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.

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.

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.

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.

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.

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.

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.

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.

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.

Nitroaromatic detection and infrared communication from wild-type plants using plant nanobionics.

Plant nanobionics aims to embed non-native functions to plants by interfacing them with specifically designed nanoparticles. Here, we demonstrate that living spinach plants (Spinacia oleracea) can be engineered to serve as self-powered pre-concentrators and autosamplers of analytes in ambient groundwater and as infrared communication platforms that can send information to a smartphone. The plants employ a pair of near-infrared fluorescent nanosensors-single-walled carbon nanotubes (SWCNTs) conjugated to the peptide Bombolitin II to recognize nitroaromatics via infrared fluorescent emission, and polyvinyl-alcohol functionalized SWCNTs that act as an invariant reference signal-embedded within the plant leaf mesophyll. As contaminant nitroaromatics are transported up the roots and stem into leaf tissues, they accumulate in the mesophyll, resulting in relative changes in emission intensity. The real-time monitoring of embedded SWCNT sensors also allows residence times in the roots, stems and leaves to be estimated, calculated to be 8.3 min (combined residence times of root and stem) and 1.9 min mm-1 leaf, respectively. These results demonstrate the ability of living, wild-type plants to function as chemical monitors of groundwater and communication devices to external electronics at standoff distances.

Observation of the Marcus Inverted Region of Electron Transfer from Asymmetric Chemical Doping of Pristine (n,m) Single-Walled Carbon Nanotubes.

The concept of electrical energy generation based on asymmetric chemical doping of single-walled carbon nanotube (SWNT) papers is presented. We explore 27 small, organic, electron-acceptor molecules that are shown to tune the output open-circuit voltage (VOC) across three types of pristine SWNT papers with varying (n,m) chirality distributions. A considerable enhancement in the observed VOC, from 80 to 440 mV, is observed for SWNT/molecule acceptor pairs that have molecular volume below 120 Å3 and lowest unoccupied molecular orbital (LUMO) energies centered around -0.8 eV. The electron transfer (ET) rate constants driving the VOC generation are shown to vary with the chirality-associated Marcus theory, suggesting that the energy gaps between SWNT and the LUMO of acceptor molecules dictate the ET process. When the ET rate constants and the maximum VOC are plotted versus the LUMO energy of the acceptor organic molecule, volcano-shaped dependencies, characteristic of the Marcus inverted region, are apparent for three distinct sources of SWNT papers with modes in diameter distributions of 0.95, 0.83, and 0.75 nm. This observation, where the ET driving force exceeds reorganization energies, allows for an estimation of the outer-sphere reorganization energies with values as low as 100 meV for the (8,7) SWNT, consistent with a proposed image-charge modified Born energy model. These results expand the fundamental understanding of ET transfer processes in SWNT and allow for an accurate calculation of energy generation through asymmetric doping for device applications.

Microscale solid-state thermal diodes enabling ambient temperature thermal circuits for energy applications.

Thermal diodes, or devices that transport thermal energy asymmetrically, analogous to electrical diodes, hold promise for thermal energy harvesting and conservation, as well as for phononics or information processing. The junction of a phase change material and phase invariant material can form a thermal diode; however, there are limited constituent materials available for a given target temperature, particularly near ambient. In this work, we demonstrate that a micro and nanoporous polystyrene foam can house a paraffin-based phase change material, fused to PMMA, to produce mechanically robust, solid-state thermal diodes capable of ambient operation with Young's moduli larger than 11.5 MPa and 55.2 MPa above and below the melting transition point, respectively. Moreover, the composites show significant changes in thermal conductivity above and below the melting point of the constituent paraffin and rectification that is well-described by our previous theory and the Maxwell-Eucken model. Maximum thermal rectifications range from 1.18 to 1.34. We show that such devices perform reliably enough to operate in thermal diode bridges, dynamic thermal circuits capable of transforming oscillating temperature inputs into single polarity temperature differences - analogous to an electrical diode bridge with widespread implications for transient thermal energy harvesting and conservation. Overall, our approach yields mechanically robust, solid-state thermal diodes capable of engineering design from a mathematical model of phase change and thermal transport, with implications for energy harvesting.

Observation of Switchable Photoresponse of a Monolayer WSe2-MoS2 Lateral Heterostructure via Photocurrent Spectral Atomic Force Microscopic Imaging.

In the pursuit of two-dimensional (2D) materials beyond graphene, enormous advances have been made in exploring the exciting and useful properties of transition metal dichalcogenides (TMDCs), such as a permanent band gap in the visible range and the transition from indirect to direct band gap due to 2D quantum confinement, and their potential for a wide range of device applications. In particular, recent success in the synthesis of seamless monolayer lateral heterostructures of different TMDCs via chemical vapor deposition methods has provided an effective solution to producing an in-plane p-n junction, which is a critical component in electronic and optoelectronic device applications. However, spatial variation of the electronic and optoelectonic properties of the synthesized heterojunction crystals throughout the homogeneous as well as the lateral junction region and the charge carrier transport behavior at their nanoscale junctions with metals remain unaddressed. In this work, we use photocurrent spectral atomic force microscopy to image the current and photocurrent generated between a biased PtIr tip and a monolayer WSe2-MoS2 lateral heterostructure. Current measurements in the dark in both forward and reverse bias reveal an opposite characteristic diode behavior for WSe2 and MoS2, owing to the formation of a Schottky barrier of dissimilar properties. Notably, by changing the polarity and magnitude of the tip voltage applied, pixels that show the photoresponse of the heterostructure are observed to be selectively switched on and off, allowing for the realization of a hyper-resolution array of the switchable photodiode pixels. This experimental approach has significant implications toward the development of novel optoelectronic technologies for regioselective photodetection and imaging at nanoscale resolutions. Comparative 2D Fourier analysis of physical height and current images shows high spatial frequency variations in substrate/MoS2 (or WSe2) contact that exceed the frequencies imposed by the underlying substrates. These results should provide important insights in the design and understanding of electronic and optoelectronic devices based on quantum confined atomically thin 2D lateral heterostructures.

Highly CO2/N2-switchable zwitterionic surfactant for pickering emulsions at ambient temperature.

Cross-linked polymer particles were prepared via surfactant-free emulsion copolymerization of 2-(diethylamino)ethyl methacrylate (DEAEMA) and sodium methacrylate (SMA) using N,N'-methylenebis(acrylamide) (MBA) as a cross-linker. Generated particles are zwitterionic, possessing unique isoelectric points in the pH range of 7.5-8.0, which is readily tunable through CO2/N2 bubbling. The particles were found to be highly responsive to CO2/N2 switching, dissolving in water with CO2 bubbling and precipitating with N2 bubbling at room temperature. Pickering emulsions of n-dodecane were prepared using these particles as the sole emulsifier. These emulsions can be rapidly demulsified with CO2 bubbling, resulting in complete oil/water phase separations. Nitrogen bubbling efficiently re-emulsifies the oil with the aid of homogenization. The rapid emulsification/demulsification using CO2/N2 bubbling at room temperature provides these cross-linked zwitterionic particles with distinct advantages as functional Pickering surfactants.

Generalized Model for Inhibitor-Modulated 2D Polymer Growth to Understand the Controlled Synthesis of Covalent Organic Frameworks.

Two-dimensional (2D) polymers, also known as 2D covalent organic frameworks (COFs), are increasingly finding use in applications such as membrane separations, catalysis, and energy conversion. Current research is focused on the development of new synthesis routes for COFs and obtaining a mechanistic understanding of the growth process to control it in a better manner. In this regard, synthesis methods such as reversible polycondensation termination use monofunctional inhibitor species to achieve a controlled growth rate for COFs. However, so far, the role of the inhibitors in modulating the kinetics of COF growth is inadequately understood. In this work, inspired by the Mayo-Lewis framework, we develop a generalized kinetic model to describe the synthesis of a 2D COF monolayer. Our model involves six parameters corresponding to the rate constants of attachment and detachment of monomer and inhibitor species, as well as enhancement factors that quantify the effect of the local coordination environment of the attaching/detaching species on the reaction kinetics. We measure the inhibitor concentration-dependent growth kinetics of the COF experimentally and fit our model to experimental yield data, with the same parameters working across multiple inhibitor concentrations. As the growth process is inherently stochastic, we use this knowledge to develop a comprehensive kinetic Monte Carlo (KMC) simulation of 2D COF synthesis, demonstrating that scaled rate constants are required in the inherently local KMC simulations rather than those obtained from the global kinetic model. The KMC simulations point to an inverse flake size-inhibitor concentration relationship, in agreement with experiments, indicating that flake sizes could be precisely regulated by changing the inhibitor concentrations. Overall, our work promises to improve the understanding of 2D COF synthesis and will help in controlling the growth process to obtain the desired flake size distribution and product morphology.

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.

Chemical and structural changes in preserved white egg during pickled by vacuum technology.

The aim of the present study was to investigate the chemical and structural changes in preserved white egg during pickled by vacuum technology. In order to evaluate the changes of preserved egg white during its salted period, sulfhydryl group, disulfide bond contents, and surface hydrophobicity were measured. Fourier transformation infrared spectroscopy and circular dichroism analysis were performed by considering changes of the secondary structural elements, the protein components of egg white was also studied in more detail by electrophoresis. Results showed that the sulfhydryl group and surface hydrophobicity were increased with increasing salting time, whereas a decrease in the disulfide bond contents was observed. Regarding the secondary structure analysis, a decrease in α-helices and ß-turns were accompanied by increases in ß-sheets and random coils, which indicating a decrease in non-random structure while ever increasing of unordered structure. Electrophoresis shows no significant differences in protein patterns among fresh egg white and samples salting for up to 4 days. Disappearance of most protein was at the 5th day. Single band of ovalbumin can be readily observed from the 6th day to the ripening period finished. Our findings reveal that high pH could induce duck egg white protein aggregation when salting eggs in strong alkaline solution for a long time.

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.

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.