Ultrafast and Radiation-Hard Lead Halide Perovskite Nanocomposite Scintillators

Abstract

The use of scintillators for the detection of ionizing radiation is a critical aspect in many fields, including medicine, nuclear monitoring, and homeland security. Recently, lead halide perovskite nanocrystals (LHP-NCs) have emerged as promising scintillator materials. However, the difficulty of affordably upscaling synthesis to the multigram level and embedding NCs in optical-grade nanocomposites without compromising their optical properties still limits their widespread use. In addition, fundamental aspects of the scintillation mechanisms are not fully understood, leaving the scientific community without suitable fabrication protocols and rational guidelines for the full exploitation of their potential. In this work, we realize large polyacrylate nanocomposite scintillators based on CsPbBr₃ NCs, which are synthesized via a novel room temperature, low waste turbo-emulsification approach, followed by their in situ transformation during the mass polymerization process. The interaction between NCs and polymer chains strengthens the scintillator structure, homogenizes the particle size distribution and passivates NC defects, resulting in nanocomposite prototypes with luminescence efficiency >90%, exceptional radiation hardness, 4800 ph/MeV scintillation yield even at low NC loading, and ultrafast response time, with over 30% of scintillation occurring in the first 80 ps, promising for fast-time applications in precision medicine and high-energy physics. Ultrafast radioluminescence and optical spectroscopy experiments using pulsed synchrotron light further disambiguate the origin of the scintillation kinetics as the result of charged-exciton and multiexciton recombination formed under ionizing excitation. This highlights the role of nonradiative Auger decay, whose potential impact on fast timing applications we anticipate via a kinetic model.

Figure 1

(a) Photograph of the turbo-emulsifier homogenized synthesis of 8 g of CsPbBr₃ NCs in a 5 L reactor. (b) STEM-HAADF image of the NC sample after washing. Inset: corresponding HRTEM image. (c) Photograph of a fabricated PMMA–PLMA nanocomposite with dimensions of 60 × 50 × 0.3 cm comprising CsPbBr₃ [NC] = 0.2 wt % under ambient illumination. (d) TEM micrographs of 70 nm thin nanocomposite section showing domains of CsPbBr₃ NCs in the polymeric matrix. (e) Powder X-ray diffraction patterns of the native NCs (top, black line) and of the NCs embedded in the nanocomposite displayed in panel c (middle, green line), together with the calculated PXRD pattern for orthorhombic CsPbBr₃ (bottom, purple line, ICSD 97851). The diffraction halo pattern associated with the polymeric host matrix in the PXRD of the nanocomposite was subtracted for clarity (Figure S6). The peaks denoted by star symbols are associated with minor crystalline impurities included in the nanocomposite (red stars, CsBr). (f) Optical absorption (top panel) and PL (bottom panel, excitation energy 3.1 eV) of 0.2 wt % dispersions of CsPbBr₃ NCs in LMA:MMA (20:80%vol) during the polymerization reaction (time evolution indicated by the black arrow). The initial spectra before the activation of the UV initiators and at the end of the process are highlighted in black and green, respectively. (g) Evolution of the PL quantum yield was observed during the polymerization process. The photographs of the liquid monomer mixture and the polymerized solid under UV illumination (3.4 eV) are reported as inset. (h) Contour plot of the spectrally resolved PL decay traces of the native NCs (top panel) and the final nanocomposite (middle panel) excited at 3.1 eV and (bottom panel) representative decay curves collected at the emission energies indicated by the dashed lines in the contour plots and the vertical bars in panel f. (i) ¹H NMR free induction decay (FID) at 333 K indicating faster relaxation for the nanocomposite (green dots, 0.2 wt %) with respect to the bare polymer matrix (black dots). The solid red lines are the fitting functions. (l) HRTEM image (top panel) and corresponding Fast Fourier Transform (FFT) pattern (bottom panel) of CsPbBr₃ NCs evolved in a solution of acrylate monomers.

Figure 2

(a) RL spectra of polyacrylate nanocomposites containing increasing concentration of CsPbBr₃ NCs together with respective photographs under ambient or UV light. (b) Corresponding LY values obtained via absolute (triangles) or relative (circles) methods. The black line is the fitting function with a power law I_RL = A × [NC]^p with p = 1.1. (c) Optical absorption (dashed lines), RL spectra (solid lines) and photographs under ambient or UV light of polyacrylate nanocomposites with [NC] = 0.1 wt % at increasing cumulative γ-ray doses from 0 Gy to 1 MGy (bottom to top). The spectra have been normalized at the emission maxima and respective absorption edge and vertically shifted for clarity. (d) LY (circles) and corresponding Φ_PL (triangles) as a function of cumulative dose. Inset: TEM micrographs of 70 nm thin nanocomposite sections before and after irradiation (scale bar 50 nm). (e) Normalized PL decay curves and (f) and FTIR transmission spectra at 0 Gy and 1 MGy showing no variation of the NC decay kinetics and no modification of the vibrational spectrum of the polymer indicating high radiation resistance.

Figure 3

(a) Scintillation decay of the five nanocomposites shown in Figure 2a. The scintillation decay is shown in a linear scale over 24 ns. The inset shows details of the ultrafast component (over 5 ns) in semilogarithmic scale; the shaded gray line represents the system IRF. The solid curves are the fit functions. (b) Transient absorption spectra (in semilogarithmic scale) at increasing time (t = 2, 7, 12, 23, 50, 200, 500, 1000, 3000 ps) after the excitation pulse for progressively larger average exciton population ⟨N⟩ showing the emergence of bi- and multiexciton spectral contributions. The respective decay traces taken at the energies indicated in the figure are shown in the right-hand panels highlighting gradually faster decay with increasing ⟨N⟩. (c) TA dynamics at increasing average exciton population ⟨N⟩. (d) Differential TA curves extracted from panel c representing single-order, charged-order, bi-order, and higher order exciton dynamics. (e) PL decay traces excited by synchrotron light at increasing energy up to E_EXC = 19 eV together with the respective single exponential fitting curves. (f) Corresponding PL decay times vs E_EXC. Inset: schematic depiction of the ionization of CsPbBr₃ NCs upon excitation with the E_EXC ≥ IE.

Figure 4

(a) Schematic representation of the possible decay pathways for single (X) and biexciton (XX) states in the presence of AR. Other nonradiative channels are neglected for simplicity. Simulated emission decay curves for ⟨N⟩ = 1.6 corresponding to n_X(t=0) = 0.32 and n_XX(t=0) = 0.26 for (b) nonionizing or (c) ionizing AR. The inset in panel b highlights the similarity between the experimental TA kinetics for ⟨N⟩ = 1.6 and the simulation for Φ_AR = 0.9 suggesting that in our case AR is mostly nonionizing. (d) Effective emission lifetime, τ_EFF, (e) integrated emission, and (f) CTR extracted from the decay curves in panels a and b. The blue (red) symbols correspond to nonionizing (ionizing) AR.