons.Compared to other all-inorganic and lead-based scintillators, the K-edge originating from the relatively low-Z component in OMH makes them tends to exhibit sufficient X-ray absorption at lower energies, but not at higher energies.[45] Hence, one of the most straightforward and effective approaches to compensate for the deficiency in X-ray absorption is to increase the material's thickness without altering its framework structure.Meanwhile, this must also be carefully managed to maintain a sufficiently high resolution.
In this work, a capillary manganese halide needle-like array scintillator with isolated light crosstalk was developed to achieve a tradeoff between thickness-dependent X-ray imaging spatial resolution and scintillation brightness.By constructing such waveguide structures, X-ray-induced scintillation photons were confined within individual pixels by the aluminum reflective cladding, enabling oriented propagation along the well-aligned needle-like scintillator.As a piece of evidence, laser diffraction experiments were conducted to validate the light confinement effect of the designed waveguide structure by comparing and analyzing the evolution of the beam spot profile in the optical field distribution before and after passing through different samples.More importantly, ultrahigh spatial resolutions of 60.8 and 51.7 lp mm −1 at a modulation transfer function of 0.2 were achieved on the scintillator screens with a thickness of 0.5 mm and even 1 mm, benefiting from the isolated lateral light-crosstalk channels and the robust light output resulting from the supplementary thickness.It is noteworthy that both attained resolutions exceed the pore diameter of the capillary arrays (Φ = 10 μm), ranking as one of the highest values among the widely investigated metal halide scintillators obtained via a portable X-ray source system without relying on a complicated system such as the synchrotron radiation accelerator system.

Results and Discussion
The light propagation mechanisms and results of conventional nonstructured scintillators and our designed structured scintillators are demonstrated in Figure 1c.More specifically, conventional scintillators typically comprise nonoriented emitters, where grain boundaries or refractive index mismatches between the emitter and matrix result in anisotropic propagation of the generated scintillation photons.Upon reaching the imaging plane, some off-track photons can cause unwanted signal crosstalk as well as low SNR in the image sensor.Since the ultimate image of a thick scintillator is a stack of multiple inhomogeneous scattering events, when the deviation induced by light scattering is larger than the imaged line width, the resultant line becomes blurred and indistinguishable.In contrast, structured scintillators consist of pixelated needle-like scintillators, which are grown in an aluminum-clad capillary glass array.It is worth noting that although metallic aluminum oxidizes in air to form a dense layer of aluminum oxide, this is known to be generally thin and has high transmittance (more than 90%) in the visible region (Figure S1, Supporting Information).The vast majority of photons will pass through the thin layer of aluminum oxide on the surface and reach the metallic aluminum layer for reflection.Aluminum reflective cladding within pore walls can provide over 90% reflectivity in the visible light range, confining the scintillation photons to a single pixel (Figure S1, Supporting Information).Consequently, the scintillation photons oriented propagate along the needle-like scintillator and arrive at the imaging plane through this waveguide structure.This isolated lateral light crosstalk results in high imaging resolution.
The classical Rayleigh criterion was used to further elucidate the resolution limit of this structured scintillator.It states that two point sources are just resolved if the diffraction maximum of one source coincides with the diffraction minimum of the other. [46,47]xtending it to the X-ray imaging case, assume that an X-rayinduced scintillator creates two discrete photons at proximate spatial locations, which correspond to two individual Airy disks upon the imaging plane.In this paradigm, if Δx is defined as the center-to-center separation between two Airy disks, the resolution limit is given by Δx = 1.22f/D,where f is the focal length of the optical system,  is the wavelength of the photon, and D is the aperture diameter of the imaging lens. [47]For the unstructured scintillators, unoriented linear propagation deviations due to photon scattering lead to a large overlap between the two Airy disks (low Δx value), making them indistinguishable, whereas, for a structured scintillator, photon scattering is strictly confined to a single pixel.On a macroscopic scale, this can be regarded as the linear propagation of photons within a single-pixel aperture.This means that two Airy disks can theoretically be distinguished as long as the pitch between the centers of adjacent pixels exceeds the Δx value (excluding the impact of the focal spot of the X-ray source and imaging system).In other words, the effective resolution of such a structured scintillator should be limited only by the pixel size, conveying that its resolution is expected to at least equal or surpass the specified pixel size within our conceptual framework.
As a demonstrative model of this work, benzyl triphenylphosphonium manganese bromide (BTP 2 MnBr 4 ) was chosen as the core scintillator to be loaded into the capillary arrays.Because the large BTP + cation provides a long Mn-Mn spacing in BTP 2 MnBr 4 , which suppresses the luminescence quenching caused by nonradiative resonance energy transfer that occurs between adjacent Mn ions, resulting in the excellent optical and scintillation properties. [7,12]Briefly, as-prepared crystals were converted to the amorphous melt state at about 200 °C, which was below the decomposition temperature revealed by thermogravimetric analysis (Figure S2, Supporting Information).These melts were then poured onto a preheated aluminum-clad capillary glass array template.Noteworthily, continuous heating was maintained for a while to ensure that the melt adequately flowed into the holes of the capillary glass array before slowly reducing to room temperature.After careful surface polishing, a capillary manganese halide needle-like array scintillator was obtained.Notably, this synthesis process is quite convenient, reproducible, and low cost, making it attractive for commercialization.As shown in Figure S3 (Supporting Information), various samples exhibited intense and uniform green luminescence under both UV and X-ray excitation (see the following section).The uniform distribution of needle-like scintillators results in oriented light propagation, enabling the background symbol to be clearly seen without any blurring or distortion (Figure S3, Supporting Information).The powder X-ray diffraction patterns of as-prepared crystals and annealed manganese halide needle-like array scintillator were in agreement with the simulated data, confirming the high phase purity of the samples (Figure S4, Supporting Information).Upon melting, the crystal was converted into an amorphous glassy state, which is also consistent with similar previous reports. [41]o facilitate the narrative, capillary manganese halide needlelike array scintillators of different thicknesses were abbreviated in the following sections as M x , where x denotes the thickness of the sample.Scanning electron microscopy (SEM) combined with energy dispersive X-ray (EDX) spectroscopy revealed that the capillary manganese halide needle-like array was highly aligned with the uniform elemental distribution, excluding the inhomogeneities caused by cross-sectional cutting during the measurement process (Figure S5, Supporting Information).Additionally, needle-like manganese halide arrays show a continuity and fill rate with few cavities or dead zones.Top-view and crosssectional EDX mappings (Figure 2a; Figure S6, Supporting Information) show well-defined complementary distributions of the corresponding elements in the needle-like manganese halide (C, P, Mn, and Br) and the capillary glass array template (mainly composed of silicon dioxide).The photoluminescence (PL) excitation and PL emission spectra show very weak self-absorption of this manganese halide, which facilitates efficient light propagation across long distances in waveguide structures (Figure 2b).The PL mapping image of the capillary manganese halide needlelike array in Figure 2c further verifies the uniformity of its luminescence distribution.Since the samples fabricated by this synthesis pathway exhibit a high homogeneity, the dimensions of the scintillator can be readily and accurately controlled by simply manipulating the size of the capillary array template.
The scintillation performances of these capillary manganese halide needle-like arrays were evaluated in the following part.Figure 2d (top) exhibits the photoelectric absorption coefficient as a function of X-ray energy for OMH and two typical commercial scintillators bismuth germanate (BGO) and cerium-doped lutetium-yttrium oxyorthosilicate (LYSO:Ce).As mentioned previously, the organic-inorganic hybrid framework leads to relatively low photoelectric absorption coefficients for OMH compared to inorganic BGO and LYSO:Ce scintillators.Fortunately, increasing thickness while maintaining oriented light propagation can greatly compensate for the lack of X-ray absorption in OMH, as shown in Figure 2d (bottom).X-ray-induced radioluminescence (RL) spectra were measured in transmission mode to compare the relative light output of capillary manganese halide needle-like array scintillators with BGO and LYSO:Ce scintillator references (Figure 2e).By integrating their X-ray-induced RL spectra and comparing their results, the relative RL light outputs of BGO (0.5 mm), LYSO:Ce (0.5 mm), M 0.5 , and M 1 are 20.8%,62.4%, 63.9%, and 100%, respectively (Figure 2f; Figure S7, Supporting Information).These results indicate that although OMH possesses a relatively low-Z composition, its high X-ray to visible light conversion efficiency enables it to yield a quite high light output compared to BGO and LYSO:Ce scintillator references.More importantly, thanks to the negligible self-absorption of the OMH and the effective oriented light propagation provided by the waveguide-type structure, the light output of the M 1 sample with increased X-ray energy deposition presents a considerable enhancement compared to the M 0.5 sample.Both M 1 and M 0.5 samples displayed a nearly linear response over a range of dose rates from 7.8 to 250 μGy air s −1 (Figure 2g).The detection limits for M 1 and M 0.5 were derived from dose rate-dependent fitted RL curves as 36.2 and 55.8 nGy air s −1 , respectively, which were approximately two orders of magnitude lower than the dose rate required for typical medical X-ray diagnoses (5.5 μGy air s −1 ).In addition, the RL signal response of a capillary manganese halide array scintillator demonstrates no obvious degradation when subjected to total X-ray irradiation for ≈25 h (with a cumulative dose of ≈23 Gy air ), implying their capacity to maintain reliable and sustained scintillation performance under prolonged radiation exposure (Figure S8, Supporting Information).These results clearly indicate that capillary manganese halide needle-like array scintillators with excellent scintillation properties are highly promising candidates for high-resolution flat-panel X-ray imaging.
To verify the light confinement of the designed capillary manganese halide needle-like array scintillators, dedicated optical paths were built to record the evolution of the laser beam spot profile in the optical field distribution before and after passing through different samples (Figure 3a).Specifically, a femtosecond-pulsed infrared (IR) laser beam (: 1030 nm, repetition rate: 2.0 MHz) was directed to a 40/60 beam splitter, and 40% of it was used for second harmonic generation to produce 515 nm green light.Neutral-density (ND) filters with ND values of 1 and 2, along with a variable ND filter, were employed to control the laser intensity and prevent sample damage.The beam was reflected by mirrors 2 and 3, and focused on the sample using the lens 1 with F = 100 mm.Subsequently, lens 2 and lens 3 re-focused the beam after interacting with the sample, directing it toward an optical beam profiler designed for wavelengths ranging from 200 to 1100 nm.Notably, all measurements, including the blank experiments, were conducted using an identical setup, ensuring consistent distances between the samples and the optical beam profiler.
As shown in Figure 3b-i,ii, the background without the laser beam and the profile of the incident primary laser spot were first recorded by the optical beam profiler, respectively.Thanks to the light confinement effect of the aluminum reflective cladding layer inside the capillary wall, the original laser spot shows weak expansion values of about 1.42 and 1.68 times after passing through the M 0.5 and M 1 with aluminum-cladding samples, respectively (Figure 3b-iii,iv).This expansion was mainly attributed to light scattering within the pore of the capillary array.The extension of the M 1 sample is slightly larger than that of M 0.5 , which may be related to the increased light scattering during longer distance propagation in the pore.As references, the profile of the laser spot becomes blurred and hard to distinguish when passing through the M 0.5 without an aluminum-cladding sample and pristine manganese halide powder @ polydimethylsiloxane (PDMS) film due to severe optical crosstalk (Figure 3b-v,vi).Furthermore, the laser diffraction patterns after passing through various samples were captured by the camera and are shown in The energy deposition efficiency of BGO, LYSO:Ce, and different thicknesses of manganese halide in relation to X-ray energy (bottom).e) X-ray-induced RL spectra of the capillary M 0.5 and M 1 needle-like array scintillator and commercial reference LYSO:Ce and BGO scintillators, an X-ray tube voltage of 30 kV.Note that the spectral response of the spectrometer was calibrated during the measurement.f) Comparison of the integrated light output intensities of the X-ray-induced RL spectra for the above-mentioned samples.g) Dose rate-dependent RL curves of the capillary M 0.5 and M 1 needle-like array scintillators ranging from 17.8 to 250 μGy air s −1 (X-ray tube voltage: 50 kV).The detection limit was derived from the fitted line at an SNR of 3.

Figure S9 (Supporting Information
).It is noteworthy to mention that the distance between different samples and the projection screen is fixed.The capillary manganese halide needle-like array with aluminum-cladding samples displays small, intense, and well-defined diffraction fringes, while the sample without alu-minum cladding exhibits decreased intensity and a widened scattering range (Figure S9, Supporting Information).These results provide clear pieces of evidence that our designed aluminumcladding capillary manganese halide needle-like array can successfully isolate light crosstalk through light confinement effects.Motivated by the excellent scintillation performance and outstanding light confinement effects of this capillary manganese halide needle-like array scintillator, its X-ray imaging applications were further investigated to prove our concepts.In the experimental setup, a laboratory portable X-ray tube was used as the imaging source.Since it is different from the highly collimated synchrotron radiation beam used in these typical previous works on micro-X-ray imaging, the effect of the focal spot of the X-ray source was also taken into account.Given its known cone angle of 86°, the X-ray source was placed far enough away from the sample to act as a quasiparallel light source.
As shown in Figure 4a, the capillary manganese halide needlelike array scintillator exhibits a strong green RL emission under X-ray excitation.The gray value mapping extracted from the region of interest in the box from Figure 4a confirms that its RL emission shows a pretty homogeneous distribution (Figure 4b).It is well known that uniform luminescence is a prerequisite for achieving high-resolution X-ray imaging.The MTF curves of aluminum clads M 0.5 and M 1 were first calculated by extracting their X-ray images for the sharp edges of the tungsten sheet (Figure 4c).As expected, ultrahigh spatial resolutions of 60.8 and 51.7 lp mm −1 at an MTF value of 0.2 were achieved for aluminum-clad M 0.5 and M 1 scintillators, respectively.It should be noted that their resolution all exceeds the pore diameter (10 μm) of the capillary template, which implies that lateral photon leakage was well isolated otherwise their resolutions would be lower than 10 μm.The variation in resolution may be related to light scattering caused by different light propagation distances in the hole, which also corresponds to the phenomena of light confinement experiments.As a comparison, X-ray imaging of different samples was measured using a standard resolution card, further supporting the high resolution of the aluminum-clad capillary manganese halide needle-like array scintillator (Figure S10, Supporting Information).Moreover, X-ray images of capillary manganese halide scintillators with and without aluminumcladding samples as well as transparent LYSO:Ce scintillator were recorded under normal exposure and overexposure (Figure S11, Supporting Information).The aluminum-cladding sample still displays sharp edges under overexposure, while the aluminumfree-cladding sample and LYSO:Ce scintillators show blurred edges because of light scattering.These further emphasize the contribution of our designed waveguide structure engineering in the isolation of light crosstalk.
To visualize and check the resolution limits more intuitively, a microresolution chart (JIMA RT RC-05B, 3-50 μm) was used for imaging.As presented in Figure 4d,e, a spatial resolution of up to 8 μm (i.e., 62.5 lp mm −1 ) can be clearly resolved in a microresolution chart from the gray value intensity along these lines for the aluminum-clad M 0.5 scintillator.The observable resolution of the aluminum-clad M 1 sample drops slightly to 9-10 μm (i.e., 50-55.5 lp mm −1 ), but still exceeds its own pore diameter (Figure S12, Supporting Information).Figure 4f-h shows the bright-field and X-ray images of a copper grid, a storage card, and a chip, respectively.Their structural details can be sharply defined with extreme clarity even for the tiny bonding wires and bonding points in the circuit.These prototype experiments for nondestructive testing of electronic devices clearly demonstrate that our capillary manganese halide needle-like array scintillator holds tremendous potential as a promising alternative to its current commercial counterpart.

Conclusion
In conclusion, we successfully developed a capillary manganese halide needle-like array scintillator with isolated light crosstalk via a facile and low-cost synthesis approach.A most fascinating feature of this structure is its capability of oriented light propagation, enabling ultrahigh resolutions of 60.8 and 51.7 lp mm −1 at an MTF of 0.2 for 0.5 mm and even 1 mm thick scintillators, respectively.This resolution stands as one of the highest for X-ray imaging, particularly among the extensively studied metal halide scintillators.Furthermore, given that both resolutions exceed the pore diameter of the capillary arrays template (Φ = 10 μm), which, along with the laser diffraction experiments results, serves as direct evidence for the light confinement effects achieved through our designed waveguide structure.As a result, the rare combination of high light output, low detection limit, eco-friendly element composition, high resolution, and costeffectiveness makes this thick scintillator highly promising for a myriad of X-ray imaging-related applications spanning scientific research to real-life fields.Our findings offer valuable insights into the effective harmonization of the scintillatorthickness-dependent X-ray imaging spatial resolution and scintillation brightness, presenting an innovative strategy for developing high-performance X-ray imaging scintillators.

Figure 2 .
Figure 2. a) Top-view (top) and cross-sectional (bottom) SEM images and corresponding EDX spectroscopy elemental mapping results for the capillary manganese halide needle-like array.b) Photoluminescence (PL) excitation ( em = 520 nm) and PL emission ( ex = 365 nm) spectra of the capillary manganese halide needle-like array.The inset shows the photograph of a capillary manganese halide needle-like array under UV excitation.c) PL mapping of the capillary manganese halide needle-like array.d) Photoelectric absorption coefficient of BGO, LYSO:Ce, and manganese halide as a function of X-ray energy (top).The energy deposition efficiency of BGO, LYSO:Ce, and different thicknesses of manganese halide in relation to X-ray energy (bottom).e) X-ray-induced RL spectra of the capillary M 0.5 and M 1 needle-like array scintillator and commercial reference LYSO:Ce and BGO scintillators, an X-ray tube voltage of 30 kV.Note that the spectral response of the spectrometer was calibrated during the measurement.f) Comparison of the integrated light output intensities of the X-ray-induced RL spectra for the above-mentioned samples.g) Dose rate-dependent RL curves of the capillary M 0.5 and M 1 needle-like array scintillators ranging from 17.8 to 250 μGy air s −1 (X-ray tube voltage: 50 kV).The detection limit was derived from the fitted line at an SNR of 3.

Figure 3 .
Figure 3. a) Optical path diagram of the light confinement effect validation experiment for capillary manganese halide needle-like array scintillators.b-i) Background image captured by an optical beam profiler without the presence of samples and a laser beam; b-ii) original laser beam spot profile without passing through samples; evolution of the laser spot distribution after passing through b-iii) M 0.5 and b-iv) M 1 with aluminum cladding, b-v) M 0.5 -without aluminum cladding, and b-vi) pristine manganese halide powder @ PDMS film.

Figure 4 .
Figure 4. a) A photograph of an M 0.5 scintillator under X-ray irradiation.b) The surface plot of the gray values' distribution extracted from the region of interest in panel (a).c) Modulation transfer function (MTF) curves for M 0.5 (green) and M 1 (cyan) with the aluminum-cladding scintillator, and M 0.5 -without aluminum-cladding scintillator (gray).d) The X-ray image of a microresolution chart (X-ray tube voltage: 20 kV, dose rate: 161 μGy air s −1 , exposure time: 1 s).The inset in the left panel shows a bright-field image of this microresolution chart.The right one is an enlarged view for the region of interest in the left one.e) The gray value profiles along the cyan and green lines extracted from the X-ray image of the microresolution chart.Note that the numbers (3-50, unit: μm) in panel (d) represent the corresponding spatial resolution, which is converted to the numbers in panel (e) (16.6-71.4)andexpressed in lp mm −1 .Bright-field (inset) and X-ray images of f) a copper grid, g) a storage card, and h) a chip, recorded before and after X-ray exposure (X-ray tube voltage: 50 kV, dose rate: 256 μGy air s −1 , exposure time: 1 s).