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All-inorganic perovskite nanocrystal scintillators

Abstract

The rising demand for radiation detection materials in many applications has led to extensive research on scintillators1,2,3. The ability of a scintillator to absorb high-energy (kiloelectronvolt-scale) X-ray photons and convert the absorbed energy into low-energy visible photons is critical for applications in radiation exposure monitoring, security inspection, X-ray astronomy and medical radiography4,5. However, conventional scintillators are generally synthesized by crystallization at a high temperature and their radioluminescence is difficult to tune across the visible spectrum. Here we describe experimental investigations of a series of all-inorganic perovskite nanocrystals comprising caesium and lead atoms and their response to X-ray irradiation. These nanocrystal scintillators exhibit strong X-ray absorption and intense radioluminescence at visible wavelengths. Unlike bulk inorganic scintillators, these perovskite nanomaterials are solution-processable at a relatively low temperature and can generate X-ray-induced emissions that are easily tunable across the visible spectrum by tailoring the anionic component of colloidal precursors during their synthesis. These features allow the fabrication of flexible and highly sensitive X-ray detectors with a detection limit of 13 nanograys per second, which is about 400 times lower than typical medical imaging doses. We show that these colour-tunable perovskite nanocrystal scintillators can provide a convenient visualization tool for X-ray radiography, as the associated image can be directly recorded by standard digital cameras. We also demonstrate their direct integration with commercial flat-panel imagers and their utility in examining electronic circuit boards under low-dose X-ray illumination.

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Fig. 1: Full-colour radioluminescence from perovskite nanocrystal scintillators.
Fig. 2: Mechanistic investigation of X-ray energy conversion by perovskite nanocrystals.
Fig. 3: Ultrasensitive X-ray sensing and radiography using CsPbBr3 nanocrystals.
Fig. 4: Prototype perovskite-nanocrystal-based flat-panel detector for digital radiography.

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Acknowledgements

This work is supported by the King Abdullah University of Science and Technology; the Singapore Ministry of Education (grants R143000627112 and R143000642112); the Agency for Science, Technology and Research (A*STAR) under contracts 122-PSE-0014 and 1231AFG028 (Singapore); the National Research Foundation, Prime Minister’s Office, Singapore under its Competitive Research Program (CRP award number NRF-CRP15-2015-03); the National Basic Research Program of China (973 Program, grant number 2015CB932200); the National Natural Science Foundation of China (21635002, 21471109, 21210001 and 21405143); and the Natural Science Foundation of Jiangsu Province (BE2015699). We thank H. Jiang, B. Deng, Z. Fang, Z. Zhou, Y. Zhang, X. Ling, M. Sun and A. Malko for technical assistance.

Reviewer information

Nature thanks R. Comin, W. Heiss and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Authors and Affiliations

Authors

Contributions

Q.C. and X.L. conceived and initiated the project. X.L., H.Y. and W.H. supervised the project and led the collaboration efforts. Q.C., X.L., H.Y. and W.H. designed the experiments. Q.C., J.W., L.L. and S.H. performed the nanocrystal synthesis. Q.C., X.O. and J.L. carried out the spectral measurements. Q.C., X.O., Y.W., Y.L., D.F., Z.Y., D.B.L.T. and A.H.L. contributed to the design and implementation of the X-ray sensing experiments. B.H., M.B. and O.F.M. carried out the theoretical calculations. J.A., A.A.Z. and O.M.B. prepared the perovskite single crystals. X.G. and T.W. fabricated the photoconductor devices and performed the photocurrent measurements. X.X. fabricated the PDMS moulds and measured the low-temperature scintillation spectra. Q.C. and X.L. wrote the manuscript. All authors discussed the results and commented on the manuscript.

Corresponding authors

Correspondence to Huanghao Yang, Wei Huang or Xiaogang Liu.

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The authors declare no competing interests.

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Extended data figures and tables

Extended Data Fig. 1 Schematic representation of scintillation mechanism and X-ray-induced luminescence in bulk inorganic scintillators.

a, In bulk inorganic materials, X-ray photons absorbed by the lattice atoms can generate hot charge carriers, followed by exciton thermalization and subsequent transport to defect sites or activator centres (‘traps’), where radiative emission occurs. VB, valence band; CB, conduction band; Vk, self-trapped hole; F, Farbe centre; νexc, photon frequency during excitonic luminescence; νA, photon frequency during activator (A) luminescence. b, Scintillation properties of CsPbBr3 QDs and commercial bulk inorganic materials under X-ray excitation. The full-width at half-maximum (FWHM) represents the spectral width of the scintillation spectra of the crystal. The insets show photographs of the corresponding samples acquired under X-ray excitation. BGO, Bi4Ge3O12.

Extended Data Fig. 2 Physical characterization of as-synthesized perovskite QDs.

a, TEM images of the as-prepared cubic-phased nanocrystals (left) and the corresponding size distribution of the nanocrystals (right). The samples are CsPbCl3, CsPb(Cl/Br)3, CsPbBr3, CsPb(Br/I)3 and CsPbI3 nanocrystals (from top to bottom). Insets are images of perovskite nanocrystals dispersed in cyclohexane, recorded under 365-nm ultraviolet light excitation. b, Powder X-ray diffraction patterns for typical ternary and mixed-halide CsPbX3 (X = Cl, Br or I) nanocrystals. All peaks are indexed in accordance with the cubic-phased CsPbBr3 structure (Joint Committee on Powder Diffraction Standards file (PDF) number 54-0752). c, d, Dark-field scanning transmission electron micrographs (STEM; JEM-F200HR) of CsPbBr3 nanocrystals. e, Elemental mapping of Br, Pb and Cs for the nanocrystals, obtained from the area marked by the rectangular box in d using energy-dispersive X-ray spectroscopy. f, Atomically resolved dark-field STEM image of a single CsPbBr3 nanocrystal, showing Cs and Pb lattice atoms. g, Energy-dispersive X-ray spectrum of the as-prepared CsPbBr3 perovskite nanocrystals, confirming the stoichiometric composition of the CsPbBr3 nanocrystals. We note that strong Cu signals come from the TEM copper grid.

Extended Data Fig. 3 Multicolour-emitting perovskite QD scintillators upon X-ray irradiation.

a, Demonstration of X-ray-induced luminescence modulation using CsPbX3 QDs of different compositions (X = Cl, Br or I). b, Multicolour X-ray scintillation from CsPb(Cl/Br)3, CsPbBr3 and CsPb(Br/I)3 nanocrystals cast on a PDMS substrate. The X-ray dose rate is 278 μGy s−1. c, Typical photographs of radioluminescence from CsPb(Cl/Br)3, CsPbBr3 and CsPb(Br/I)3 QDs under X-ray excitation. d, e, Multicolour visualization of as-developed perovskite QD scintillators using X-rays (d) and the corresponding bright-field image (e).

Extended Data Fig. 4 Comparison of X-ray-induced luminescence intensity for QDs.

ac, Radioluminescence spectra of carbon dots (a), CdTe QDs (b) and CsPbBr3 nanocrystals (NCs; c) under X-ray excitation at 5.0 μGy s−1 and 278 μGy s−1. d, Comparison of radioluminescence intensity for the as-prepared carbon dots, CdTe QDs and CsPbBr3 nanocrystals under 278 μGy s−1 X-ray excitation. e, Scintillation performance and maximal K-edge energy of lead halide perovskite nanocrystals, carbon dots and CdTe QDs.

Extended Data Fig. 5 Comparison of X-ray-induced luminescence for lead halide perovskite materials.

ac, Radioluminescence spectra of bulk single-crystal CH3NH3PbBr3 (a) and CsPbBr3 (b) and of CsPbBr3 nanocrystals (c) under X-ray excitation at 5.0 μGy s−1 and 278 μGy s−1. The insets are photographs of CsPbBr3 bulk single crystal (b) and nanocrystal powders (c) taken under ambient light (top) and X-ray illumination (bottom). The X-ray dose used for the experiments was 278 μGy s−1. d, Comparison of radioluminescence intensity for three types of perovskite material under 278 μGy s−1 X-ray excitation. e, PLQY and exciton binding energy of perovskite materials at 300 K31,37. We note that the thermal energy at 300 K is kBT ≈ 25 meV. Nanocrystalline perovskites are highly luminescent materials, whereas bulk perovskite crystals are most suitable for the generation of free charge carriers owing to their low exciton binding energy.

Extended Data Fig. 6 Measurement of exciton binding energy and synchrotron-radiation-induced radioluminescence of CsPbBr3 nanocrystals.

a, Temperature-dependent scintillation spectra of the CsPbBr3 nanocrystals at 77–300 K under X-ray illumination at 50 μGy s−1. b, Arrhenius plot of the X-ray-induced luminescence intensities of the CsPbBr3 nanocrystals at 532 nm. c, Experimental setup for the synchrotron-radiation-induced radioluminescence measurements at the X-ray beamline of the Shanghai Synchrotron Radiation Facility (SSRF). The X-ray energy is 10–38 keV. d, The electronic edge energies for Pb L, Cs K and Br K.

Extended Data Fig. 7 Electronic structure and scintillation mechanism of the CsPbBr3 nanocrystal along selected reciprocal high-symmetry points within the Brillouin zone.

a, The Brillouin zone of the cubic-phased crystal lattice in CsPbBr3, calculated using relativistic corrections. Γ, M, R and X denote high-symmetry points within the reciprocal space (blue). b, c, Calculated electron density associated with the valence band (b) and the conduction band (c) of the cubic CsPbBr3. We note that the halide ions contribute to the changes of the bandgap in the perovskite nanocrystal through the influence of the valence orbitals. Cs, purple; Pb, grey; Br, orange; electron orbital, blue; hole orbital, green. dg, Localized electronic and hole levels for VBr and VPb at different charge states. h, i, Schematic diagram of energy transfer for radioluminescence induced by intrinsic lattice defects and the quenching effect caused by evident ion movement (Cs+). \({{\rm{V}}}_{{\rm{P}}{\rm{b}}}^{\delta -}\) denotes a Pb vacancy with small (|δ| < 1) charge transfer. j, Thermodynamic transition levels of the perovskite nanocrystal. Ec, maximum valence band energy; Ev, minimum conduction band energy.

Extended Data Fig. 8 Theoretical studies of the surface electronic properties of CsPbBr3 nanocrystals.

a, Simplified model of CsPbBr3 structure composed of 293 atoms with a particle size of 12.06 Å. b, Calculated orbital contour plots of a CsPbBr3 nanocrystal, showing the HOMO (blue), LUMO (green) and SVIC trapping (red) states. The SVIC states are formed owing to unsaturated p orbitals of surface Br sites. c, PDOS of the CsPbBr3 QD. The SVIC state is located near the Fermi level (EF). d, The relative energy of the SVIC state to the valence band (VB) maximum as a function of inter-particle distance, d. The red line is calculated by fitting with the Gaussian distribution function. The top inset shows the simulation model used to calculate the energy evolution of the CsPbBr3 nanocrystal as a function of particle distance. CB, conduction band.

Extended Data Fig. 9 Characterization of absorption cross-section and transient luminescence spectra of the CsPbBr3 nanocrystals.

a, Absorption spectra of CsPbBr3 nanocrystals dispersed in cyclohexane at different concentrations. b, Absorption as a function of concentration for CsPbBr3 nanocrystals. The molar extinction coefficient, ε, was determined by applying the Beer–Lambert law A = εcL, where A is the absorbance, c is the molar concentration (mol l−1) and L is the optical path length (1 cm) through the sample. The absorption cross-section, σ, was determined by ε = NAσ/(1,000 × ln10), where NA is Avogadro’s number. c, Photoluminescence (PL) lifetime of a single CsPbBr3 perovskite nanocrystal. d, Second-order correlation function, g2(τ), of the nanocrystal. The value g2(0) = 0.16 confirms the single-quantum-emitter nature of the photon emission. e, Fluorescence intermittency trace recorded for a single CsPbBr3 perovskite nanocrystal. The recorded photoluminescence intensity reaches more than 2,000 counts per 20-ms bin.

Extended Data Fig. 10 Performance of X-ray detection system using CsPbBr3 nanocrystal scintillators.

a, Normalized radioluminescence intensity of CsPbBr3 nanocrystals of different thickness under X-ray excitation at a voltage of 50 kV. b, Normalized radioluminescence intensity as a function of perovskite nanocrystal film thickness. c, X-ray output spectra recorded at 10, 20, 30, 40 and 50 kV. d, Kinetic measurement of radioluminescence intensity at 532 nm in response to an X-ray dose rate of 0.013–278 μGy s−1. e, Emission spectra of CsPbBr3 nanocrystal scintillator in response to an X-ray dose rate of 0.013–278 μGy s−1. f, Radioluminescence (RL) intensity as a function of the X-ray dose rate shown in e.

Extended Data Fig. 11 Direct X-ray imaging and multiplexed labelling for in vivo optical imaging using perovskite nanocrystal scintillators.

ad, A flexible flat cable (a) and needle-implanted pork tissue (b) were imaged with bright-field and X-ray imaging (c, d). We note that the CsPbBr3 nanocrystal scintillator platform shown in Fig. 3d was used for the X-ray phase contrast imaging. In both cases, the X-ray images clearly reveal the presence of metallic wires embedded in the cable and pork tissue. e, Synthesis of CsPbBr3/SiO2 core-shell nanoparticles with a hydrophobic surface for protection against moisture. RT, room temperature. f, TEM image of the as-prepared CsPbBr3/SiO2 nanoparticles. g, Multicolour luminescence spectra of the perovskite nanocrystals under X-ray irradiation at a voltage of 50 kV. The materials’ compositions are CsPbBr3, CsPbBr1.5I1.5 and CsPbBr1.2I1.8 for green (G), orange (O) and red (R) emissions, respectively. We note that the risk of lead toxicity must be considered during experimentation. h, Bright-field and multicolour luminescent in vivo imaging in mice under X-ray excitation at a voltage of 50 kV. The X-ray-induced luminescence was recorded by a charge-coupled-device camera equipped with three optical filters at 530 nm, 630 nm and 670 nm.

Extended Data Table 1 Scintillation characteristics for different materials
Extended Data Table 2 Properties of perovskite nanocrystals and bulk crystals used for X-ray detection

Supplementary information

Video 1

Multicolour X-ray scintillation by perovskite nanocrystals. Two types of perovskite nanocrystal (CsPbBr3 and CsPbBr1.2I1.8) were cast onto a flexible substrate made of a PDMS device. This substrate was then irradiated under an X-ray source (50 kV, 80 μA) for multicolour display demonstration. The video was recorded by repeatedly switching on/off of X-rays for three times. Scale bar is 5 mm.

Video 2

Radioluminescence of perovskite nanocrystals under synchrotron radiation. A layer of CsPbBr3 nanocrystals was coated onto a PDMS substrate. The resulting nanocrystal scintillator film was illuminated using a beam source of synchrotron radiation with an energy of 16 keV.

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Chen, Q., Wu, J., Ou, X. et al. All-inorganic perovskite nanocrystal scintillators. Nature 561, 88–93 (2018). https://doi.org/10.1038/s41586-018-0451-1

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Keywords

  • Perovskite Nanocrystals
  • Radioluminescence
  • Bulk Scintillator
  • PDMS Substrate
  • Perovskite QDs

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