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High-resolution X-ray luminescence extension imaging

Abstract

Current X-ray imaging technologies involving flat-panel detectors have difficulty in imaging three-dimensional objects because fabrication of large-area, flexible, silicon-based photodetectors on highly curved surfaces remains a challenge1,2,3. Here we demonstrate ultralong-lived X-ray trapping for flat-panel-free, high-resolution, three-dimensional imaging using a series of solution-processable, lanthanide-doped nanoscintillators. Corroborated by quantum mechanical simulations of defect formation and electronic structures, our experimental characterizations reveal that slow hopping of trapped electrons due to radiation-triggered anionic migration in host lattices can induce more than 30 days of persistent radioluminescence. We further demonstrate X-ray luminescence extension imaging with resolution greater than 20 line pairs per millimetre and optical memory longer than 15 days. These findings provide insight into mechanisms underlying X-ray energy conversion through enduring electron trapping and offer a paradigm to motivate future research in wearable X-ray detectors for patient-centred radiography and mammography, imaging-guided therapeutics, high-energy physics and deep learning in radiology.

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Fig. 1: Characterization of lanthanide-doped persistent luminescent nanoscintillators.
Fig. 2: Photophysical studies of X-ray irradiation on lanthanide-doped nanoscintillators.
Fig. 3: Mechanistic investigations of X-ray energy trapping in lanthanide-doped nanoscintillators.
Fig. 4: High-resolution Xr-LEI.

Data availability

The data that support the findings of this study are available from the corresponding authors upon reasonable request.

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Acknowledgements

We thank L. Ma, Y. Huang, X. Wang and B. Hou for technical assistance. This work is supported by the National Key and Program of China (grant number 2018YFA0902600), the National Natural Science Foundation of China (grant numbers 21635002, 21771135, 21871071 and 21771156), the Early Career Scheme fund (grant number PolyU 253026/16P) from the Research Grant Council in Hong Kong, Research Institute for Smart Energy of the Hong Kong Polytechnic University, Agency for Science, Technology and Research (grant numbers A1883c0011 and A1983c0038), NUS NanoNash Programme (NUHSRO/2020/002/NanoNash/LOA and R143000B43114) and National Research Foundation, the Prime Minister’s Office of Singapore under its NRF Investigatorship Programme (award number NRF-NRFI05-2019-0003).

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Authors

Contributions

X.O. and H.Y. initiated the project. Q.C. and X.L. conceived the concept of X-ray luminescence extension imaging. X.L., H.Y. and Q.C. supervised the project and organized the collaboration. X.O., X.L., H.Y. and Q.C. designed the experiments. X.O., Q.W., X.C., Z.H. and J.Z. performed nanocrystal synthesis. X.O., Q.W., J.Z. and L.X. performed luminescence measurements and X-ray imaging. X.O., Z.Y. and H.B. performed flexible X-ray imaging. X.Q. and B.H. carried out theoretical calculations. J.L., H.B. and Y.W. fabricated PDMS moulds and measured low-temperature scintillation spectra. X.O., H.Y., Q.C. and X.L. wrote the manuscript. All authors discussed the results and commented on the manuscript.

Corresponding authors

Correspondence to Qiushui Chen, Huanghao Yang or Xiaogang Liu.

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The authors declare no competing interests.

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Peer review information Nature thanks Christophe Dujardin, Oscar Malta and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Synthesis and characterization of Tb3+-doped nanocrystals.

a, Schematic for the synthesis of NaLuF4:Ln/Gd (Ln = Pr3+, Sm3+, Ho3+, Er3+, Tb3+, Dy3+, Tm3+ and Nd3+) nanocrystals. In a typical procedure, lanthanide acetate salts (Ln(Ac)3xH2O) were added to a flask containing OA and ODE. The mixture was heated at 150 °C to form lanthanide–oleate coordination complexes. Nucleation of NaLuF4:Ln/Gd nanocrystals was triggered by injecting a methanol solution of NaOH and NH4F. Subsequently, the reaction solution was heated at 300 °C for 1 h. The final product was precipitated with ethanol. OA was used as a surface ligand to control the particle size and stabilize as-synthesized nanocrystals. bf, Low-resolution TEM images of as-synthesized hexagonal-phase nanocrystals (top) and corresponding size distributions (bottom). These samples are NaLuF4:Tb/Gd (15/35 mol%) (b), NaLuF4:Tb/Gd (15/25 mol%) (c), NaLuF4:Tb/Gd (15/15 mol%) (d), NaLuF4:Tb/Gd (15/5 mol%) (e) and NaLuF4:Tb (15 mol%) (f). Scale bars, 200 nm. g, Particle size as a function of the lutetium doping ratio. h, Powder X-ray diffraction patterns for NaLuF4:Tb/Gd (15/x mol%; x = 0–35) nanocrystals. All peaks are consistent with the hexagonal-phase NaLuF4 structure (Joint Committee on Powder Diffraction Standards (PDF) file number 27-0726). i, Corresponding persistent radioluminescence decay curves of NaLuF4:Tb/Gd (15/x mol%; x = 0–35) nanocrystals monitored at 546 nm as a function of time. Spectra were obtained after X-ray excitation at a power density of 278 μGy s−1 for 5 min at room temperature (298 K).

Extended Data Fig. 2 Chemical composition analysis of Tb3+-doped fluoride nanocrystals.

a, Energy-dispersive X-ray element mapping of as-prepared NaLuF4:Tb/Gd (15/5 mol%) nanocrystals (Na, green; Lu, red; Gd, yellow; F, purple; Tb, green blue). bf, EDX spectra of NaLuF4:Tb/Gd (15/35 mol%) (b), NaLuF4:Tb/Gd (15/25 mol%) (c), NaLuF4:Tb/Gd (15/15 mol%) (d), NaLuF4:Tb/Gd (15/5 mol%) (e) and NaLuF4:Tb (15 mol%) (f) nanocrystals. g, Corresponding stoichiometric composition of NaLuF4:Tb/Gd (15/x mol%; x = 0–35) nanocrystals.

Extended Data Fig. 3 Afterglow characterizations of the Tb3+-doped nanocrystals.

a, Powder X-ray diffraction patterns of NaLuF4:Tb/Gd (x/(20 − x) mol%; x = 2–20) nanocrystals, showing that all peaks are consistent with hexagonal-phase NaLuF4 (Joint Committee on Powder Diffraction Standards file number 27-0726). b, Radioluminescence decay curves of NaLuF4:Tb3+/Gd3+ (x/(20 − x) mol%; x = 2–20) nanocrystals monitored at 546 nm after cessation of X-rays (dose rate, 278 μGy s−1; excitation time, 300 s; temperature, 298 K). c, Radioluminescence intensities, monitored at 546 nm as a function of time, of as-synthesized NaLuF4:Tb (15 mol%) and NaLuF4:Tb(15 mol%)@NaYF4 nanocrystals upon continuous X-ray irradiation. d, Luminescent decay profiles of NaLuF4:Tb(15 mol%)@NaYF4 nanocrystals. The luminescence intensity was monitored at 546 nm as a function of time, recorded upon turning off X-rays or ultraviolet–visible excitation at 273, 369, 487, 530, 620 and 750 nm for 5 min, respectively. All measurements were performed at room temperature.

Extended Data Fig. 4 Morphology and radioluminescent afterglow performance of various persistent phosphors upon X-ray excitation.

a-d, Representative TEM images of SrAl2O4:Eu2+/Dy3+ powder (a), mechanically grounded SrAl2O4:Eu2+/Dy3+ nanoparticles (b), ZnGa2O4:Cr3+ (ZGO:Cr) nanoparticles prepared by hydrothermal synthesis at 220 °C (c) and ZGO:Cr nanoparticles calcinated at 950 °C (d). e, Radioluminescence spectra of NaLuF4:Tb(15 mol%)@NaYF4 nanoparticles, SrAl2O4:Eu2+/Dy3+ bulk powder, ZnS:Cu2+/Co2+ bulk powder, SrAl2O4:Eu2+/Dy3+ nanoparticles (after grinding), ZnGa1.995O4:Cr0.005 (ZGO:Cr) nanoparticles (before and after calcination), CaAl2O4:Eu2+/Nd3+ nanoparticles (after grinding) and Sr2MgSi2O7:Eu2+/Nd3+ nanoparticles (after grinding). Insets show corresponding photographs of the samples under X-ray excitation. f, Radioluminescence intensity profiles of various persistent luminescent materials upon continuous X-ray irradiation as a function of time (accelerating voltage, 50 kV; temperature, 298 K). g, Comparison of afterglow intensities of various persistent phosphors. Afterglow intensities were recorded after cessation of X-rays, following 300 s of X-ray excitation. h, Corresponding SEM images of compressed samples. All samples were prepared by a tablet machine without the PDMS matrix.

Extended Data Fig. 5 Characterization of persistent luminescent nanocrystals doped with different lanthanide activators.

a, TEM images of NaLuF4:Pr/Gd (0.5/19.5 mol%), NaLuF4:Sm/Gd (0.5/19.5 mol%), NaLuF4:Ho/Gd (1/19 mol%), NaLuF4:Er/Gd (1/19 mol%), NaLuF4:Tb/Gd (15/5 mol%), NaLuF4:Dy/Gd (0.5/19.5 mol%), NaLuF4:Tm/Gd (1/19 mol%) and NaLuF4:Nd/Gd (1/19 mol%) nanocrystals. b, Powder X-ray diffraction patterns of NaLuF4:Ln/Gd (Ln = Pr3+, Sm3+, Ho3+, Er3+, Tb3+, Dy3+, Tm3+ and Nd3+) nanocrystals. All peaks are indexed in accordance with the hexagonal-phase NaLuF4 structure (Joint Committee on Powder Diffraction Standards file number 27-0726). c, Room-temperature afterglow spectra of NaLuF4:Pr/Gd (0.5/19.5 mol%), NaLuF4:Sm/Gd (0.5/19.5 mol%), NaLuF4:Ho/Gd (1/19 mol%), NaLuF4:Er/Gd (1/19 mol%), NaLuF4:Tb/Gd (15/5 mol%), NaLuF4:Dy/Gd (0.5/19.5 mol%), NaLuF4:Tm/Gd (1/19 mol%) and NaLuF4:Nd/Gd (1/19 mol%) nanocrystals. All spectra were recorded after turning off X-rays (dose rate, 278 μGy s−1; excitation time, 300 s). d, Corresponding commission Internationale de l’Eclairage chromaticity coordinates of persistent luminescence. e, Room-temperature afterglow decay curves of NaLuF4:Ln/Gd (Ln = Pr3+, Sm3+, Ho3+, Er3+, Dy3+, Tm3+ and Nd3+) nanocrystals monitored at 606, 594, 542, 543, 573, 453 and 385 nm, respectively (dose rate, 278 μGy s−1; excitation time, 300 s).

Extended Data Fig. 6 Physical investigation of X-ray-induced luminescence on lanthanide-doped fluoride nanocrystals.

a, Emission spectra of NaLuF4:Eu (15 mol%) nanocrystals with and without X-ray irradiation. b, Luminescence intensity of NaLuF4:Eu and NaLuF4:Tb nanocrystals as a function of time upon switching on/off X-rays. c, X-ray absorption near-edge structure (XANES) spectra of Tb LIII-edge recorded for NaLuF4:Eu (15 mol%) nanocrystals and Eu2O3 and EuTiO3 references. d, Room-temperature emission spectra of NaYF4:Tb (15 mol%), NaGdF4:Tb (15 mol%), and NaLuF4:Tb (15 mol%) nanocrystals. e, X-ray-induced luminescence intensity of NaYF4:Tb (15 mol%), NaGdF4:Tb (15 mol%) and NaLuF4:Tb (15 mol%), monitored at 546 nm. All samples were excited with X-ray irradiation at 50 kV (dose rate, 278 μGy s−1; temperature, 298 K). f, Luminescence decay curves of NaLuF4:Er/Gd (1/x mol%; x = 0–49) nanocrystals after X-ray excitation is ceased. g, Emission spectra of NaLuF4:Tb/Gd (15/5 mol%) nanocrystals with and without X-ray irradiation, showing energy migration from Gd3+ to Tb3+. h, Luminescence decay curves of the NaGdF4:Tb (15 mol%) core and NaGdF4:Tb(15 mol%)@NaYF4 core–shell nanoparticles after cessation of X-rays. i, Schematic of NaGdF4:Tb crystal lattice and the energy level diagram of Gd3+ and Tb3+. The excitation energy dissipates non-radiatively to quenching sites through energy migration.

Extended Data Fig. 7 Calculated electronic structures of NaLuF4-based systems.

a, Schematic illustrating creation of Frenkel-related trap states in NaLuF4 crystal lattices upon high-energy X-ray irradiation. Small fluoride ions (F) are then displaced from lattice to interstitial sites. This leads to many fluoride vacancies (VF) and interstitials (IF), along with trapping of energetic electrons (e) at anion defects. b, Structural configuration of closely and distantly paired defects in the NaLuF4 lattice. Fluorine atoms are ejected from their original lattice sites to interstitial sites upon X-ray irradiation, followed by either spontaneous or stimulated self-recovery. c, Calculated electron and atom relaxation speed of defect pairs featuring different separation distances. d, Density of states of pristine β-NaLuF4 (top), VF–IF-contained β-NaLuF4 (middle) and VF–IF-contained β-NaLuF4:Tb (bottom). Green dashed lines indicate the position of Fermi levels. Localized states due to F displacement are marked with arrows. Note that the values of the 4f-resolved density of states are scaled up (tenfold) for comparison purposes. e, The corresponding spatial distribution of partial charge densities of VF- and IF-induced localized states within the bandgap. Light purple and orange iso-surfaces are used for occupied and unoccupied localized states, respectively.

Extended Data Fig. 8 Characterization of electronic trap depth in NaLuF4:Tb/Gd (15/x mol%; x = 0–35) nanocrystals.

ae, Density distribution of electronic trap depths in NaLuF4:Tb3+/Gd3+ (15/35 mol%) (a), NaLuF4:Tb/Gd (15/25 mol%) (b), NaLuF4:Tb/Gd (15/15 mol%) (c), NaLuF4:Tb/Gd (15/5 mol%) (d) and NaLuF4:Tb (15 mol%) (e) nanocrystals. f, Measured electronic trap depth of Tb3+-doped nanocrystals as a function of the doping ratio of lutetium in the material host. Data were calculated from the measured results of ae. g, Luminescence profile of NaLuF4:Tb(15 mol%)@NaYF4 nanocrystals under X-ray and after cessation of excitation, followed by cycled near-infrared stimulation with a 980-nm laser. h, Radioluminescence intensity of nanocrystals under repeated X-ray irradiation and thermal stimulation. Samples were excited with an X-ray source at 50 kV for 300 s. Radioluminescent afterglow decays quickly upon heating. i, Recycling performance evaluation of NaLuF4:Tb(15 mol%)@NaYF4 nanocrystals under X-ray irradiation and heating at 80 °C for 14 cycles.

Extended Data Fig. 9 Xr-LEI based on persistent radioluminescent nanocrystals.

a, Schematic showing the microscopy setup for X-ray imaging. b, c, Bright-field photos (top) and X-ray images (bottom) of an X-ray dosimeter (b) and a computer mouse (c). df, Bright-field photos (top) and X-ray images (bottom) of a 3D electronic circuit board, conforming and adhering to the X-ray detector (d, e) or placed on the top of the X-ray detector as a control (f). g, Photograph (left) and corresponding X-ray images (right) of an encapsulated metallic spring, recorded with a digital camera at time intervals from 1 s to 15 days. The Xr-LEI was performed by heating the flexible detector at 80 °C after cessation of X-rays (50 kV).

Extended Data Fig. 10 X-ray imaging of an electronic circuit board using a PDMS thin film containing NaLuF4:Tb(15 mol%)@NaYF4 nanoparticles.

The X-ray exposure was controlled from 1 to 15 s, and the nanoparticle concentration in the PDMS film was controlled between 0.4 and 2.5 wt%.

Extended Data Fig. 11 Characterization of the stretchable X-ray detector.

a, Material parameters were obtained by fitting the stress–strain curve of the elastomer using the Mooney–Rivlin model. Experimental results and analysis derived a tensile elastic modulus (Et) of 10 psi (0.0689 MPa), a tensile strength (σt) of 200 psi (1.379 MPa), a Poisson ratio (μ) of 0.35 and a bulk modulus (D) of 0.0766 MPa (C10 = 0.065 MPa, C01 = 0.36 MPa). b, Stress–strain curve of the film in 10 cyclic stress-strain tests, with a sample width of 10 mm, a thickness of 1 mm, a gauge length of 50 mm and a loading rate of 100 mm min–1. c, Finite element simulation of strain distribution over the stretchable X-ray detector as the local strain increases to 500%. d, Light intensity function of pixels (along the blue line below and the full-width at half-maximum taken as the resolution) and X-ray imaging of a line-pair mask.

Supplementary information

Supplementary Information

This file contains Supplementary Figs 1-4, and Supplementary Table 1. It includes additional information on scintillation properties, experimental apparatus, synchrotron radiation characterization, structures of a commercial flat-panel X-ray detector, and comparison of physical parameters of various persistent materials to support the conclusions of the main text.

Video 1

Characterization of persistent radioluminescent nanocrystals Terbium (Tb3+)-doped NaLuF4 nanoscintillator powders were irradiated by X-rays at 50 kV/80 A for 5 min, and then the corresponding radioluminescence afterglow was recorded after stoppage of X-rays. After 2 hours, the powders were heated to 378 K and maintained for 10 mins for thermally stimulated luminescence.

Video 2

X-ray luminescence extension imaging (Xr-LEI) of a 3D electronic circuit board The acquired X-ray image of the circuit board by Xr-LEI was projected in 3D using graphical simulations.

Video 3

X-ray luminescence extension imaging (Xr-LEI) using a stretchable detector Experimental demonstration of Xr-LEI using a flexible X-ray detector. The X-ray detector was heated at 80 oC on a hotplate to render the recorded image.

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Ou, X., Qin, X., Huang, B. et al. High-resolution X-ray luminescence extension imaging. Nature 590, 410–415 (2021). https://doi.org/10.1038/s41586-021-03251-6

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