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

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 μGyair s−1 to 145 μGyair s−1 and a detection limit as low as 5.83 nGyair 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.


Introduction
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][5][6][7]. However, their practical applications are restricted by the toxicity of the Pb 2+ , 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][9][10][11]. Especially, the high emission efficiency resulting from the strong exciton confinement effect and negligible self-absorption of Ln 2+ ion-based metal halides benefit high light yield (LY). The intrinsic short excited-state lifetimes of Ln 2+ ion-based metal halides enable a fast response (~ns) under high-energy irradiation [12]. Recently, inorganic Cs 4 EuX 6 (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, BA 10 EuI 12 (BA denotes C 4 H 9 NH 2 ), by a facile solution method for the first time. In BA 10 EuI 12 crystal, the photoactive sites, [EuI 6 ] 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 (P2 1 /c). BA 10 EuI 12 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 BA 10 EuI 12 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
BA 10 EuI 12 crystals were synthesized by a one-step reaction from BAI and EuI 2 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 BA 10 EuI 12 is determined by single-crystal x-ray diffraction (SCXRD). It crystallizes in a monoclinic space group of P2 1 /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 Å 3 (Table S1). As shown in the crystal structure of BA 10 EuI 12 , the photoactive sites, [EuI 6 ] 4− octahedra, were isolated by BA + cations to form a 0D crystal structure (Fig. 1B). These isolated [EuI 6 ] 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 [EuI 6 ] 4− octahedra [14]. Different from EuI 2 , BA 10 EuI 12 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 BA 10 EuI 12 , suggesting the absence of a secondary phase and high purity of BA 10 EuI 12 (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 (Eu 2+ ) and demonstrated the interaction between the BA + cations and the central Eu 2+ . 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). BA 10 EuI 12 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 Eu 2+ -based organic-inorganic hybrid halides.
First, we used density functional theory (DFT) calculations to understand the electronic structure and the luminous mechanism of BA 10 EuI 12 . The emission was caused by parity-allowed electron transitions from the 4f 6 5d 1 excited states into the 4f 7 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 [EuI 6 ] 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 Eu 2+ 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 Eu 2+ and the spectroscopic polarizability α sp , respectively [7,12]. The weak ligand field effect in [EuI 6 ] 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, BA 10 EuI 12 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 [EuI 6 ] octahedra, consequently guaranteeing negligible peak overlap (Fig. 3B) [22]. The negligible selfabsorption property together with the high theoretical exciton utilization efficiency origin from the strong exciton's confinement effect of BA 10 EuI 12 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   -nm excitation by time-resolved PL measurement, and a biexponential function fitting was conducted to reveal a short PL lifetime (108 ns) of BA 10 EuI 12 (Fig. 3C). The short lifetime boosts the potential of BA 10 EuI 12 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 BA 10 EuI 12 was calculated by fitting data to Eq. 1: where E a is the thermal activation energy, T is the test temperature, I 0 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, BA 10 EuI 12 demonstrated identical decay traces of the PL spectra intensity across the entire emission spectra range, indicating only one emission center for BA 10 EuI 12 . Moreover, the linear correlation between PL and incident light intensity and the constant lifetimes at different incident wavelengths indicated that the luminescence of BA 10 EuI 12 was not from defect states, which further verified that the luminescence mechanism is the d-f transition ( Fig. 4D and Figs. S7 and S8). Ln 2+ has low stability against water and oxygen, among which Eu 2+ has relatively high stability and low reduction potential (Eu 2+ /Eu 3+ = 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 LaBr 3 encapsulated before their practical x-ray sensing and imaging application.
The high PLQY and short decay lifetime on the nanosecond scale make the BA 10 EuI 12 crystal an excellent candidate for an x-ray scintillator [24,25]. The LY of BA 10 EuI 12 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 BA 10 EuI 12 crystals (1) 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 BA 10 EuI 12 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 eff 4 /E 3 . Accordingly, BA 10 EuI 12 is an organic-inorganic hybrid material, which contains many light atoms such as C, H, and N. The lack of heavy elements in BA 10 EuI 12 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 BA 10 EuI 12 is not as good as in inorganic materials such as CsPbBr 3 , but higher than the representative organic scintillators, anthracene (Fig. 5B). The BA 10 EuI 12 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 BA 10 EuI 12 crystals have LY equivalent to ~27,000 photons MeV −1 . There are many excellent Eu 2+ -based scintillators for gamma-ray spectroscopy with high LY. Compared with Eu 2+doped scintillators, BA 10 EuI 12 has a facile fabrication method and a short fluorescence lifetime (Table S3) [12,[26][27][28][29][30][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 BA 10 EuI 12 with several representative scintillators (Table S4 and Fig. S10) [30][31][32][33][34][35]. Although the FoM of BA 10 EuI 12 scintillator is 179 photons MeV −1 ns −1 and lower than LaBr 3 : Ce (LaBr 3 : 5% Ce 4,000 photons MeV −1 ns −1 ; LaBr 3 : 0.2% Ce 1,867 photons MeV −1 ns −1 ), it is higher than most scintillators [23,34,[36][37][38]. As shown in Fig. 5C, the dosedependent response of BA 10 EuI 12 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 BA 10 EuI 12 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 BA 10 EuI 12 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, BA 10 EuI 12 has good thermal and irradiation stability with negligible loss of emission intensity of BA 10 EuI 12 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 Eu 2+ emitter scintillators, BA 10 EuI 12 is prepared through the facile solution method (Table S4).
Inspired by the promising scintillation property of BA 10 EuI 12 , 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 BA 10 EuI 12 scintillators with polystyrene (PS) to obtain a free-standing film with BA 10 EuI 12 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 BA 10 EuI 12 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 BA 10 EuI 12 /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 BA 10 EuI 12 /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 BA 10 EuI 12 /PS film was comparable with the reported good-performing scintillators (Table S5) [36,[40][41][42][43][44][45][46][47][48]. We believe that these results will stimulate further work on Eu-based scintillators for x-ray imaging.

Discussion
In summary, we have successfully developed the BA 10 EuI 12 scintillator through a facile solution processing method. BA 10 EuI 12 showed blue emission with negligible self-absorption, a high PLQY of 72.5%, and a short excited-state lifetime (108 ns). The BA 10 EuI 12 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
Chemicals N-butyl amine (C 4 H 9 NH 2 ·HI, BAI, 99.5%) was purchased from Xi'an Polymer Light Technology Co. Ltd. Europium (II) iodide (EuI 2 , 99.9 wt%) was purchased from Tianjin Novcare Biotechnology Co. Ltd. Dried methanol (CH 3 OH, 99.0%) was purchased from J and K Scientific Reagent Co. Ltd. All reagents and solvents were used without further purification.

BA 10 EuI 12 single-crystal synthesis
The BA 10 EuI 12 single crystals were synthesized by a one-step reaction between BAI and EuI 2 through a solvent evaporation process followed by a slow cooling process. BAI (0.5 mol l −1 ) and EuI 2 (0.5 mol l −1 ) are solved in anhydrous methanol in glass bottles. Then, the vial containing the stoichiometric amount of BAI and EuI 2 was stirred and heated at 50 °C under an inert N 2 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 . BA 10 EuI 12 was crystallized gradually with the decrease of solubility during the cooling process, and the BA 10 EuI 12 crystals were washed with antisolvent anhydrous methylbenzene and followed by vacuumed drying.

BA 10 EuI 12 /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, BA 10 EuI 12 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 BA 10 EuI 12 single crystal was encapsulated by using vacuum grease during the SCXRD test. For luminescent properties and x-ray imaging test, the BA 10 EuI 12 and BA 10 EuI 12 /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 BA 10 EuI 12 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 BA 10 EuI 12 . 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 projectionaugmented 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 BA 10 EuI 12 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 BA 10 EuI 12 crystal, which leads the scintillation light to come out from LYSO without coupling into SiPM because of the optical waveguide effect. Moreover, we encapsulated BA 10 EuI 12 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 BA 10 EuI 12 is obtained by taking the known LY of LYSO and different irradiation areas into account. In detail, the LY of BA 10 EuI 12 can be obtained from Eq. 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 BA 10 EuI 12 , respectively. As for the measurement of the linear range, the samples generated light proportionally under x-ray irradiation with

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 BA 10 EuI 12 /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 μm 2 .