Solution-Processed Hybrid Europium (II) Iodide Scintillator for Sensitive X-Ray Detection

Solution-Processed Hybrid Europium (II) Iodide Scintillator for Sensitive X-Ray Detection

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Xue Zhao¹^,², Pengfei Fu², Pan Li², Hainan Du², Jinsong Zhu², Ciyu Ge², Longbo Yang², Boxiang Song², Haodi Wu²^,^*, Tong Jin², Qingxun Guo², Liang Wang², Jinghui Li², Zewen Xiao²^,^*, Jingjing Chang¹, Guangda Niu²^,³, Jiajun Luo²^,^*, Jiang Tang²^,³^,⁴

Research. Vol 6 Article ID 0125

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Research. Vol 6 Article ID 0125

• Research Article •

Solution-Processed Hybrid Europium (II) Iodide Scintillator for Sensitive X-Ray Detection

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Affiliations

¹School of Microelectronics, Xidian University, Xi’an 710071, China.

²Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China.

³China Optics Valley Laboratory, Wuhan 430074, China.

⁴School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China.

About Author:

^† These authors contributed equally to this work.

Published: 2023-05-20 doi: 10.34133/research.0125

Outline

Abstract

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Lead halide perovskite nanocrystals have recently demonstrated great potential as x-ray scintillators, yet they still suffer toxicity issues, inferior light yield (LY) caused by severe self-absorption. Nontoxic bivalent europium ions (Eu²⁺) with intrinsically efficient and self-absorption-free d–f transition are a prospective replacement for the toxic Pb²⁺. Here, we demonstrated solution-processed organic–inorganic hybrid halide BA₁₀EuI₁₂ (BA denotes C₄H₉NH₄⁺) single crystals for the first time. BA₁₀EuI₁₂ was crystallized in a monoclinic space group of P2₁/c, with photoactive sites of [EuI₆]⁴⁻ octahedra isolated by BA⁺ cations, which exhibited high photoluminescence quantum yield of 72.5% and large Stokes shift of 97 nm. These properties enable an appreciable LY value of 79.6% of LYSO (equivalent to ~27,000 photons per MeV) for BA₁₀EuI₁₂. Moreover, BA₁₀EuI₁₂ shows a short excited-state lifetime (151 ns) due to the parity-allowed d–f transition, which boosts the potential of BA₁₀EuI₁₂ for use in real-time dynamic imaging and computer tomography applications. In addition, BA₁₀EuI₁₂ demonstrates a decent linear scintillation response ranging from 9.21 μGy_air s⁻¹ to 145 μGy_air s⁻¹ and a detection limit as low as 5.83 nGy_air s⁻¹. The x-ray imaging measurement was performed using BA₁₀EuI₁₂ polystyrene (PS) composite film as a scintillation screen, which exhibited clear images of objects under x-ray irradiation. The spatial resolution was determined to be 8.95 lp mm⁻¹ at modulation transfer function = 0.2 for BA₁₀EuI₁₂/PS composite scintillation screen. We anticipate that this work will stimulate the exploration of d–f transition lanthanide metal halides for sensitive x-ray scintillators.

Cite this Article

Xue Zhao, Pengfei Fu, Pan Li, Hainan Du, Jinsong Zhu, Ciyu Ge, Longbo Yang, Boxiang Song, Haodi Wu, Tong Jin, Qingxun Guo, Liang Wang, Jinghui Li, Zewen Xiao, Jingjing Chang, Guangda Niu, Jiajun Luo, Jiang Tang. Solution-Processed Hybrid Europium (II) Iodide Scintillator for Sensitive X-Ray Detection[J]. Research, 2023 , 6 (5) : 0125 - . DOI: 10.34133/research.0125

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Introduction

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X-ray detectors are widely used for high-energy space physics, security inspection, oil drilling exploration, medical radiography, industrial control, and so on [1,2]. Scintillator-based indirect x-ray detection is the mainstream technology in x-ray detection applications [3]. Lead (Pb) halide perovskite nanocrystals have recently emerged as affordable alternatives to conventional ceramic scintillators [4–7]. However, their practical applications are restricted by the toxicity of the Pb²⁺, the severe self-absorption, and the complex synthesis processes.

Lanthanide (Ln: Ce–Lu) ion-based metal halides with 5d–4f transition demonstrate great promise to address the issues mentioned above because of their low toxicity, large stokes shift, high luminescence efficiency, and short decay lifetime [8–11]. Especially, the high emission efficiency resulting from the strong exciton confinement effect and negligible self-absorption of Ln²⁺ ion-based metal halides benefit high light yield (LY). The intrinsic short excited-state lifetimes of Ln²⁺ ion-based metal halides enable a fast response (~ns) under high-energy irradiation [12]. Recently, inorganic Cs₄EuX₆ (X = Br, I) single crystals have been reported as self-activated γ-ray scintillators with superior performance [13]. However, the crystals were prepared by the complex and expensive Bridgman method. Therefore, it is highly desired to develop Eu-based metal halide scintillators, which can be fabricated by a facile, cost-effective process.

In this work, we designed and synthesized an organic–inorganic hybrid europium (II) halide, BA₁₀EuI₁₂ (BA denotes C₄H₉NH₂), by a facile solution method for the first time. In BA₁₀EuI₁₂ crystal, the photoactive sites, [EuI₆]⁴⁻ octahedra, were isolated by BA⁺ cations to form a zero-dimensional (0D) crystal structure, which belonged to a monoclinic system with a space group of Pnma (P2₁/c). BA₁₀EuI₁₂ exhibits blue emission peaked at 462 nm with narrow full width at half-maximum (FWHM) of 31 nm, a Stokes shift of 97 nm, a high photoluminescence quantum yield (PLQY) of 72.5%, and a short excited-state lifetime of 151 ns. When applied as x-ray scintillators, the BA₁₀EuI₁₂ crystals demonstrate a high LY value of 79.6% of LYSO (equivalent to ~27,000 photons per MeV) and possess excellent detection linear range and low detection limit under x-ray radiation.

Results

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BA₁₀EuI₁₂ crystals were synthesized by a one-step reaction from BAI and EuI₂ through a facile solvent evaporation and temperature cooling process. Methanol was used as the solvent, in which the solubility of perovskite increases as temperature rises. The crystals were collected by first preparing a saturated solution and then slowly decreasing its temperature to enter into the oversaturation zone (Fig. 1A). The crystal structure of BA₁₀EuI₁₂ is determined by single-crystal x-ray diffraction (SCXRD). It crystallizes in a monoclinic space group of P2₁/c with lattice parameters of a = 14.71 Å, b = 16.51 Å, c = 17.73 Å, α = 90°, β = 108.5°, γ = 90°, and V = 4,082.5 Å³ (Table S1). As shown in the crystal structure of BA₁₀EuI₁₂, the photoactive sites, [EuI₆]⁴⁻ octahedra, were isolated by BA⁺ cations to form a 0D crystal structure (Fig. 1B). These isolated [EuI₆]⁴⁻ octahedra are separated by layers of BA⁺ cations parallel to the (100) plane. As shown in Fig. 1C, the BA⁺ cations are larger in volume, which separate and slightly distort the [EuI₆]⁴⁻ octahedra [14]. Different from EuI₂, BA₁₀EuI₁₂ is 0D luminescent metal halide, which has unique crystallographic and electronic structure with fascinating optical characteristics (Fig. S1) [15,16]. Furthermore, the measured powder x-ray diffraction patterns fitted well with the simulated patterns derived from the single-crystal structure of BA₁₀EuI₁₂, suggesting the absence of a secondary phase and high purity of BA₁₀EuI₁₂ (Fig. S2). The full width at half maximum (FWHM) of the strongest diffraction peak was 0.11°, revealing good crystallinity. The x-ray photoelectron spectrum (XPS) confirmed that the europium is in the divalent state (Eu²⁺) and demonstrated the interaction between the BA⁺ cations and the central Eu²⁺. The I ion is not oxidized with the 3d_5/2 and 3d_3/2 peak position and doublet separation in good agreement with I⁻ (Fig. S3 and Table S2). BA₁₀EuI₁₂ had a low thermal decomposition temperature, which made it difficult to be prepared through the high-temperature and long-time molten salt method (Fig. S4) [17]. As a result, the facile solution method we put forward is expected to promote the development of Eu²⁺-based organic–inorganic hybrid halides.

First, we used density functional theory (DFT) calculations to understand the electronic structure and the luminous mechanism of BA₁₀EuI₁₂. The emission was caused by parity-allowed electron transitions from the 4f⁶5d¹ excited states into the 4f⁷ ground states. As shown in Fig. 2A and B, the flat valence band and conduction band were attributed to the intrinsic confined 4f and 5d orbitals of Eu (II) cations and the ionic bonding nature of the Eu–I interaction. The energy of optical absorption onset resulting from excitonic absorption was slightly lower (3.06 eV), as shown in the Tauc plot (Fig. S5). The valence band maximum (VBM) is mainly attributed to the Eu-4f orbitals, while the conductive band minimum (CBM) is derived from Eu-5d orbitals (Fig. 2C and D). The organic cations do not contribute to the band edges, therefore forming a type I band alignment between [EuI₆]⁴⁻ units and organic matrices, which favors efficient light emissions [18]. The electronic structure calculation reveals that the localized nature of the VBM is crucial in obtaining efficient narrow emission.

The emission spectra of Eu²⁺ are affected by the combined effect of crystal field splitting (ε_cfs) and centroid shift (ε_c), which are mainly determined by the coordinating ligands (I) around Eu²⁺ and the spectroscopic polarizability α_sp, respectively [7,12]. The weak ligand field effect in [EuI₆] octahedra leads to little energy splitting of Eu-5d orbitals, which results in a large energy gap between Eu-4f and Eu-5d orbitals, and, namely, large excitation and emission energies (Fig. 3A) [19]. As shown in Fig. 3B, BA₁₀EuI₁₂ exhibits highly efficient pure blue photoluminescence (PL) at 462 nm with narrow FWHM (31 nm). The photoluminescence excitation (PLE) centered at 365 nm, indicating a large Stokes shift (97 nm, 0.59 eV; Fig. S6A) [20]. The low dielectric constant of the organic layers poorly screens the attraction between electrons and holes in the inorganic layers, and the inorganic layers confine the exciton’s wave function to zero dimension [21]. Thereby, the large Stokes shift results from the confined 5d-4f transitions in [EuI₆] octahedra, consequently guaranteeing negligible peak overlap (Fig. 3B) [22]. The negligible self-absorption property together with the high theoretical exciton utilization efficiency origin from the strong exciton’s confinement effect of BA₁₀EuI₁₂ endows it with a high PLQY of 72.5%, which benefits high light output for scintillators (Fig. S6B). The room temperature PL decay curve was measured under 340-nm excitation by time-resolved PL measurement, and a biexponential function fitting was conducted to reveal a short PL lifetime (108 ns) of BA₁₀EuI₁₂ (Fig. 3C). The short lifetime boosts the potential of BA₁₀EuI₁₂ for use in high-energy physics and computer tomography (CT) applications. For example, modern CT requires the decay time of the scintillator to be lower than 10 μs to match the sampling rates ≥10 kHz [23].

To investigate the photophysical properties, the temperature-dependent PL spectra and time-resolved fluorescence spectra were further explored (Fig. 4A and B). The plot of PL integrated intensity [I(T)] as a function of reciprocal temperature from 80 to 380 K showed a decrease in the value of I(T) with the elevation of the temperature (Fig. 4C). The E_a value of BA₁₀EuI₁₂ was calculated by fitting data to Eq. 1:

I T = I 0 1 + Aexp − E a / k B T

(1)

where E_a is the thermal activation energy, T is the test temperature, I₀ is I(T) at T = 0 K, and k_B is the Boltzmann constant. The E_a value is estimated to be 242 meV, indicating that thermal quenching is negligible at room temperature. As shown in Fig. 4B, BA₁₀EuI₁₂ demonstrated identical decay traces of the PL spectra intensity across the entire emission spectra range, indicating only one emission center for BA₁₀EuI₁₂. Moreover, the linear correlation between PL and incident light intensity and the constant lifetimes at different incident wavelengths indicated that the luminescence of BA₁₀EuI₁₂ was not from defect states, which further verified that the luminescence mechanism is the d–f transition (Fig. 4D and Figs. S7 and S8). Ln²⁺ has low stability against water and oxygen, among which Eu²⁺ has relatively high stability and low reduction potential (Eu²⁺/Eu³⁺ = 0.35 V). Fortunately, encapsulation technology can help keep the scintillators stable against water and oxygen, which has been successfully demonstrated by commercial CsI and LaBr₃ encapsulated before their practical x-ray sensing and imaging application.

The high PLQY and short decay lifetime on the nanosecond scale make the BA₁₀EuI₁₂ crystal an excellent candidate for an x-ray scintillator [24,25]. The LY of BA₁₀EuI₁₂ was measured by using LYSO as the reference. In each test, the response of the sample was recorded by coupling it with a silicon photomultiplier (SiPM) in the integrating sphere under identical test conditions (Fig. 5A). In addition, due to the existence of heavier elements, Eu (Z = 63) and I (Z = 53), the BA₁₀EuI₁₂ crystals have a high x-ray attenuation ability. According to the calculated result from the software named “Auto Z_eff,” the mean effective atomic number (Z_eff) of BA₁₀EuI₁₂ is 26.65 when the incident energy is set as a 60-keV x-ray (Fig. S9) [26]. The absorption coefficient of a scintillator to x-ray is described by α∝ρZ_eff⁴/E³. Accordingly, BA₁₀EuI₁₂ is an organic–inorganic hybrid material, which contains many light atoms such as C, H, and N. The lack of heavy elements in BA₁₀EuI₁₂ results in its low absorption to x-ray with the absorption coefficient inferior to the traditional scintillators such as CsI: Tl and LYSO. The x-ray absorption of BA₁₀EuI₁₂ is not as good as in inorganic materials such as CsPbBr₃, but higher than the representative organic scintillators, anthracene (Fig. 5B). The BA₁₀EuI₁₂ crystals demonstrate LY as high as 79.6% of LYSO under irradiation with an Amptek Mini-X tube at 50-kV working voltage and 140-μA current (dose rate: 5.30 mGy_air s⁻¹). According to a previous study, LYSO has an LY of 33,900 photons per MeV (photons MeV⁻¹) [27]. Ruling out the possible impact of SiPM, the BA₁₀EuI₁₂ crystals have LY equivalent to ~27,000 photons MeV⁻¹. There are many excellent Eu²⁺-based scintillators for gamma-ray spectroscopy with high LY. Compared with Eu²⁺-doped scintillators, BA₁₀EuI₁₂ has a facile fabrication method and a short fluorescence lifetime (Table S3) [12,26–31]. Lifetime is also an important factor affecting the applications of scintillators. For example, long-life scintillators have optical memory advantages to develop flat-panel-free x-ray detectors [33] and short-life scintillators are suitable for evaluating fast/real-time x-ray imaging. We define the LY value to decay time ratio as the figure of merit (FoM), also known as the initial photon rate, to evaluate the use of scintillators for fast/real-time x-ray imaging [34]. We compared BA₁₀EuI₁₂ with several representative scintillators (Table S4 and Fig. S10) [30–35]. Although the FoM of BA₁₀EuI₁₂ scintillator is 179 photons MeV⁻¹ ns⁻¹ and lower than LaBr₃: Ce (LaBr₃: 5% Ce 4,000 photons MeV⁻¹ ns⁻¹; LaBr₃: 0.2% Ce 1,867 photons MeV⁻¹ ns⁻¹), it is higher than most scintillators [23,34,36–38]. As shown in Fig. 5C, the dose-dependent response of BA₁₀EuI₁₂ was measured, demonstrating a linear response to the x-ray dose ranging from 9.21 μGy_air s⁻¹ to 145 μGy_air s⁻¹. As shown in Fig. 5D, the detection limit of BA₁₀EuI₁₂ is as low as 5.83 nGy_air s⁻¹, which is much lower than that of LYSO (3.5 μGy_air s⁻¹) [27]. The low detection limit of BA₁₀EuI₁₂ is approximately 1,000 times lower than the dosage for a standard medical diagnostic (5.5 μGy a time), which could reduce the radiation dosage used for medical examination and thus alleviate the carcinogenic risks during diagnosis [39]. Moreover, BA₁₀EuI₁₂ has good thermal and irradiation stability with negligible loss of emission intensity of BA₁₀EuI₁₂ PL strength under continuous heating at 85 °C in the glove box and continuous x-ray irradiation (Fig. 5E and F and Fig. S11). Compared with the common Eu²⁺ emitter scintillators, BA₁₀EuI₁₂ is prepared through the facile solution method (Table S4).

Inspired by the promising scintillation property of BA₁₀EuI₁₂, we investigated the possibility of demonstrating practical x-ray imaging through a homemade optical system (Fig. S12). To make a free-standing scintillator screen, we mixed the BA₁₀EuI₁₂ scintillators with polystyrene (PS) to obtain a free-standing film with BA₁₀EuI₁₂ crystals embedded inside the PS matrix. The PS is the host matrix to fabricate a uniform thick film without causing emission quenching and the encapsulation to protect BA₁₀EuI₁₂ from atmospheric water and oxygen (Fig. 6A). The photographs and x-ray images of a stainless nail and spring are shown in Fig. 6B and C, respectively. To evaluate the spatial resolution, we took images of the pattern plate for the standard x-ray resolution test (Fig. 6D), showing that the observation limit was between ≈8 and 10 lp mm⁻¹. To further confirm those values of BA₁₀EuI₁₂/PS, we determined the modulation transfer function (MTF; expressed in units of line pairs per millimeter) through the slanted-edge method for a quantitative resolution value. The spatial resolution of the BA₁₀EuI₁₂/PS film was determined to be 8.95 lp mm⁻¹ at MTF = 0.2, consistent with its x-ray resolution pattern test result (Fig. 6E). The high resolution of BA₁₀EuI₁₂/PS film was comparable with the reported good-performing scintillators (Table S5) [36,40–48]. We believe that these results will stimulate further work on Eu-based scintillators for x-ray imaging.

Discussion

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In summary, we have successfully developed the BA₁₀EuI₁₂ scintillator through a facile solution processing method. BA₁₀EuI₁₂ showed blue emission with negligible self-absorption, a high PLQY of 72.5%, and a short excited-state lifetime (108 ns). The BA₁₀EuI₁₂ scintillators had the characteristics of nontoxicity, easy fabrication, decent LY value of 79.6% of LYSO (equivalent to ~27,000 photons MeV⁻¹), excellent linear response to x-ray dose rate, and a low detection limit. As a result, high-resolution x-ray imaging of 8.95 lp mm⁻¹ was realized. It is anticipated that this work could inspire the d–f transition Ln ion-based metal halides for promising x-ray scintillators and neutrons and γ-ray detection in the future.

Materials and Methods

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Chemicals

N-butyl amine (C₄H₉NH₂·HI, BAI, 99.5%) was purchased from Xi'an Polymer Light Technology Co. Ltd. Europium (II) iodide (EuI₂, 99.9 wt%) was purchased from Tianjin Novcare Biotechnology Co. Ltd. Dried methanol (CH₃OH, 99.0%) was purchased from J and K Scientific Reagent Co. Ltd. All reagents and solvents were used without further purification.

BA₁₀EuI₁₂ single-crystal synthesis

The BA₁₀EuI₁₂ single crystals were synthesized by a one-step reaction between BAI and EuI₂ through a solvent evaporation process followed by a slow cooling process. BAI (0.5 mol l⁻¹) and EuI₂ (0.5 mol l⁻¹) are solved in anhydrous methanol in glass bottles. Then, the vial containing the stoichiometric amount of BAI and EuI₂ was stirred and heated at 50 °C under an inert N₂ atmosphere until forming a saturation precursor solution. The clear solution was then cooled to room temperature (25 °C) at a rate of 5 °C h⁻¹. BA₁₀EuI₁₂ was crystallized gradually with the decrease of solubility during the cooling process, and the BA₁₀EuI₁₂ crystals were washed with antisolvent anhydrous methylbenzene and followed by vacuumed drying.

BA₁₀EuI₁₂/PS composite film synthesis

First, 2 g of PS (average M_w ~ 280,000) was dissolved in 10 ml of anhydrous toluene by stirring at room temperature to form a transparent solution. Second, BA₁₀EuI₁₂ single-crystal powders were added to the solution and stirred thoroughly. Finally, the uniform composites were coated on glass and vacuum-dried.

Inert protection

The BA₁₀EuI₁₂ single crystal was encapsulated by using vacuum grease during the SCXRD test. For luminescent properties and x-ray imaging test, the BA₁₀EuI₁₂ and BA₁₀EuI₁₂/PS films were encapsulate by 2 pieces of thin glass (0.2 mm in thickness) followed by the application of ultraviolet (UV)-curable adhesive.

Characterizations

SCXRD data of BA₁₀EuI₁₂ were collected using an XtaLAB PRO MM007HF diffractometer with Cu Kα radiation. The crystal was mounted in the sample holder surrounded with vacuum silicone and held at 110 K for data collection. The subsequent crystal structure determination and refinement were carried out using CRYSTALS. Structural characterization was conducted by using powder X-ray diffraction (PXRD) recorded on a Philips diffractometer (X pert pro-MRD) operating at 40 kV and 40 mA with Cu Kα radiation. The XPS spectra measurements were conducted using the Thermo Scientific K-Alpha+ from ThermoFisher. The thermogravimetric analysis and differential scanning calorimetry (TGA-DSC) was performed with a PerkinElmer Instruments, Diamond TG/DSC6300, to determine the thermal stability of BA₁₀EuI₁₂. The UV–visible absorption spectra were collected with a UV–vis–near-IR spectrophotometer (Shimadzu Instruments, SolidSpec-3700). The PLE, PL, and time-resolved PL spectra were measured by the Edinburgh FLS920 system. PLQY was obtained through Hamamatsu Quantaurus-QY, with the excitation wavelength at 365 nm. The PL decay spectra were recorded using the Edinburgh FLS920 system with a 340-nm laser beam as the light source at room temperature. The temperature-dependent PL spectra were also measured using the Edinburgh FLS920 system. The selected single crystal was put into the sample room (Linkam attachment, temperature range: 77 to 600 K).

Computational methods

DFT calculations were performed using the Vienna Ab initio Simulation Package (VASP) 6.1 code [49] with the projection-augmented wave (PAW) method. The plane-wave cutoff energy was set to 400 eV. The structural optimization was performed using the Heyd-Scuseria-Ernzerhof (HSE) [50,51] hybrid functionals with a mixing parameter of 25%. Only Γ point was employed for sampling the Brillouin zones, and crystal structures were fully relaxed until the total force on each atom was <0.03 eV/Å.

Calculation of Z_eff

The Z_eff value of BA₁₀EuI₁₂ was determined using Auto-Zeff software developed by Taylor et al. [26]. After imputing the formula as parameter and setting a certain incident energy, the Z_eff value can be obtained.

X-ray scintillation measurement

An Amptek Mini-X tube (Au target, 10 W, Newton Scientific M237), an integrating sphere, a SiPM (JSP-TN3050-SMT), and an oscilloscope (Keysight) were used. The typical scintillator LYSO was selected as the reference for the scintillator property calculation. The samples were placed in the integrating sphere to measure the emission intensity and obtain the LY accurately. In this way, we can eliminate test errors due to more severe light scattering of LYSO than the BA₁₀EuI₁₂ crystal, which leads the scintillation light to come out from LYSO without coupling into SiPM because of the optical waveguide effect. Moreover, we encapsulated BA₁₀EuI₁₂ with a polymer film, made it have the same shape as LYSO, and placed the samples in the same position to obtain the LY value accurately.

According to the proportion between the LY and the test signal, the LY of BA₁₀EuI₁₂ is obtained by taking the known LY of LYSO and different irradiation areas into account. In detail, the LY of BA₁₀EuI₁₂ can be obtained from Eq. 2:

LY BA 10 EuI 12 LY LYSO = R BA 10 EuI 12 R LYSO × ∫ I LYSO λ S λ d λ / ∫ I LYSO λ d λ ∫ I BA 10 EuI 12 λ S λ d λ / ∫ I BA 10 EuI 12 λ d λ × S LYSO S BA 10 EuI 12

(2)

where R is the voltage response, I (λ) is the radioluminescence (RL) intensity, and S (λ) is the detection efficiency of SiPM. S_LYSO and S_BA10EuI12 are the area exposed to x-ray of LYSO and BA₁₀EuI₁₂, respectively. As for the measurement of the linear range, the samples generated light proportionally under x-ray irradiation with a series of intensities. The dose rate is calibrated by an ion chamber dosimeter (MagicMax from IBA DOSIMETRY).

X-ray imaging

We assembled a homemade imaging system in a black lead box. The x-ray source used 435 in the system was M237 (50 kV, Newton Scientific) with Au target. The BA₁₀EuI₁₂/PS film was placed on a reflector (CCM1-G01, Thorlabs), and the scintillation light was deflected to a 437 CMOS camera (C13440, Hamamatsu) with a pixel size of 6.5 × 6.5 μm².

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Year 2023 volume 6 Issue 5

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doi: 10.34133/research.0125

Receive Date：2022-12-16
Online Date：2026-10-28
Published：2023-05-20

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Received：2022-12-16
Accepted：2023-03-29

Affiliations

¹School of Microelectronics, Xidian University, Xi’an 710071, China.

²Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China.

³China Optics Valley Laboratory, Wuhan 430074, China.

⁴School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China.

Corresponding:

^* Address correspondence to: luojiajun@hust.edu.cn (J.L.)

zwxiao@hust.edu.cn (Z.X.)

whd18501@163.com (H.W.)

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表12种不同金属材料的力学参数

科 Family	属数 Number of genus	种数 Number of species	占总种数比例 Percentage of total species (%)	属 Genus	种数 Number of species	占总种数比例 Percentage of total species (%)
鹅膏菌科Amanitaceae	2	11	5.26	鹅膏菌属 Amanita	10	4.78
小菇科 Mycenaceae	2	12	5.74	丝盖伞属 Inocybe	5	2.39
多孔菌科 Polyporaceae	8	14	6.70	蜡蘑属 Laccaria	5	2.39
红菇科 Russulaceae	3	23	11.00	小皮伞属 Marasmius	6	2.87
				小菇属 Mycena	11	5.26
				光柄菇属 Pluteus	5	2.39
				红菇属 Russula	17	8.13
				栓菌属 Trametes	5	2.39

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Fig. 1. (A) Synthesis process of BA₁₀EuI₁₂ single crystals. (B) Crystal structure of BA₁₀EuI₁₂ projected along the b axis. Cyan octahedra correspond to [EuI₆]⁴⁻, in the center of which an Eu atom (light cyan) was located and in the corners of which the I atoms (navy blue) were located. (C) Ball-and-stick model of the BA⁺ cation, in which the small orange, brown, and pink balls in the crystal structure indicate N, C, and H atoms, respectively. (D) Bond lengths of the twisted [EuI₆]⁴⁻ octahedron.

Fig. 2. Electronic structures of BA₁₀EuI₁₂. (A) Calculated band structures of BA₁₀EuI₁₂. (B) Projected densities of states (DOS) of BA₁₀EuI₁₂. (C and D) Isosurface plots of the wave function |Ψ|² of CBM and VBM.

Fig. 3. Emission mechanism and optical characterization. (A) Orbital energy diagram and the energy transition of Eu²⁺ d–f transition in the octahedral crystal field. (B) Absorbance and PL spectra of BA₁₀EuI₁₂. The inset presents the bright blue emission of the BA₁₀EuI₁₂ film under 254-nm irradiation. (C) PL lifetime of BA₁₀EuI₁₂. The decay curve can be well-fitted by a biexponential function.

Fig. 4. Photophysical properties of BA₁₀EuI₁₂. Pseudocolor map of the temperature-dependent PL spectra ranging from 80 to 380 K (A) and time-resolved fluorescence spectra (B) of BA₁₀EuI₁₂. (C) PL intensity derived from the temperature-dependent PL spectra as a function of temperature. The inset presents the photograph of the BA₁₀EuI₁₂ crystal under daylight and 254-nm irradiation. (D) PL intensity as a function of the incident intensity from 3.98 × 106 μJ cm² to 1.19 × 108 μJ cm².

Fig. 5. (A) Scheme of scintillator performance measurement system. (B) Absorption coefficients of BA₁₀EuI₁₂, CsI: Tl, LYSO, and anthracene as a function of photon energy from 1 keV (soft x-rays) to 100 MeV according to the photon cross-section database. (C) Linear response of BA₁₀EuI₁₂ covers an extensive range. The voltage responses of the SiPM coupled to the integrating sphere are proportional to the photon number produced by the BA₁₀EuI₁₂ scintillator. (D) Signal-to-noise ratio value versus the dose rate. The PL intensity of BA₁₀EuI₁₂ under continuous heating at 85 °C (E) and x-ray irradiation at a dose rate of 20 mGy s⁻¹ with a total dosage of 864 Gy_air (F).

Fig. 6. (A) Schematic illustration of mixing BA₁₀EuI₁₂ and PS to fabricate scintillator thick films. The green atoms denote benzene rings of PS, and the cyan octahedra correspond to [EuI₆]⁴⁻. The inset presents the photograph of the BA₁₀EuI₁₂/PS film under daylight and 254-nm irradiation. (B and C) Photographs and x-ray images of a stainless nail and a spring. Scale bars, 5 mm. (D) X-ray images of a partial region (from 8 to 11 lp mm⁻¹) of the standard x-ray test pattern (x-ray tube voltage: 60 kV, 200 mA; dose rate: 2.86 mGy_air s⁻¹). (E) MTFs of x-ray image obtained from the BA₁₀EuI₁₂/PS scintillator film. The spatial resolution (when the MTF value equals 0.2) is 8.95 lp mm⁻¹.

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