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.

Scintillation Properties of CsPbBr3 Nanocrystals Prepared by Ligand-Assisted Reprecipitation and Dual Effect of Polyacrylate Encapsulation toward Scalable Ultrafast Radiation Detectors

“…Before proceeding with such a detailed analysis, we performed time-resolved scintillation and gamma irradiation experiments. The timeresolved RL of the CsPbBr 3 NCs in PMMA is reported in Figure 1g showing, in agreement with recent reports, 11,19,74 a largely ultrafast kinetics (average lifetime ⟨τ⟩ = 1.81 ns) with a prompt decay (limited by the ∼75 ps time response of our setup) and a ∼600 ps contribution respectively ascribed to the recombination of multi-and charged-excitons formed under ionizing radiation, followed by a ∼7 ns tail matching the fast PL component of neutral band-edge excitons (see Supporting Information for the fitting procedure). Such a fast scintillation kinetics confirms the potential of LARP-synthesized LHP-NCs for fast timing applications further corroborated by the mass scalability of the LARP method.…”

“…As counterevidence, NCs embedded in PS, which has no acrylic functionalities, show PL dynamics essentially identical to those of NCs in solution (Figure S2). Consistent with previous results, the room temperature radioluminescence (RL) spectra (Figure 1f) of both the NCs and the nanocomposite are slightly redshifted with respect to the corresponding PL, owing to the contribution of attractive biexcitons 19 and excitons trapped in bromine surface vacancies 73 at the NCs surfaces that are likely preferentially populated by electrons resulting from the thermalization of highly energetic carriers generated upon the primary interaction with X-rays. 20 Further compelling evidence for the substantial passivation effect of the PMMA embedding of NCs surface defects comes from low temperature PL and RL spectra, showing the suppression of shallow defect RL contribution for the nanocomposite.…”

“…The RL intensity shows stronger intensification (Figure 2b), as further emphasized by the temperature evolution of the I RL /I PL ratio shown in Figure 2f, suggesting that the RL process is influenced by other nonradiative pathways such as trapping of diffusely generated free carriers in deep trap sites (also in line with the observed redshift of the RL vs PL spectra discussed above), that are also suppressed at cryogenic temperature. Moreover, the evidence of shallow trapped excitons at low temperature indicates that the optical properties improvement upon polymer encapsulation are not due to restricted degrees of freedom of the surface ligand shell causing nonradiative losses, as these would be also suppressed at T < 50 K. Crucially, the incorporation of NCs into PMMA restores the band-edge origin of RL (Figure 2d) which now matches the corresponding PL except for the visible low energy shoulder, ascribed to the biexcitonic contribution to the scintillation emission, 11,19 and shows a more pronounced enhancement with temperature decreasing (Figure 2e). These observations support the partial passivation of surface defects due to the coordination of the electron-rich acrylic groups of PMMA with uncoordinated Pb sites on the NCs surfaces.…”

“…22 To date, most studies have been devoted to understanding and optimizing the scintillation of LHP-NCs prepared by the hot-injection method, 20,23 which produces the most luminescent materials. Specifically, recent studies of CsPbBr 3 NCs have shown that the scintillation emission is largely multiexcitonic 19 with substantial contributions by shallow trapped excitons in surface bromine vacancies, 20,23,24 which can be suppressed by various resurfacing techniques, 25−29 resulting in enhanced light yield and exceptionally high radiation stability. 20,30 Despite such advantages, hot-injection synthesis is performed at high temperature and is hardly scalable to the very large quantities required for radiation detection.…”

“…The detection of ionizing radiation is critical in many applications including nuclear and homeland security, , high-energy physics, , and medicine. , To date, massive inorganic scintillator crystals account for the largest share of the market but suffer from high production costs and limited customization. , Plastic scintillators, on the other hand, have faster kinetics and can be fabricated in custom size and shape at low cost but suffer from poor radiation stability and lower detection efficiency. In an attempt to combine the advantages of these two classes of scintillators, increasing attention is being paid to the development of nanocomposite scintillators consisting of polymeric matrices doped or coated with nanocrystals (NCs) of high atomic number compounds prepared by scalable solution-based processes. − Metal halide NCs, and in particular lead halide perovskites (LHP), have emerged as promising scintillators, ,− prized for their solution synthesis, efficient color-tunable radioluminescence, and high radiation hardness . To date, most studies have been devoted to understanding and optimizing the scintillation of LHP-NCs prepared by the hot-injection method, , which produces the most luminescent materials.…”