Effect of repeated x-ray exposure on the resolution of amorphous selenium based x-ray imagers

Monolithic integration of perovskite heterojunction on TFT backplanes through vapor deposition for sensitive and stable x-ray imaging

Metal halide perovskites exhibit substantial potential for advancing next-generation x-ray detection. However, fabricating high-performance pixelated imaging arrays remains challenging due to the substantial dark current density and stability issues associated with common organic-inorganic hybrid perovskites. Here, we develop a vapor deposition method to create the first all-inorganic perovskite heterojunction film. The heterojunction introduction effectively reduces the dark current density of detectors to about 0.8 nA·cm-2, satisfying thin-film transistor (TFT) integration standards, while also increases sensitivity to above 2.6 × 104 μC·Gyair-1·cm-2, thus giving rise to a record low detection limit of <1 nGyair·s-1 among all polycrystalline perovskite-based x-ray detectors. The devices also demonstrate remarkable stability across multifarious demanding working conditions. Last, through monolithic integration of the heterojunction film with a 64 × 64 pixelated TFT array, we have achieved high-resolution real-time x-ray imaging, which paves the way for the application of all-inorganic perovskite in low-dose flat-panel x-ray detection.

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.

All-Inorganic Manganese-Based CsMnCl3 Nanocrystals for X-Ray Imaging

Lead-based halide perovskite nanomaterials with excellent optical properties have aroused great attention in the fields of solar cells, light-emitting diodes, lasing, X-ray imaging, etc. However, the toxicity of lead prompts researchers to develop alternatives to cut down the usage of lead. Herein, all-inorganic manganese-based perovskite derivatives, CsMnCl₃ nanocrystals (NCs), with uniform size and morphology have been synthesized successfully via a modified hot-injection method. These NCs have a direct bandgap of 4.08 eV and a broadband emission centered at 660 nm. Through introducing modicum lead (1%) into the CsMnCl₃ NCs, the photoluminescence intensity greatly improves, and the quantum yield (PLQY) increases from 0.7% to 21%. Furthermore, the CsMnCl₃ :1%Pb NCs feature high-efficiency of X-ray absorption and radioluminescence, which make these NCs promising candidates for X-ray imaging.

Effects of Charge Carrier Trapping on Image Resolution of Multilayer Photoconductive Detectors: Application to Amorphous Selenium X-ray Detectors

The effects of charge carrier trapping on image resolution in multilayer photoconductive imaging detectors was analyzed by developing an analytical model for calculating the modulation transfer function (MTF) of the imaging detectors. The MTF model was developed by considering the charge carrier trapping/recombination in the blocking layer and the distributed trapping in the bulk photoconductor layer of multilayer photoconductive imaging detectors. The relative importance of the bulk and blocking layer trapping, as well as the K-fluorescence reabsorption on the resolution, was also analyzed. The model was applied to amorphous selenium- (a-Se) based multilayer detectors for both chest radiographic and mammographic applications. The theoretical model agreed well with experimental results. The charge carrier trapping/recombination in the blocking layer and K-fluorescence reabsorption mainly controlled the resolution of the a-Se mammographic detectors. In addition to these two effects, the bulk trapping also had a significant effect on the resolution of chest radiographic detectors. The blocking layer thickness had more of a profound effect on the resolution than the amount of trap centers in the blocking layer. The resolution deteriorated drastically with increased blocking layer thickness.

Highly efficient copper halide scintillators for high-performance and dynamic X-ray imaging

Progress towards high performance X-ray detection and dynamic imaging applications, including nondestructive inspection, homeland security, and medical diagnostics, requires scintillators with a high light yield, a reasonable decay time, low cost, and eco-friendliness. Recently, copper halide scintillators have drawn tremendous attention due to their outstanding radioluminescence performance. Here, we first employed β-Cs₃Cu₂Cl₅ as a high-performance scintillator, with a photoluminescence quantum yield (PLQY) of 94.6%, a radioluminescence light yield of 34 000 ± 4000 photons per MeV, a low detection limit of 81.7 nGy_air s^-1, and good operational stability under a total X-ray dose of 174.6 Gy_air in air. In addition, this scintillator presents a high spatial resolution of 9.6 lp mm^-1 at the modulation transfer function of 0.2 and a superb performance at 60 frames per second in our X-ray imaging system. Overall, this highly efficient scintillator demonstrates outstanding comprehensive performance and shows great potential for broad applications in X-ray detection and dynamic imaging.

Low-dose real-time X-ray imaging with nontoxic double perovskite scintillators

X-rays are widely used in probing inside information nondestructively, enabling broad applications in the medical radiography and electronic industries. X-ray imaging based on emerging lead halide perovskite scintillators has received extensive attention recently. However, the strong self-absorption, relatively low light yield and lead toxicity of these perovskites restrict their practical applications. Here, we report a series of nontoxic double-perovskite scintillators of Cs2Ag0.6Na0.4In1-yBiyCl6. By controlling the content of the heavy atom Bi3+, the X-ray absorption coefficient, radiative emission efficiency, light yield and light decay were manipulated to maximise the scintillator performance. A light yield of up to 39,000 ± 7000 photons/MeV for Cs2Ag0.6Na0.4In0.85Bi0.15Cl6 was obtained, which is much higher than that for the previously reported lead halide perovskite colloidal CsPbBr3 (21,000 photons/MeV). The large Stokes shift between the radioluminescence (RL) and absorption spectra benefiting from self-trapped excitons (STEs) led to a negligible self-absorption effect. Given the high light output and fast light decay of this scintillator, static X-ray imaging was attained under an extremely low dose of ∼1 μGyair, and dynamic X-ray imaging of finger bending without a ghosting effect was demonstrated under a low-dose rate of 47.2 μGyair s-1. After thermal treatment at 85 °C for 50 h followed by X-ray irradiation for 50 h in ambient air, the scintillator performance in terms of the RL intensity and X-ray image quality remained almost unchanged. Our results shed light on exploring highly competitive scintillators beyond the scope of lead halide perovskites, not only for avoiding toxicity but also for better performance.

High-Performance Next-Generation Perovskite Nanocrystal Scintillator for Nondestructive X-Ray Imaging

Readily commercializable and cost-effective next-generation CsPbBr3 perovskite nanocrystals (PNCs) based X-ray detectors are demonstrated. The PNCs-based X-ray detector exhibits higher spatial resolution (9.8 lp mm-1 at modulation transfer function (MTF) = 0.2 and 12.5-8.9 lp mm-1 for a linear line chart), faster response time (≈200 ns), and comparable stability (>40 Gyair s-1 of X-ray exposure) compared with the commercialized terbium-doped gadolinium oxysulfide (GOS)-based detectors (spatial resolution = 6.2 lp mm-1 at MTF = 0.2 and 6.3 lp mm-1 for a linear line chart, response time = ≈1200 ns) because the PNCs-based scintillator has ≈5.6-fold faster average photoluminescence lifetime and stronger emission than the GOS-based one.

Amorphous and polycrystalline photoconductors for direct conversion flat panel x-ray image sensors

In the last ten to fifteen years there has been much research in using amorphous and polycrystalline semiconductors as x-ray photoconductors in various x-ray image sensor applications, most notably in flat panel x-ray imagers (FPXIs). We first outline the essential requirements for an ideal large area photoconductor for use in a FPXI, and discuss how some of the current amorphous and polycrystalline semiconductors fulfill these requirements. At present, only stabilized amorphous selenium (doped and alloyed a-Se) has been commercialized, and FPXIs based on a-Se are particularly suitable for mammography, operating at the ideal limit of high detective quantum efficiency (DQE). Further, these FPXIs can also be used in real-time, and have already been used in such applications as tomosynthesis. We discuss some of the important attributes of amorphous and polycrystalline x-ray photoconductors such as their large area deposition ability, charge collection efficiency, x-ray sensitivity, DQE, modulation transfer function (MTF) and the importance of the dark current. We show the importance of charge trapping in limiting not only the sensitivity but also the resolution of these detectors. Limitations on the maximum acceptable dark current and the corresponding charge collection efficiency jointly impose a practical constraint that many photoconductors fail to satisfy. We discuss the case of a-Se in which the dark current was brought down by three orders of magnitude by the use of special blocking layers to satisfy the dark current constraint. There are also a number of polycrystalline photoconductors, HgI₂ and PbO being good examples, that show potential for commercialization in the same way that multilayer stabilized a-Se x-ray photoconductors were developed for commercial applications. We highlight the unique nature of avalanche multiplication in a-Se and how it has led to the development of the commercial HARP video-tube. An all solid state version of the HARP has been recently demonstrated with excellent avalanche gains; the latter is expected to lead to a number of novel imaging device applications that would be quantum noise limited. While passive pixel sensors use one TFT (thin film transistor) as a switch at the pixel, active pixel sensors (APSs) have two or more transistors and provide gain at the pixel level. The advantages of APS based x-ray imagers are also discussed with examples.