into visible light that are further captured and converted into electrical signals by a following photomultiplier. [7][8][9][10][11] Scintillators have been actively utilized for radiation detection applications in many fields, like nondestructive inspection, medical imaging, and space exploration. Scintillator-based X-ray detectors are advantageous in terms of cost and stability than direct X-ray detectors (a-Se), and the current market of X-ray detectors is dominated by scintillators.The light yield of scintillators, as one of the most important figures of merit, determines the X-ray conversion efficiency and detection contrast. Liu and co-workers reported the good X-ray imaging properties from CsPbBr 3 nanocrystals [8] and Zhang et al. evaluated the light yield for CsPbBr 3 nanocrystals as 21 000 photons per MeV. [11] Such value is still much lower than traditional scintillators like Lu 1.8 Y 0.2 SiO 5 -Ce (LYSO, 33 200 photons per MeV), [12] CsI-Tl (54 000 photons per MeV) [12] and Gd 2 O 2 S-Tb (GOS, 60 000 photons per MeV) [13] etc. The major reason is that the small Stokes shift and the self-absorption effect for lead halide perovskites would severely restrict the light outcoupling efficiency in films and crystals, which require large thickness for complete X-ray attenuations. For scintillators, large Stokes shift and high photoluminescence efficiency are required to obtain high scintillation light yield. The recently emerged self-trapped exciton emissions from low dimensional perovskites exhibit large stokes shift and high PLQY, and may provide efficient X-ray scintillations, but have scarcely been studied. [14][15][16] Another severe issue restricting the applications of lead halide perovskite scintillators is the toxicity of lead element. The ionic nature of halide perovskites and high solubility in water may seriously harm the human health as well as the environment. It is thus of great significance to find lead-free perovskites or halide scintillators.Here we present 1D structured Rb 2 CuBr 3 as one new member of scintillators with exceptionally high light yield. Rb 2 CuBr 3 is obtained by direct reaction between RbBr and CuBr with phase-purity, high quality, and good stability. Its 1D crystal structure and soft crystal lattice facilitate the formation of self-trapped exciton, which emits at 385 nm with a large Stokes shift of 85 nm (0.91 eV) and 98.6% photoluminescence quantum yield. The high emission efficiency, large Stokes shift, strong X-ray attenuation, and good spectrum matching with the photomultiplier tube (PMT) or silicon photomultiplier Scintillators are widely utilized for radiation detections in many fields, such as nondestructive inspection, medical imaging, and space exploration. Lead halide perovskite scintillators have recently received extensive research attention owing to their tunable emission wavelength, low detection limit, and ease of fabrication. However, the low light yields toward X-ray irradiation and the lead toxicity of these perovskites severely restricts their practical ...

Se; [8] Park and co-workers reported sensitivities of CH 3 NH 3 PbI 3 X-ray detectors as high as 11 000 µC Gy air −1 cm −2 . [9] On the other hand, inorganic lead halide perovskites have demonstrated high carrier mobilities, good stability, and low ionic migrations compared to organic-inorganic hybrid perovskite [11] and, therefore, provide higher detection sensitivity toward X-rays; however, they have rarely been studied as X-ray detectors.Another challenge is the fabrication of a thick perovskite film with controlled orientations. A thickness of hundreds of micrometers to millimeters is the prerequisite for complete X-ray attenuation. The uniform orientation of perovskite films is beneficial for charge transport and collection along the electric field direction, thereby requiring a quasi-monocrystalline film. Here, we define a quasi-monocrystalline as the crystal with the same orientation and without grain boundaries along the vertical direction. Current perovskite film-based X-ray detectors face the issue of random orientation and, thus, inefficient charge collection. [9] A new method to fabricate thick quasimonocrystalline perovskite films for X-ray detection is urgently needed.For the first time, here, we employ a hot-pressing method to fabricate thick quasi-monocrystalline inorganic perovskite CsPbBr 3 films. The thickness of the film reaches hundreds of micrometers, which guarantees complete X-ray attenuation. The good orientation of the films is tuned to promote carrier transport close to that of the single-crystal samples. Most importantly, as-fabricated CsPbBr 3 X-ray detectors demonstrate a high sensitivity of 55 684 µC Gy air −1 cm −2 as a result of high carrier mobility, large µτ product, and photoconductive gain effect. Additionally, CsPbBr 3 detectors also exhibit relatively fast response speed, negligible baseline drift, and good stability, which together make the CsPbBr 3 X-ray detector extremely competitive in high-contrast X-ray detection and imaging applications.Currently, the synthesis of high-quality millimeter-thick perovskite films is still a great challenge, because the typical solution-based process often results in many pinholes when the solvent is evaporated out of the film. [9] Here, we propose and demonstrate a hot-pressing method to obtain thick perovskite films. The hot-pressing method avoids the use of any solvent, An X-ray detector with high sensitivity would be able to increase the generated signal and reduce the dose rate; thus, this type of detector is beneficial for applications such as medical imaging and product inspection. The inorganic lead halide perovskite CsPbBr 3 possesses relatively larger density and a higher atomic number in contrast to its hybrid counterpart. Therefore, it is expected to provide high detection sensitivity for X-rays; however, it has rarely been studied as a direct X-ray detector. Here, a hot-pressing method is employed to fabricate thick quasi-monocrystalline CsPbBr 3 films, and a record sensitivity of 55 684 µC Gy air −1 cm −2 is achieved, surpas...

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Metal halide perovskites and derivatives exhibit a high sensitivity and low detection limit as direct X‐ray detectors. Inorganic 2D bismuth halide perovskites are promising for X‐ray detections, but have not been reported. Moreover, the quantitative relationship between the structural dimensionality of A3B2X9 perovskites and their compositions has never been investigated, and the underlying mechanism is unclear. Here, the key structural descriptors for 2D A3B2X9 perovskite derivatives are reported: i) octahedral factor μ, 0.377 < μ < 0895; ii) tolerance factor t, 0.8 < t < 1.06; iii) (rA‐0.55)/t < 1.48 Å. Accordingly, a new 2D A3B2X9 perovskite derivative, Rb3Bi2I9, with high X‐ray attenuation coefficients is found. The assembled X‐ray detector exhibits a high μτ product of 2.51 × 10−3 cm2 V−1, good sensitivity for 159.7 μC Gyair−1 cm−2, and a record low detection limit of 8.32 nGyair s−1 among all direct and indirect perovskite X‐ray detectors. The device also exhibits good stability toward external bias and continuous gamma ray radiations (480 000 Gy). This work provides crystal structural insights to rationally design 2D perovskites for new types of radiation detectors.

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Lead halide perovskites have recently shown great potential as X-ray scintillators; however, the toxicity of the lead element seriously restricts their applications. Herein we report a new lead-free and self-absorption-free scintillator based on Rb 2 CuCl 3 metal halide. The Rb 2 CuCl 3 exhibits a near-unity photoluminescence quantum yield (99.4%) as well as a long photoluminescence lifetime (11.3 μs). Furthermore, Rb 2 CuCl 3 demonstrates an appreciable light yield of 16 600 photons per megaelectronvolt and a large scintillation response with a linear range from 48.6 nGy air s −1 to 15.7 μGy air s −1 . Notably, the detection limit is as low as 88.5 nGy air s −1 , enabling a reduced radiation dose to the human body when a medical and security check is conducted. In addition, Rb 2 CuCl 3 exhibits good stability against the atmosphere, continuous ultraviolet light, as well as X-ray irradiation. The combination of the decent scintillation performance, low toxicity and good stability suggests the Rb 2 CuCl 3 could be a possible promising X-ray scintillator.

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Traditional password based authentication schemes are mostly considered in single server environments. They are unfitted for the multi-server environments from two aspects. On the one hand, users need to register in each server and to store large sets of data, including identities and passwords. On the other hand, servers are required to store a verification table containing user identities and passwords. Recently, On the base on Sood et al.'s protocol(2011), Li et al. proposed an improved dynamic identity based authentication and key agreement protocol for multi-server architecture(2012). Li et al. claims that the proposed scheme can make up the security weaknesses of Sood et al.'s protocol. Unfortunately, our further research shows that Li et al.'s protocol contains several drawbacks and can not resist some types of known attacks, such as replay attack, Deny-of-Service attack, internal attack, eavesdropping attack, masquerade attack, and so on. In this paper, we further propose a light dynamic pseudonym identity based authentication and key agreement protocol for multi-server architecture. In our scheme, service providing servers don't need to maintain verification tables for users. The proposed protocol provides not only the declared security features in Li et al.'s paper, but also some other security features, such as traceability and identity protection.