HgBr2: an easily growing wide-spectrum birefringent crystal.

Birefringent materials are of great significance to the development of modern optical technology; however, research on halide birefringent crystals with a wide transparent range remains limited. In this work, mercuric bromide (HgBr2) has been investigated for the first time as a promising birefringent material with a wide transparent window spanning from ultraviolet (UV) to far-infrared (far-IR) spectral regions (0.34-22.9 µm). HgBr2 has an exceptionally large birefringence (Δn, 0.235 @ 546 nm), which is 19.6 times that of commercial MgF2. The ordered linear motif [Br-Hg-Br] with high polarizability anisotropy within the molecule is the inherent source of excellent birefringence, making it an efficient building block for birefringent materials. In addition, HgBr2 can be easily grown under mild conditions and remain stable in air for prolonged periods. Studying the birefringent properties of HgBr2 crystals would provide new ideas for future exploration of wide-spectrum birefringent materials.

Efficient Direct X-ray Detection and Imaging Based on a Lead-Free Electron Donor-Acceptor MOF.

Metal-organic frameworks (MOFs) have recently gained extensive attention as potential materials for direct radiation detection due to their strong radiation absorption, long-range order, and chemical tunability. However, it remains challenging to develop a practical MOF-based X-ray direct detector that possesses high X-ray detection efficiency, radiation stability, and environmental friendliness. The integration of donor-acceptor (D-A) pairs into crystalline MOFs is a powerful strategy for the precise fabrication of multifunctional materials with unique optoelectronic properties. Herein, a new lead-free MOF, Cu2I2(TPPA) (CuI-TPPA, TPPA = tris[4-(pyridine-4-yl)phenyl]amine), with a 6-fold interpenetrated structure is designed and synthesized based on the electron donor-acceptor strategy. CuI-TPPA has a large mobility-lifetime (µτ) product of 5.8 × 10-4 cm2 V-1 and a high detection sensitivity of 73.1 µC Gyair-1 cm-2, surpassing that of commercial α-Se detectors. Moreover, the detector remains fairly stable with only a 2% reduction in photocurrent under continuous bias irradiation conditions with a total dose of over 42.83 Gyair. The CuI-TPPA/poly(vinylidene fluoride) flexible composite X-ray detector films are successfully manufactured with different thicknesses. Through multifaceted assessments, the optimal thickness is found with a high detection sensitivity of up to 143.6 µC Gyair-1 cm-2. As proof-of-concept, 11 × 9 pixelated X-ray detectors are fabricated on the same composite film to realize X-ray direct imaging. This work opens up potential applications of MOFs in environmentally friendly and wearable devices for direct X-ray detection and imaging.

Design Strategy for Improving Detection Sensitivity in a Bromoplumbate Photochromic Semiconductor.

Reducing the dark current of photodetectors is an important strategy for enhancing the detection sensitivity, but hampered by the manufacturing cost due to the need for controlling the complex material composition and processing intricate interface. This study reports a new single-component photochromic semiconductor, [(HDMA)4(Pb3Br10)(PhSQ)2]n (1, HDMA = dimethylamine cation, PhSQ = 1-(4-sulfophenyl)-4,4'-bipyridinium), by introducing a redox-active monosubstituted viologen zwitterion into inorganic semiconducting skeleton. It features yellow to green coloration after UV irradiation with the sharply dropping intrinsic conductivity of 14.6-fold, and the photodetection detection sensitivity gain successfully doubles. The reason of decreasing conductivity originates from the increasing the band gap of the inorganic semiconducting component and formation of Frenkel excitons with strong Coulomb interactions, thereby decreasing the concentration of thermally excited intrinsic carriers.

Excellent Nonlinear Optical M[M4 Cl][Ga11 S20 ] (M = A/Ba, A = K, Rb) Achieved by Unusual Cationic Substitution Strategy.

The typical chalcopyrite AgGaQ2 (Q = S, Se) are commercial infrared (IR) second-order nonlinear optical (NLO) materials; however, they suffer from unexpected laser-induced damage thresholds (LIDTs) primairy due to their narrow band gaps. Herein, what sets this apart from previously reported chemical substitutions is the utilization of an unusual cationic substitution strategy, represented by [[SZn4 ]S12 + [S4 Zn13 ]S24 + 11ZnS4 ⇒ MS12 + [M4 Cl]S24 + 11GaS4 ], in which the covalent Sx Zny units in the diamond-like sphalerite ZnS are synergistically replaced by cationic Mx Cly units, resulting in two novel salt-inclusion sulfides, M[M4 Cl][Ga11 S20 ] (M = A/Ba, A = K, 1; Rb, 2). As expected, the introduction of mixed cations in the GaS4 anionic frameworks of 1 and 2 leads to wide band gaps (3.04 and 3.01 eV), which exceeds the value of AgGaS2 , facilitating the improvement of high LIDTs (9.4 and 10.3 × AgGaS2 @1.06 µm, respectively). Furthermore, compounds 1 and 2 exhibit moderate second-harmonic generation intensities (0.84 and 0.78 × AgGaS2 @2.9 µm, respectively), mainly originating from the orderly packing tetrahedral GaS4 units. Importantly, this study demonstrates the successful application of the cationic substitution strategy based on diamond-like structures to provide a feasible chemical design insight for constructing high-performance NLO materials.

Thermally Activated Delayed Fluorescence (TADF)-active Coinage-metal Sulfide Clusters for High-resolution X-ray Imaging.

The study of facile-synthesis and low-cost X-ray scintillators with high light yield, low detection limit and high X-ray imaging resolution plays a vital role in medical and industrial imaging fields. However, the optimal balance between X-ray absorption, decay lifetime and excitonic utilization efficiency of scintillators to achieve high-resolution imaging is extremely difficult due to the inherent contradiction. Here two thermally activated delayed fluorescence (TADF)-actived coinage-metal clusters M6 S6 L6 (M=Ag or Cu) were synthesized by simple solvothermal reaction, where the cooperation of heavy atom-rich character and TADF mechanism supports strong X-ray absorption and rapid luminescent collection of excitons. Excitingly, Ag6 S6 L6 (SC-Ag) displays a high photoluminescence quantum yield of 91.6 % and scintillating light yield of 17420 photons MeV-1 , as well as a low detection limit of 208.65 nGy s-1 that is 26 times lower than the medical standard (5.5 µGy s-1 ). More importantly, a high X-ray imaging resolution of 16 lp/mm based on SC-Ag screen is demonstrated. Besides, rigid core skeleton reinforced by metallophilicity endows clusters M6 S6 L6 strong resistance to humidity and radiation. This work provides a new view for the design of efficient scintillators and opens the research door for silver clusters in scintillation application.

Nonlinear Optical Phosphide CuInSi2P4: The Inaugural Member of Diamond-Like Family I-III-IV2-V4 Inspired by ZnGeP2.

Noncentrosymmetric phosphides have garnered significant attention as promising systems of infrared (IR) nonlinear optical (NLO) materials. Herein, a new quaternary diamond-like phosphide family I-III-IV2-V4 and its inaugural member, namely, CuInSi2P4 (CISP), were successfully fabricated by isovalent and aliovalent substitution based on ZnGeP2. First-principles calculations revealed that CISP has a large NLO coefficient (d14 = 110.8 pm/V), which can be attributed to the well-aligned tetrahedral [CuP4], [InP4], and [SiP4] units. Remarkably, the extremely small thermal expansion anisotropy (0.09) of CISP enables it to exhibit a considerable laser-induced damage threshold (LIDT, 5.0 × AgGaS2@1.06 µm) despite the relatively narrow band gap (0.81 eV). This work improves the chemical diversity of inorganic phosphide and promotes the development of phosphide systems, which may provide valuable perspectives for future exploration of IR NLO materials.

The first polyanion-substitution-driven centrosymmetric-to-noncentrosymmetric structural transformation realizing an excellent nonlinear optical supramolecule [Cd4P2][CdBr4].

Crystallographically, noncentrosymmetricity (NCS) is an essential precondition and foundation of achieving nonlinear optical (NLO), pyroelectric, ferroelectric, and piezoelectric materials. Herein, structurally, octahedral [SmCl6]3- is substituted by the acentric tetrahedral polyanion [CdBr4]2-, which is employed as a templating agent to induce centrosymmetric (CS)-to-NCS transformation based on the new CS supramolecule [Cd5P2][SmCl6]Cl (1), thereby providing the NCS supramolecule [Cd4P2][CdBr4] (2). Meanwhile, this replacement further results in the host 2D ∞2[Cd5P2]4+ layers converting to yield the twisted 3D ∞3[Cd4P2]2+ framework, which promotes the growth of bulk crystals. Additionally, phase 2 possesses well-balanced NLO properties, enabling considerable second-harmonic generation (SHG) responses (0.8-2.7 × AgGaS2) in broadband spectra, the thermal expansion anisotropy (2.30) together with suitable band gap (2.37 eV) primarily leading to the favorable laser-induced damage threshold (3.33 × AgGaS2), broad transparent window, and sufficient calculated birefringence (0.0433) for phase-matching ability. Furthermore, the first polyanion substitution of the supramolecule plays the role of templating agent to realize the CS-to-NCS transformation, which offers an effective method to rationally design promising NCS-based functional materials.

Significant increase of the photoresponse range and conductivity for a chalcogenide semiconductor by viologen coating through charge transfer.

Widening the photoresponse range while enhancing the electrical properties of semiconductors could reduce the complexity and cost of photodetectors or increase the power conversion efficiency of solar cells. Surface doping through charge transfer with organic species is one of the most effective and widely used approaches to achieve this aim. It usually features easier preparation over other doping methods but is still limited by the low physicochemical stability and high cost of the used organic species or low improvement of electrical properties. This work shows unprecedented surface doping of semiconductors with highly stable, easily obtained, and strong electron-accepting viologen components, realizing the significant improvement of both the photoresponse range and conductivity. Coating the chalcogenide semiconductor KGaS2 with dimethyl viologen dichloride (MV) yields a charge-transfer complex (CTC) on the surface, which broadens the photoresponse range by nearly 300 nm and improves the conductivity by 5 orders of magnitude. The latter value surpasses all records obtained by surface doping through charge transfer with organic species.

Photoinduced large magnetic change at room temperature and radical-quenched spin glass in a cyanide-bridged MnII-FeIII compound.

By the coordination assembly of a redox photoactive functional motif and a cyanide-bridged moiety, a cyanide-bridged MnII-FeIII compound with large photoinduced magnetic change at room-temperature due to photoinduced electron transfer was obtanied. This compound also shows unprecedented radical-quenched spin glass in molecule based magnets.

Selectivity Control in the Direct CO Esterification over Pd@UiO-66: The Pd Location Matters.

The selectivity control of Pd nanoparticles (NPs) in the direct CO esterification with methyl nitrite toward dimethyl oxalate (DMO) or dimethyl carbonate (DMC) remains a grand challenge. Herein, Pd NPs are incorporated into isoreticular metal-organic frameworks (MOFs), namely UiO-66-X (X=-H, -NO2 , -NH2 ), affording Pd@UiO-66-X, which unexpectedly exhibit high selectivity (up to 99 %) to DMC and regulated activity in the direct CO esterification. In sharp contrast, the Pd NPs supported on the MOF, yielding Pd/UiO-66, displays high selectivity (89 %) to DMO as always reported with Pd NPs. Both experimental and DFT calculation results prove that the Pd location relative to UiO-66 gives rise to discriminated microenvironment of different amounts of interface between Zr-oxo clusters and Pd NPs in Pd@UiO-66 and Pd/UiO-66, resulting in their distinctly different selectivity. This is an unprecedented finding on the production of DMC by Pd NPs, which was previously achieved by Pd(II) only, in the direct CO esterification.

Bottom-Up Photosynthesis of an Air-Stable Radical Semiconductor Showing Photoconductivity to Full Solar Spectrum and X-Ray.

Single-component semiconductors with photoresponse to full solar spectrum are highly desirable to simplify the device structure of commercial photodetectors and to improve solar conversion or photocatalytic efficiency but remain scarce. This work reports bottom-up photosynthesis of an air-stable radical semiconductor using BiI3 and a photochromism-active benzidine derivative as a photosensitive functional motif. This semiconductor shows photoconductivity to full solar spectrum contributed by radical and non-radical forms of the benzidine derivative. It has also the potential to detect X-rays because of strong X-ray absorption coefficient. This finding opens up a new synthetic method for radical semiconductors and may find applications on extending photoresponsive ranges of perovskites, transition metal sulfides, and other materials.

Cs4 Zn5 P6 S18 I2 : the Largest Birefringence in Chalcohalide Achieved by Highly Polarizable Nonlinear Optical Functional Motifs.

Chalcohalides not only keep the balance between the nonlinear optical (NLO) coefficient and wide band gap, but also provide a promising solution to achieve sufficient birefringence for phase-matching ability in NLO crystals. In this study, a novel chalcohalide, Cs4 Zn5 P6 S18 I2 (1) is successfully synthesized, by incorporating the highly electropositive Cs and the large electronegative I element into the zinc thiophosphate. Its 3D open framework features an edge-shared by distorted [ZnS4 ], ethanol-like [P2 S6 ], and unusual [ZnS2 I2 ] polyhedrons, which is inconsistent with the soft-hard-acids-bases theory. Remarkably, compound 1 simultaneously exhibits the large second-harmonic generation (SHG, 1.1×AgGaS2 , @1.3 µm) and a wide band gap (3.75 eV) toward a high laser-induced damage threshold (16.7×AgGaS2 , @1.06 µm), satisfying the rigorous requirements for a prominent infrared NLO material with concurrent SHG intensity (≥0.5×AGS) and band gap (≥3.5 eV). Moreover, to the best of the knowledge, the experimental result shows that phase 1 has the largest birefringence (0.108, @546 nm) in chalcohalide and meets phase-matching behavior demand originating from the polarizable anisotropy of NLO-functional motifs. This finding may provide great opportunities for designing birefringent chalcohalides.

Highly Sensitive Direct X-Ray Detector Based on Copper(II) Coordination Polymer Single Crystal with Anisotropic Charge Transport.

Anisotropic charge transport plays a pivotal role in clarifying the conductivity mechanism in direct X-ray detection to improve the detection sensitivity. However, the anisotropic photoelectric effect of semiconductive single crystal responsive to X-ray is still lacking of theoretical and experimental proof. The semiconductive coordination polymers (CPs) with designable structures, adjustable functions, and high crystallinity provide a suitable platform for exploring the anisotropic conductive mechanism. Here,the study first reveals a 1D conductive transmission path for direct X-ray detection from the perspective of structural chemistry. The semiconductive copper(II)-based CP 1 single crystal detector exhibits unique anisotropic X-ray detection performance. Along the 1D π-π stacking direction, the single crystal device (1-SC-a) shows a superior sensitivity of 2697.15 µCGyair -1  cm-2 and a low detection limit of 1.02 µGyair  s-1 among CPs-based X-ray detectors. This study provides beneficial guidance and deep insight for designing high-performance CP-based X-ray detectors.

Breaking the bottleneck of simultaneously wide band gap and large nonlinear optical coefficient by a "pore reconstruction" strategy in a salt-inclusion chalcogenide.

The template-based design of the crystal structure is a direct and highly efficient method to achieve optimal nonlinear optical (NLO, meaning second-order NLO) performances. The structural flexibility of porous salt-inclusion chalcogenides (SICs) provides an alternative platform for modulating the enlargement of the band gap (that is generally positive with laser-induced damage threshold) and second harmonic generation (SHG) response simultaneously. By applying the "pore reconstruction" strategy to SIC [K3Cl][Mn2Ga6S12] (1), a new derivative K3Rb3[K3Cl][Li2Mn4Ga12S27] (2) is successfully isolated, which unusually features a heterologous nanopore framework with inner diameters of 8.90 and 9.16 Å. Guided by such a strategy, compound 2 possesses the widest band gap (3.31 eV) among the magnetic NLO chalcogenides; this finding is dominantly attributed to the porous structure and the "dimensional deduction" effect. Moreover, phase 2 displays a remarkable phase-matchable SHG intensity (1.1 × AgGaS2 at the incident laser of 1910 nm) that originated from the oriented alignment of NLO-functional motifs, as well as the rich terminal S atoms in the nanopore structure. Furthermore, the "pore reconstruction" strategy offers an efficient pathway to explore potential NLO candidates with excellent comprehensive performances; in particular, it settles the conflicting issue of enhancing the band gap (>3.0 eV) and SHG intensity (>1.0 × AgGaS2) concurrently.

Improving X-ray Scintillating Merits of Zero-Dimensional Organic-Manganese(II) Halide Hybrids via Enhancing the Ligand Polarizability for High-Resolution Imaging.

Luminescent metal halides have been exploited as a new class of X-ray scintillators for security checks, nondestructive inspection, and medical imaging. However, the charge traps and hydrolysis vulnerability are always detrimental to the three-dimensional ionic structural scintillators. Here, the two zero-dimensional organic-manganese(II) halide coordination complexes 1-Cl and 2-Br were synthesized for improvements in X-ray scintillation. The introduction of a polarized phosphine oxide can help to increase the stabilities, especially the self-absorption-free merits of these Mn-based hybrids. The X-ray dosage rate detection limits reached up to 3.90 and 0.81 µGyair/s for 1-Cl and 2-Br, respectively, superior to the medical diagnostic standard of 5.50 µGyair/s. The fabricated scintillation films were applied to radioactive imaging with high spatial resolutions of 8.0 and 10.0 lp/mm, respectively, holding promise for use in diagnostic X-ray medical imaging.

Nonlinear Optical Sulfides LiMGa8 S14 (M = Rb/Ba, Cs/Ba) Created by Li+ Driven 2D Centrosymmetric to 3D Noncentrosymmetric Transformation.

Cations that can regulate the configuration of anion group are greatly important but regularly unheeded. Herein, the structural transformation from 2D CS to 3D noncentrosymmetric (NCS, which is the prerequisite for second-order NLO effect) is rationally designed to newly afford two sulfides LiMGa8 S14 (M = Rb/Ba, 1; Cs/Ba, 2) by introducing the smallest alkali metal Li+ cation into the interlamination of 2D centrosymmetric (CS) RbGaS2 . The unusual frameworks of 1 and 2 are constructed from C2 -type [Ga4 S11 ] supertetrahedrons in a highly parallel arrangement. 1 and 2 display distinguished NLO performances, including strong phase-matchable second-harmonic generation (SHG) intensities (0.8 and 0.9 × AgGaS2 at 1910 nm), wide optical band gaps (3.24 and 3.32 eV), and low coefficient of thermal expansion for favorable laser-induced damage thresholds (LIDTs, 4.7, and 7.6 × AgGaS2 at 1064 nm), which fulfill the criteria of superior NLO candidates (SHG intensity >0.5 × AGS and band gap >3.0 eV). Remarkably, 1 and 2 melt congruently at 873.8 and 870.5 °C, respectively, which endows them with the potential of growing bulk crystals by the Bridgeman-Stockbarge method. This investigated system provides a new avenue for the structural evolution from layered CS to 3D NCS of NLO materials.

Open honeycomb frameworks of sulphides AHg4Ga5S12 (A = Rb, Cs) exhibiting infrared nonlinear optical properties.

A crystal structure with a diamond-like anionic framework belongs to a non-centrosymmetric macrostructure due to the aligned arrangement of tetrahedral units, meeting the premise of second-order nonlinear optical (NLO) materials. Herein, two new Hg-based sulphides, namely RbHg4Ga5S12 (1) and CsHg4Ga5S12 (2), which are isostructural and crystallise in the trigonal space group R3, are successfully isolated in sealed silica tubes by a solid-state reaction. The features of their three-dimensional open honeycomb frameworks are attributed to the parallel alignment of tetrahedral MS4 (M is disordered by 0.444 Hg and 0.555 Ga) building motifs, accompanied by Rb+ (or Cs+) reseating in the cavities. Notably, although the band gap values of 1 and 2 are 2.30 and 2.36 eV, separately, their thermal expansion anisotropies (0.15 and 0.41, respectively) are favourable for achieving laser-induced damage thresholds (5.6 and 5.8 times that of AgGaS2 for 1 and 2, respectively). In addition, the strong polarisability of tetrahedral MS4 building motifs in the diamond-like anionic structures is responsible for the promising second-harmonic generation (SHG) intensities (1.1 and 1.8 times that of AgGaS2 for 1 and 2, respectively) in the particle size range of 50-75 µm with non-phase-matchable behaviour at 1910 nm. Furthermore, theoretical investigation elaborates that electron transitions in compounds 1 and 2 mainly occur from valence band S-3p to conduction band Hg-6s and Ga-4s states, demonstrating that the linear and nonlinear optical properties originate primarily from the synergy of tetrahedral MS4 units.

[K2 PbX][Ga7 S12 ] (X = Cl, Br, I): The First Lead-Containing Cationic Moieties with Ultrahigh Second-Harmonic Generation and Band Gaps Exceeding the Criterion of 2.33 eV.

In contrast to anionic group theory of nonlinear optical (NLO) materials that second-harmonic generation (SHG) responses mainly originate from anionic groups, structural regulation on the cationic groups of salt-inclusion chalcogenides (SICs) is performed to make them also contribute to the NLO effects. Herein, the stereochemically active lone-electron-pair Pb2+ cation is first introduced to the cationic groups of NLO SICs, and the resultant [K2 PbX][Ga7 S12 ] (X = Cl, Br, I) are isolated via solid-state method. The features of their three-dimensional structures comprise highly oriented [Ga7 S12 ]3- and [K2 PbX]3+ frameworks derived from AgGaS2 , which display the largest phase-matching SHG intensities (2.5-2.7 × AgGaS2 @1800 nm) among all SICs. Concurrently, three compounds manifest band gap values of 2.54, 2.49, and 2.41 eV (exceeding the criterion of 2.33 eV), which can avoid two-photon absorption under the fundamental laser of 1064 nm, along with the relatively low anisotropy of thermal expansion coefficients, leading to improved laser-induced damage thresholds (LIDTs) values of 2.3, 3.8, and 4.0 times that of AgGaS2 . In addition, the density of states and SHG coefficient calculations demonstrate that the Pb2+ cations narrow the band gaps and benefit SHG responses.

Material research from the viewpoint of functional motifs.

As early as 2001, the need for the 'functional motif theory' was pointed out, to assist the rational design of functional materials. The properties of materials are determined by their functional motifs and how they are arranged in the materials. Uncovering functional motifs and their arrangements is crucial in understanding the properties of materials and rationally designing new materials of desired properties. The functional motifs of materials are the critical microstructural units (e.g. constituent components and building blocks) that play a decisive role in generating certain material functions, and can not be replaced with other structural units without the loss, or significant suppression, of relevant functions. The role of functional motifs and their arrangement in materials, with representative examples, is presented. The microscopic structures of these examples can be classified into six types on a length scale smaller than ∼10 nm with maximum subatomic resolution, i.e. crystal, magnetic, aperiodic, defect, local and electronic structures. Functional motif analysis can be employed in the function-oriented design of materials, as elucidated by taking infrared non-linear optical materials as an example. Machine learning is more efficient in predicting material properties and screening materials with high efficiency than high-throughput experimentation and high-throughput calculations. In order to extract functional motifs and find their quantitative relationships, the development of sufficiently reliable databases for material structures and properties is imperative.

AAg3Ga8Se14 (A = Rb, Cs): second-harmonic generation responses realized through the parallel arrangement of AgSe4 and GaSe4 tetrahedrons.

Two new quaternary selenides AAg3Ga8Se14, (A = Rb, 1; Cs, 2) were synthesised via solid-state reaction in sealed silica tubes. Compounds 1 and 2 crystallised in the monoclinic space group Cm (no. 8) and their three-dimensional [Ag3Ga8Se14]- anionic frameworks were comprised of AgSe4 and GaSe4 tetrahedrons. Their UV-Vis-near infrared diffuse reflectance spectra showed that 1 and 2 possessed wide band gaps of 2.17 and 2.10 eV, respectively. Notably, under incident laser irradiation at 1910 nm, compounds 1 and 2 presented moderate second-harmonic generation responses of 0.6 and 0.7 × AgGaS2, respectively, with phase-matching behaviours due to the parallel arrangement of nonlinear optical (NLO) functional tetrahedral AgSe4 and GaSe4 units. The laser-induced damage thresholds of 1 and 2 were estimated to be 25.4 and 18.0 MW cm-2, respectively, which were 2.1 and 1.5 times the threshold of AgGaS2. This study revealed that the title selenides, which were constructed from tetrahedral units arranged in a parallel array, are promising infrared NLO materials.