Ultrastable 3D cage-like metal-organic frameworks constructed using polydentate cyanurate ligands and their gas adsorption properties.

Stability is a key factor that restricts the practical applications of metal-organic framework (MOF) materials. In this work, we report an ultrastable three-dimensional cage-like MOF, SrCu(HC3N3O3)2, constructed by a polydentate cyanurate ligand and two kinds of different metal nodes. A high ratio of coordination sites in organic ligands, specific coordination of strong acid with a strong base and weak acid with a weak base and double independent completed coordination networks endow SrCu(HC3N3O3)2 with outstanding thermal stability (up to 300 °C) and acid/alkali resistance (pH = 2-14). Moreover, SrCu(HC3N3O3)2 possesses the highest porosity up to 36.7% among cyanuric acid-based MOF materials and exhibits differentiated adsorption of C3H4 (63 cm3 g-1) and C3H6 (51 cm3 g-1). The breakthrough experiment further verified that efficient C3H4/C3H6 separation can be achieved under dynamic conditions by SrCu(HC3N3O3)2.

Dual Resistive Switching Performance Derived from Ionic Migration in Halide Perovskite Based Memory.

Herein, we report an environmentally stable and friendly halide perovskite based resistive random access memory device with an Ag/PMMA/(PMA)2CuBr4/FTO (PMMA = poly(methyl methacrylate); PMA = C6H5CH2NH3) architecture. The device exhibits the coexistence of two bipolar resistive switching modes, including counterclockwise and clockwise switching characteristics. The devices with both switching modes show stable endurance (>100 cycles) and long retention performance (>104 s). By applying a suitable electrical stimulation, the counterclockwise and clockwise switching behaviors are interconvertible. Furthermore, the Au/PMMA/(PMA)2CuBr4/FTO and Ag/(PMA)2CuBr4/FTO devices were fabricated to verify the origin of dual resistive switching behaviors. The similar dual resistive switching behaviors after electroforming processes of three types of memory devices suggest that the interconvertible dual resistive switching characteristics could be attributed to the ionic migration in the (PMA)2CuBr4 perovskite layer.

High-Performance Resistive Random Access Memories Based on Two-Dimensional HAPbI4 Organic-Inorganic Hybrid Perovskite.

Organic-inorganic hybrid perovskites have attracted extensive attention for potential memory applications because of their excellent properties, such as high charge carrier mobility and fast ion migration. Herein, the two-dimensional HAPbI4 perovskite with an octahedral structure and high stability was prepared by a facile solution method. Moreover, the resistive random access memory (RRAM) with the Ag/PMMA/HAPbI4/ITO structure has been successfully fabricated by spin coating and vacuum thermal evaporation. The as-prepared RRAM device based on HAPbI4 demonstrated superior resistive switching performance. The on/off ratio is as high as 105, and the corresponding retention of the device exceeds 10 000 s; furthermore, the RRAM device could be kept stable after being kept in the air for 24 weeks.

Halide Perovskites for Resistive Switching Memory.

Resistive switching random access memory (RRAM), also known as memristor, is regarded as an emerging nonvolatile memory and computing-in-memory technology to address the intrinsic physical limitations of conventional memory and the bottleneck of von Neumann architecture. In particular, halide perovskite RRAMs have attracted widespread attention in recent years because of their ionic migration nature and excellent photoelectric properties. This Perspective first provides a condensed overview of halide perovskite RRAMs based on materials, device performance, switching mechanism, and potential applications. Moreover, this Perspective attempts to detail the challenges, such as the quality of halide perovskite films, the compatible processing of device fabrication, the reliability of memory performance, and clarification of the switching mechanism, and further discusses how the outstanding challenges of halide perovskite RRAMs could be met in future research.

K4Cu3(C3N3O3)2X (X = Cl, Br): strong anisotropic layered semiconductors containing mixed p-p and d-p conjugated π-bonds.

2D materials are gaining more and more interest owing to their promising applications in future electronic industry. Here, two new quasi-2D metal cyanurates, K4Cu3(C3N3O3)2X (X = Cl, Br), were grown and characterized for the first time. They belong to the trigonal P3[combining macron]m1 space group and feature an infinite layer, constructed by p-p conjugation in the (C3N3O3) planar six-membered-rings and d-p conjugation in the N-Cu-N linear chains. Moreover, they are indirect semiconductors with suitable bandgaps of 3.5 eV, locating between g-C3N4 and h-BN. The electronic states and anisotropic optical responses were also studied through theoretical calculations.

"Old dog, new tricks": the lone pair effect inducing divergent optical responses in lead cyanurates containing π-bonds.

The stereochemically active lone pair (SCALP) effect of Pb2+ plays a vital role in enhancing the nonlinear optical response and engineering the band gap of lead-based materials with intriguing properties. Herein, we combined SCALP Pb2+ cations and π-conjugated (HC3N3O3)2- anions into one structure to investigate the tunable optical properties. Two lead cyanurates, namely, Pb3(HC3N3O3)2(OH)2 (I) and its partly cadmium-substituted analog Pb2Cd(HC3N3O3)2(OH)2 (II) were grown by the mild hydrothermal method. They exhibited greatly narrowed bandgaps and enhanced optical anisotropy resulting from the SCALP effect. In addition, divergent optical responses induced from the involved Pb (6s26p2) and Cd (4d105s2) were elaborated by a combination of experimental and theoretical analyses.

Two rare-earth-based quaternary chalcogenides EuCdGeQ4 (Q = S, Se) with strong second-harmonic generation.

Two new rare-earth-based chalcogenides EuCdGeQ4 (Q = S, Se) have been designed and constructed by using Eu2+ and the classical NLO-active SBUs of [CdQ4] and [GeQ4]. They crystallize in a non-centrosymmetric Ama2 (no. 40) space group. Benefiting from the synergistic effects of [GeQ4] and highly distorted [CdQ4] tetrahedra, both compounds possess type-I phase-matching behaviour and large powder second harmonic generation (SHG) effects at 2.09 µm (2.6 and 3.8 × AgGaS2 for sulfide and selenide), as well as large direct band gaps (2.5 eV and 2.25 eV). Besides, they melt congruently at relatively low temperatures (997 °C for EuCdGeS4 and 882 °C for EuCdGSe4), which is suitable for bulk crystal growth by the Bridgman method. In addition, their electronic structures and some optical coefficients are calculated by first-principles.

"Two in one": an unprecedented mixed anion, Ba2(C3N3O3)(CNO), with the coexistence of isolated sp and sp2π-conjugated groups.

The combination of different types of π-conjugated anions into one structure can lead to multifunctional materials with intriguing properties. Herein, we successfully developed a new mixed anion compound, Ba2(C3N3O3)(CNO) (I), containing two types of π-conjugated groups, planar (C3N3O3)3- six-membered rings and linear (CNO)- units, for the first time. The compound I exhibits the wide bandgap of 4.82 eV and high absorption coefficients in the wavelength range of 200-280 nm; this indicates that it is a potential optical detection material in the solar-blind region.

Facile Growth of an Ultraviolet Hydroisocyanurate Crystal with Strong Nonlinearity and a Wide Phase-Matching Region from π-Conjugated (HC3N3O3)2- Groups.

Mixed-alkali-metal hydroisocyanurate nonlinear-optical crystal RbLi(HC3N3O3)·2H2O was grown by a facile aqueous solution method. It exhibits a short phase-matching wavelength (λPM = 239 nm) and a strong second-harmonic-generation response (deff = 2.1KDP) benefitting from the well-aligned planar π-conjugated (HC3N3O3)2- anions, as confirmed by the first-principles calculations. Moreover, solar blind ultraviolet radiation can be reached using the title crystal via a fourth-harmonic-generation technique of a Nd-based laser, i.e., 1064 nm/4 = 266 nm.

Ba2M(C3N3O3)2 (M = Sr, Pb): Band Engineering from p-π Interaction via Homovalent Substitution in Metal Cyanurates Containing Planar π-Conjugated Groups.

The two new metal cyanurates Ba2M(C3N3O3)2 (M = Sr, Pb) were successfully grown by a solid-state cyclotrimerization reaction. The electronic energy bands of Ba2M(C3N3O3)2 are totally divergent in spite of their same structures and similar interlayer distances. Theoretical calculations show the narrowing band gap of Ba2Pb(C3N3O3)2 stems from the strong interaction between Pb 6p orbitals and anti-π orbitals in (C3N3O3)3- groups.

A rich structural chemistry in π-conjugated hydroisocyanurates: layered structures of A2B(H2C3N3O3)4·nH2O (A = K, Rb, Cs; B = Mg, Ca; n = 4, 10) with high ultraviolet transparency and strong optical anisotropy.

Ultraviolet (UV) transparent birefringent crystals are indeed indispensable for modern optoelectronics and polarizer devices. Herein, mixed alkali/alkali-earth metal hydroisocyanurates A2B(H2C3N3O3)4·nH2O (A = K, Rb, Cs; B = Mg, Ca; n = 4, 10) were synthesized and their thermal, vibrational and optical properties were characterized in detail. Although they crystallize in different crystallographic point groups, all compounds feature quasi-two-dimensional layered structures built by the 2∞[H2C3N3O3]- ribbons through hydrogen bonds, separated by water molecules and cations. Benefiting from the delocalized π-conjugated bonds and planar structural configuration of (H2C3N3O3)- groups, they show a concurrently short ultraviolet absorption edge (λcut-off ≈ 230 nm) and strong optical anisotropy (Δn ≈ 0.37 at 800 nm), twice larger than that of the benchmark UV birefringent crystal calcite.

Parallel Alignment of π-Conjugated Anions in Hydroisocyanurates Enhancing Optical Anisotropy.

Birefringent crystals with optical anisotropy, which are employed to modulate the polarization of light, play a vital role in many fields of the optical industry. In this Communication, two mixed alkali metal hydroisocyanurates, RbLi(H2C3N3O3)2·2H2O (I) and CsLi(H2C3N3O3)2·2H2O (II), were synthesized by a simple aqueous solution method, and they feature layered structures composed of ∞2[H2C3N3O3]- ribbons through hydrogen bonds with the separation of water molecules and alkali cations. They exhibit a simultaneously short ultraviolet cutoff and giant birefringence, resulting from the perfect parallel alignment of π-conjugated (H2C3N3O3)- anions. Interestingly, the first-principles calculations elucidated that the electronic states and optical properties are slightly different owing to the distinct p-π interaction between alkaline metals and the (H2C3N3O3)- group despite the isostructural crystallographic lattice.