Metal-free perovskites have arisen great interest for their chemical diversity and eco-friendly properties, and recently expanded into X-ray detection for superior carrier behavior.  However, the size and shape complexity of organic components results in the difficulties on evaluating their stability from theoretical considerations in high-energy radiation. Herein, we introduce multiple hydrogen-bonds metal-free PAZE-NH4X3·H2O perovskite, where H2O leads to more hydrogen bonds appearing between organic molecules and perovskite host. Suggested by the theoretical calculations, multiple hydrogen bonds promote the stiffness of lattice, and increase the diffusion barrier to inhibit ionic migration. Then, low trap density, high μτ products and structural flexibility of PAZE-NH4Br3·H2O impel us to expand it into the flexible X-ray detector with the highest sensitivity of 3708 μC Gyair-1 cm-2, ultra-low detection limit of 0.19 μGyair-1 s-1 and superior spatial resolution of 5.0 lp mm-1.

Metal-Free PAZE-NH4X3·H2O Perovskite for Flexible Transparent X-ray Detection and Imaging.

The detection of ionizing radiation such as X-ray, γ-ray, α-particle, and neutrons has been widely required in medical and industrial areas. Perovskite semiconductor detectors are arousing widespread interest and achieving significant progresses in this area. Unique material strengths, such as large attenuation coefficient, defect tolerant nature, balanced carrier mobilities, and feasible preparation techniques, make perovskite semiconductor detector a strong competitor to existing commercial products. Inspired by the booming development of this area, we review the current achievements of ionizing radiation detection with perovskite semiconductors. Firstly, a comprehensive summary of the device preparation techniques is provided, followed by a profound discussion of the detector physics. Then, the broad applications of perovskite semiconductor detectors are illustrated. Finally, we give an outlook of the current challenges pending to be tackled for commercialized applications. We believe the perovskite semiconductor detectors will be new blood to the current ionizing radiation detection market. 

Perovskite semiconductors for ionizing radiation detection

Metal halide perovskites are promising for direct X-ray detection applications thanks to their high X-ray absorption capability and excellent charge carrier transport. Single-pixelated perovskite X-ray detectors have hitherto demonstrated excellent device performance, including high sensitivity and detectivity. On the other hand, multipixel flat-panel detectors (FPDs) that enable 2D X-ray imaging are undeveloped due to challenges in scalable deposition processes. In this perspective, we summarize promising scalable perovskite deposition processes and discuss the challenges and opportunities toward direct X-ray FPD applications. 

Scalable Fabrication of Metal Halide Perovskites for Direct X-ray Flat-Panel Detectors: A Perspective

Exploring halogen engineering is of great significance for reducing the density of defect states in crystals of organic-inorganic hybrid perovskites and hence improving the crystal quality. Herein, high-quality single crystals of PEA2PbX4 (X = Cl, Br, I) and their para-F (p-F) substitution analogs are prepared using the facile solution method to study the effects of both p-F substitution and halogen anion engineering. After p-F substitution, the triclinic PEA2PbX4 (X = Cl, Br) and cubic PEA2PbX4 (X = I) crystals unifies to monoclinic crystal structure for p-F-PEA2PbX4 (X = Cl, Br, I) crystals. The p-F substitution and halogen engineering, together with crystal structure variation, enable the tunability of optoelectrical properties. Experimentally, after the p-F substitution, the energy levels are lowered with increased Fermi levels, and the bandgaps of p-F-PEA2PbX4 (X = Cl, Br, I) are slightly reduced. Benefitting from the enhancement of the charge transfer and the reduced trap density by p-F substitution and halogen anion engineering, the average carrier lifetime of the p-F-PEA2PbX4 is obviously reduced. Compared with PEA2PbI4, the X-ray detector based on p-F-PEA2PbI4 perovskite single-crystal has a higher sensitivity of 119.79 μC Gyair−1·cm−2. Moreover, the X-ray detector based on p-F-PEA2PbI4 single crystals exhibits higher radiation stability under high-dose X-ray irradiation, implying long-term operando stability.

https://downloads.spj.sciencemag.org/research/2022/9768019.pdf

Low Trap Density Para-F Substituted 2D PEA2PbX4 (X = Cl, Br, I) Single Crystals with Tunable Optoelectrical Properties and High Sensitive X-Ray Detector Performance

Intrinsically high attenuation of X‐rays and excellent optoelectronic properties endow Cs3Bi2I9 with great promise for high‐performance X‐ray detectors. However, preparation of high quality Cs3Bi2I9 single crystal with large area and millimeter‐thick remains challenging and time‐consuming. A size scalable growth technology, a mechanical sintering process, which is applicable for the integration in image detectors is proposed in this work. Compared with the Cs3Bi2I9 wafer, the Cs3Bi2I9/MXene composite wafer exhibits higher carrier transfer efficiency and mobility‐lifetime product through incorporation of Ti3C2Tx MXene multilayer nanoflakes (MXene nanoflakes). The Cs3Bi2I9/MXene composite wafer X‐ray detector shows an excellent linearity of X‐ray response, a fast response speed, and a low limit of detection of 231 nGyair s−1. The sensitivity of 368 µC Gyair−1 cm−2 under 120 V mm−1 is achieved, which is 15.4% enhanced over the Cs3Bi2I9 wafer detector. The sensitivity of Cs3Bi2I9/MXene wafer device enhances ≈20–40% at low applied voltage regime. In addition, the Cs3Bi2I9/MXene wafer detector exhibits excellent operational stability after storage for 12 months in ambient air. This work provides an alternative approach for realization of next‐generation high‐performance perovskite X‐ray detectors.

Enhanced Carrier Transport in X‐Ray Detector Based on Cs3Bi2I9/MXene Composite Wafers

Halide perovskites are promising candidates for soft X-ray detection (<80 keV) owing to their high X-ray absorption coefficient, resistivity, and mobility lifetime product. However, the lack of large high-quality single crystals (SCs) renders it challenging to manufacture robust hard X-ray imaging systems (>100 keV) with a low detection limit and stable dark current. Herein, high-quality inch-size two-dimensional (2D) Cs3Bi2Br9 (CBB) single crystals are grown from a melt via the Bridgman method. The crystal quality is enhanced by eliminating inclusions of CsBr-rich phases and restraining the trap-state density, leading to an enhanced resistivity of 1.41 × 1012 Ω cm and a mobility lifetime product of 8.32 × 10-4 cm2 V-1. The Au/CBB/Au single-crystal device exhibits a high sensitivity of 1705 μC Gyair-1 cm-2 in all-inorganic bismuth-based perovskites and an ultralow detection limit of 0.58 nGyair s-1 in all of the bismuth-based perovskites for 120 keV hard X-ray detection. The CBB detector exhibits high work stability with an ultralow dark current drift of 2.8 × 10-10 nA cm-1 s-1 V-1 and long-term air environment reliability under a high electric field of 10 000 V cm-1 owing to the ultrahigh ionic activation energy of the 2D structure. The proposed robust imaging system based on CBB SC is a promising tool for X-ray medical imaging and diagnostics.

Ultralow Detection Limit and Robust Hard X-ray Imaging Detector Based on Inch-Sized Lead-Free Perovskite Cs3Bi2Br9 Single Crystals.

Metal halide perovskite single crystals are promising for diverse optoelectronic applications due to their outstanding properties. In comparison to the bulk, the crystal surface suffers from high defect density and is moisture sensitive; however, surface modification strategies of perovskite single crystals are relatively deficient. Herein, solar cells based on methylammonium lead triiodide (MAPbI3) thin single crystals are selected as a prototype to improve single‐crystal perovskite devices by surface modification. The surface trap passivation and protection against moisture of MAPbI3 thin single crystals are achieved by one bifunctional molecule 3‐mercaptopropyl(dimethoxy)methylsilane (MDMS). The sulfur atom of MDMS can coordinate with bare Pb2+ of MAPbI3 single crystals to reduce surface defect density and nonradiative recombination. As a result, the modified devices show a remarkable efficiency of 22.2%, which is the highest value for single‐crystal MAPbI3 solar cells. Moreover, MDMS modification mitigates surface ion migration, leading to enhanced reverse‐bias stability. Finally, the cross‐link of silane molecules forms a protective layer on the crystal surface, which results in enhanced moisture stability of both materials and devices. This work provides an effective way for surface modification of perovskite single crystals, which is important for improving the performance of single‐crystal perovskite solar cells, photodetectors, X‐ray detectors, etc.

Mitigating Surface Deficiencies of Perovskite Single Crystals Enables Efficient Solar Cells with Enhanced Moisture and Reverse‐Bias Stability

Annealing Modification and defect analysis of Hg2Br2 crystal

The investigation of the anisotropic X-ray detection performance of bulk CsPbBr3 single crystals grown by the vertical Bridgman method.

Anisotropic X-ray detection performance of melt-grown CsPbBr3 single crystals

BACKGROUND Guillain-Barré syndrome (GBS) is an autoimmune-mediated peripheral neuropathy characterized by symmetric weakness. Asymmetric weakness in GBS is uncommon and may be easily confused with other differential diagnoses. We herein present three cases of asymmetric GBS and review the literature on this atypical subtype of GBS in order to describe the characteristics of asymmetric GBS and to provide experience for clinicians. CASE SUMMARY Different from patients in the previous reports, our patients showed persistent asymmetric limb weakness from the onset to recovery phase. All three patients were serologically positive for antecedent infections. Two of the three cases had IgG antibodies against ganglioside GM1. Two patients received immunotherapy including intravenous immunoglobulin and plasma exchange, while one patient received only supportive treatment. Autoantibodies against gangliosides, asymmetry of congenital development of blood-nerve barrier and limb use may contribute to the development of asymmetric limb weakness in GBS. CONCLUSION Asymmetric GBS may be a rare clinical variant and should be considered when a patient develops acute and progressive asymmetric limb weakness. The differences in clinical features and prognosis between asymmetric GBS and classic GBS deserve further investigation in a large study.

https://doi.org/10.12998/wjcc.v10.i6.1896

Guodong Zhang

Papers

Ultralow Detection Limit and Robust Hard X-ray Imaging Detector Based on Inch-Sized Lead-Free Perovskite Cs3Bi2Br9 Single Crystals.

Mitigating Surface Deficiencies of Perovskite Single Crystals Enables Efficient Solar Cells with Enhanced Moisture and Reverse‐Bias Stability

Annealing Modification and defect analysis of Hg2Br2 crystal

Anisotropic X-ray detection performance of melt-grown CsPbBr3 single crystals

Asymmetric limb weakness in Guillain-Barré syndrome: Three case reports