All-inorganic perovskite nanocrystals: next-generation scintillation materials for high-resolution X-ray imaging

Ligand Equilibrium Influences Photoluminescence Blinking in CsPbBr3: A Change Point Analysis of Widefield Imaging Data

Photoluminescence intermittency remains one of the biggest challenges in realizing perovskite quantum dots (QDs) as scalable single photon emitters. We compare CsPbBr3 QDs capped with different ligands, lecithin, and a combination of oleic acid and oleylamine, to elucidate the role of surface chemistry on photoluminescence intermittency. We employ widefield photoluminescence microscopy to sample the blinking behavior of hundreds of QDs. Using change point analysis, we achieve the robust classification of blinking trajectories, and we analyze representative distributions from large numbers of QDs (Nlecithin = 1308, Noleic acid/oleylamine = 1317). We find that lecithin suppresses blinking in CsPbBr3 QDs compared with oleic acid/oleylamine. Under common experimental conditions, lecithin-capped QDs are 7.5 times more likely to be nonblinking and spend 2.5 times longer in their most emissive state, despite both QDs having nearly identical solution photoluminescence quantum yields. We measure photoluminescence as a function of dilution and show that the differences between lecithin and oleic acid/oleylamine capping emerge at low concentrations during preparation for single particle experiments. From experiment and first-principles calculations, we attribute the differences in lecithin and oleic acid/oleylamine performance to differences in their ligand binding equilibria. Consistent with our experimental data, density functional theory calculations suggest a stronger binding affinity of lecithin to the QD surface compared to oleic acid/oleylamine, implying a reduced likelihood of ligand desorption during dilution. These results suggest that using more tightly binding ligands is a necessity for surface passivation and, consequently, blinking reduction in perovskite QDs used for single particle and quantum light experiments.

Cold-Sintered All-Inorganic Perovskite Bulk Composite Scintillators for Efficient X-ray Imaging

Cost-effective bulk scintillators with high density, large-area, and long-term stability are desirable for high-energy radiation detections. Conventional bulk polycrystalline or single-crystal scintillators are generally synthesized by high-temperature approaches, and it is challenging to realize simultaneously high detectivity/responsivity, spatial resolution, and rapid time response. Here, we report the cold sintering of bulk scintillators (at 90 °C) based on an "emitter-in-matrix" principle, in which emissive CsPbBr3 nanocrystals are embedded in a durable and transparent Cs4PbBr6 matrix. These bulk scintillators exhibit high light yield (33,800 photons MeV-1), low detection limit (79 nGyair s-1), fast decay time (9.8 ns), and outstanding spatial resolution of 8.9 lp mm-1 to X-ray radiation and an energy resolution of 19.3% for γ-ray (59.6 keV) detection. The composite scintillator also shows exceptional stability against environmental degradation and cyclic X-ray radiation. Our results demonstrate a cost-effective strategy for developing perovskite-based bulk transparent scintillators with exceptional performance and high radioluminescence stability for high-energy radiation detection and imaging.

Cs2NaGdCl6:Tb3+─A Highly Luminescent Rare-Earth Double Perovskite Scintillator for Low-Dose X-ray Detection and Imaging

Rare-earth-based double perovskite (DP) X-ray scintillators have gained significant importance with low detection limits in medical imaging and radiation detection owing to their high light yield (LY) and remarkable spatial resolution. Herein, we report the synthesis of 3D double perovskite (DP) crystals, namely, Cs2NaGdCl6 and Tb3+-Cs2NaGdCl6 using hydrothermal reaction. Cs2NaGdCl6 DP single crystals exhibited a blue self-trapped exciton (STE) emission at 470 nm under ultraviolet (265 nm) excitation with a photoluminescence quantum yield (PLQY) of 8.4%. Introducing Tb3+ ions into Cs2NaGdCl6 has resulted in quenching of STE emission and enhancing green emission at 549 nm attributed to the 5D4 → 7F5 transition of Tb3+, suggesting efficient energy transfer (ET) from STE to Tb3+. This ET process is evidenced by the appearance of Tb3+ bands in the excitation spectra of the host, the shortening of the STE lifetimes in the presence of Tb3+ ions, and the enhancement of PLQY (72.6%). Furthermore, Cs2NaGdCl6:5%Tb3+ films of various thicknesses (0.1-0.6 mm) were synthesized and their X-ray scintillating performance has been examined. The Cs2NaGdCl6:5%Tb3+ film with 0.4 mm thickness has exhibited an excellent linear response to the X-ray dose rate with a low detection limit of 41.32 nGyair s-1, an LY of 39,100 photons MeV-1, and excellent radiation stability. Benefiting from the strong X-ray excited luminescence (XEL) of Cs2NaGdCl6:5%Tb3+, we developed a Cs2NaGdCl6:5%Tb3+ X-ray scintillator screen with a least thickness (0.1 mm), exhibiting remarkable imaging ability with a spatial resolution of 10.75 lp mm-1. These results suggest that Cs2NaGdCl6:Tb3+ can be a potential candidate for low-dose and X-ray imaging applications.

Colloidal Synthesis of Hollow Double Perovskite Nanocrystals and Their Applications in X-ray Imaging

Rare earth-based halide double perovskites are regarded as an emerging class of X-ray scintillation materials. However, the majority of related scintillator applications are still focused on single crystal and powder systems; the application of nanocrystal (NC) scintillators is rarely reported. Here, we present the synthesis of high-purity Cs2NaTbCl6 NCs by an improved hot-injection method. Interestingly, hollow Cs2NaTbCl6 NCs are observed, the monitoring of the growth process indicates that micrometer-sized NaCl is the initial product, and then the NaCl would convert into Cs2NaTbCl6 NCs through the diffusion of Cs+ and Tb3+ into NaCl lattice, and the faster outward diffusion of Na+ results in the formation of hollow NCs. The double perovskite NCs exhibit green light emission, and the photoluminescence intensity can be significantly enhanced through Ce3+ doping. In particular, the Cs2NaTbCl6:5%Ce3+ scintillator exhibits a linear response and a low detection limit of 79.09 nGy/s when exposed to X-rays. Furthermore, a flexible scintillator film for X-ray imaging is prepared by mixing NCs with polymer, showing a high spatial resolution imaging capability of 10 lp/mm. This work provides a new strategy for hollow perovskite NCs and may shed light on the synthesis of related hollow NCs and their applications in X-ray detection.

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

Lead halide perovskite nanocrystals (LHP-NCs) embedded in polymeric hosts are gaining attention as scalable and low-cost scintillation detectors for technologically relevant applications. Despite rapid progress, little is currently known about the scintillation properties and stability of LHP-NCs prepared by the ligand assisted reprecipitation (LARP) method, which allows mass scalability at room temperature unmatched by any other type of nanostructure, and the implications of incorporating LHP-NCs into polyacrylate hosts are still largely debated. Here, we show that LARP-synthesized CsPbBr3 NCs are comparable to particles from hot-injection routes and unravel the dual effect of polyacrylate incorporation, where the partial degradation of LHP-NCs luminescence is counterbalanced by the passivation of electron-poor defects by the host acrylic groups. Experiments on NCs with tailored surface defects show that the balance between such antithetical effects of polymer embedding is determined by the surface defect density of the NCs and provide guidelines for further material optimization.

Stable and luminescent cesium copper halide nanocrystals embedded in flexible polymer fibers for fabrication of down-converting WLEDs

Recently, CsPbX3 (X = I, Br, Cl) perovskite nanocrystals (NCs) have drawn wide attention owing to their outstanding photophysical and optoelectronic properties. However, the toxicity of such NCs remained a big challenge for further commercialization. Herein, we adopt facile methods for synthesizing green-emissive Cs3Cu2Cl5 and blue-emissive Cs3Cu2Br2.5I2.5 perovskite NCs that exhibit broad emission spectra with large Stokes shifts. These NCs showed photoluminescence quantum yields (PLQY) up to 65% (Cs3Cu2Cl5 NCs) and 32% (Cs3Cu2Br2.5I2.5 NCs) with limited stabilities. To further improve the stability, the NCs were blended with a hydrophobic polymer poly-methylmethacrylate (PMMA) and embedded inside the polymer fiber by an electrospinning process to form composite fibers. The as-prepared Cs3Cu2Cl5@PMMA and Cs3Cu2Br2.5I2.5@PMMA fiber films demonstrated good surface coverage and better thermal stability, and even retained their emission properties when dispersed in water. The emissive fibers were also deposited on flexible polyethylene terephthalate (PET) substrates that displayed high resistance towards bending and twisting with no signs of breakage, damage, or loss of optical properties. Finally, UV-pumped phosphor-converted WLEDs fabricated by using these blue and green-emitting fibers revealed CIE chromaticity coordinates at (0.27, 0.33) with a maximum luminous efficiency of 69 Lm W-1 and correlated color temperature (CCT) value of 8703 K. These outcomes can be beneficial for the development of futuristic flexible display technologies.

High-Efficiency Narrow-Band Green-Emitting Manganese(II) Halide for Multifunctional Applications

Zero-dimensional (0D) Mn2+-based metal halides used as luminescent materials and scintillators have become a research hotspot in the field of photoelectric materials and devices due to their unique composition, structure, and fluorescence properties. It is of great value to explore new Mn2+-based metal halides to achieve multifunctional applications. Herein, the novel 0D Mn2+-based metal halide single crystal (BPTP)2MnBr4 is synthesized by a simple solvent-antisolvent recrystallization method. Under excitation at 468 nm, the (BPTP)2MnBr4 single crystal shows a pronounced narrow-band green luminescence centered at 515 nm derived from the d-d transition of the Mn2+ ion. This emission has a relatively narrow full width at half maximum of 43 nm and a high photoluminescence quantum yield (PLQY) of 82%. In addition, (BPTP)2MnBr4 exhibits good thermal stability at 393 K with a retention of 79% of the initial photoluminescence intensity at 298 K. Benefiting from its strong blue light excitation, high PLQY, and good thermal stability, we manufacture an ideal white light-emitting diode (LED) device using a 460 nm blue LED chip, green-emitting (BPTP)2MnBr4, and commercial K2SiF6:Mn4+ red phosphor. Under 20 mA drive current, the LED shows a high luminous efficiency of 112 lm/W and a wide color gamut of 110.8%, according to the National Television System Committee standard. In addition, (BPTP)2MnBr4 crystals show a strong X-ray absorption. Based on the commercial Lu3Al5O12:Ce3+ scintillator, the calculated light yield of (BPTP)2MnBr4 reaches up to about 136,000 photons/MeV and the detection limit reaches 0.282 μGyair s-1. Additionally, a melt quenching approach is used to construct a (BPTP)2MnBr4 clear glass scintillation screen, realizing a spatial resolution of 10.1 lp/mm. The proper performances of (BPTP)2MnBr4 as phosphor-converted LED materials and the X-ray scintillator with the addition of eco-friendly, low-cost solution processability make 0D Mn2+-based metal halides potential luminescent materials for multifunctional applications.

Surfactant-Dependent Bulk Scale Mechanochemical Synthesis of CsPbBr3 Nanocrystals for Plastic Scintillator-Based X-ray Imaging

We report a facile, solvent-free surfactant-dependent mechanochemical synthesis of highly luminescent CsPbBr3 nanocrystals (NCs) and study their scintillation properties. A small amount of surfactant oleylamine (OAM) plays an important role in the two-step ball milling method to control the size and emission properties of the NCs. The solid-state synthesized perovskite NCs exhibit a high photoluminescence quantum yield (PLQY) of up to 88% with excellent stability. CsPbBr3 NCs capped with different amounts of surfactant were dispersed in toluene and mixed with polymethyl methacrylate (PMMA) polymer and cast into scintillator discs. With increasing concentration of OAM during synthesis, the PL yield of CsPbBr3/PMMA nanocomposite was increased, which is attributed to reduced NC aggregation and PL quenching. We also varied the perovskite loading concentration in the nanocomposite and studied the resulting emission properties. The most intense PL emission was observed from the 2% perovskite-loaded disc, while the 10% loaded disc exhibited the highest radioluminescence (RL) emission from 50 kV X-rays. The strong RL yield may be attributed to the deep penetration of X-rays into the composite, combined with the large interaction cross-section of the X-rays with the high-Z atoms within the NCs. The nanocomposite disc shows an intense RL emission peak centered at 536 nm and a fast RL decay time of 29.4 ns. Further, we have demonstrated the X-ray imaging performance of a 10% CsPbBr3 NC-loaded nanocomposite disc.

Radioluminescent Cu-Au Metal Nanoclusters: Synthesis and Self-Assembly for Efficient X-ray Scintillation and Imaging

Zero-dimensional (0D) scintillation materials have drawn tremendous attention due to their inherent advantages in the fabrication of flexible high-energy radiation scintillation screens by solution processes. Although considerable progress has been made in the development of 0D scintillators, such as the current leading lead-halide perovskite nanocrystals and quantum dots, challenges still persist, including potential issues with self-absorption, air stability, and eco-friendliness. Here, we present a strategy to overcome those limitations by synthesis and self-assembly of a new class of scintillators based on metal nanoclusters. We demonstrate the gram-scale synthesis of an atomically precise nanocluster with a Cu-Au alloy core exhibiting high phosphorescence quantum yield, aggregation-induced emission enhancement (AIEE) behavior, and intense radioluminescence. By controlling solvent interactions, the AIEE-active nanoclusters were self-assembled into submicron spherical superparticles in solution, which we exploited as a novel building block for flexible particle-deposited scintillation films with high-resolution X-ray imaging performance. This work reveals metal nanoclusters and their self-assembled superstructures as a promising class of scintillators for practical applications in high-energy radiation detection and imaging.

Review of nanomaterial advances for ionizing radiation dosimetry

There are a wide range of applications with ionizing radiation and a common theme throughout these is that accurate dosimetry is usually required, although many newer demands are provided by improved features in higher range, multi-spectral and particle type detected. Today, the array of dosimeters includes both offline and online tools, such as gel dosimeters, thermoluminescence (TL), scintillators, optically stimulated luminescence (OSL), radiochromic polymeric films, gels, ionization chambers, colorimetry, and electron spin resonance (ESR) measurement systems. Several future nanocomposite features and interpretation of their substantial behaviors are discussed that can lead to improvements in specific features, such as (1) lower sensitivity range, (2) less saturation at high range, (3) overall increased dynamic range, (4) superior linearity, (5) linear energy transfer and energy independence, (6) lower cost, (7) higher ease of use, and (8) improved tissue equivalence. Nanophase versions of TL and ESR dosimeters and scintillators each have potential for higher range of linearity, sometimes due to superior charge transfer to the trapping center. Both OSL and ESR detection of nanomaterials can have increased dose sensitivity because of their higher readout sensitivity with nanoscale sensing. New nanocrystalline scintillators, such as perovskite, have fundamentally important advantages in sensitivity and purposeful design for key new applications. Nanoparticle plasmon coupled sensors doped within a lower Zeff material have been an effective way to achieve enhanced sensitivity of many dosimetry systems while still achieving tissue equivalency. These nanomaterial processing techniques and unique combinations of them are key steps that lead to the advanced features. Each must be realized through industrial production and quality control with packaging into dosimetry systems that maximize stability and reproducibility. Ultimately, recommendations for future work in this field of radiation dosimetry were summarized throughout the review.

Energy Partitioning in Multicomponent Nanoscintillators for Enhanced Localized Radiotherapy

Multicomponent nanomaterials consisting of dense scintillating particles functionalized by or embedding optically active conjugated photosensitizers (PSs) for cytotoxic reactive oxygen species (ROS) have been proposed in the last decade as coadjuvant agents for radiotherapy of cancer. They have been designed to make scintillation-activated sensitizers for ROS production in an aqueous environment under exposure to ionizing radiations. However, a detailed understanding of the global energy partitioning process occurring during the scintillation is still missing, in particular regarding the role of the non-radiative energy transfer between the nanoscintillator and the conjugated moieties which is usually considered crucial for the activation of PSs and therefore pivotal to enhance the therapeutic effect. We investigate this mechanism in a series of PS-functionalized scintillating nanotubes where the non-radiative energy transfer yield has been tuned by control of the intermolecular distance between the nanotube and the conjugated system. The obtained results indicate that non-radiative energy transfer has a negligible effect on the ROS sensitization efficiency, thus opening the way to the development of different architectures for breakthrough radiotherapy coadjutants to be tested in clinics.

A versatile laboratory setup for high resolution X-ray phase contrast tomography and scintillator characterization

BACKGROUND

X-ray micro-tomography (μCT) is a powerful non-destructive 3D imaging method applied in many scientific fields. In combination with propagation-based phase-contrast, the method is suitable for samples with low absorption contrast. Phase contrast tomography has become available in the lab with the ongoing development of micro-focused tube sources, but it requires sensitive and high-resolution X-ray detectors. The development of novel scintillation detectors, particularly for microscopy, requires more flexibility than available in commercial tomography systems.

OBJECTIVE

We aim to develop a compact, flexible, and versatile μCT laboratory setup that combines absorption and phase contrast imaging as well as the option to use it for scintillator characterization. Here, we present details on the design and implementation of the setup.

METHODS

We used the setup for μCT in absorption and propagation-based phase-contrast mode, as well as to study a perovskite scintillator.

RESULTS

We show the 2D and 3D performance in absorption and phase contrast mode, as well as how the setup can be used for testing new scintillator materials in a realistic imaging environment. A spatial resolution of around 1.3μm is measured in 2D and 3D.

CONCLUSIONS

The setup meets the needs for common absorption μCT applications and offers increased contrast in phase contrast mode. The availability of a versatile laboratory μCT setup allows not only for easy access to tomographic measurements, but also enables a prompt monitoring and feedback beneficial for advances in scintillator fabrication.

Compositionally Disordered Crystalline Compounds for Next Generation of Radiation Detectors

The review is devoted to the analysis of the compositional disordering potential of the crystal matrix of a scintillator to improve its scintillation parameters. Technological capabilities to complicate crystal matrices both in anionic and cationic sublattices of a variety of compounds are examined. The effects of the disorder at nano-level on the landscape at the bottom of the conduction band, which is adjacent to the band gap, have been discussed. The ways to control the composition of polycationic compounds when creating precursors, the role of disorder in the anionic sublattice in alkali halide compounds, a positive role of Gd based matrices on scintillation properties, and the control of the heterovalent state of the activator by creation of disorder in silicates have been considered as well. The benefits of introducing a 3D printing method, which is prospective for the engineering and production of scintillators at the nanoscale level, have been manifested.

Structural and Optical Properties of High Entropy (La,Lu,Y,Gd,Ce)AlO3 Perovskite Thin Films

Mixtures of Ce-doped rare-earth aluminum perovskites are drawing a significant amount of attention as potential scintillating devices. However, the synthesis of complex perovskite systems leads to many challenges. Designing the A-site cations with an equiatomic ratio allows for the stabilization of a single-crystal phase driven by an entropic regime. This work describes the synthesis of a highly epitaxial thin film of configurationally disordered rare-earth aluminum perovskite oxide (La_0.2 Lu_0.2 Y_0.2 Gd_0.2 Ce_0.2 )AlO₃ and characterizes the structural and optical properties. The thin films exhibit three equivalent epitaxial domains having an orthorhombic structure resulting from monoclinic distortion of the perovskite cubic cell. An excitation of 286.5 nm from Gd³⁺ and energy transfer to Ce³⁺ with 405 nm emission are observed, which represents the potential for high-energy conversion. These experimental results also offer the pathway to tunable optical properties of high-entropy rare-earth epitaxial perovskite films for a range of applications.

Lanthanide-doped nanoscintillators

Lanthanide-doped nanoscintillators are taking the lead in several important fields including radiation detection, biomedicine, both at the level of diagnosis and therapy, and information encoding.

Design Principles of Hybrid Nanomaterials for Radiotherapy Enhanced by Photodynamic Therapy

Radiation (RT) remains the most frequently used treatment against cancer. The main limitation of RT is its lack of specificity for cancer tissues and the limited maximum radiation dose that can be safely delivered without damaging the surrounding healthy tissues. A step forward in the development of better RT is achieved by coupling it with other treatments, such as photodynamic therapy (PDT). PDT is an anti-cancer therapy that relies on the light activation of non-toxic molecules—called photosensitizers—to generate ROS such as singlet oxygen. By conjugating photosensitizers to dense nanoscintillators in hybrid architectures, the PDT could be activated during RT, leading to cell death through an additional pathway with respect to the one activated by RT alone. Therefore, combining RT and PDT can lead to a synergistic enhancement of the overall efficacy of RT. However, the involvement of hybrids in combination with ionizing radiation is not trivial: the comprehension of the relationship among RT, scintillation emission of the nanoscintillator, and therapeutic effects of the locally excited photosensitizers is desirable to optimize the design of the hybrid nanoparticles for improved effects in radio-oncology. Here, we discuss the working principles of the PDT-activated RT methods, pointing out the guidelines for the development of effective coadjutants to be tested in clinics.