Nano Organic Co‐Crystal Scintillator for X‐ray Imaging

Traditional‐metal‐containing scintillators are widely used in X‐ray imaging due to their efficient X‐ray absorption and output of visible light. However, they suffer from heavy‐metal toxicity, environmental stability, harsh preparation, and afterglow. Metal‐free organic scintillators show a rising momentum, especially organic‐halogen‐containing molecules. Halogens are introduced to improve their X‐ray absorption, but the resulting increase in spin–orbit coupling leads to significant delayed fluorescence or phosphorescence, affecting the response speed to X‐rays. Moreover, there is still insufficient practice in fabricating microstructured organic scintillators for high spatial resolution of imaging. Herein, the preparation of nano organic co‐crystals (t‐Bpe‐IFB co‐crystal, abbreviated as BIC, t‐Bpe for trans‐1,2‐bis(4‐pyridyl)ethylene, and IFB for 1,3,5‐trifluoro‐2,4,6‐triiodobenzene) and its application in X‐ray imaging are explored. In contrast to previous single organic‐halogen‐containing molecules, BIC generates nanosecond‐scale fluorescence through the charge‐transfer state of the donor–acceptor. Its high iodine content ensures large X‐ray absorption, strong radioluminescence, and a low detection limit of 85 nGyair s−1. The composite film made of nano‐sized BICs and polydimethylsiloxane exhibits a high spatial resolution of 16.7 lp mm−1. Herein, the application of organic co‐crystals is expanded and ideas are provided for the development of new scintillators.

Traditional-metal-containing scintillators are widely used in X-ray imaging due to their efficient X-ray absorption and output of visible light.However, they suffer from heavy-metal toxicity, environmental stability, harsh preparation, and afterglow.Metal-free organic scintillators show a rising momentum, especially organichalogen-containing molecules.Halogens are introduced to improve their X-ray absorption, but the resulting increase in spin-orbit coupling leads to significant delayed fluorescence or phosphorescence, affecting the response speed to X-rays.Moreover, there is still insufficient practice in fabricating microstructured organic scintillators for high spatial resolution of imaging.Herein, the preparation of nano organic co-crystals (t-Bpe-IFB co-crystal, abbreviated as BIC, t-Bpe for trans-1,2-bis(4-pyridyl)ethylene, and IFB for 1,3,5-trifluoro-2,4,6-triiodobenzene) and its application in X-ray imaging are explored.In contrast to previous single organichalogen-containing molecules, BIC generates nanosecond-scale fluorescence through the charge-transfer state of the donor-acceptor.Its high iodine content ensures large X-ray absorption, strong radioluminescence, and a low detection limit of 85 nGy air s À1 .The composite film made of nano-sized BICs and polydimethylsiloxane exhibits a high spatial resolution of 16.7 lp mm À1 .Herein, the application of organic co-crystals is expanded and ideas are provided for the development of new scintillators.absorption of X-ray since α ∝ Z 3 (α is X-ray absorption cross section), resulting in poor X-ray response; [7a] 2) thermally activated delayed fluorescence molecules and phosphorescent molecules utilize triplet states to improve the luminous efficiency and counteract the aforementioned shortcomings, but bring unfavorable an extension of luminescence lifetime (microseconds or milliseconds) [7a,b,e,8] ; 3) although halogencontaining organic scintillators reported earlier increase the absorption of X-rays caused by halogens, they introduce large spin-orbit coupling caused by halogens and corresponding afterglow (phosphorescence or delayed fluorescence) as well; [7a,e] 4) furthermore, given harsh conditions and long periods for growing large-sized crystal, organic-scintillator-based uniform screens require other simple processes, for example, using microstructured scintillators to achieve high spatial resolution. [2,9]rganic co-crystals are crystalline single-phase materials composed of two or more different compounds in a stoichiometric ratio, which could achieve target properties through a collaborative strategy in distinct constituent unit. [10]Given multi requirements of scintillators for X-ray imaging, organic halogen-bonded co-crystals [11] may be good candidates by simultaneously introducing chromophores with bright emission and halogens with strong X-Ray absorption, especially iodine (I).7a] However, the co-crystals they chose show phosphorescent emission and long afterglows (on the order of milliseconds).Our previously reported t-Bpe/IFB co-crystal (abbreviated as BIC, t-Bpe for trans-1,2-bis(4-pyridyl)ethylene, and IFB for 1,3,5-trifluoro-2,4,6-triiodobenzene) is expected to be a good candidate because high-iodine-containing IFB promotes strong absorption of X-rays and a charge-transfer (CT) state of donor-acceptor induces prompt fluorescence. [12]Moreover, its convenient solution processability offers possibility to prepare largescale size-uniform nano-BICs for fabricating microstructured screens by the reverse saturation recrystallization (RSR) method. [13]he size of BICs can also be adjusted with experimental conditions (e.g., concentration of precursor, surfactant). [14]erein, we prepared nano-BICs by RSR method and fabricated BICs/polydimethylsiloxane (PDMS) composite films for X-ray imaging (Scheme 1).Factors affecting the size of BICs were investigated and BICs with lengths of 400 nm-%30 μm were prepared.High-iodine BICs absorb X-rays significantly, resulting in strong XRL and a low LoD of 85 nGy air s À1 , and its nanosecond fluorescence lifetime ensures a fast response to X-rays.Moreover, nano-sized BICs/PDMS composite film exhibits excellent X-ray imaging with a high resolution of 16.7 lp mm À1 .This work demonstrates the feasibility of microstructured organic halogen-bonded co-crystal scintillator for X-ray imaging, providing new insight into the development of ideal scintillators.

Preparation and Characterization of Nano-BICs
Preparing size-uniform nano-BICs by a convenient method is the first step of its X-ray imaging application, because scintillation screens are usually uniform films that fabricated by direct dripping of nanocrystals or compositing them with other transparent matrices. [2,9]Our previous work has demonstrated the successful preparation of BICs by the drop-casting method. [12]The precursor was prepared by dissolving equimolar raw materials in "good solvent" (ethanol for this work), and dripped onto a clean glass slide, then BICs formed after the solvent evaporated under ambient conditions (Figure 1a).Here, we found that the evaporation rate of ethanol determines the size of BICs.When the evaporation was deliberately slowed down by covering the droplet Scheme 1. Illustration of nano organic co-crystal scintillators for X-ray imaging.with a small beaker, millimeter-scale rod-shaped BICs were obtained until the solvent evaporated (Figure S1a,b, Supporting Information).Without controlling the volatilization rate, the size of rodlike BICs decreased to about 150 μm (Figure S1c,d, Supporting Information).The size of BICs decreased with the increase of solvent evaporation rate due to the dominant nucleation process, which confirms the good crystallization behavior of BICs and provides an idea for our subsequent preparation of nano-sized BICs by accelerating the dissociation of ethanol from precursor.In addition, the driving force for the self-assembly of organic co-crystals originates from the interactions between donor and acceptor including hydrogen bonds, halogen bonds, etc., which can be evaluated by the electrostatic surface potential (ESP) of molecules. [15]The ESP values of t-Bpe and IFB were calculated by Multiwfn software, respectively (Figure 1b, Tables S1 and S2, Supporting Information). [16]or donor (t-Bpe), the minimum value of ESP is located near the nitrogen atom, about À42.68 kcal mol À1 , and for acceptor (IFB), the maximum value of ESP is located near the iodine atom, about 28.82 kcal mol À1 .The significant ESP difference between donor and acceptor indicates that they will undergo strong intermolecular interactions and rapid assembly processes once released from good solvent.
The aforementioned growth characteristic of BICs allows us to obtain their nanocrystals by the RSR method, a method commonly used to prepare nanocrystals of ionic compounds. [13]As shown in Figure 1c, equimolar raw materials are first dissolved in ethanol (as a good solvent) to obtain a high-concentration precursor, and then a small amount of precursor is quickly added to water (as a poor solvent) under vigorous stirring.The solubility of t-Bpe and IFB in the mixed solvent drops sharply, resulting in a fast kinetic process and rapid precipitation, which is equivalent to the "rapid evaporation" of solvent.According to the classical growth theory, rapid solute precipitation leads to a nucleation-dominated process and ultimately promotes the formation of small crystals.Moreover, surfactants are necessary to obtain monodisperse nanoparticles and prevent their further growth.Several typical surfactants, anion-typed sodium dodecyl sulfate (SDS), cation-typed cetyltrimethylammonium chloride (CTAC), and nonionic polyvinylpyrrolidone (PVP) were selected in this work.The fast preparation process as predicted is shown in Figure S2 and Videos S1 and S2, Supporting Information.Within seconds of the addition of the precursor, the solution rapidly changed from clear to white under day light (colorless to bright blue under UV) indicating the formation of BICs.Indeed, the concentrations of precursor and surfactant have a synergistic effect on the size and morphology of BICs (Figure 2a).For low-concentration precursor, the size of BICs decreases (%30 μm to 3 μm) with increasing concentrations of surfactant because more surfactants effectively prevent particles from getting bigger.While smaller BICs (less than 5 μm) are dominated by high-concentration precursors, since higher concentrations provide more robust kinetics during the RSR process, favoring the formation of smaller crystals.The smallest BICs are about 400 nm under the protection of PVP-K10.In addition, with the protection of surfactants, the micro/nano-BICs tend to be monodisperse and uniform (Figures 2b-e and S3, Supporting Information).Micrographs (Figure 2b-e) and X-ray diffraction (XRD) patterns (Figures 2f and S4, Supporting Information) of representative micro/nano-BICs reveal a rodlike crystal habit until the axial dimension decreases to about 400 nm and more crystal planes are exposed (Figure 2f ).The combination of donor and acceptor in BICs mainly depends on the intermolecular halogen bond (Figure 2g).Due to the larger interaction between donor and acceptor, this process can easily form co-crystals rather than retain monomers.Surfactants have electron pair donor or acceptor, thus they can bind to acceptor molecule or donor molecules exposed on the crystal surface, breaking the formation of halogen bonds and resulting in small, uniform BICs.Moreover, this RSR method is easily applied to the preparation of large-scale micro/nano-BICs with the proportional expansion of precursor and anti-solvent (Figure S5, Supporting Information).

Optical and Scintillation Properties of BICs
The optical properties of BICs are significantly changed compared to the single-component chromophore.Under UV, t-Bpe powder exhibits two main emissions in blue (340-440 nm) and green region (440-540 nm), and its crystal only exhibits UV emission (320-400 nm).BICs show different blue emissions (360-530 nm), which has been confirmed to be a CT state from donor to acceptor (Figure S6, Supporting Information). [12]This remarkable blue emission is also an intuitive basis for the generation of BICs during RSR progress (Figure S2 and Video S2, Supporting Information).Furthermore, the macroscopic-sized rodlike BICs exhibit bright ends at both ends under a fluorescence microscope (Figure S1, Supporting Information), which is ascribed to their optical waveguide (OW) properties. [12]As the size of BICs decreases, the attenuation of light along the radial direction is effectively suppressed, resulting in an insignificant OW property, which is beneficial to the optical uniformity of the microstructured screen and the improvement of subsequent imaging quality.
The X-ray absorption cross section of scintillator strongly depends on its element composition and can be queried from XCOM. [17]Although IFB is nonluminous, it provides a highiodine content (%55 wt% of BIC).Thus, the X-ray absorption cross section of BIC is much larger than that of fluorescent monomeric t-Bpe with only light atoms (Figure 3a).The photophysical process of the scintillator to generate fluorescence emission after absorbing X-ray is shown in Figure 3b.The scintillator generates high-energy photoelectrons excited by X-rays through Compton effect or photoelectric effect.These photoelectrons then release energy in the scintillator and cause a large number of secondary electrons.After thermalization, electrons will be distributed in the excited states.Therefore, larger cross section indicates that they can absorb more X-rays, convert them into more secondary electrons inside the material, and finally generate more fluorescence emission along the schematic diagram in Figure 3b.In previous report, we confirmed that the fluorescence of BICs was ascribed to the CT interaction between donor and acceptor by theoretical calculations. [12]This fluorescence mechanism of BICs still works under the excitation of X-rays, as its XRL spectrum exhibits similar fluorescence emission under UV (Figure 3c).Fluorescence intensity of t-Bpe powder and BICs is comparable under UV as shown in their photoluminescence (PL) spectra and photographs (Figures 3c and S6, Supporting Information); however, the XRL intensity of BICs is nearly 10 times higher than that of t-Bpe (Figure 3c, comparison of equimolar samples).This significant difference is attributed to the strong absorption of X-rays by iodine atoms and is a direct experimental evidence for energy transfer between donor and acceptor as well.Due to dominant X-ray absorption, its relative light yield is equivalent to that of strong fluorescent anthracene and inorganic scintillator CsI (Figure S7, Supporting Information, comparison of equimolar samples).In addition, the XRL of BICs gradually increased with the intensity of incident X-rays (Figure S8, Supporting Information).Fitting results show that the peak value of XRL exhibits a linear relationship with the X-ray dose rate (Figure 3d).The LoD of BICs is 85 nGy air s À1 according to 3σ theory, [18] which is much lower than the current medical standard of 5.5 μGy air s À1 (only about 1/65 of this standard). [19]he fluorescence lifetime of BICs calculated from the fluorescence decay curve is only 2.71 ns (Figure 3e), which is attributed into its CT interaction.Compared with traditional inorganic scintillators and reported halogen-containing organic scintillators, BIC shows its advantages, namely, introducing halogen to increase X-ray absorption and having nanosecond fluorescence lifetime based on CT interaction.Consistent with its fluorescence lifetime, BICs exhibit optical pulses of about 15 ns under irradiation of high-energy photons with different energies (Figure 3f ).These narrow pulses are consistent with the scintillation decay under pulsed X-ray (Figure S9, Supporting Information), implying its fast response to X-rays.This fast X-ray response is also reflected in the boxlike curves with X-ray on and off (Figure 3g).Under the continuous 166 on/off cycles (%10 000 s) of X-ray (X-ray intensity is 34.75 μGy air s À1 ), the XRL intensity of BICs can still reach 93% of the initial intensity (Figure 3g), Which confirmed that its stability is good but slightly inferior to commercial CsI:Tl (Figure S10, Supporting Information).

X-ray Imaging of BICs/PDMS Films
To verify the feasibility of micro/nano-BICs as microstructures for X-ray imaging, %1 mm thick BICs/PDMS composite films containing BICs with size of 5 μm (S-1) or 400 nm (S-2) were comparatively studied.These films were fabricated by adding 5 wt% BICs into PDMS (see Experimental Section and Figure S11, Supporting Information, for details).Particularly, to avoid the introduction of water and particle agglomeration, BICs were collected by freeze-drying instead of drying after filtration or centrifugation.The as-prepared BICs/PDMS films are flexible and stretchable and exhibit blue emission under X-ray irradiation (Figure 4a).The schematic diagram of test equipment for X-ray imaging is shown in Figure S12, Supporting Information.Collimated X-rays irradiate the object to be tested, then the unabsorbed part stimulates BICs/PDMS film to produce radioluminescence.The rear objective lens collects radioluminescence and enlarges the field of view, and CMOS camera captures optical signal and records it as the result of X-ray imaging.We used metal masks to explore the spatial resolution of X-ray imaging (Figure 4b).Each group of scribe lines in the mask has the same width and spacing, 250, 200, 150, 100, 75, 50, and 30 μm from bottom to top (the resolutions corresponding to each group of scribe lines are 2, 2.5, 3.3, 5, 6.7, 10, and 16.7 lp mm À1 ).X-ray imaging results show that the clarity of S-2 is significantly better than that of S-1 (Figure 4b).When the logarithm is 16.7, the light and dark areas are still clear (area A2 in Figure 4b), which is a competitive result compared to previous reports (Table 1), confirming the effectiveness of microstructured scintillators for high spatial resolution.The values of pixel array within the A1-A4 area were extracted for further numerical analysis.We drew the pixel fluctuations of each area (Figure 4c) with the average value of each row as ordinate, and the actual distance of each row from the first row as abscissa.These curves numerically reflect the contrast and light-dark changes in each area.Consistent with the intuitive vision in Figure 4b, the pixel values of A2 and A4 corresponding to S-2 fluctuate greatly.These fluctuations of A2 and A4 are about 1.5 times of those of A1 and A3 in value, indicating better imaging quality of small-sized BICs (Figure 4c).We tested X-ray imaging of typical items with S-2 film.For a capsule containing a wire, the imaging results show that the capsule and its invisible wire can be identified clearly by the contrast of light and dark (Figure 4d), thanks to their ) Photoluminescence (PL, λ ex = 275 nm) and X-ray radioluminescence (XRL, the dose rate of incident X-ray is 278 μGy air s À1 ) spectra of t-Bpe powder and BIC.d) XRL intensity as a function of X-ray dose rate.Inset is XRL at low dose rate.The limit of detection (LoD) was calculated to be 85 nGy air s À1 , according to 3σ theory.The voltage of the X-ray tube is 50 kV.The dose rate is controlled by adjusting the tube current.e) Fluorescence lifetime curve and fitting results of BIC.f ) Optical pulse signals of BIC under the irradiation of various photon energies (59.5 keV for Am-241; 661.7 keV for Cs-137; 1.17 and 1.33 MeV for Co-60).g) Stability test using X-ray (silver target, tube voltage is 50 kV, tube current is 10 μA) with a dose rate of 34.75 μGy air s À1 .The time interval between each X-ray on and off is 30 s.
difference in the absorption of X-rays.Details such as the wall of capsule (about 65 μm) and the gap between two walls (about 40 μm) are clearly visible.Moreover, we also imaged an ant encapsulated in resin and an unknown electronic component using S-2 film (Figure S13, Supporting Information), and their detailed features can also be clearly reflected.The results indicate that the BICs/PDMS film is a candidate for indirect X-ray imaging.

Conclusion
In conclusion, this paper demonstrated the preparation of nano-BICs and their application as a scintillator in X-ray imaging.The high-iodine-containing IFB enhances the absorption of X-rays, and the CT interaction between donor-acceptors ensures a nanosecond fluorescence emission, resulting in a low LoD of 85.5 nGy air s À1 and a fast X-ray response.The spatial resolution of X-ray imaging as high as 16.7 lp mm À1 was obtained by constructing the nano-BICs/PDMS composite film, showing the contribution of the microstructured scintillator.This work provides new insights into the development of novel scintillators and expands the practical application of organic co-crystals.
Preparation of BICs by Drop-Casting Method: The precursor solution was obtained by dissolving 0.1 mmol (0.0182 g) t-Bpe and 0.1 mmol (0.0509 g)  IFB in 10 mL ethanol.The 0.2 mL of the precursor was dripped onto a clean glass slide with covering a small beaker to slow down the evaporation of ethanol.Millimeter-sized BICs could be obtained after the solvent was completely evaporated.In addition, without covering the beaker, the evaporation of the solvent was faster, resulting in BICs with a size of tens of micrometers.
Preparation of Nano-BICs by Reverse Saturation Recrystallization Method: Equimolar t-Bpe and IFB were dissolved in 10 mL of ethanol to obtain a precursor solution, wherein the amount of t-Bpe and IFB used was determined as needed.A certain mass of surfactant was dissolved in 10 mL of deionized water as an anti-solvent.Then, 1 mL of precursor was quickly added to deionized water, and the reaction was completed after continuous rapid stirring for several seconds.For the preparation of large-scale micro/nano-BICs, it was only necessary to expand the amount of precursor and anti-solvent in the same proportion.The product could be obtained by freeze-drying or by centrifugation.
Preparation of BICs/PDMS Composite Films: PDMS and curing agent were mixed uniformly in a mass ratio of 10:1, and stored at 4 °C for 6 h.Then, the preprepared micro/nano-BICs were added to PDMS and mixed well by thorough stirring.The mass ratio of added micro/nano-BICs to PDMS was 1:19 (5 wt% of BICs).The appropriate volume of the mixture was added to the high-temperature-resistant plastic mold and kept at a constant temperature of 80 °C for 12 h in a vacuum state.In particular, to uniformize the thickness of the film, it was necessary to ensure that the mold was placed horizontally.
Characterization: Optical microscope (OM) and fluorescence images were pictured using a Leica 2700M OM.Scanning electron microscope (SEM) images were characterized by Hitachi SU8010 field-emission SEM and the surface of sample was sprayed with gold for improving imaging quality.X-ray diffractometer spectra were recorded by Rigaku SmartLab X-ray diffractometer with a power of (9 kW), a monochromatic radiation of Cu Kα (λ = 1.5406Å), and a parallel beam measurement mode.PL and PL excitation spectra were acquired by F-7000 fluorescence spectrophotometer.Fluorescence emission decay curve was done with picosecond diode lasers (Horiba Jobin Yvon Instruments) using time-correlated single-photon counting attached onto a PL Spectrometer FLS 1000.XRL spectra measurement was performed using an Edinburgh FS5 fluorescence spectrophotometer where X-ray photons were generated from a Mini-X X-ray tube (Amptek, Inc., USA).The optical pulse signal of BICs under high-energy rays was acquired by a digitizer (CAEN, DT5730, 500 MS s À1 ) matched with a photomultiplier tube (Hamamatsu R7899) and high-voltage power supply.The scintillation decay curve of BICs was measured using a pulsed X-ray tube (N5084) excited by a picosecond laser (405 nm, PLP-10, Hamamatsu Photon) and a fluorescence lifetime tester (C11367-31, Hamamatsu Photon).The X-ray absorption cross sections of IFB, t-Bpe, and BIC were obtained from the photon cross-section database. [17]All tests were operated at room temperature and ambient atmosphere.Decay curves were fitted by the following formula where I(t) is the PL intensity at time t, A is background, B i is amplitude, and τ i is decay time constant.Fluorescence lifetime τ is calculated by Calculation: The electrostatic potential involved in the analyses was evaluated by Multiwfn based on the highly effective algorithm proposed in ref. [20].
X-ray Imaging Experiments: X-ray imaging experiments were performed at BL13W1 X-ray Imaging and Medical Application Station of Shanghai Synchrotron Radiation Facility.Monochromatic X-rays with an energy of 18 keV and a CMOS detector with a pixel size of 1.625 μm were used for imaging.The acquisition time of X-ray imaging was 100 ms.BICs/PDMS film was attached to the metal window in front of an objective lens and used as a scintillation screen.The distance between the object to be measured and the detector was about 0.5 m.Since the imaging field of view (longitudinal size) was only 3.328 mm, the imaging of objects with larger longitudinal size needed to be completed in steps, and finally the imaging results were artificially combined into one picture.

Figure 2 .
Figure 2. Characterization of micro/nano-BICs prepared by RSR method.a) Size of BICs as function of surfactant and raw materials.b-d) Representative fluorescence micrographs, and c,e) corresponding scanning electron microscope (SEM) images of BICs.b,c) The 0.033 mol mL À1 IFB/t-Bpe and 0.005 g mL À1 sodium dodecyl sulfate (SDS) for (b,c); 0.033 mol mL À1 IFB/t-Bpe and 0.01 g mL À1 PVP-K10 for (d,e).f ) X-ray diffraction (XRD) pattern of representative BICs and calculated XRD pattern according to the crystal data of BICs (CCDC No. 293 753).The broad peak at 20°-30°is caused by the substrate.g) Crystal structure of BIC.Scale bar is 10 μm for (b,d), 1 μm for (c,e).

Figure 3 .
Figure 3. Optical and scintillation properties of BICs.a) The X-ray absorption cross sections of IFB, t-Bpe, and BIC.b) Schematic diagram of the luminescence mechanism of BIC under X-ray irradiation.c) Photoluminescence (PL, λ ex = 275 nm) and X-ray radioluminescence (XRL, the dose rate of incident X-ray is 278 μGy air s À1 ) spectra of t-Bpe powder and BIC.d) XRL intensity as a function of X-ray dose rate.Inset is XRL at low dose rate.The limit of detection (LoD) was calculated to be 85 nGy air s À1 , according to 3σ theory.The voltage of the X-ray tube is 50 kV.The dose rate is controlled by adjusting the tube current.e) Fluorescence lifetime curve and fitting results of BIC.f ) Optical pulse signals of BIC under the irradiation of various photon energies (59.5 keV for Am-241; 661.7 keV for Cs-137; 1.17 and 1.33 MeV for Co-60).g) Stability test using X-ray (silver target, tube voltage is 50 kV, tube current is 10 μA) with a dose rate of 34.75 μGy air s À1 .The time interval between each X-ray on and off is 30 s.

Figure 4 .
Figure 4. X-Ray imaging application of BIC/polydimethylsiloxane (PDMS) films.a) Photographs of representative BICs/PDMS film under day light (left) and X-ray (right).b) Resolution evaluation of BICs/PDMS films.From left to right: the photomicrograph of the mask, the X-ray imaging result of S-1 film, S-2 film, and its local magnification.c) Average of pixel values along the transverse direction of areas A1-A4 marked in (b).d) X-ray imaging of capsule and metal using S-2 film.Scale bars are 1 cm for (a), 500 μm for (b), and 2 mm for (d).The incident X-ray is generated from synchrotron radiation with an energy of 18 keV.See Experiment Section for more details.

Table 1 .
Performance comparison of organic scintillators for X-ray imaging in the literature.
b) Measured use mask with clustered line pairs and modulation transfer function.c) Measured use mask with parallel line pairs.