The CRISPR-Cas systems, as exemplified by CRISPR-Cas9, are RNA-guided adaptive immune systems used by bacteria and archaea to defend against viral infection. The CRISPR-Cpf1 system, a new class 2 CRISPR-Cas system, mediates robust DNA interference in human cells. Although functionally conserved, Cpf1 and Cas9 differ in many aspects including their guide RNAs and substrate specificity. Here we report the 2.38 Å crystal structure of the CRISPR RNA (crRNA)-bound Lachnospiraceae bacterium ND2006 Cpf1 (LbCpf1). LbCpf1 has a triangle-shaped architecture with a large positively charged channel at the centre. Recognized by the oligonucleotide-binding domain of LbCpf1, the crRNA adopts a highly distorted conformation stabilized by extensive intramolecular interactions and the (Mg(H2O)6)(2+) ion. The oligonucleotide-binding domain also harbours a looped-out helical domain that is important for LbCpf1 substrate binding. Binding of crRNA or crRNA lacking the guide sequence induces marked conformational changes but no oligomerization of LbCpf1. Our study reveals the crRNA recognition mechanism and provides insight into crRNA-guided substrate binding of LbCpf1, establishing a framework for engineering LbCpf1 to improve its efficiency and specificity for genome editing.

X-ray detectors are extensively utilized, including in medical diagnosis, scientific research, and security screening. So far, X-ray detectors have been developed mainly on the basis of metal-based semiconductors. Recently, in addition to traditional Si, Cd(Zn)Te and Ge, crystals based on metal halide perovskites have emerged as a new generation of semiconductors for radiation detection due to their high-Z elements Pb, Bi, and Br. [1-3] However, the requirements for practical wearable materials to be lightweight, economically inexpensive, and environmentally friendly motivate the exploration for nontoxic, low-cost, and simple organic compounds. [4] Lightweight semiconductors based on conjugated molecules or polymers have been demonstrated in a proof-of-principle manner for direct X-ray detection, including 4-hydroxycyanobenzene (4HCB), 1,8-naphthaleneimide (NTI), 1,5-dinitronaphthalene, and rubrene. [5-7] However, the fabrication of large-scale crystals with exceptionally Metal-free halide perovskites, as a specific category of the perovskite family, have recently emerged as novel semiconductors for organic ferroelectrics and promise the wide chemical diversity of the ABX 3 perovskite structure with mechanical flexibility, light weight, and eco-friendly processing. However, after the initial discovery 17 years ago, there has been no experimental information about their charge transport properties and only one brief mention of their optoelectronic properties. Here, growth of large single crystals of metalfree halide perovskite DABCO-NH 4 Br 3 (DABCO = N-N′-diazabicyclo[2.2.2] octonium) is reported together with characterization of their instrinsic optical and electronic properties and demonstration, of metal-free halide perovskite optoelectronics. The results reveal that the crystals have an unusually large semigap of ≈16 eV and a specific band nature with the valence band maximum and the conduction band minimum mainly dominated by the halide and DABCO 2+ , respectively. The unusually large semigap rationalizes extremely long lifetimes approaching the millisecond regime, leading to very high charge diffusion lengths (tens of µm). The crystals also exhibit high X-ray attenuation as well as being lightweight. All these properties translate to high-performance X-ray imaging with sensitivity up to 173 µC Gy air −1 cm −2. This makes metal-free perovskites novel candidates for the next generation of optoelectronics.

No abstract

Solution‐processed metal‐based halide perovskites have taken a dominant position for perovskite optoelectronics including light emission and X‐ray detection; however, the toxicity of the included heavy metals severely restricts their applications for wearable, lightweight, and transient optoelectronic devices. Here, the authors describe investigations of large (4 × 6 × 2 mm3) 3D metal‐free perovskite MDABCO‐NH4I3 (MDBACO = methyl‐N′‐diazabicyclo[2.2.2]octonium) single crystal and its charge recombination and extraction behavior for light emission and X‐ray detection. Unlike conventional 3D metal‐based perovskites, this lightweight and biocompatible perovskite large crystal is processed from aqueous solution at room temperature, and can achieve both an extremely long carrier lifetime up to ≈1.03 µs and the formation of self‐trapped excited states for luminescence. These features contribute to a photoluminescence quantum yield (PLQY) as high as ≈53% at room temperature and an X‐ray sensitivity up to 1997 ± 80 μC Gy cm−2 at 50 V bias (highest among all metal‐free detectors). The ability to tune the perovskite band gap by modulating the structure under high pressure is also demonstrated, which opens up applications for the crystal as colored emitters. These attributes make it a molecular alternative to metal‐based perovskites for biocompatible and transient optoelectronics.

Even though the perovskite solar cell has been so popular for its skyrocketing power conversion efficiency, its further development is still roadblocked by its overall performance, in particular long-term stability, large-area fabrication and stable module efficiency. In essence, the soft component and ionic–electronic nature of metal halide perovskites usually chaperonage large number of anion vacancy defects that act as recombination centers to decrease both the photovoltaic efficiency and operational stability. Herein, we report a one-stone-for-two-birds strategy in which both anion-fixation and associated undercoordinated-Pb passivation are in situ achieved during crystallization by using a single amidino-based ligand, namely 3-amidinopyridine, for metal-halide perovskite to overcome above challenges. The resultant devices attain a power conversion efficiency as high as 25.3% (certified at 24.8%) with substantially improved stability. Moreover, the device without encapsulation retained 92% of its initial efficiency after 5000 h exposure in ambient and the device with encapsulation retained 95% of its initial efficiency after >500 h working at the maximum power point under continuous light irradiation in ambient. It is expected this one-stone-for-two-birds strategy will benefit large-area fabrication that desires for simplicity.

Ion migration is a key root-cause of photocurrent instability in perovskite X-ray detectors. Although 2D perovskite single crystal (PSC) is a good candidate to suppress ion migration compared with its 3D counterpart, its intrinsic stability still needs to be improved. In this work, it is first envisioned to conquer the ion migration by enhanced chemical bonding; the proposal is further confirmed by density functional theory calculations, in which the bonds are made stronger by introducing a fluorine atom into the ortho position of phenethylamine (o-F-PEA) to shorten the distance between adjacent organic cations in the crystal lattice for enhanced electrostatic interactions between the F atom and the neighboring benzene ring. It is further demonstrated experimentally that the activation energy for ion migration (AEIM) of (o-F-PEA) 2 PbI 4 PSC is increased compared with that of (PEA) 2 PbI 4 PSC. The improved AEIM is also confirmed by enhanced thermal stability. Consequently, the dark current in the (o-F-PEA) 2 PbI 4 2D PSC X-ray detectors is reduced by two times compared with the (PEA) 2 PbI 4 reference. Furthermore, the detector shows the sensitivity of 1724.5 μC Gy air −1 cm −2 at 1250 V mm −1 , and much improved photocurrent stability.

2D Dion−Jacobson (DJ) perovskite single crystals (PSCs) usually demonstrate better X‐ray detection performance than Ruddlesden‐Popper (RP) PSCs. However, the mechanism of the improved performance is still elusive. Here, by the aid of strong interactions between dimethylbiguanide (DGA) and PbI2, a novel DJ‐perovskitoid (DGA)PbI4 is designed. From the comparison of (DGA)PbI4 to other 2D PSCs, it is discovered that the tiniest lattice distortion and increased hydrogen bonds in the atom‐scaled analysis strengthen lattice rigidity and weaken electron‐phonon coupling to suppress disordered scattering of carriers, resulting in significantly improved carrier transport and stability. Therefore, high carrier mobility (78.1 cm2 V−1 s−1) and a pronounced sensitivity of 4869.0 µC Gyair−1cm−2 are achieved using (DGA)PbI4, which are the best in 2D Pb‐based PSC devices to date. Finally, the (DGA)PbI4 devices exhibit good spatial resolution in X‐ray imaging and excellent long‐term stability to work as a promising candidate for medical diagnostics and nondestructive determination.

Modulating lasing wavelength flexibly and repeatedly on a single rod is essential to the practical applications of micro/nanorod lasers. In this paper, a structure that decouples the gain medium and optical cavity is proposed, where the corresponding mechanism for the lasing wavelength shift is explained. Based on the above structure, one kind of wavelength continuously variable lasers is achieved on a single GaN/InGaN core-shell microrod without modifying the geometry of the resonant cavity or cutting the microrod. By using this method, lasing wavelength can be modulated from 372 to 408 nm flexibly and repeatedly in a 10 μm facilely synthesized microrod. This approach demonstrates a big application potential in numerous fields consisting of optical telecommunication and environmental monitoring.