Engineering Charge Density Waves by Stackingtronics in Tantalum Disulfide.

In the realm of condensed matter physics and materials science, charge density waves (CDWs) have emerged as a captivating way to modulate correlated electronic phases and electron oscillations in quantum materials. However, collectively and efficiently tuning CDW order is a formidable challenge. Herein, we introduced a novel way to modulate the CDW order in 1T-TaS2 via stacking engineering. By introducing shear strain during the electrochemical exfoliation, the thermodynamically stable AA-stacked TaS2 consecutively transform into metastable ABC stacking, resulting in unique 3a × 1a CDW order. By decoupling atom coordinates, we atomically deciphered the 3D subtle structural variations in trilayer samples. As suggested by density functional theory (DFT) calculations, the origin of CDWs is presumably due to collective excitations and charge modulation. Therefore, our works shed light on a new avenue to collectively modulate the CDW order via stackingtronics and unveiled novel mechanisms for triggering CDW formation via charge modulation.

Phase and Composition Engineering of Self-Intercalated 2D Metallic Tantalum Sulfide for Second-Harmonic Generation.

Self-intercalation in two-dimensional (2D) materials is significant, as it offers a versatile approach to modify material properties, enabling the creation of interesting functional materials, which is essential in advancing applications across various fields. Here, we define ic-2D materials as covalently bonded compounds that result from the self-intercalation of a metal into layered 2D compounds. However, precisely growing ic-2D materials with controllable phases and self-intercalation concentrations to fully exploit the applications in the ic-2D family remains a great challenge. Herein, we demonstrated the controlled synthesis of self-intercalated H-phase and T-phase Ta1+xS2 via a temperature-driven chemical vapor deposition (CVD) approach with a viable intercalation concentration spanning from 10% to 58%. Atomic-resolution scanning transmission electron microscopy-annular dark field imaging demonstrated that the self-intercalated Ta atoms occupy the octahedral vacancies located at the van der Waals gap. The nonperiodic Ta atoms break the centrosymmetry structure and Fermi surface properties of intrinsic TaS2. Therefore, ic-2D T-phase Ta1+xS2 consistently exhibit a spontaneous nonlinear optical (NLO) effect regardless of the sample thickness and self-intercalation concentrations. Our results propose an approach to activate the NLO response of centrosymmetric 2D materials, achieving the modulation of a wide range of optoelectronic properties via nonperiodic self-intercalation in the ic-2D family.

Carbon-Extraction-Induced Biaxial Strain Tuning of Carbon-Intercalated Iridium Metallene for Hydrogen Evolution Catalysis.

Metallene materials with atomic thicknesses are receiving increasing attention in electrocatalysis due to ultrahigh surface areas and distinctive surface strain. However, the continuous strain regulation of metallene remains a grand challenge. Herein, taking advantage of autocatalytic reduction of Cu2+ on biaxially strained, carbon-intercalated Ir metallene, we achieve control over the carbon extraction kinetics, enabling fine regulation of carbon intercalation concentration and continuous tuning of (111) in-plane (-2.0%-2.6%) and interplanar (3.5%-8.8%) strains over unprecedentedly wide ranges. Electrocatalysis measurements reveal the strain-dependent activity toward hydrogen evolution reaction (HER), where weakly strained Ir metallene (w-Ir metallene) with the smallest lattice constant presents the highest mass activity of 2.89 A mg-1Ir at -0.02 V vs reversible hydrogen electrode (RHE). Theoretical calculations validated the pivotal role of lattice compression in optimizing H binding on carbon-intercalated Ir metallene surfaces by downshifting the d-band center, further highlighting the significance of strain engineering for boosted electrocatalysis.

Carbon Nanotube-Directed 7 GPa Heterocyclic Aramid Fiber and Its Application in Artificial Muscles.

Poly(p-phenylene-benzimidazole-terephthalamide) (PBIA) fibers with excellent mechanical properties are widely used in fields that require impact-resistant materials such as ballistic protection and aerospace. The introduction of heterocycles in polymer chains increases their flexibility and makes it easier to optimize the fiber structure. However, the inadequate orientation of polymer chains is one of the main reasons for the large difference between the measured and theoretical mechanical properties of PBIA fibers. Herein, carbon nanotubes (CNTs) are selected as an orientation seed. Their structural features allow CNTs to orient during the spinning process, which can induce an orderly arrangement of polymers and improve the orientation of the fiber microstructure. To ensure the complete 1D topology of long CNTs (≈10 µm), PBIA is used as an efficient dispersant to overcome dispersion challenges. The p-CNT/PBIA fibers (10 µm single-walled carbon nanotube 0.025 wt%) exhibit an increase of 22% in tensile strength and 23% in elongation, with a maximum tensile strength of 7.01 ± 0.31 GPa and a reinforcement efficiency of 893.6. The artificial muscle fabricated using CNT/PBIA fibers exhibits a 34.8% contraction and a 25% lifting of a 2 kg dumbbell, providing a promising paradigm for high-performance organic fibers as high-load smart actuators.

Oriented attachment interfaces of zeolitic imidazolate framework nanocrystals.

Understanding the growth and coarsening mechanisms of metal-organic framework (MOF) nanoparticles is crucially important for the design and fabrication of MOF materials with diverse functionalities and controllable stability. Oriented attachment (OA) growth is a common manner of MOF nanocrystal coarsening and agglomeration, but the underlying molecular mechanisms have not been well understood to date. Here we report the molecular-scale characterization of the OA interfaces of zeolitic imidazolate framework (ZIF) crystals by state-of-the-art low-dose aberration-corrected transmission electron microscopy. A series of OA interfaces with different molecular structures are captured, implying that multiple kinetic steps are involved in the OA growth of ZIF crystals from non-directional physical attractions between primary nanocrystals, lattice-aligned attachment of the ligand-capped nanocrystals, to coherent interfaces with perfect lattice alignment or stacking faults. It was found that the surface-capping organic ligands not only play an essential role in crystal lattice alignment by near-field directional interactions, but also dominate the interfacial reaction kinetics by interfacial diffusion-controlled elimination of excess surface-capping ligands. These observations provide molecular-scale insights into the OA growth mechanisms of ZIF crystals, which is important for engineering MOF crystal growth pathways by designing surface-capping ligands.

Atomically Unveiling an Atlas of Polytypes in Transition-Metal Trihalides.

Transition-metal trihalides MX3 (M = Cr, Ru; X = Cl, Br, and I) belong to a family of novel two-dimensional (2D) magnets that can exhibit topological magnons and electromagnetic properties, thus affording great promises in next-generation spintronic devices. Rich magnetic ground states observed in the MX3 family are believed to be strongly correlated to the signature Kagome lattice and interlayer van der Waals coupling raised from distinct stacking orders. However, the intrinsic air instability of MX3 makes their direct atomic-scale analysis challenging. Therefore, information on the stacking-registry-dependent magnetism for MX3 remains elusive, which greatly hinders the engineering of desired phases. Here, we report a nondestructive transfer method and successfully realize an intact transfer of bilayer MX3, as evidenced by scanning transmission electron microscopy (STEM). After surveying hundreds of MX3 thin flakes, we provide a full spectrum of stacking orders in MX3 with atomic precision and calculated their associated magnetic ground states, unveiled by combined STEM and density functional theory (DFT). In addition to well-documented phases, we discover a new monoclinic C2/c phase in the antiferromagnetic (AFM) structure widely existing in MX3. Rich stacking polytypes, including C2/c, C2/m, R3̅, P3112, etc., provide rich and distinct magnetic ground states in MX3. Besides, a high density of strain soliton boundaries is consistently found in all MX3, combined with likely inverted structures, allowing AFM to ferromagnetic (FM) transitions in most MX3. Therefore, our study sheds light on the structural basis of diverse magnetic orders in MX3, paving the way for modulating magnetic couplings via stacking engineering.

A general thermodynamics-triggered competitive growth model to guide the synthesis of two-dimensional nonlayered materials.

Two-dimensional (2D) nonlayered materials have recently provoked a surge of interest due to their abundant species and attractive properties with promising applications in catalysis, nanoelectronics, and spintronics. However, their 2D anisotropic growth still faces considerable challenges and lacks systematic theoretical guidance. Here, we propose a general thermodynamics-triggered competitive growth (TTCG) model providing a multivariate quantitative criterion to predict and guide 2D nonlayered materials growth. Based on this model, we design a universal hydrate-assisted chemical vapor deposition strategy for the controllable synthesis of various 2D nonlayered transition metal oxides. Four unique phases of iron oxides with distinct topological structures have also been selectively grown. More importantly, ultra-thin oxides display high-temperature magnetic ordering and large coercivity. MnxFeyCo3-x-yO4 alloy is also demonstrated to be a promising room-temperature magnetic semiconductor. Our work sheds light on the synthesis of 2D nonlayered materials and promotes their application for room-temperature spintronic devices.

Composition-Controllable Syntheses and Property Modulations from 2D Ferromagnetic Fe5 Se8 to Metallic Fe3 Se4 Nanosheets.

Exploring new-type 2D magnetic materials with high magnetic transition temperature and robust air stability has attracted wide attention for developing innovative spintronic devices. Recently, intercalation of native metal atoms into the van der Waals gaps of 2D layered transition metal dichalcogenides (TMDs) has been developed to form 2D non-layered magnetic TMDs, while only succeeded in limited systems (e.g., Cr2 S3 , Cr5 Te8 ). Herein, composition-controllable syntheses of 2D non-layered iron selenide nanosheets (25% Fe-intercalated triclinic Fe5 Se8 and 50% Fe-intercalated monoclinic Fe3 Se4 ) are firstly reported, via a robust chemical vapor deposition strategy. Specifically, the 2D Fe5 Se8 exhibits intrinsic room-temperature ferromagnetic property, which is explained by the change of electron spin states from layered 1T'-FeSe2 to non-layered Fe-intercalated Fe5 Se8 based on density functional theory calculations. In contrast, the ultrathin Fe3 Se4 presents novel metallic features comparable with that of metallic TMDs. This work hereby sheds light on the composition-controllable synthesis and fundamental property exploration of 2D self-intercalation induced novel TMDs compounds, by propelling their application explorations in nanoelectronics and spintronics-related fields.

Synthesis of Fluoride-Substituted Layered Perovskites ZnMoO4 with an Enhanced Photocatalytic Activity.

Oxyfluorides possess considerable attention for their multiple excellent properties, but the conventional high-temperature solid-state syntheses have seen bottlenecks in the synthesis of new compounds. Herein, we report a novel layered oxyfluoride ZnMoO4:F, which is prepared by a facile hydrothermal method using ZnF2 as the fluoride source. The fluoride anions are successfully introduced into the oxygen sublattice, which is confirmed by a combined analysis using XRD, STEM, and TGA techniques. The as-synthesized ZnMoO4:F has an absorption edge at around 550 nm, indicating a red shift of Eg to the visible region compared to the oxide counterpart. The layered oxyfluoride exhibits an enhanced photocatalytic active for hydrogen evolution under simulated sunlight (λ > 350 nm), and the activity of ZnMoO4:F (651.9 µmol g-1) was 2 times higher than that of ZnMoO4 (309.7 µmol g-1). Further electrochemical analysis has shown that the conduction band position plays a critical role in the high performances of ZnMoO4:F. This work sheds new light on the future design and synthesis of novel fluoride-doped materials for photocatalysis applications.

The Universal Growth of Ultrathin Perovskite Single Crystals.

Perovskites have engaged significant attention owing to rich species and remarkable physical properties as well as optoelectronic applications. Compared to bulk counterparts, ultrathin perovskites exhibit more available compositions due to the breaking of bulk lattice limitation. Coupled with crystal lattice relaxation and quantum confinement, infinite intriguing properties of ultrathin perovskites deserve to be explored. Developing ultrathin perovskites with alterable composition and structure is a necessity to fully explore this versatile family. Herein, a universal strategy is conceived via constructing oriented solvent microenvironment induced by the interfacial electric field originated from the charge separation between solid and liquid phases, which is conducive to controlling the precursor distribution and makes crystals preferentially nucleate and grow in the preferentially lateral mode. From layered to nonlayered, organic to inorganic, and toxic to low-toxic lead-free perovskite, a full-range synthesis is achieved of ultrathin perovskites. This work opens up opportunities both for ultrathin perovskite exploration through compositional engineering and for device miniaturization in energy conversion applications.

Ultrafast Single-Crystal-to-Single-Crystal Transformation from Metal-Organic Framework to 2D Hydroxide.

Single-crystal-to-single-crystal (SCSC) transformations have received considerable interest in crystal engineering, owing to providing a key platform for creating new materials. However, because of the limited tolerance of chemical bonds against the lattice strains, it is challenging to maintain the crystallinity when the structure changes dramatically. Here, a peculiar SCSC transformation from organic crystals to inorganic crystals, simultaneously achieving a drastic change in structure, connectivity, and dimension, is reported. As a demonstration, after reacting with liquid gallium, zeolitic imidazolate framework-8 (ZIF-8) can easily transform to 2D hydroxide single crystals. Interestingly, long-range ordered metallic atoms of hydroxide inherited from the ordered atomic arrangement of ZIF-8, but the connectivity is distinct. With good universality and extensibility, this transformation vastly expands the research scope of the SCSC transformations and provides a novel pathway for the synthesis of crystalline materials.

Visualizing the {110} surface structure of equilibrium-form ZIF-8 crystals by low-dose Cs-corrected TEM.

The properties of zeolitic imidazolate framework (ZIF) crystals highly depend on the structures of the low-energy surfaces, such as {110} of ZIF-8. However, the atomic/molecular configurations of the ZIF-8 {110} surfaces remain debated. In this study, the near-atomic-scale characterization of {110} surfaces of ZIF-8 is conducted by low-dose aberration-corrected transmission electron microscopy (TEM). The real-space images with mitigated surface delocalization by minimized spherical aberration of TEM, together with the solvent corrected surface energy calculations, demonstrate that the {110} surfaces of ZIF-8 crystals with an equilibrium-form rhombic morphology have a zigzag-type termination. This study provides experimental evidence to clarify the debated structure of {110} ZIF-8 surfaces and has important implications in understanding the crystal growth and surface related properties of ZIF-8.

Structures and Structural Evolution of Sublayer Surfaces of Metal-Organic Frameworks.

The structural characterization of sublayer surfaces of MIL-101 is reported by low-dose spherical aberration-corrected high-resolution transmission electron microscopy (HRTEM). The state-of-the-art microscopy directly images atomic/molecular configurations in thin crystals from charge density projections, and uncovers the structures of sublayer surfaces and their evolution to stable surfaces regulated by inorganic Cr3 (µ3 -O) trimers. This study provides compelling evidence of metal-organic frameworks (MOFs) crystal growth via the assembly of sublayer surfaces and has important implications in understanding the crystal growth and surface-related properties of MOFs.

Hyperpolarized Xe NMR signal advancement by metal-organic framework entrapment in aqueous solution.

We report hyperpolarized Xe signal advancement by metal-organic framework (MOF) entrapment (Hyper-SAME) in aqueous solution. The 129Xe NMR signal is drastically promoted by entrapping the Xe into the pores of MOFs. The chemical shift of entrapped 129Xe is clearly distinguishable from that of free 129Xe in water, due to the surface and pore environment of MOFs. The influences from the crystal size of MOFs and their concentration in water are studied. A zinc imidazole MOF, zeolitic imidazole framework-8 (ZIF-8), with particle size of 110 nm at a concentration of 100 mg/mL, was used to give an NMR signal with intensity four times that of free 129Xe in water. Additionally, Hyper-SAME is compatible with hyperpolarized 129Xe chemical exchange saturation transfer. The 129Xe NMR signal can be amplified further by combining the two techniques. More importantly, Hyper-SAME provides a way to make detection of hyperpolarized 129Xe in aqueous solution convenient and broadens the application area of MOFs.

Confinement Effect of Mesopores: In Situ Synthesis of Cationic Tungsten-Vacancies for a Highly Ordered Mesoporous Tungsten Phosphide Electrocatalyst.

Engineering defects in crystalline electrocatalysts is an effective approach to tailor the electronic structure and number of active sites, which are essential for the intrinsic activity of the hydrogen evolution reaction (HER). Unlike previously reported methods, we demonstrate a confinement effect using a mesoporous template for in situ fabrication of cationic W vacancies in as-prepared ordered mesoporous tungsten phosphide (WP) nanostructures by adjusting the nonstoichiometric ratio of the precursor elements. With a plenty of W vacancies and ordered mesoporosity, the as-prepared catalyst WP-Mesop exhibits better catalytic performance than the catalysts without mesopores and/or vacancies. The WP-Mesop shows an ultralow overpotential of 175 mV in acid and 229 mV in alkaline at 100 mA cm-2 and stability of 48 h without structural collapse in both acid and alkaline media. Meanwhile, density functional theory calculations further reveal that the activation barrier for HER can be lowered by introducing cationic W vacancies. This strategy can be extended to generate cationic defects in other transition metal phosphides to improve their HER activities.

Unveiling Electronic Properties in Metal-Phthalocyanine-Based Pyrazine-Linked Conjugated Two-Dimensional Covalent Organic Frameworks.

π-Conjugated two-dimensional covalent organic frameworks (2D COFs) are emerging as a novel class of electroactive materials for (opto)electronic and chemiresistive sensing applications. However, understanding the intricate interplay between chemistry, structure, and conductivity in π-conjugated 2D COFs remains elusive. Here, we report a detailed characterization for the electronic properties of two novel samples consisting of Zn- and Cu-phthalocyanine-based pyrazine-linked 2D COFs. These 2D COFs are synthesized by condensation of metal-phthalocyanine (M = Zn and Cu) and pyrene derivatives. The obtained polycrystalline-layered COFs are p-type semiconductors both with a band gap of ∼1.2 eV. A record device-relevant mobility up to ∼5 cm2/(V s) is resolved in the dc limit, which represents a lower threshold induced by charge carrier localization at crystalline grain boundaries. Hall effect measurements (dc limit) and terahertz (THz) spectroscopy (ac limit) in combination with density functional theory (DFT) calculations demonstrate that varying metal center from Cu to Zn in the phthalocyanine moiety has a negligible effect in the conductivity (∼5 × 10-7 S/cm), charge carrier density (∼1012 cm-3), charge carrier scattering rate (∼3 × 1013 s-1), and effective mass (∼2.3m0) of majority carriers (holes). Notably, charge carrier transport is found to be anisotropic, with hole mobilities being practically null in-plane and finite out-of-plane for these 2D COFs.

A Phthalocyanine-Based Layered Two-Dimensional Conjugated Metal-Organic Framework as a Highly Efficient Electrocatalyst for the Oxygen Reduction Reaction.

Layered two-dimensional (2D) conjugated metal-organic frameworks (MOFs) represent a family of rising electrocatalysts for the oxygen reduction reaction (ORR), due to the controllable architectures, excellent electrical conductivity, and highly exposed well-defined molecular active sites. Herein, we report a copper phthalocyanine based 2D conjugated MOF with square-planar cobalt bis(dihydroxy) complexes (Co-O4 ) as linkages (PcCu-O8 -Co) and layer-stacked structures prepared via solvothermal synthesis. PcCu-O8 -Co 2D MOF mixed with carbon nanotubes exhibits excellent electrocatalytic ORR activity (E1/2 =0.83 V vs. RHE, n=3.93, and jL =5.3 mA cm-2 ) in alkaline media, which is the record value among the reported intrinsic MOF electrocatalysts. Supported by in situ Raman spectro-electrochemistry and theoretical modeling as well as contrast catalytic tests, we identified the cobalt nodes as ORR active sites. Furthermore, when employed as a cathode electrocatalyst for zinc-air batteries, PcCu-O8 -Co delivers a maximum power density of 94 mW cm-2 , outperforming the state-of-the-art Pt/C electrocatalysts (78.3 mW cm-2 ).