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.
A scintillator is a radiosensitive luminescent material that can convert the energy of incident radiation (high-energy photons or particles) into ultraviolet (UV)/visible light and constitutes a critical part of the radiation detector1. A wide range of applications have been developed for the scintillator in medical imaging, homeland security, and high-energy physics2,3,4,5,6,7. 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 crystal8. The gadolinium oxysulfide scintillator is relatively cheap and can be easily made into a flexible material2. 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 energy9. 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 applications10,11,12. Among the MHP family, zero-dimensional (0D) perovskite-related Cs4PbBr6 shows a completely isolated octahedron and cation in the structure13,14,15. 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 performance16,17. Cs4PbBr6 SCs have been demonstrated to have a high photoluminescence (PL) quantum yield (PLQY) at room temperature and have been used for light-emission applications18,19.
Due to the high-Z atom, efficient light emission, and solution processability, Cs4PbBr6-based solid powder has been developed as a scintillator for X(γ)-ray detection20,21,22,23,24,25. Haibo Zeng and colleagues fabricated a large-area film based on CsPbBr3@Cs4PbBr6 mixed perovskite with excellent scintillation properties and achieved high contrast X-ray imaging24. In our previous publication, CsPbBr3/Cs4PbBr6 nanocrystals (NCs) exhibited desirable scintillation properties with a fast decay time (<10 ns), a fine energy resolution (3.0 ± 0.1% at 59.6 keV) and a high light yield (64,000 photons/MeV)25. However, for high-energy X(γ)-ray detection applications, these NC-based thin film scintillators cannot provide sufficient X(γ)-ray absorption. A thick, bulk single-crystal scintillator is expected for high-energy radiation applications, especially for high-energy X-ray imaging.
In this article, centimeter-sized bulk Cs4PbBr6 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 nGyair/s was extracted from the radiation dose-dependent X-ray radioluminescence (RL) spectra, showing the high sensitivity of Cs4PbBr6 SCs to 40 keV X-rays. Subsequently, scintillation time-response characteristics of Cs4PbBr6 SCs were obtained through a time-correlated single-photon counting (TCSPC) system under alpha-particle excitation, and conceptual X-ray imaging and alpha-particle pulse height spectrum experiments were also carried out. Our study suggests that low-cost Cs4PbBr6 SCs have potential in indirect radiation applications, including X-ray imaging and high-count-rate radiation detection.
Results and discussion
Cs4PbBr6 SCs were crystallized by a hydrobromic acid-assisted slow cooling method via reaction of CsBr with PbBr219. Figure 1a displays the unit cell of Cs4PbBr6 in which the [PbBr6]4− anions are spatially isolated by Cs+ cations. Centimeter-sized bulk Cs4PbBr6 SCs were harvested from the solution, as shown in the inset in Fig. 1b. The powder X-ray diffraction pattern demonstrates that Cs4PbBr6 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 literature26,27,28.
XPS spectra were obtained to further investigate the elemental composition of the samples. As shown in Fig. 1c, only Cs, Pb, and Br are observed in the crystal. The high-resolution XPS spectra of Cs 3d, Pb 4f, and Br 3d are 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 [PbBr6]4− of Cs4PbBr6 and to the Br atoms of CsBr impurities or corner-sharing [PbBr6]4− of CsPbBr3, respectively, which is consistent with the previous reports25.
In previous reports, the color of Cs4PbBr6 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 PLQY19. Currently, there are still many disputes about the origin of their strong bright green light29,30. Several researchers attribute this phenomenon to the CsPbBr3 nanoparticles (~2.4 eV bandgap) embedded in the Cs4PbBr6 matrix undergoing downconversion emission28,31. Others suggest that it should be attributed to some defect states originating from halogen (Br) vacancies29,30,31. As shown in Fig. S1, the PL excitation spectra of Cs4PbBr6 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 Pb2+ ions that occupy Cs+ sites32. 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 CsPbBr3/Cs4PbBr6 NCs25, 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 I0 is the integrated RL intensity at 0 K, Eb is the exciton binding energy, and kb 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 Cs4PbBr6 SCs is ~126.3 meV, which is larger than that of 0D (C4H14N2)2In2Br10 SCs or CsPbBr3 NCs33,34.
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 effect35. 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, Cs4PbBr6, CsPbBr3, LYSO, and MAPbBr3 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 I0 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 Cs4PbBr6 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 CsPbBr3 and MAPbBr3 perovskite scintillators22,37. PLQY measurements continue to be developed to quantify the luminous efficiency of Cs4PbBr6 SCs (Fig. 3c). The inset in Fig. 3c shows the bright green fluorescence of Cs4PbBr6 SCs under ultraviolet light (375 nm)13,38. The high PLQY of Cs4PbBr6 SCs can ensure effective radiative recombination when the photocarriers are transferred to the recombination centers during the scintillation process39.
As shown in Fig. 3d, the RL spectral peak of the Cs4PbBr6 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 Cs4PbBr6 SCs exhibit high sensitivity to 40 keV X-rays, 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 nGyair/s, which is comparable to the reported value for lead-free Cs2Ag0.6Na0.4In0.85Bi0.15Cl6 scintillator wafers39. In addition, the integrated RL intensity of Cs4PbBr6 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), Cs4PbBr6, and EJ-228 excited by the alpha particles from a 241Am radiation source at room temperature. The results of the scintillation decay time fitted by a biexponential function are tabulated in Table 1. The Cs4PbBr6 SCs exhibit a very fast time response (τav = 1.46 ns) to alpha particles. 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). A comparison of the key scintillation parameters of Cs4PbBr6 SCs and other reported scintillator materials is shown in Table S1. The fast decay time characteristic demonstrates that the Cs4PbBr6 SCs can be used for applications such as fast X(γ)-ray counting.
Figure 4b shows the pulse height spectrum acquired with Cs4PbBr6 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 241Am 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 Cs4PbBr6 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 surface41,42
As seen from the above results, Cs4PbBr6 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 Cs4PbBr6 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 μGyair/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 μGyair/s. The results indicate that the Cs4PbBr6 SC scintillator is expected to be a promising low-dose X-ray imaging material with high sensitivity, high resolution, and fast response.
In conclusion, we have successfully synthesized centimeter-sized lead-halide Cs4PbBr6 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 Cs4PbBr6 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 Cs4PbBr6 SCs is needed to provide a material basis for possible radiation detection applications.
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This work was funded by the National Natural Science Foundation of China (Grant Nos. 11705090, 11875166, and 11435010).
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Li, Y., Shao, W., Chen, L. et al. Lead-halide Cs4PbBr6 single crystals for high-sensitivity radiation detection. NPG Asia Mater 13, 40 (2021). https://doi.org/10.1038/s41427-021-00308-w