Halide perovskites scintillators: unique promise and current limitations

One-Dimensional Copper-Doped Rb2AgI3 with Efficient Sky-Blue Emission as a High-Performance X-ray Scintillator

Scintillators have garnered heightened attention for their diverse applications in medical imaging and security inspection. Nonetheless, commercial scintillators encounter challenges with costly rare-earth metals and toxic elements like thallium (Tl), driving the need for sustainable, cost-effective, and eco-friendly alternatives to meet contemporary X-ray detection demands. This study focuses on exploring the potential of Cu+-doped Rb2AgI3 as an effective metal halide (MH) scintillator. One-dimensional (1D) Rb2AgI3 and Cu+-doped Rb2AgI3 single crystals (SCs) were synthesized by using the conventional temperature-lowering crystallization method. When excited by UV light, Cu+-doped SCs emitted a broad sky-blue light at 490 nm with a high photoluminescence (PL) quantum yield (PLQY) of 76.48%. Remarkably, under X-ray excitation, these Cu+-doped SCs demonstrated an outstanding light yield of 36,293 photons MeV-1, a relatively low detection threshold of 1.022 μGyair s-1, and a rapid scintillation decay time of 465 ns. The prepared translucent scintillation film has good uniformity and flexibility, with a high spatial resolution of 10.2 lp mm-1. These results position Cu+-doped Rb2AgI3 as a leading candidate among promising X-ray scintillators, offering superior scintillation light yield, excellent stability, and nontoxicity.

Bright and durable scintillation from colloidal quantum shells

Efficient, fast, and robust scintillators for ionizing radiation detection are crucial in various fields, including medical diagnostics, defense, and particle physics. However, traditional scintillator technologies face challenges in simultaneously achieving optimal performance and high-speed operation. Herein we introduce colloidal quantum shell heterostructures as X-ray and electron scintillators, combining efficiency, speed, and durability. Quantum shells exhibit light yields up to 70,000 photons MeV-1 at room temperature, enabled by their high multiexciton radiative efficiency thanks to long Auger-Meitner lifetimes (>10 ns). Radioluminescence is fast, with lifetimes of 2.5 ns and sub-100 ps rise times. Additionally, quantum shells do not exhibit afterglow and maintain stable scintillation even under high X-ray doses (>109 Gy). Furthermore, we showcase quantum shells for X-ray imaging achieving a spatial resolution as high as 28 line pairs per millimeter. Overall, efficient, fast, and durable scintillation make quantum shells appealing in applications ranging from ultrafast radiation detection to high-resolution imaging.

Metal-Halide Perovskite Submicrometer-Thick Films for Ultra-Stable Self-Powered Direct X-Ray Detectors

Metal-halide perovskites are revolutionizing the world of X-ray detectors, due to the development of sensitive, fast, and cost-effective devices. Self-powered operation, ensuring portability and low power consumption, has also been recently demonstrated in both bulk materials and thin films. However, the signal stability and repeatability under continuous X-ray exposure has only been tested up to a few hours, often reporting degradation of the detection performance. Here it is shown that self-powered direct X-ray detectors, fabricated starting from a FAPbBr3 submicrometer-thick film deposition onto a mesoporous TiO2 scaffold, can withstand a 26-day uninterrupted X-ray exposure with negligible signal loss, demonstrating ultra-high operational stability and excellent repeatability. No structural modification is observed after irradiation with a total ionizing dose of almost 200 Gy, revealing an unexpectedly high radiation hardness for a metal-halide perovskite thin film. In addition, trap-assisted photoconductive gain enabled the device to achieve a record bulk sensitivity of 7.28 C Gy-1 cm-3 at 0 V, an unprecedented value in the field of thin-film-based photoconductors and photodiodes for "hard" X-rays. Finally, prototypal validation under the X-ray beam produced by a medical linear accelerator for cancer treatment is also introduced.

Halide Perovskites and Their Derivatives for Efficient, High-Resolution Direct Radiation Detection: Design Strategies and Applications

The past decade has witnessed a rapid rise in the performance of optoelectronic devices based on lead-halide perovskites (LHPs). The large mobility-lifetime products and defect tolerance of these materials, essential for optoelectronics, also make them well-suited for radiation detectors, especially given the heavy elements present, which is essential for strong X-ray and γ-ray attenuation. Over the past decade, LHP thick films, wafers, and single crystals have given rise to direct radiation detectors that have outperformed incumbent technologies in terms of sensitivity (reported values up to 3.5 × 106 µC Gyair -1 cm-2 ), limit of detection (directly measured values down to 1.5 nGyair s-1 ), along with competitive energy and imaging resolution at room temperature. At the same time, lead-free perovskite-inspired materials (e.g., methylammonium bismuth iodide), which have underperformed in solar cells, have recently matched and, in some areas (e.g., in polarization stability), surpassed the performance of LHP detectors. These advances open up opportunities to achieve devices for safer medical imaging, as well as more effective non-invasive analysis for security, nuclear safety, or product inspection applications. Herein, the principles behind the rapid rises in performance of LHP and perovskite-inspired material detectors, and how their properties and performance link with critical applications in non-invasive diagnostics are discussed. The key strategies to engineer the performance of these materials, and the important challenges to overcome to commercialize these new technologies are also discussed.

Experimental study of the fast scintillating behavior of ZnO:Ga crystal under MeV electron beam

An experimental study on the scintillating behavior of ZnO:Ga crystals was conducted using a 7-MeV electron accelerator. The ZnO:Ga crystals were grown using both hydrothermal and chemical vapor transport methods. One of the ZnO:Ga crystals grown via the hydrothermal method exhibited a 1.5-ns afterglow time when excited by an MeV electron beam. The ultra-short scintillation was captured using both an ultrafast streak camera and a framed camera. The experimental results also confirm its potential for diagnosing electron beam trains with a repetition rate of hundreds of MHz.

In Situ Intrinsic Self-Healing of Low Toxic Cs2 ZnX4 (X = Cl, Br) Metal Halide Nanoparticles

This study reports on the intrinsic and fast self-healing ability of all inorganic, low-toxic Cs2 ZnX4 (X = Cl, Br) metal halide nanoparticles (NPs) when subjected to local heating by electron beam irradiation in high-resolution transmission electron microscopy (HR-TEM). The local heating induces the creation of nanoshells (NSs) following the template of the corresponding NPs, which are subsequently healed back to their original state within several minutes. Energy dispersive spectroscopy (EDS) and fast Fourier transform (FFT) analysis reveal that the composition, phase, and crystallographic structure of the original NPs are restored during the self-healing process, with a thin crystalline layer observed at the bottom of the NSs acting as the healing template. The inelastic scattering of the electron beam energy generates local heat that causes rapid atomic displacement, resulting in atomic mobility that lowers the density of the material and leads to NS formation. A unique insitu TEM heating stage measurement demonstrates the appearance of identical damage and self-healing to those induced by the electron beam. The NPs exhibit excellent stability under ambient conditions for up to a month, making them suitable for self-healing scintillators and other optoelectronic applications that require atomic-scale stability and healing.

Light Management of Metal Halide Scintillators for High-Resolution X-Ray Imaging

The ever-growing need to inspect matter with hyperfine structures requires a revolution in current scintillation detectors, and the innovation of scintillators is revived with luminescent metal halides entering the scene. Notably, for any scintillator, two fundamental issues arise: Which kind of material is suitable and in what form should the material exist? The answer to the former question involves the sequence of certain atoms into specific crystal structures that facilitate the conversion of X-ray into light, whereas the answer to the latter involves assembling these crystallites into particular material forms that can guide light propagation toward its corresponding pixel detector. Despite their equal importance, efforts are overwhelmingly devoted to improving the X-ray-to-light conversion, while the material-form-associated light propagation, which determines the optical signal collected for X-ray imaging, is largely overlooked. This perspective critically correlates the reported spatial resolution with the light-propagation behavior in each form of metal halides, combing the designing rules for their future development.

Transient High-Harmonic Spectroscopy in an Inorganic-Organic Lead Halide Perovskite

High-harmonic generation is the frequency upconversion of an intense femtosecond infrared laser in a material. In condensed-phase high-harmonic generation, laser-driven currents of coherently excited charge carriers map the electronic structure onto the emitted light. This promises a thus far scarcely explored potential of condensed-phase time-resolved high-harmonic spectroscopy for probing carrier dynamics. Here, we realize this potential and use time-resolved solid-state high-harmonic spectroscopy from a laser-excited methylammonium lead bromide (MAPbBr3) thin film, a key material in perovskite solar cells, for measuring carrier cooling and relaxation on femto- and picosecond time scales. Through comparison with transient absorption, we show the links between carrier dynamics and experimental observables of generated harmonics. By highlighting and understanding the interplay of these dynamics, we demonstrate transient optical control over the emission of solid-state high-harmonic generation in MAPbBr3.

Imaging Light-Induced Migration of Dislocations in Halide Perovskites with 3D Nanoscale Strain Mapping

In recent years, halide perovskite materials have been used to make high-performance solar cells and light-emitting devices. However, material defects still limit device performance and stability. Here, synchrotron-based Bragg coherent diffraction imaging is used to visualize nanoscale strain fields, such as those local to defects, in halide perovskite microcrystals. Significant strain heterogeneity within MAPbBr3 (MA = CH3 NH3 + ) crystals is found in spite of their high optoelectronic quality, and both 〈100〉 and 〈110〉 edge dislocations are identified through analysis of their local strain fields. By imaging these defects and strain fields in situ under continuous illumination, dramatic light-induced dislocation migration across hundreds of nanometers is uncovered. Further, by selectively studying crystals that are damaged by the X-ray beam, large dislocation densities and increased nanoscale strains are correlated with material degradation and substantially altered optoelectronic properties assessed using photoluminescence microscopy measurements. These results demonstrate the dynamic nature of extended defects and strain in halide perovskites, which will have important consequences for device performance and operational stability.

Solvent-antisolvent interactions in metal halide perovskites

The fabrication of metal halide perovskite films using the solvent-engineering method is increasingly common. In this method, the crystallisation of the perovskite layer is triggered by the application of an antisolvent during the spin-coating of a perovskite precursor solution. Herein, we introduce the current state of understanding of the processes involved in the crystallisation of perovskite layers formed by solvent engineering, focusing in particular on the role of antisolvent properties and solvent-antisolvent interactions. By considering the impact of the Hansen solubility parameters, we propose guidelines for selecting the appropriate antisolvent and outline open questions and future research directions for the fabrication of perovskite films by this method.

Appealing perspectives of the structural, electronic, elastic and optical properties of LiRCl3 (R = Be and Mg) halide perovskites: a DFT study

To enhance the effectiveness of materials, we are motivated to investigate lithium-based halide perovskites LiRCl3 (where R = Be and Mg) using first-principles techniques based on density functional theory (DFT), implemented in the WIEN2K code. In this study, the research makes use of the WIEN2K simulation code, employing the plane-wave and self-consistent (PWSCF) approach. The cut-off energy, responsible for distinguishing core and valence states, is established at -6.0 Ry. To guarantee well-converged solutions with 2000 K points, parameters of RMT × Kmax = 7.0 are selected, where RMT represents the smallest muffin-tin radius and Kmax denotes the plane wave cut-off. Convergence is determined to be attained when the overall energy of the system remains unchanged during self-consistent calculations, reaching a threshold of 0.001 Ry. We observe structural stability of these materials using the Birch-Murnaghan fit, tolerance factor and formation energy. The tolerance factor for LiMgCl3 and LiBeCl3 are 1.03 and 0.857, while the formation energy for LiMgCl3 and LiBeCl3 are -7.39 eV and -8.92 eV respectively, confirming these to be stable structurally. We evaluate the electronic properties of the current materials, shedding light on their nature, by using the suggested modified Becke-Johnson potential. It turns out that they are indirect insulators, with calculated band gaps of 4.02 and 4.07 eV for LiMgCl3 and LiBeCl3, respectively. For both materials, we also calculate the density of states (DOS), and our findings regarding the band gap energies are consistent with the band structure. It is observed that both materials exhibit transparency to low-energy photons, with absorption and optical conduction occurring in the UV range. These compounds are mechanically stable, according to the elastic investigation, however LiBeCl3 shows higher resistance to compressive and shear loads as well as resistance to shape change. On the other hand, LiMgCl3 exhibits weaker resistance to changes in volume. Furthermore, we discovered that none of the compounds are entirely isotropic, and specifically, LiMgCl3 and LiBeCl3 are brittle in nature. These materials appear to be potential candidates for use in optoelectronic devices based on our analysis of their optical properties. Our findings may provide comprehensive insight, invoking experimental studies for further investigations.

Highly Efficient, Flexible, and Eco-Friendly Manganese(II) Halide Nanocrystal Membrane with Low Light Scattering for High-Resolution X-ray Imaging

Scintillators enable invisible X-ray to be converted into ultraviolet (UV)/visible light that can be collected using a sensor array and is the core component of the X-ray imaging system. However, combining the excellent properties of high light output, high spatial resolution, flexibility, non-toxicity, and cost effectiveness into a single X-ray scintillator remains a great challenge. Herein, a novel scintillator based on benzyltriphenylphosphonium manganese(II) bromide (BTP₂MnBr₄) nanocrystal (NC) membranes was developed by the in situ fabrication strategy. The long Mn-Mn distance provided by the large BTP cation allows the nonradiative energy dissipation in this manganese(II) halide to be significantly suppressed. As a result, the flexible BTP₂MnBr₄ NC scintillator shows an excellent linear response to the X-ray dose rate, a high light yield of ∼71,000 photon/MeV, a low detection limit of 86.2 nGy_air/s at a signal-to-noise ratio of 3, a strong radiation hardness, and a long-term thermal stability. Thanks to the low Rayleigh scattering associated with the dense distribution of nanometer-scale emitters, light cross-talk in X-ray imaging is greatly suppressed. The impressively high-spatial resolution X-ray imaging (23.8 lp/mm at modulation transfer function = 0.2 and >20 lp/mm for a standard pattern chart) was achieved on this scintillator. Moreover, well-resolved 3D dynamic rendering X-ray projections were also successfully demonstrated using this scintillator. These results shed light on designing efficient, flexible, and eco-friendly scintillators for high-resolution X-ray imaging.

Synthesis, Photoluminescence and Vibrational Properties of Aziridinium Lead Halide Perovskites

Three-dimensional lead halide perovskites are known for their excellent optoelectronic properties, making them suitable for photovoltaic and light-emitting applications. Here, we report for the first time the Raman spectra and photoluminescent (PL) properties of recently discovered three-dimensional aziridinium lead halide perovskites (AZPbX₃, X = Cl, Br, I), as well as assignment of vibrational modes. We also report diffuse reflection data, which revealed an extended absorption of light of AZPbX₃ compared to the MA and FA counterparts and are beneficial for solar cell application. We demonstrated that this behavior is correlated with the size of the organic cation, i.e., the energy band gap of the cubic lead halide perovskites decreases with the increasing size of the organic cation. All compounds show intense PL, which weakens on heating and shifts toward higher energies. This PL is red shifted compared to the FA and MA counterparts. An analysis of the PL data revealed the small exciton binding energy of AZPbX₃ compounds (29-56 meV). Overall, the properties of AZPbX₃ are very similar to those of the well-known MAPbX₃ and FAPbX₃ perovskites, indicating that the aziridinium analogues are also attractive materials for light-emitting and solar cell applications.

Examining computationally the structural, elastic, optical, and electronic properties of CaQCl3 (Q = Li and K) chloroperovskites using DFT framework

This study presents the investigations of structural, elastic, optical, and electronic properties of CaQCl3 (Q = Li and K) chloroperovskites for the first time using the DFT framework. The WIEN2K and IRelast packages are used in which the exchange-correlation potential of the modified Becke-Johnson potential (TB-mBJ) is used for obtaining better results. The optimized crystal structural parameters comprising the lattice constant, optimum volume, ground state energy, bulk modulus, and the pressure derivative of bulk modulus are computed by fitting the primitive unit cell energy versus primitive unit cell volume using the Birch-Murnaghan equation of state. The elastic properties which consist of cubic elastic constants, Poisson's ratio, elastic moduli, anisotropy factor, and the Pugh ratio are computed using the very precise IRelast package incorporated inside WIEN2K. The electronic properties are analyzed from the computation of electronic bands structure and density of states (DOS), and it is concluded that an indirect band gap of 4.6 eV exists for CaLiCl3 and a direct band gap of 3.3 eV for CaKCl3 which confirms that CaLiCl3 is an insulator while CaKCl3 is a wide band gap semiconductor. The analysis of DOS shows that the greater contribution to the conduction band (CB) occurs because of the "Ca" element whereas in the valence band the major contribution is from the "Cl" element. The spectral curves of various parameters of optical properties from 0 eV up to 42 eV incident photon energies are observed and it is found that the CaQCl3 (Q = Li and K) chloroperovskites are optically active having a high absorption coefficient, optical conductivity, optical reflectivity, and energy loss function from 25 eV to 35 eV incident photon energies. The applications of these materials can be deemed to alter or control electromagnetic radiation in the ultraviolet (UV) spectral regions. In summary, the results for selected CaQCl3 (Q = Li and K) chloroperovskites depict that these are important compounds and can be used as scintillators, and energy storage devices, and in many modern electronic gadgets.

Extreme-Ultraviolet Excited Scintillation of Methylammonium Lead Bromide Perovskites

Inorganic-Organic lead halide materials have been recognized as potential high-energy X-ray detectors because of their high quantum efficiencies and radiation hardness. Surprisingly little is known about whether the same is true for extreme-ultraviolet (XUV) radiation, despite applications in nuclear fusion research and astrophysics. We used a table-top high-harmonic generation setup in the XUV range between 20 and 45 eV to photoexcite methylammonium lead bromide (MAPbBr₃) and measure its scintillation properties. The strong absorbance combined with multiple carriers being excited per photon yield a very high carrier density at the surface, triggering photobleaching reactions that rapidly reduce the emission intensity. Concurrent to and in spite of this photobleaching, a recovery of the emission intensity as a function of dose was observed. X-ray photoelectron spectroscopy and X-ray diffraction measurements of XUV-exposed and unexposed areas show that this recovery is caused by XUV-induced oxidation of MAPbBr₃, which removes trap states that normally quench emission, thus counteracting the rapid photobleaching caused by the extremely high carrier densities. Furthermore, it was found that preoxidizing the sample with ozone was able to prolong and improve this intensity recovery, highlighting the impact of surface passivation on the scintillation properties of perovskite materials in the XUV range.

A-site cation influence on the conduction band of lead bromide perovskites

Hot carrier solar cells hold promise for exceeding the Shockley-Queisser limit. Slow hot carrier cooling is one of the most intriguing properties of lead halide perovskites and distinguishes this class of materials from competing materials used in solar cells. Here we use the element selectivity of high-resolution X-ray spectroscopy and density functional theory to uncover a previously hidden feature in the conduction band states, the σ-π energy splitting, and find that it is strongly influenced by the strength of electronic coupling between the A-cation and bromide-lead sublattice. Our finding provides an alternative mechanism to the commonly discussed polaronic screening and hot phonon bottleneck carrier cooling mechanisms. Our work emphasizes the optoelectronic role of the A-cation, provides a comprehensive view of A-cation effects in the crystal and electronic structures, and outlines a broadly applicable spectroscopic approach for assessing the impact of chemical alterations of the A-cation on perovskite electronic structure.

Ytterbium-Doped Cesium Lead Chloride Perovskite as an X-ray Scintillator with High Light Yield

Ytterbium-doped cesium lead halides are quantum cutting materials with exceptionally high photoluminescence quantum yields, making them promising materials as scintillators. In this work, we report ytterbium-doped cesium lead chloride (Yb³⁺:CsPbCl₃) with an X-ray scintillation light yield of 102,000 photons/MeV at room temperature, which is brighter than the current state-of-the-art commercial scintillators. The high light yield was achieved based on a novel method of synthesizing Yb³⁺:CsPbCl₃ powders using water and low-temperature processing. The combination of high light yield and the simple and inexpensive manufacturing method reported in this work demonstrates the great potential of Yb³⁺:CsPbCl₃ for scintillation applications.

Recent advances in lead-free double perovskites for x-ray and photodetection

Over the last decade, lead halide perovskites have attracted significant research attention in the field of photovoltaics, light-emitting devices, photodetection, ionizing radiation detection, etc, owing to their outstanding optoelectrical properties. However, the commercial applications of lead-based perovskite devices are restricted due to the poor ambient stability and toxicity of lead. The encapsulation of lead-based devices can reduce the possible leakage of lead. However, it is hard to ensure safety during large-scale production and long-term storage. Recently, considerable efforts have been made to design lead-free perovskites for different optoelectronic applications. Metal halide double perovskites with the general formula of A₂M^IM^IIIX₆or A₂M^IVX₆could be potentially considered as green and stable alternatives for different optoelectronic applications. In this review article, we focus on the recent progress and findings on lead-free halide double perovskites for x-ray and UV-vis photodetection applications. Lead-free halide double perovskite has recently drawn a great deal of attention for superior x-ray detection due to its high absorption coefficient, large carrier mobility-lifetime product, and large bulk resistance. In addition, these materials exhibit good performance in photodetection in the UV-vis region due to high photocarrier generation and efficient carrier separation. In this review, first, we define the characteristics of lead-free double perovskite materials. The fundamental characteristics and beneficial properties of halide perovskites for direct and indirect x-ray detection are then discussed. We comprehensively review recent developments and efforts on lead-free double perovskite for x-ray detection and UV-vis photodetection. We bring out the current challenges and opportunities in the field and finally present the future outlook for developing lead-free double perovskite-based x-ray and UV-vis photodetectors for practical applications.

Scintillation Response Enhancement in Nanocrystalline Lead Halide Perovskite Thin Films on Scintillating Wafers

Lead halide perovskite nanocrystals of the formula CsPbBr₃ have recently been identified as potential time taggers in scintillating heterostructures for time-of-flight positron emission tomography (TOF-PET) imaging thanks to their ultrafast decay kinetics. This study investigates the potential of this material experimentally. We fabricated CsPbBr₃ thin films on scintillating GGAG:Ce (Gd_2.985Ce_0.015Ga_2.7Al_2.3O₁₂) wafer as a model structure for the future sampling detector geometry. We focused this study on the radioluminescence (RL) response of this composite material. We compare the results of two spin-coating methods, namely the static and the dynamic process, for the thin film preparation. We demonstrated enhanced RL intensity of both CsPbBr₃ and GGAG:Ce scintillating constituents of a composite material. This synergic effect arises in both the RL spectra and decays, including decays in the short time window (50 ns). Consequently, this study confirms the applicability of CsPbBr₃ nanocrystals as efficient time taggers for ultrafast timing applications, such as TOF-PET.

On the Role of Cs4PbBr6 Phase in the Luminescence Performance of Bright CsPbBr3 Nanocrystals

CsPbBr3 nanocrystals have been identified as a highly promising material for various optoelectronic applications. However, they tend to coexist with Cs4PbBr6 phase when the reaction conditions are not controlled carefully. It is therefore imperative to understand how the presence of this phase affects the luminescence performance of CsPbBr3 nanocrystals. We synthesized a mixed CsPbBr3-Cs4PbBr6 sample, and compared its photo- and radioluminescence properties, including timing characteristics, to the performance of pure CsPbBr3 nanocrystals. The possibility of energy transfer between the two phases was also explored. We demonstrated that the presence of Cs4PbBr6 causes significant drop in radioluminescence intensity of CsPbBr3 nanocrystals, which can limit possible future applications of Cs4PbBr6-CsPbBr3 mixtures or composites as scintillation detectors.