Lead-halide Cs4PbBr6 single crystals for high-sensitivity radiation detection

Low-dimensional perovskite materials and their derivatives with excellent optical performance are promising candidates for light-emission applications. Herein, centimeter lead-halide Cs4PbBr6 single crystals (SCs), which have been used for radiation detection with the indirect conversion method, were synthesized by a facile solution process. The Cs4PbBr6 scintillator exhibits bright green emission peaking at 525 nm and a high photoluminescence quantum yield (up to 86.7%) under 375 nm laser excitation. The Cs4PbBr6 SCs exhibit high sensitivity to 40 keV X-rays, with a favorable linearity with the X-ray exposure dose rate, and the detection limit is as low as 64.4 nGyair/s. The scintillation time-response performance of the Cs4PbBr6 SCs was acquired by a time-correlated single-photon counting system under alpha-particle excitation. The Cs4PbBr6 SCs exhibit a very fast time response (τav = 1.46 ns) to alpha particles from a 241Am radiation source. This value is comparable to that of the commercial plastic scintillator EJ-228 (τav = 1.31 ns) and much faster than that of the LYSO(Ce) scintillator (τav = 36.17 ns). Conceptual X-ray imaging and alpha-particle pulse height spectroscopy experiments were also performed. These results demonstrated the potential of Cs4PbBr6 SCs for radiation detection applications, including X-ray imaging and charged particle detection with fast scintillation decay time and high sensitivity. An improved technique for producing scintillators, materials that light up in the presence of ionizing radiation, shows promise for low-dose X-ray imaging applications. Commercial scintillators typically need to be fabricated through a series of high-temperature purification steps to become sufficiently sensitive to radiation. Qiang Xu and Xiaoping Ouyang from the Nanjing University of Aeronautics and Astronautics in China and co-workers now demonstrate that cesium–lead–bromide scintillators with ideal single-crystal structures can be synthesized through an inexpensive process involving the slow cooling of liquefied precursors. The team’s experiments revealed that these crystals could be produced at centimeter scales with favorable characteristics for radiation detection, including nanosecond-quick response times. Clear X-ray images were obtained at dosages much lower than those in standard chest computed tomography (CT) scans. We investigated the scintillation performance of centimeter lead-halide Cs4PbBr6 single crystal synthesized by a facile solution process. Cs4PbBr6 single crystal have been demonstrated with fast scintillation decay time, low detection limit, and without hygroscopic, which makes it ideal for indirection radiation detection applications. The alpha pulse height spectroscopy deconvoluted into two Gaussian functions were obtained. The clear X-ray imaging of a standard pattern plate with 600 μm interval width under a low dose rate below 3.3 μGyair/s was collected. All these results indicate that this low-cost Cs4PbBr6 SCs scintillator is expected to be a promising low-dose X-ray imaging material.


Introduction
A scintillator is a radiosensitive luminescent material that can convert the energy of incident radiation (highenergy photons or particles) into ultraviolet (UV)/visible light and constitutes a critical part of the radiation detector 1 . A wide range of applications have been developed for the scintillator in medical imaging, homeland security, and high-energy physics [2][3][4][5][6][7] . Due to their excellent scintillation performance, typical bulk scintillators such as NaI:Tl and CsI:Tl are widely used in the mature commercial market and generally involve complex, rigorous, and high-temperature fabrication procedures (Kyropoulos and Czochralski methods) 8 . The disadvantage of these methods is that during the high-temperature procedure (>1700°C), the generated weak oxidizing atmosphere may contaminate the melt and form inclusion defects in the crystal 8 . The gadolinium oxysulfide scintillator is relatively cheap and can be easily made into a flexible material 2 . However, the long decay time of this scintillator results in a relatively long X-ray exposure time. Plastic scintillators usually have the advantages of a fast decay time and a low production cost, but the relatively low atomic number makes it difficult to fully deposit radiant energy 9 . Herein, an ideal scintillation material with desirable properties, such as high light yield, fast decay time, appreciable emission wavelength (matching the quantum efficiency of the photomultiplier tube), and low cost, is expected.
Recently, solution-processable metal halide perovskites (MHPs) have been demonstrated to be a promising class of materials for optoelectronic applications [10][11][12] . Among the MHP family, zero-dimensional (0D) perovskite-related Cs 4 PbBr 6 shows a completely isolated octahedron and cation in the structure [13][14][15] . The potential role of this structure is to localize charge carriers and provide a very strong exciton confinement, which is considered to be beneficial for light-emission performance 16,17 . Cs 4 PbBr 6 SCs have been demonstrated to have a high photoluminescence (PL) quantum yield (PLQY) at room temperature and have been used for light-emission applications 18,19 .
In this article, centimeter-sized bulk Cs 4 PbBr 6 SCs were synthesized by a facile solution process. Due to its unique material structure, this 0D scintillator material exhibits a high PLQY of up to 86.7%. A detection limit as low as 64.4 nGy air /s was extracted from the radiation dosedependent X-ray radioluminescence (RL) spectra, showing the high sensitivity of Cs 4 PbBr 6 SCs to 40 keV X-rays. Subsequently, scintillation time-response characteristics of Cs 4 PbBr 6 SCs were obtained through a time-correlated single-photon counting (TCSPC) system under alphaparticle excitation, and conceptual X-ray imaging and alpha-particle pulse height spectrum experiments were also carried out. Our study suggests that low-cost Cs 4 PbBr 6 SCs have potential in indirect radiation applications, including X-ray imaging and high-count-rate radiation detection.

Results and discussion
Cs 4 PbBr 6 SCs were crystallized by a hydrobromic acidassisted slow cooling method via reaction of CsBr with PbBr 2 19 . Figure 1a displays the unit cell of Cs 4 PbBr 6 in which the [PbBr 6 ] 4− anions are spatially isolated by Cs + cations. Centimeter-sized bulk Cs 4 PbBr 6 SCs were harvested from the solution, as shown in the inset in Fig. 1b. The powder X-ray diffraction pattern demonstrates that Cs 4 PbBr 6 is in the trigonal space group R3c with lattice parameters a = b = 13.83 Å and c = 17.45 Å. The position of the diffraction peaks in Fig. 1b is well aligned with previous literature [26][27][28] .
XPS spectra were obtained to further investigate the elemental composition of the samples. As shown in   Fig. 1d-f. The peak positions located at 736.8 and 722.9 eV correspond to Cs 3d 5/2 and 3d 3/2, respectively. The peaks centered at 137.6 and 142.7 eV are assigned to Pb 4f 7/2 and 4f 5/2, respectively. The peak in the range 67-70 eV correlated with Br 3d can be well fitted with two Gaussian equations. These two peaks (68.3 and 67.3 eV) can be attributed to the Br atoms in isolated [PbBr 6 ] 4− of Cs 4 PbBr 6 and to the Br atoms of CsBr impurities or corner-sharing [PbBr 6 ] 4− of CsPbBr 3 , respectively, which is consistent with the previous reports 25 .
In previous reports, the color of Cs 4 PbBr 6 SCs harvested from solution during different growth stages ranged from transparent to bright green. Their PL emission band underwent an evolution from blue to green, accompanied by a sharp increase in the PLQY 19 . Currently, there are still many disputes about the origin of their strong bright green light 29,30 . Several researchers attribute this phenomenon to the CsPbBr 3 nanoparticles (~2.4 eV bandgap) embedded in the Cs 4 PbBr 6 matrix undergoing downconversion emission 28,31 . Others suggest that it should be attributed to some defect states originating from halogen (Br) vacancies [29][30][31] . As shown in Fig. S1, the PL excitation spectra of Cs 4 PbBr 6 SCs exhibit a bright green luminescence peak at a wavelength of 525 nm when the excitation wavelength is in the range 340-500 nm. Notably, as shown in Fig. 2a, a weak peak at~375 nm (peak I) is observed in the temperature-dependent PL spectra, which can be attributed to two overlapping emission bands originating from different optical transitions related to Pb 2+ ions that occupy Cs + sites 32 . With increasing temperature, the position of peak II exhibits a blueshift, and the full-width at half-maximum increases from~9 to 21 nm (Fig. S2). The evolution trend of peak II with temperature is consistent with our previously reported optical properties of CsPbBr 3 /Cs 4 PbBr 6 NCs 25 , revealing their consistent possibly analogical downconversion luminescence mechanism. The exciton binding energy is a key factor to be considered in measuring the luminescence performance of materials, which can be extracted by the following Arrhenius relation: 25 where I 0 is the integrated RL intensity at 0 K, E b is the exciton binding energy, and k b is the Boltzmann constant.
The relationship between the integrated PL intensity of peak II and temperature can be well fitted by formula (1), as plotted in Fig. 2b. Due to the 0D crystal structure and low electronic dimensionality, the extracted binding energy of Cs 4 PbBr 6 SCs is~126.3 meV, which is larger than that of 0D (C 4 H 14 N 2 ) 2 In 2 Br 10 SCs or CsPbBr 3 NCs 33,34 .
The RL of a scintillator under relatively low-energy X-rays comes from the interaction between the incident radiation and heavy atoms of the material through the photoelectric effect 35 . In this process, a large number of electrons are generated and ejected from the lattice. The attenuation power of a material for different energy photons is related to its cross-section, which is highly dependent on the effective atomic number and density of its constituent atoms. Figure 3a shows the mass attenuation coefficients of CsI, Cs 4 PbBr 6 , CsPbBr 3 , LYSO, and MAPbBr 3 versus the photon energy according to the XCOM database. The attenuation efficiency for 40 keV X-rays can be extracted from Fig. 3b and calculated by an exponential function: 36 where I 0 is the intensity of the incident photons, μ is the linear attenuation coefficient, and x is the penetration depth in the material. By comparison, although slightly lower than that of advanced CsI scintillators, the attenuation efficiency of Cs 4 PbBr 6 is higher than that of the commercial LYSO scintillator. Because of the increase in effective atomic number, the attenuation efficiency is also higher than that of the previously reported CsPbBr 3 and MAPbBr 3 perovskite scintillators 22,37 . PLQY measurements continue to be developed to quantify the luminous efficiency of Cs 4 PbBr 6 SCs (Fig. 3c). The inset in Fig. 3c shows the bright green fluorescence of Cs 4 PbBr 6 SCs under ultraviolet light (375 nm) 13,38 . The high PLQY of Cs 4 PbBr 6 SCs can ensure effective radiative recombination when the photocarriers are transferred to the recombination centers during the scintillation process 39 . As shown in Fig. 3d, the RL spectral peak of the Cs 4 PbBr 6 SCs is located at~520 nm under 40 keV X-ray excitation. A single sharp absorption edge at 525 nm can also be observed from the transmission profiles (Fig. 3d). The Cs 4 PbBr 6 SCs exhibit high sensitivity to 40 keV Xrays, and the response is linear over a large dose rate range (Fig. 3e). Thus, it is foreseeable that the dose rate and scintillation response should be highly correlated. The detection limit can be extracted from the fitted curve as~64.4 nGy air /s, which is comparable to the reported value for lead-free Cs 2 Ag 0.6 Na 0.4 In 0.85 Bi 0.15 C l6 scintillator wafers 39 . In addition, the integrated RL intensity of Cs 4 PbBr 6 SCs shows no apparent reduction after storage at room temperature for 20 days, which demonstrates excellent air stability (Fig. 3f).
The decay time of a scintillator represents its response rate and counting ability for incident radiation particles, which is a key factor in practical applications. To achieve high time resolution performance, a scintillator with a fast decay time is required. Therefore, a self-built TCSPC system was used to accurately obtain the scintillation time-response properties (Fig. S3) 40 . Figure 4a shows the scintillation temporal profile of LYSO(Ce), Cs 4 PbBr 6 , and EJ-228 excited by the alpha particles from a 241 Am radiation source at room temperature. The results of the scintillation decay time fitted by a biexponential function are tabulated in Table 1. The Cs 4 PbBr 6 SCs exhibit a very fast time response (τ av = 1.46 ns) to alpha particles. This value is comparable to that of the commercial plastic scintillator EJ-228 (τ av = 1.31 ns) and much faster than that of the LYSO(Ce) scintillator (τ av = 36.17 ns). A comparison of the key scintillation parameters of Cs 4 PbBr 6 SCs and other reported scintillator materials is shown in Table S1. The fast decay time characteristic demonstrates that the Cs 4 PbBr 6 SCs can be used for applications such as fast X(γ)-ray counting. Calculated attenuation efficiency plot of the above materials for 40 keV X-rays. c Photoluminescence quantum yield (PLQY) measurement using an integrating sphere; the inset shows the luminescence of Cs 4 PbBr 6 SCs under UV light (375 nm). d RL spectrum under 40 keV X-ray excitation, and transmission spectra. e Dose rate versus integrated RL spectra excited with 40 keV X-rays. The inset shows a schematic of the measurement, which consists of Cs 4 PbBr 6 SCs, a photomultiplier tube (PMT) and an X-ray dosimeter. f Stability of the RL intensity of Cs 4 PbBr 6 SCs stored in air. Figure 4b shows the pulse height spectrum acquired with Cs 4 PbBr 6 SCs. This Gaussian-shaped peak is attributed to the two groups of alpha particles with similar energy (5.44 and 5.49 MeV) released by the 241 Am radiation source. The fitted Gaussian-shaped peak was deconvoluted into two Gaussian functions. Peak 1 (black dash-dotted) can be ascribed to the energy released by the 5.49 MeV alpha particles, and the energy resolution is estimated to be~58.7%. Peak 2 (cyan dash-dotted) can be attributed to the 5.44 MeV alpha particles, with an energy resolution of~51.6%. The relatively poor energy resolution of Cs 4 PbBr 6 SCs may be due to the incomplete collection of scintillation photons, which is mainly affected by the photon scattering from unpolished crystals with uneven surfaces. This can be further improved by the construction of a photonic crystal structure or the polishing of the crystal surface 41,42 As seen from the above results, Cs 4 PbBr 6 SCs feature fast time-response properties and a low detection limit, which are of great significance in reducing the radiation dose and exposure time suffered by patients in practical medical imaging applications. To perform an X-ray radiography experiment with Cs 4 PbBr 6 as a scintillator, a homemade X-ray imaging system was constructed, as shown in Fig. 5a. The applied voltage and current on the X-ray tube and the measured dose rate are shown in Table S2. Figure 5b shows a photograph of a standard pattern plate, and the interval width between the lines is 1 mm (i) and 0.6 mm (ii). The X-ray-induced RL intensity exhibits a great linear relationship with the X-ray dose rate, which has been proven in the above results. Based on this, clear X-ray images with different doses can be obtained by adjusting the current of the X-ray source (Fig.  5c). It should be noted that the dose rate of X-rays applied in this demonstrative experiment is much lower than that in practical chest computed tomography (CT) 2,24 . The interval widths of 1 and 0.6 mm in the image can be resolved at dose rates ranging from 3.3 to 9.2 μGy air /s, and the contour profile of the standard pattern plate at a dose rate of 9.2 μGyair/s was extracted to provide more details (Fig. 5d). Due to the limitation of the experimental conditions, the estimated spatial resolution is at least 600 μm under a low dose rate <3.3 μGy air /s. The results indicate that the Cs 4 PbBr 6 SC scintillator is expected to be a promising low-dose X-ray imaging material with high sensitivity, high resolution, and fast response.

Conclusions
In conclusion, we have successfully synthesized centimeter-sized lead-halide Cs 4 PbBr 6 SCs by a facile solution process. This 0D scintillator is demonstrated to have a high PLQY and a large exciton binding energy under UV light excitation. Furthermore, under X-ray exposure, it exhibits a favorable linear detection range, a low detection limit, a fast decay time, and air stability. Notably, the fast scintillation decay time of Cs 4 PbBr 6 is greatly advantageous for reducing the energy accumulation in pulse amplitude spectrum experiments and decreasing the radiation exposure time in X-ray imaging applications. Future research focusing on optimization of the crystal surface quality and size of Cs 4 PbBr 6 SCs is  needed to provide a material basis for possible radiation detection applications.