Thermally Activated Photoluminescence Induced by Tunable Interlayer Interactions in Naturally Occurring van der Waals Superlattice SnS/TiS2.

van der Waals (vdW) superlattices, comprising different 2D materials aligned alternately by weak interlayer interactions, offer versatile structures for the fabrication of novel semiconductor devices. Despite their potential, the precise control of optoelectronic properties with interlayer interactions remains challenging. Here, we investigate the discrepancies between the SnS/TiS2 superlattice (SnTiS3) and its subsystems by comprehensive characterization and DFT calculations. The disappearance of certain Raman modes suggests that the interactions alter the SnS subsystem structure. Specifically, such structural changes transform the band structure from indirect to direct band gap, causing a strong PL emission (∼2.18 eV) in SnTiS3. In addition, the modulation of the optoelectronic properties ultimately leads to the unique phenomenon of thermally activated photoluminescence. This phenomenon is attributed to the inhibition of charge transfer induced by tunable intralayer strains. Our findings extend the understanding of the mechanism of interlayer interactions in van der Waals superlattices and provide insights into the design of high-temperature optoelectronic devices.

A High-Rigidity Organic-Inorganic Metal Halide Hybrid Enabling Reversible and Enhanced Self-Trapped Exciton Emission under High Pressure.

Zero-dimensional organic-inorganic metal halide hybrids provide ideal bulk-crystal platforms for exploring the pressure engineering of electron-phonon coupling (EPC) and self-trapped exciton (STE) emission at the molecular level. However, the low stiffness of inorganic clusters hinders the reversible tuning of these physical properties. Herein, we designed a Sb3+-doped metal halide with a high emission yield (89.4%) and high bulk modulus (35 GPa) that enables reversible and enhanced STE emission (20-fold) under pressure. The high lattice rigidity originates from the corner-shared cage-structured inorganic tetramers and ring-shaped organic ligands. Further, we reveal that the pressure-enhanced emission regime below 4.5 GPa is owing to the lattice hardening and preferably EPC strength reducing, while the pressure-insensitive emission regime within 4.5-8.5 GPa results from the enhanced intercluster Coulombic attraction force that resists intracluster compression. These results provide insights into the structure-property relation and molecular engineering of zero-dimensional metal halides toward wide-band and pressure-sensitive light sources.

Multi-Mode Photoluminescence Regulation in a Zero-Dimensional Organic-Inorganic Hybrid Metal Halide Perovskite─[(CH3)4N]2SnCl6.

Metal ion-doped zero-dimensional halide perovskites provide good platforms to generate broadband emission and explore the fundamental dynamics of emission regulations. Recently, Sb3+-doped zero-dimensional halide perovskites have attracted considerable attention for the high quantum yield of yellow emission; however, the triplet state recombination is activated and the singlet state emission is usually absent. Herein, we fabricate an Sb3+-doped zero-dimensional [(CH3)4N]2SnCl6 perovskite that can induce singlet and triplet emission. Density functional theory calculation shows that there are some overlaps between the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals, which may induce a large energy separation between the lowest excited triplet states (T1) and the lowest excited singlet states (S1) [ΔE(S1 - T1)], impeding all the carriers' transfer from the singlet state to the triplet state. As a result, the reserved singlet emission together with the triplet emission can be regulated by excitation wavelength in situ. In addition, different Bi3+ ratios are co-doped into Sb3+@[(CH3)4N]2SnCl6, resulting in a photoluminescence ex situ regulation. Single-phase white light LED and optical anti-counterfeiting are developed further.

Supramolecular-Interactions-Modulated Photoluminescence in Indium Bromide-Based Isomers.

Here, two isomeric ionic zero-dimensional indium bromide crystals of α (1)/ß (2)-[OPy][InBr4(Phen)] (OPy = N-octylpyridinium; Phen = 1,10-phenanthroline) have been isolated simply by changing the cooling conditions in solvothermal syntheses. Structural comparisons indicate their different supramolecular interactions, which can be confirmed by Hirshfeld surface analyses. The crystal 2 has additional hydrogen bonds and π-π interactions; as a result, the more compact stacking of 2 could result in a 10-fold higher photoluminescence (PL) quantum yield (PLQY) than that of 1. Density functional theory calculations confirm the electron transition from the inorganic moiety to the organic ligand, which provides a further understanding of the optical process. This work provides a new idea for designing PL indium-based halides by understanding the structure-PL relationship.

Mussel-Inspired Two-Dimensional Halide Perovskite Facilitated Dopamine Polymerization and Self-Adhesive Photoelectric Coating.

Polydopamine (PDA) is a good adhesion agent for lots of gels inspired by the mussel, whereas hybrid organic-inorganic perovskites (HOIPs) usually exhibit extraordinary optoelectronic performance. Herein, mussel-inspired chemistry has been integrated with two-dimensional HOIPs first, leading to the preparation of new crystal (HDA)2PbBr4 (1) (DA = dopamine). The organic cation dopamine can be introduced into PDA resulting in a thin film of (HPDA)2PbBr4 (PDA-1). The dissolved inorganic components of layered perovskite in DMF solution together with H2O2 addition can facilitate DA polymerization greatly. More importantly, PDA-1 can inherit an excellent semiconductor property of HOIPs and robust adhesion of the PDA hydrogel resulting in a self-adhesive photoelectric coating on various interfaces.

Luminescence Enhancement and Temperature Sensing Properties of Hybrid Bismuth Halides Achieved via Tuning Organic Cations.

Bismuth-halide-based inorganic-organic hybrid materials (Bi-IOHMs) are desirable in luminescence-related applications due to their advantages such as low toxicity and chemical stability. Herein, two Bi-IOHMs of [Bpy][BiCl4(Phen)] (1, Bpy = N-butylpyridinium, Phen = 1,10-phenanthroline) and [PP14][BiCl4(Phen)]·0.25H2O (2, PP14 = N-butyl-N-methylpiperidinium), containing different ionic liquid cations and same anionic units, have been synthesized and characterized. Single-crystal X-ray diffraction reveals that compounds 1 and 2 crystallize in the monoclinic space group of P21/c and P21, respectively. They both possess zero-dimensional ionic structures and exhibit phosphorescence at room temperature upon excitation of UV light (375 nm for 1, 390 nm for 2), with microsecond lifetime (24.13 µs for 1 and 95.37 µs for 2). Hirshfeld surface analysis has been utilized to visually exhibit the different packing motifs and intermolecular interactions in 1 and 2. The variation in ionic liquids makes compound 2 have a more rigid supramolecular structure than 1, resulting in a significant enhancement in photoluminescence quantum yield (PLQY), that is, 0.68% for 1 and 33.24% for 2. In addition, the ratio of the emission intensities for compounds 1 and 2 shows a correlation with temperature. This work provides new insight into luminescence enhancement and temperature sensing applications involving Bi-IOHMs.

Reversible Luminescent Switching Induced by Heat/Water Treatment in a Zero-Dimensional Hybrid Antimony(â ¢) Chloride.

Recently zero-dimensional (0-D) inorganic-organic metal halides (IOMHs) have become a promising class of optoelectronic materials. Herein, we report a new photoluminescent (PL) 0-D antimony(III)-based IOMH single crystal, namely [H2BPZ][SbCl5]·H2O (BPZ = benzylpiperazine). Photophysical characterizations indicate that [H2BPZ][SbCl5]·H2O exhibits singlet/triplet dual-band emission. Density functional theory (DFT) calculations suggest that [H2BPZ][SbCl5]·H2O has the large energy difference between singlet and triplet states, which might induce the dual emission in this compound. Temperature-dependent PL spectra analyses suggest the soft lattice and strong electron-phonon coupling in this compound. Thermogravimetric analysis shows that the water molecules in the lattice of the title crystal could be removed by thermal treatment, giving rise to a dehydrated phase of [H2BPZ][SbCl5]. Interestingly, such structural transformation is accompanied by a reversible PL emission transition between red light (630 nm, dehydrated phase) and yellow light (595 nm, water-containing phase). When being exposed to an environment with 77% relative humidity, the emission color of the dehydrated phase was able to change from red to yellow within 20 s, and the red emission could be restored after reheating. The red to yellow emission switching could be achieved in acetone with water concentration as low as 0.2 vol%. The reversible PL transition phenomenon makes [H2BPZ][SbCl5]·H2O a potential material for luminescent water-sensing.

Single-Crystal Superstructures via Hierarchical Assemblies of Giant Rubik's Cubes as Tertiary Building Units.

Intricate superstructures possess unusual structural features and promising applications. The preparation of superstructures with single-crystalline nature are conducive to understanding the structure-property relationship, however, remains an intriguing challenge. Herein we put forward a new hierarchical assembly strategy towards rational and precise construction of intricate single-crystal superstructures. Firstly, two unprecedented superclusters in Rubik's cube's form with a size of ≈2×2×2 nm3 are constructed by aggregation of eight {Pr4 Sb12 } oxohalide clusters as secondary building units (SBUs). Then, the Rubik's cubes further act as isolable tertiary building units (TBUs) to assemble diversified single-crystal superstructures. Importantly, intermediate assembly states are captured, which helps illustrate the evolution of TBU-based superstructures and thus provides a profound understanding of the assembly process of superstructures at the atomic level.

Lithium Storage Performance Boosted via Delocalizing Charge in Znx Co1- x PS3 /CoS2 of 2D/3D Heterostructure.

A promising anode material consisting of bimetallic thiophosphate Znx Co1- x PS3 and CoS2 with 2D/3D heterostructure is designed and prepared by an effective chemical transformation. Density functional theory calculations illustrate that the Zn2+ can effectively modulate the electrical ordering of Znx Co1- x PS3 on the nanoscale: the reduced charge distribution emerging around the Zn ions can enhance the local built-in electric field, which will accelerate the ions migration rate by Coulomb forces and provide tempting opportunities for manipulating Li+ storage behavior. Moreover, the merits of the large planar size enable Znx Co1- x PS3 to provide abundant anchoring sites for metallic CoS2 nanocubes, generating a 2D/3D heterostructure with a strong electric field. The resultant Znx Co1- x PS3 /CoS2 can offer the combined advantages of bimetallic alloying and heterostructure in lithium storage applications, leading to outstanding performance as an anode material for lithium-ion batteries. Consequently, a high capacity of 794 mA h g-1 can be retained after 100 cycles at 0.2 A g-1 . Even at 3.0 A g-1 , a satisfactory capacity of 465 mA h g-1 can be delivered. The appealing alloying-heterostructure and electrochemical performance of this bimetallic thiophosphate demonstrate its great promise for applications in practical rechargeable batteries.

ZrF2(IO3)2 and RbGaF3(IO3): Two Promising Birefringent Crystals Featuring 1D Metal-Fluoride Cationic Chains and Wide Bandgaps.

Herein, two novel metal iodate-fluorides, ZrF2(IO3)2 and RbGaF3(IO3), have been prepared via hydrothermal reactions. In the structure of ZrF2(IO3)2, neighboring Zr atoms are connected via a pair of F atoms into the one-dimensional (1D) [ZrF2]2+ cationic chain, which further attaches two iodate groups forming 1D ZrF2(IO3)2, whereas the 1D cationic chain in RbGaF3(IO3) is [GaF]2+, formatted by the bridging of Ga3+ cations and axial F- anions, which further connect with one iodate group and two terminal F- anions into 1D [GaF3(IO3)]- anionic chains. According to the experimental characterization and theoretical calculations, both of them exhibit wide bandgaps (4.24 and 4.61 eV), short absorption edges (242 and 216 nm), and large birefringence (Δncal = 0.329 @1064 nm and Δncal = 0.174 @1064 nm for ZrF2(IO3)2 and RbGaF3(IO3), respectively), indicating that they can be candidates as UV birefringent materials.

Multi-Dopant Engineering in Perovskite Cs2SnCl6: White Light Emitter and Spatially Luminescent Heterostructure.

Bi3+/Te4+ co-doped Cs2SnCl6 with dual emission spectrum (i.e., 450 and 575 nm) was achieved by a modified solution method, which can overcome the phase separation in the previous method for Cs2SnCl6 crystal growth. The two emission peaks arising from the two dopants Bi3+ and Te4+ have distinct photoluminescence (PL) lifetimes. Thus, the control of dopant ratio or PL delay time will regulate the PL intensity ratio between 450 and 575 nm peaks leading to adjustable emission color. The energy transfer between the two emission centers, which is confirmed by the optical spectra and PL lifetime, has a critical distance around 7.8 nm with a maximum of 50% transfer efficiency. The Bi3+/Te4+ co-doped Cs2SnCl6 with superior stability in water and aqua regia was fabricated into a single-phase white light-emitting diode. In the meantime, various luminescent heterostructures were obtained by epitaxial Cs2SnCl6 crystal growth with different dopants, which can broaden the study of composition engineering in halide perovskites.

Selective Luminescence Response of a Zero-Dimensional Hybrid Antimony(III) Halide to Solvent Molecules: Size-Effect and Supramolecular Interactions.

Zero-dimensional (0D) metal halides with solid-state luminescence switching (SSLS) have attracted attention as sensors and luminescent anticounterfeiting. Herein, selective solvent molecule response and accordingly luminescence switching were discovered in 0D [EtPPh3]2[SbCl5] (1, EtPPh3 = ethyltriphenylphosphonium). More than a dozen kinds of solvent molecules have been tested to find out the selection rule for molecule absorption in 1, which is demonstrated to be the size effect of guest molecules. Confirmed by crystal structural analysis, only the solvents with molecular volume less than 22.3 Å3 could be accommodated in 1 leading to the solvatochromic photoluminescence (PL). The mechanism of solvatochromic PL was also deeply studied, which was found to be closely related to the supramolecular interactions between solvent molecules and the host material. Different functional groups of the solvent molecule can affect its strength of hydrogen bonding with [SbCl5]2-, which is crucial for the distortion level of [SbCl5]2- unit and thus results in not only distinct solvatochromic PL but also distinct thermochromic PL. In addition, they all show typical self-trapped exciton triplet emissions. The additional supramolecular interactions from guest molecules can enhance the photoluminescence quantum yield to be as high as 95%.

Se4P4nanoparticles confined within porous carbon as a lithium-ion battery anode with superior electrochemical performance.

The exploration of advanced anode materials through rational structure/phase design is the key to developing high-performance rechargeable batteries. Herein, tetraphosphorus tetraselenide (Se4P4) nanoparticles confined within porous carbon (named SeP@C) are developed for lithium-ion batteries. The designed SeP@C shows a set of structural/compositional advantages as lithium-ion battery anodes including high electrical conductivity, low ion diffusion barrier, and relieved lithiation stress. Consequently, the SeP@C electrode displays superior comprehensive lithium storage performance, e.g., high reversible capacity (640.8 mA h g-1at 0.1 A g-1), excellent cycling stability (500 cycles with respective capacity retention of over or nearly 100%), and good rate capability, representing a comparable lithium storage performance in reported phosphide-based anodes. More significantly, it shows excellent energy storage properties in lithium-ion full cells which can light up 85 red LEDs for over 3.2 h. This work offers an advanced electrode construction guidance of phosphorous-based anodes for the development of high-performance energy storage devices.

Modulation of the Structure and Photoluminescence of Bismuth(III) Chloride Hybrids by Altering the Ionic-Liquid Cations.

Two bismuth(III) halides hybrids with room-temperature phosphorescence (RTP), namely, [BPy]2[Bi2Cl8(bpym)] (1, BPy = N-butylpyridinium) and [EPy]2[Bi2Cl8(bpym)] (2, EPy = N-ethylpyridinium), were synthesized and characterized. Structural comparison reveals that 1 and 2 possess similar anionic zigzaglike chain of [Bi2Cl8(bpym)]n2n-; however, different packing modes of anion/cations and thus different weak interactions. Interestingly, the utilization of pyridinium cations with different length of alkyl chain could tune the RTP behaviors efficiently. The RTP quantum yield (QY) is increased more than 5-fold from 1 to 2 probably due to more rigid structure of 2 arising from the additional H-bond and anion-π interactions, as confirmed by Hirshfeld surfaces analyses and PLATON calculations. Moreover, additional π-π interactions in 1 could stabilize the triplet excitons, leading to an average lifetime of 1 (11.36 ms at 77 K and 1.407 ms at 298 K) being higher than 2 (0.3618 ms at 77 K and 0.07511 ms at 298 K). Density functional theory (DFT) calculations confirm that inorganic moiety to organic ligand charge-transfer (IOCT) is involved in the phosphorescence process. The present work provides a new sight into the design of RTP metal halides through studying the structure-RTP relationship.

Tunable Semiconductors: Control over Carrier States and Excitations in Layered Hybrid Organic-Inorganic Perovskites.

For a class of 2D hybrid organic-inorganic perovskite semiconductors based on π-conjugated organic cations, we predict quantitatively how varying the organic and inorganic component allows control over the nature, energy, and localization of carrier states in a quantum-well-like fashion. Our first-principles predictions, based on large-scale hybrid density-functional theory with spin-orbit coupling, show that the interface between the organic and inorganic parts within a single hybrid can be modulated systematically, enabling us to select between different type-I and type-II energy level alignments. Energy levels, recombination properties, and transport behavior of electrons and holes thus become tunable by choosing specific organic functionalizations and juxtaposing them with suitable inorganic components.

Intrinsic Instability of Cs2In(I)M(III)X6 (M = Bi, Sb; X = Halogen) Double Perovskites: A Combined Density Functional Theory and Experimental Study.

Recently, there has been substantial interest in developing double-B-cation halide perovskites, which hold the potential to overcome the toxicity and instability issues inherent within emerging lead halide-based solar absorber materials. Among all double perovskites investigated, In(I)-based Cs2InBiCl6 and Cs2InSbCl6 have been proposed as promising thin-film photovoltaic absorber candidates, with computational examination predicting suitable materials properties, including direct bandgap and small effective masses for both electrons and holes. In this study, we report the intrinsic instability of Cs2In(I)M(III)X6 (M = Bi, Sb; X = halogen) double perovskites by a combination of density functional theory and experimental study. Our results suggest that the In(I)-based double perovskites are unstable against oxidation into In(III)-based compounds. Further, the results show the need to consider reduction-oxidation (redox) chemistry when predicting stability of new prospective electronic materials, especially when less common oxidation states are involved.

Layered A2Sn3S7·1.25H2O (A = Organic Cation) as Efficient Ion-Exchanger for Rare Earth Element Recovery.

Exploring new ion-exchangers for the recovery of rare earth elements (REEs) and recycling is worthwhile for the high-tech industry and an eco-friendly sustainable economy. The efficient enrichment of low concentration REE from complex aqueous solutions containing large excess of competitive ions is challenging. Here we present a chalcogenide example as a superior REE ion-exchanger efficiently removing them from very complex aqueous solutions, (Me2NH2)1.33(Me3NH)0.67 Sn3S7·1.25H2O (FJSM-SnS). The material exhibits fast and efficient ion exchange behavior with short equilibrium time (<5 min), high adsorption capacity (139 mg/g for Eu, 147 mg/g for Tb, 126 mg/g for Nd), wide pH resistance (1.9-8.5), the largest distribution coefficient (Kd) value of 6.5 × 106 mL/g, good selectivity against Al3+, Fe3+, and Na+ ions, and high recovery rate (>99%) at low concentrations. Moreover, after ion-exchange, the REE in corresponding exchanged products could be easily recovered by elution. FJSM-SnS has superior capacity and faster absorption kinetics than other states of the artificial REE sorbents such as Al2O3/EG, clay minerals, zeolite, and activated carbon.

Two-Dimensional Lead(II) Halide-Based Hybrid Perovskites Templated by Acene Alkylamines: Crystal Structures, Optical Properties, and Piezoelectricity.

A series of two-dimensional (2D) hybrid organic-inorganic perovskite (HOIP) crystals, based on acene alkylamine cations (i.e., phenylmethylammonium (PMA), 2-phenylethylammonium (PEA), 1-(2-naphthyl)methanammonium (NMA), and 2-(2-naphthyl)ethanammonium (NEA)) and lead(II) halide (i.e., PbX42-, X = Cl, Br, and I) frameworks, and their corresponding thin films were fabricated and examined for structure-property relationship. Several new or redetermined crystal structures are reported, including those for (NEA)2PbI4, (NEA)2PbBr4, (NMA)2PbBr4, (PMA)2PbBr4, and (PEA)2PbI4. Non-centrosymmetric structures from among these 2D HOIPs were confirmed by piezoresponse force microscopy-especially noteworthy is the structure of (PMA)2PbBr4, which was previously reported as centrosymmetric. Examination of the impact of organic cation and inorganic layer choice on the exciton absorption/emission properties, among the set of compounds considered, reveals that perovskite layer distortion (i.e., Pb-I-Pb bond angle between adjacent PbI6 octahedra) has a more global effect on the exciton properties than octahedral distortion (i.e., variation of I-Pb-I bond angles and discrepancy among Pb-I bond lengths within each PbI6 octahedron). In addition to the characteristic sharp exciton emission for each perovskite, (PMA)2PbCl4, (PEA)2PbCl4, (NMA)2PbCl4, and (PMA)2PbBr4 exhibit separate, broad "white" emission in the long wavelength range. Piezoelectric compounds identified from these 2D HOIPs may be considered for future piezoresponse-type energy or electronic applications.

Chemical Origin of the Stability Difference between Copper(I)- and Silver(I)-Based Halide Double Perovskites.

Recently, CuI - and AgI -based halide double perovskites have been proposed as promising candidates for overcoming the toxicity and instability issues inherent within the emerging Pb-based halide perovskite absorbers. However, up to date, only AgI -based halide double perovskites have been experimentally synthesized; there are no reports on successful synthesis of CuI -based analogues. Here we show that, owing to the much higher energy level for the Cu 3d10 orbitals than for the Ag 4d10 orbitals, CuI atoms energetically favor 4-fold coordination, forming [CuX4 ] tetrahedra (X=halogen), but not 6-fold coordination as required for [CuX6 ] octahedra. In contrast, AgI atoms can have both 6- and 4-fold coordinations. Our density functional theory calculations reveal that the synthesis of CuI halide double perovskites may instead lead to non-perovskites containing [CuX4 ] tetrahedra, as confirmed by our material synthesis efforts.