Validated enhancement and temperature modulated absorbance of a WS2 monolayer based on a planar structure.

Transition-metal dichalcogenides (TMDCs), as emerging optoelectronic materials, necessitate the establishment of an experimentally viable system to study their interaction with light. In this study, we propose and analyze a WS2/PMMA/Ag planar Fabry-Perot (F-P) cavity, enabling the direct experimental measurement of WS2 absorbance. By optimizing the structure, the absorbance of A exciton of WS2 up to 0.546 can be experimentally achieved, which matches well with the theoretical calculations. Through temperature and thermal expansion strain induced by temperature, the absorbance of the A exciton can be tuned in situ. Furthermore, temperature-dependent photocurrent measurements confirmed the consistent absorbance of the A exciton under varying temperatures. This WS2/PMMA/Ag planar structure provides a straightforward and practical platform for investigating light interaction in TMDCs, laying a solid foundation for future developments of TMDC-based optoelectronic devices.

Meta-Structured Covalent Organic Framework Nanocoatings with Active and Angle-Independent Structural Coloration.

High-performance multifunctional nanocoatings not only protect and enhance substrate materials but also offer additional functionalities. This demands a sophisticated coordination of the coating's inherent properties and microstructural features. Here, a multifunctional active nanocoating via meta-structural engineering of covalent organic framework (COF) deposition materials is presented. This COF nanocoating, characterized by well-defined micropores (1-2 nm), meta-structured textures (30-300 nm), tailored thickness (100-300 nm), and good uniformness, showcases a unique combination of angle-independent structural coloration and ultrafast responsiveness to gaseous stimuli. Remarkably, it demonstrates good compatibility with a wide range of inert substrate materials, from rigid ones like glass and metal to flexible elastomers and nanomaterial films of various shapes and sizes. This versatility enables the facile development of devices that can optically report information about their environments. Examples include chemically active coatings with ultrafast (≈10 ms) color-changing behaviors and programmable actuation behaviors upon exposure to gaseous stimuli, and mechanically active coatings that can detect substrate strain up to 50% yet maintain structural robustness and consistent coloration hue. It is believed that meta-structural engineering of COF nanocoatings on inert substrates can enable them to respond to environmental stimuli, potentially indicating a new trend in developing multifunctional materials and smart devices.

Re-Designing Cellulosic Core-Shell Composite Fibers for Advanced Photothermal and Thermal-Regulating Performance.

Flexible fibers and textiles featuring photothermal conversion and storage capacities are ideal platforms for solar-energy utilization and wearable thermal management. Other than using fossil-fuel-based synthetic fibers, re-designing natural fibers with nanotechnology is a sustainable but challenging option. Herein, advanced core-shell structure fibers based on plant-based nanocelluloses are obtained using a facile co-axial wet-spinning process, which has superior photothermal and thermal-regulating performances. Besides serving as the continuous matrix, nanocelluloses also have two other important roles: dispersing agent when exfoliating molybdenum disulfide (MoS2), and stabilizer for phase change materials (PCM) in the form of Pickering emulsion. Consequently, the shell layer contains well-oriented nanocelluloses and MoS2, and the core layer contains a high content of PCM in a leak-proof encapsulated manner. Such a hierarchical cellulosic supportive structure leads to high mechanical strength (139 MPa), favorable flexibility, and large latent heat (92.0 J g-1), surpassing most previous studies. Furthermore, the corresponding woven cloth demonstrates satisfactory thermal-regulating performance, high solar-thermal conversion and storage efficiency (78.4-84.3%), and excellent long-term performance. In all, this work paves a new way to build advanced structures by assembling nanoparticles and polymers for functional composite fibers in advanced solar-energy-related applications.

Gradual conductance modulation by defect reorganization in amorphous oxide memristors.

Amorphous oxides show great prospects in revolutionizing memristors benefiting from their abundant non-stoichiometric composition. However, an in-depth investigation of the memristive characteristics in amorphous oxides is inadequate and the resistive switching mechanism is still controversial. In this study, aiming to clearly understand the gradual conductance modulation that is deeply bound to the evolution of defects-mainly oxygen vacancies, forming-free memristors based on amorphous ZnAlSnO are fabricated, which exhibit high reproducibility with an initial low-resistance state. Pulse depression reveals the logarithmic-exponential mixed relaxation during RESET owing to the diffusion of oxygen vacancies in orthogonal directions. The remnants of conductive filaments formed through aggregation of oxygen vacancies induced by high-electric-field are identified using ex situ TEM. Especially, the conductance of the filament, including the remnant filament, is larger than that of the hopping conductive channel derived from the diffusion of oxygen vacancies. The Fermi level in the conduction band rationalizes the decay of the high resistance state. Rare oxidation-migration of Au occurs upon device failure, resulting in numerous gold nanoclusters in the functional layer. These comprehensive revelations on the reorganization of oxygen vacancies could provide original ideas for the design of memristors.

Combinatorial Synthesis of Covalent Organic Framework Particles with Hierarchical Pores and Their Catalytic Application.

Precise tailoring of the aggregation state of covalent organic frameworks (COFs) to form a hierarchical porous structure is critical to their performance and applications. Here, we report a one-pot and one-step strategy of using dynamic combinatorial chemistry to construct imine-based hollow COFs containing meso- and macropores. It relies on a direct copolymerization of three or more monomers in the presence of two monofunctional competitors. The resulting particle products possess high crystallinity and hierarchical pores, including micropores around 0.93 nm, mesopores widely distributed in the range of 3.1-32 nm, and macropores at about 500 nm, while the specific surface area could be up to 748 m2·g-1, with non-micropores accounting for 60% of the specific surface area. The particles demonstrate unique advantages in the application as nanocarriers for in situ loading of Pd catalysts at 93.8% loading efficiency in the copolymerization of ethylene and carbon monoxide. The growth and assembly of the copolymer could thus be regulated to form flower-shaped particles, efficiently suppressing the fouling of the reactor. The copolymer's weight-average molecular weight and the melting temperature are also highly improved. Our method provides a facile way of fabricating COFs with hierarchical pores for advanced applications in catalysis.

Covalent Organic Framework-Supported Metallocene for Ethylene Polymerization.

The loading of homogeneous catalysts with support can dramatically improve their performance in olefin polymerization. However, the challenge lies in the development of supported catalysts with well-defined pore structures and good compatibility to achieve high catalytic activity and product performance. Herein, we report the use of an emergent class of porous material-covalent organic framework material (COF) as a carrier to support metallocene catalyst-Cp2 ZrCl2 for ethylene polymerization. The COF-supported catalyst demonstrates a higher catalytic activity of 31.1×106  g mol-1 h-1 at 140 °C, compared with 11.2×106  g mol-1 h-1 for the homogenous one. The resulting polyethylene (PE) products possess higher weight-average molecular weight (Mw ) and narrower molecular weight distribution (Ð) after COF supporting, that is, Mw increases from 160 to 308 kDa and Ð drops from 3.3 to 2.2. The melting point (Tm ) is also increased by up to 5.2 °C. Moreover, the PE product possesses a characteristic filamentous microstructure and demonstrates an increased tensile strength from 19.0 to 30.7 MPa and elongation at break from 350 to 1400 % after catalyst loading. We believe that the use of COF carriers will facilitate the future development of supported catalysts for highly efficient olefin polymerization and high-performance polyolefins.

Mechanistic Understanding of Efficient Polyethylene Hydrocracking over Two-Dimensional Platinum-Anchored Tungsten Trioxide.

Chemical upcycling of polyethylene (PE) can convert plastic waste into valuable resources. However, engineering a catalyst that allows PE decomposition at low temperatures with high activity remains a significant challenge. Herein, we anchored 0.2 wt.% platinum (Pt) on defective two-dimensional tungsten trioxide (2D WO3 ) nanosheets and achieved hydrocracking of high-density polyethylene (HDPE) waste at 200-250 °C with a liquid fuel (C5-18 ) formation rate up to 1456 gproducts ⋅ gmetal species -1 ⋅ h-1 . The reaction pathway over the bifunctional 2D Pt/WO3 is elucidated by quasi-operando transmission infrared spectroscopy, where (I) well-dispersed Pt immobilized on 2D WO3 nanosheets trigger the dissociation of hydrogen; (II) adsorption of PE and activation of C-C cleavage on WO3 are through the formation of C=O/C=C intermediates; (III) intermediates are converted to alkane products by the dissociated H. Our study directly illustrates the synergistic role of bifunctional Pt/WO3 catalyst in the hydrocracking of HDPE, paving the way for the development of high-performance catalysts with optimized chemical and morphological properties.

Preparation of covalent organic framework-based nanofibrous films with temperature-responsive release of thymol for active food packaging.

Thymol (THY) is commonly used in active food packaging, however because of its high volatility, poor water solubility, and strong aromatic odor, the application of THY is facing challenges. Herein, covalent organic frameworks (COFs) were synthesized in room temperature by asymmetric monomer exchange method for THY encapsulation, and solution blow spinning was used to fabricate the THY@COF/polycaprolactone (PCL) nanofibrous films. The synthesized COFs had a large specific surface area, porous structure, and loading capacity of 30.35% for THY, and THY@COFs possessed good thermal stability. Characterization analysis showed that THY@COFs were successfully incorporated into the PCL films and increased the barrier property of the films. Besides, the films showed good biocompatibility and antibacterial activity. Moreover, THY@COF/PCL films exhibited temperature-responsive THY release profiles, which is important for practical preservation applications, especially for preserving food in warm environments. Overall, THY@COF/PCL films possess promising potential in active food packaging.

Discretized hexagonal boron nitride quantum emitters and their chemical interconversion.

Quantum emitters in two-dimensional hexagonal boron nitride (hBN) are of significant interest because of their unique photophysical properties, such as single-photon emission at room temperature, and promising applications in quantum computing and communications. The photoemission from hBN defects covers a wide range of emission energies but identifying and modulating the properties of specific emitters remain challenging due to uncontrolled formation of hBN defects. In this study, more than 2000 spectra are collected consisting of single, isolated zero-phonon lines (ZPLs) between 1.59 and 2.25 eV from diverse sample types. Most of ZPLs are organized into seven discretized emission energies. All emitters exhibit a range of lifetimes from 1 to 6 ns, and phonon sidebands offset by the dominant lattice phonon in hBN near 1370 cm-1. Two chemical processing schemes are developed based on water and boric acid etching that generate or preferentially interconvert specific emitters, respectively. The identification and chemical interconversion of these discretized emitters should significantly advance the understanding of solid-state chemistry and photophysics of hBN quantum emission.

Vertical Growth of 2D Covalent Organic Framework Nanoplatelets on a Macroporous Scaffold for High-Performance Electrodes.

Hierarchically structural engineering of electrodes is critical to achieving high energy density and high power density in electrochemical energy storage (EES). However, rational regulation of the mesoscopic structure that coordinates microscopic and macroscopic structural features simultaneously remains a significant challenge. Here, the construction of electrodes with well-defined hierarchical pores spanning multiple length scales from 1 nm to 50 µm is reported. Vertically aligned 2D covalent organic framework (COF) nanoplatelets with a thickness around 30 nm are in situ grown on macroporous graphene aerogel scaffold by a reversible polycondensation-termination strategy. The obtained electrode thus combines abundant accessible active sites and efficient transport expressways for both ions and electrons. When used for supercapacitors, a superior gravimetric capacitance of 289 F g-1  as well as outstanding capacitance retention at both high charge/discharge rates of 77% from 0.5 to 50 A g-1  and high mass loading of 74% from 1.2 to 10.4 mg cm-2  are achieved. Hierarchical engineering of mesostructured 2D COF units on the macroporous scaffold will bring unprecedented structural designability and performance enhancement for EES electrodes.

Surface deposition of 2D covalent organic frameworks for minimizing nanocatalyst sintering during hydrogenation.

A strategy of in situ depositing 2D COFs on heterogeneous catalysts was reported for the first time to suppress the agglomeration and sintering of the supported metal nanoparticles during hydrogenation processes. The COF-decorated nanocatalysts exhibited excellent stability in various hydrogenation reactions including the reduction of dimethyl oxalate (DMO), furfural, and other chemicals.

Pesticide detection with covalent-organic-framework nanofilms at terahertz band.

Development of rapid molecular detection technologies is critical for the safer and smarter urban agriculture, medicine, and pro-environment. The emergent terahertz (THz) spectroscopy has its distinct advantages of being non-destructive, label-free and able to trace intermolecular and intramolecular vibrations, yet it suffers from the low performance of sensing materials available and their high fabrication cost. Here, we introduce a reticular material -- two dimensional (2D) covalent organic frameworks (COFs) and prepare their nanofilms as the lossy layer for THz absorbers. The COF film can be directly deposited on the dielectric layer of THz absorbers via an in-situ wet-chemistry growth. It possesses designable hierarchical structures, high specific areas of 736-971 m2/g, and precise nanopores of 1.6-2.1 nm, depending on its 2D COF constituents. The resulting THz absorber has been tested for pesticide detection. It presents a limit of detection at 2.2 ng and a selective response of 2.7-7.8 times that of interferents such as saccharides, antibiotics, and dyes, satisfying the need for practical application. Such flexible filmy sensor can measure the pesticide residue on the surface of apple for practical application. The THz sensor also demonstrates high stability over 1000 cycles of bending. Use of reticular nanofilm as the responsive layer may permit the future development of high-performance THz absorbers and other sensors for rapid molecular recognition.

Grafting Hollow Covalent Organic Framework Nanoparticles with Thermal-Responsive Polymers for the Controlled Release of Preservatives.

Covalent organic frameworks (COFs) hold great potential in various applications because of their well-defined pore structures and morphologies. However, most COF materials demonstrate poor dispersibility in solvents that significantly limits their processing and applications. Herein, we report the synthesis of COF-based hollow nanoparticles (h-NPs) with good water dispersibility, high capacity, and thermal responsiveness to load essential oil molecules for longer-term preservation of fruits. Imine-based COF h-NPs possessing a pore width of 1.3 nm, inner/outer diameters of ∼150/239 nm, and high crystallinity were synthesized and grafted with water-soluble polymers such as polyethylene glycol or poly(N-isopropylacrylamide) (PNIPAM) with molecular weights of 1-3 kDa. The h-NP products with grafting densities of 0.6-2.1 nm-2 can be well dispersed in water at room temperature. PNIPAM-grafted ones are temperature-responsive in that they can precipitate out from the dispersion at 40 °C and redisperse at 25 °C for at least 15 cycles. The h-NPs are used as nanocarriers to load essential oils such as hexanal and trans-2-hexenal with a high capacity of 1.1 g/g for fruit fresh-keeping, and the encapsulated preservatives can be released controllably at 25-40 °C as regulated by the grafted polymers. As a result, the storage time of cherry tomatoes can be prolonged by 4 days compared to the control run. Moreover, these h-NPs can be recycled and reused. Our work highlights the potential of COF nanomaterials grafting with stimuli-responsive polymers for controlled release application in various food preservation.

Irreversible synthesis of an ultrastrong two-dimensional polymeric material.

Polymers that extend covalently in two dimensions have attracted recent attention1,2 as a means of combining the mechanical strength and in-plane energy conduction of conventional two-dimensional (2D) materials3,4 with the low densities, synthetic processability and organic composition of their one-dimensional counterparts. Efforts so far have proven successful in forms that do not allow full realization of these properties, such as polymerization at flat interfaces5,6 or fixation of monomers in immobilized lattices7-9. Another frequently employed synthetic approach is to introduce microscopic reversibility, at the cost of bond stability, to achieve 2D crystals after extensive error correction10,11. Here we demonstrate a homogenous 2D irreversible polycondensation that results in a covalently bonded 2D polymeric material that is chemically stable and highly processable. Further processing yields highly oriented, free-standing films that have a 2D elastic modulus and yield strength of 12.7 ± 3.8 gigapascals and 488 ± 57 megapascals, respectively. This synthetic route provides opportunities for 2D materials in applications ranging from composite structures to barrier coating materials.

Toward Covalent Organic Framework Metastructures.

The bottom-up assembly of periodically ordered structures provides a scalable way for producing metastructured materials with exotic optical and mechanical properties. However, direct self-assembly of small molecules into such metastructures beyond the nanoscale remains an unresolved issue. Here we demonstrate that metastructured assemblies of two-dimensional (2D) polymers, specifically 2D covalent organic frameworks (COFs), can be directly synthesized in solution. We applied 2D COF monomer polycondensation to prepare flower-shaped particles consisting of highly crystalline "petals" with sizes larger than 20 µm. The petal comprises periodically arranged COF nanoflake units with tunable lengths of 490-850 nm, thicknesses about 20 nm, interflake spacing around 14 nm, and Hermans orientation factors up to 0.998. Such a metastructure is mechanically robust and remains almost intact even after full pyrolysis at 900 °C. It also demonstrates unique birefringence and polarization-dependent resonances under visible-near-infrared light not observed in its constituents, 2D COF polycrystals, and with well-defined nanopores of 1.8 nm and the high surface area of 1576 m2/g. Such metastructured particles with nanopores are well-suited as novel particulate optical devices for collecting and storing information about their surroundings that can be easily read out by polarization imaging with high sensitivity, as demonstrated by their explosive detection and anticounterfeiting applications. Self-assembly of 2D polymers into metastructures may become an important method for developing functional materials with unprecedented properties and extensive applications.

Hierarchical Assembly of Two-Dimensional Polymers into Colloidosomes and Microcapsules.

Hierarchical assembly of two-dimensional (2D) polymers to 3D microstructures provides new means of creating functional materials with exotic properties for extensive applications. Herein, we report an approach of assembling 2D covalent organic framework (COF) colloidosomes or microcapsules from small molecules. We polymerized monomers to produce narrowly distributed COF particles with average particle sizes greater than 490 nm, which were further used as stabilizers to prepare various water-in-oil Pickering emulsions with droplet sizes of 10-120 µm on average. The emulsion droplets were subsequently applied as templates for interfacial polymerization of the same monomers. The COF microcapsules with varied diameters and shell thicknesses of 0.2-3.1 µm were thus obtained, which possessed good stability, high crystallinity, and surface areas no less than 540 m2/g. The approach also permits facile loading of water-soluble substances such as salts, dyes, or proteins. The loaded molecules demonstrated different permeability against the shell, in which 98% of the encapsulated salts could be released in 1 h while only 18% of dye molecules and almost none of the fluorescent proteins diffused out from the microcapsules. Such an assembling approach may greatly extend the applications of 2D polymers and their microcapsules.

Stiffening, strengthening, and toughening of biodegradable poly(butylene adipate-co-terephthalate) with a low nanoinclusion usage.

Simultaneous stiffening, strengthening, and toughening of biodegradable polymers, such as poly(butylene adipate-co-terephthalate) (PBAT) and others, is necessary for their use in packaging and agriculture applications. However, a high content of nanoinclusions is usually required, leading to a tradeoff between composite toughness and strength or stiffness in the reinforcement. Herein, we report an iterative reinforcement strategy that uses one nanocomposite to reinforce PBAT. An in-situ grafting polymerized cellulose nanocrystal (CNC)/PBAT (CNC-g-PBAT) nanocomposite consisting of ungrafted/free PBAT (PBATf) was used as an inclusion directly to reinforce a commercial PBAT. At an exceptionally low CNC usage of 0.02 wt.%, we achieved a simultaneous enhancement of the Young's modulus by 26 %, tensile strength by 27 %, elongation at break by 37 %, and toughness by 56 % over those for PBAT. To the best of our knowledge, such reinforcement efficiency is the highest among similar biodegradable polymer nanocomposites reported in the literature. The rheology, differential scanning calorimetry, and wide-angle X-ray diffraction measurements confirmed the mechanical reinforcement attributed to a synergistic contribution from PBATf and CNC-g-PBAT. In particular, the use of PBATf enhanced both stiffness and toughness of the composites, while the CNC-g-PBAT interacted within the polymer matrix and increased the crystallinity of the polymer matrix, leading to the strengthening and toughening effect. The strategy proposed here is greatly beneficial to producing high-performance biodegradable polymer nanocomposite films for packaging and agricultural applications using a very low amount of nanoinclusion.

Highly Ordered Two-Dimensional MoS2 Archimedean Scroll Bragg Reflectors as Chromatically Adaptive Fibers.

Nanostructured fibers provide a basis for a unique class of multifunctional textiles, composites, and membrane applications, including those capable of chromatic modulating because of their high aspect ratio, surface area, and processing capability. Here in, we utilize two-dimensional (2D) materials including molybdenum disulfide (MoS2) and hexagonal boron nitride (hBN) to generate single layer Archimedean scroll fibers, possessing cross sections formed from a single 2D molecular layer. Chemical vapor deposited (CVD) monolayer MoS2 (0.29-0.33% in volume) and 226-259 nm-thick poly(methyl methacrylate) (PMMA) were used to create Bragg reflector fibers, exploiting the anisotropic function, exhibiting reflection at 630-709 nm, and verifying the highly ordered nanoinclusions. The Bragg reflectors show a memory response to heating and cooling, which switches the reflection wavelength from 629 to 698 nm. We simulate the reflection and transmission spectra of MoS2/PMMA and MoS2/polydimethylsiloxane layered composites to provide the design of scroll fiber composites using the transfer matrix methods. Moreover, we demonstrate the incorporation of a few-layer CVD hBN into the scroll fiber composite that emits photons at 576 nm. The highly oriented layered structures extend the capability of the fiber nanocomposites to take advantage of anisotropic optical, electrical, and thermal properties unique to 2D materials.

Synthesis of block cationic polyacrylamide precursors using an aqueous RAFT dispersion polymerization.

Synthesis of cationic polyacrylamides (CPAMs) by introducing cationic polymer precursors followed by chain extension of acrylamide (AM) homopolymer blocks via RAFT polymerization is a promising approach for engineering high-performance CPAMs. However, the aqueous solution polymerization of AM usually leads to high viscosity, thus limiting the solid content in the polymerization system. Herein a novel approach is introduced that uses a random copolymer of AM and methacryloxyethyltrimethyl ammonium chloride (DMC) as a macro RAFT chain transfer agent (mCTA) and stabilizer for aqueous RAFT dispersion polymerization of AM. The AM/DMC random copolymers synthesized by RAFT solution polymerization, having narrow dispersities (D s) at different molecular weights and cationic degrees (C s), could serve as the mCTA, which was confirmed by mCTA chain extension in aqueous solution polymerization of AM under different C s, solid contents, AM addition contents, extended PAM block lengths, and mCTA chain lengths. The block CPAMs had a D value of less than 1.2. A model was developed using the method of moments with consideration of the diffusion control effect, for further understanding the chain extension kinetics. Predicted polymerization kinetics provided an accurate fit of the experimental data. The AM/DMC random copolymers were further used for aqueous RAFT dispersion polymerization of AM under different polymerization temperatures, C s, and mCTA chain lengths. The resulting products had a milky appearance, and the block copolymers had D s of less than 1.3. Higher C s and longer chain lengths on mCTAs were beneficial for stabilizing the polymerization systems and produced smaller particle sizes and less particle aggregation. The products remained stable at room temperature storage for more than a month. The results indicate that aqueous RAFT dispersion polymerization using random copolymers of AM and DMC at moderate cationic degrees as a stabilizer and mCTA is a suitable approach for synthesizing CPAM block precursors at an elevated solid content.

Autoperforation of 2D materials for generating two-terminal memristive Janus particles.

Graphene and other two-dimensional materials possess desirable mechanical, electrical and chemical properties for incorporation into or onto colloidal particles, potentially granting them unique electronic functions. However, this application has not yet been realized, because conventional top-down lithography scales poorly for producing colloidal solutions. Here, we develop an 'autoperforation' technique that provides a means of spontaneous assembly for surfaces composed of two-dimensional molecular scaffolds. Chemical vapour deposited two-dimensional sheets can autoperforate into circular envelopes when sandwiching a microprinted polymer composite disk of nanoparticle ink, allowing liftoff into solution and simultaneous assembly. The resulting colloidal microparticles have two independently addressable, external Janus faces that we show can function as an intraparticle array of vertically aligned, two-terminal electronic devices. Such particles demonstrate remarkable chemical and mechanical stability and form the basis of particulate electronic devices capable of collecting and storing information about their surroundings, extending nanoelectronics into previously inaccessible environments.