Unit-cell-thick zeolitic imidazolate framework films for membrane application.

Zeolitic imidazolate frameworks (ZIFs) are a subset of metal-organic frameworks with more than 200 characterized crystalline and amorphous networks made of divalent transition metal centres (for example, Zn2+ and Co2+) linked by imidazolate linkers. ZIF thin films have been intensively pursued, motivated by the desire to prepare membranes for selective gas and liquid separations. To achieve membranes with high throughput, as in ångström-scale biological channels with nanometre-scale path lengths, ZIF films with the minimum possible thickness-down to just one unit cell-are highly desired. However, the state-of-the-art methods yield membranes where ZIF films have thickness exceeding 50 nm. Here we report a crystallization method from ultradilute precursor mixtures, which exploits registry with the underlying crystalline substrate, yielding (within minutes) crystalline ZIF films with thickness down to that of a single structural building unit (2 nm). The film crystallized on graphene has a rigid aperture made of a six-membered zinc imidazolate coordination ring, enabling high-permselective H2 separation performance. The method reported here will probably accelerate the development of two-dimensional metal-organic framework films for efficient membrane separation.

Bottom-up synthesis of graphene films hosting atom-thick molecular-sieving apertures.

Incorporation of a high density of molecular-sieving nanopores in the graphene lattice by the bottom-up synthesis is highly attractive for high-performance membranes. Herein, we achieve this by a controlled synthesis of nanocrystalline graphene where incomplete growth of a few nanometer-sized, misoriented grains generates molecular-sized pores in the lattice. The density of pores is comparable to that obtained by the state-of-the-art postsynthetic etching (1012 cm-2) and is up to two orders of magnitude higher than that of molecular-sieving intrinsic vacancy defects in single-layer graphene (SLG) prepared by chemical vapor deposition. The porous nanocrystalline graphene (PNG) films are synthesized by precipitation of C dissolved in the Ni matrix where the C concentration is regulated by controlled pyrolysis of precursors (polymers and/or sugar). The PNG film is made of few-layered graphene except near the grain edge where the grains taper down to a single layer and eventually terminate into vacancy defects at a node where three or more grains meet. This unique nanostructure is highly attractive for the membranes because the layered domains improve the mechanical robustness of the film while the atom-thick molecular-sized apertures allow the realization of large gas transport. The combination of gas permeance and gas pair selectivity is comparable to that from the nanoporous SLG membranes prepared by state-of-the-art postsynthetic lattice etching. Overall, the method reported here improves the scale-up potential of graphene membranes by cutting down the processing steps.

Unraveling the Oxidation of a Graphitic Lattice: Structure Determination of Oxygen Clusters.

Unraveling the oxidation of graphitic lattice is of great interest for atomic-scale lattice manipulation. Herein, we build epoxy cluster, atom by atom, using Van der Waals' density-functional theory aided by Clar's aromatic π-sextet rule. We predict the formation of cyclic epoxy trimers and its linear chains propagating along the armchair direction of the lattice to minimize the system's energy. Using low-temperature scanning tunneling microscopy on oxidized graphitic lattice, we identify linear chains as bright features that have a threefold symmetry, and which exclusively run along the armchair direction of the lattice confirming the theoretical predictions.

Water dynamics in the hydration shell of hyper-branched poly-ethylenimine.

We performed THz and GHz dielectric relaxation spectroscopy to investigate the reorientational dynamics of water molecules in the hydration shell of amphiphilic hyper-branched poly-ethylenimine (HPEI). Four Debye equations were employed to describe four types of water in the hydration shell, including bulk-like water, under-coordinated water, slow water (water molecules hydrating the hydrophobic groups and water molecules accepting hydrogen bonds from the NH2 groups) and super slow water (water molecules donating hydrogen bonds to and accepting hydrogen bonds from NH groups). The time scales of undercoordinated and bulk-like water show a slight decline from 0.4 to 0.1 ps and from 8 to 2 ps, respectively. Because of hydrophilic amino groups, HPEI molecules exhibit a strong retardation effect, where the time scales of slow and super slow water increase with concentration from 17 to 39.9 ps and from 88 to 225 ps, respectively.

Exceptional point in a metal-graphene hybrid metasurface with tunable asymmetric loss.

Observation of exceptional points (EPs) in non-Hermitian parity-time (PT) symmetric systems has led to various nontrivial physics and exotic phenomena. Here, a metal-graphene hybrid non-Hermitian metasurface is proposed in the terahertz regime, whose unit cell is composed of two orthogonally oriented split-ring resonators (SRRs) with identical dimensions but only one SRR containing a graphene patch at the gap. An EP in polarization space is theoretically observed at a certain Fermi level of the graphene patch, where the induced asymmetric loss and the coupling strength between the two SRRs match a certain relation predicted by a coupled mode theory. The numerical fittings using the coupled mode theory agree well with the simulations. Besides, an abrupt phase flip around the EP frequency is observed in the transmission in circular polarization basis, which can be very promising in ultra-sensitive sensing applications.

A novel degradation mechanism of the elastic modulus of wet polymer substrates under nanoindentation.

We demonstrated that the formation and solidification of a continuous confined water film played a very important role in changing the elastic modulus of the wet polymer substrate in a nanoindentation process by a coarse-grained molecular dynamics simulation of this process. It was found that as the water content increased, the elastic modulus of the wet polymer substrate showed a non-monotonic change. Relative to the dry polymer substrate, the elastic modulus of the wet polymer first decreased. This is because the appearance of a confined water film caused the force between the polymer substrate and the indenter to change from repulsion to attraction. Subsequently, as the confined water film gradually solidified and then weakened, the elastic modulus of the wet polymer slowly increased and then rapidly increased due to a large number of interstitial water molecules gradually penetrating the polymer substrate. Therefore, it is unreasonable to explain the wet polymer degradation during nanoindentation only from the plasticization and anti-plasticization effects based on the hydrogen bond breaking and formation during stretching. The above-mentioned results will help to more comprehensively understand the degradation mechanism of the polymers' encounter with water, thus promoting further practical applications for polymers.

Polarization-insensitive tunable terahertz polarization rotator.

A rotation-angle variable polarization rotator is proposed and demonstrated using an all-dielectric metasurface doublet. Such a transmissive device can conveniently rotate the polarization of incident light by any desired angles by mechanically changing the relative angle of the double metasurface layers regardless of the incident state of polarization. Under certain conditions the device acts as a full phase modulator for the circularly polarized incident wave. Hence, it has a promising application in polarization and phase control.

Terahertz electric field modulated mode coupling in graphene-metal hybrid metamaterials.

Taking advantage of the tunable conductivity of graphene under high terahertz (THz) electric field, a graphene-metal hybrid metamaterial consisting of an array of three adjoined orthogonally oriented split-ring resonators (SRRs) is proposed and experimentally demonstrated to show a maximum modulation depth of 23% in transmission when the THz peak field reaches 305 kV/cm. The transmission of the sample is dominated by the antisymmetric and symmetric resonant modes originating from the strong magneto-inductive and conductive coupling among the three SRRs, respectively. Numerical simulations and model calculations based on a coupled oscillator theory were performed to explain the modulation process. It is found that the graphene coating impairs the resonances by increasing the damping of the modes and decreasing the coupling between the SRRs whereas the strong THz field restores the resonances by decreasing the conductivity of graphene.

Polarization-dependent electromagnetic responses in an A-shape metasurface.

We numerically and experimentally demonstrate polarization-dependent terahertz responses in a proposed metasurface of A-shape resonators. With the horizontal polarization incidence, the observed transmission window is formed by two resonance dips, corresponding to the inductive-capacitive resonance at the lower frequency and the high-order antisymmetric resonance at a higher frequency, respectively. When the incident wave is perpendicularly polarized, the transmission window arises from the plasmon-induced transparency spectral response. The origin of the polarization-sensitive resonance properties is revealed by mapping the electric field and terahertz-induced surface current in the proposed metamaterials. Moreover, the influence of the geometry of the A-shape microstructures on the transmission spectra is analyzed. These polarization-dependent metamaterials may provide more degrees of freedom in tuning the electromagnetic responses, thus offering a path toward robust metamaterials design.

Terahertz surface plasmon polariton waveguiding with periodic metallic cylinders.

We demonstrated a structure with periodic cylinders arranged bilaterally and a thin dielectric layer covered inside that supports bound modes of surface plasmon polaritons at terahertz frequencies. This structure can confine the surface plasmon polaritons in the lateral direction, and at the same time reduce the field expansion into space. We examined and explored the characteristics of several different structures using scanning near-field terahertz microscopy. The proposed designs pave a novel way to terahertz waveguiding and may have important applications in the development of flexible, wideband and compact photonic circuits operating at terahertz frequencies.

Unexpectedly Enhanced Solubility of Aromatic Amino Acids and Peptides in an Aqueous Solution of Divalent Transition-Metal Cations.

We experimentally observed considerable solubility of tryptophan (Trp) in a CuCl_{2} aqueous solution, which could reach 2-5 times the solubility of Trp in pure water. Theoretical studies show that the strong cation-π interaction between Cu^{2+} and the aromatic ring in Trp modifies the electronic distribution of the aromatic ring to enhance significantly the water affinity of Trp. Similar solubility enhancement has also been observed for other divalent transition-metal cations (e.g., Zn^{2+} and Ni^{2+}), another aromatic amino acid (phenylalanine), and three aromatic peptides (Trp-Phe, Phe-Phe, and Trp-Ala-Phe).

Molecular recognition and interaction between uracil and urea in solid-state studied by terahertz time-domain spectroscopy.

Using terahertz time-domain spectroscopy characterization, we observe that urea is able to recognize and interact with uracil efficiently even in the solid phase without involving water or solvents. A cocrystal configuration linked by a pair of hydrogen bonds between uracil and urea was formed. The terahertz absorption spectrum of the cocrystal shows a distinct new absorption at 0.8 THz (26.7 cm(-1)), which originates from the intermolecular hydrogen bonding. Both mechanical milling and heating can accelerate the reaction efficiently. Density functional theory was adopted to simulate the vibrational modes of the cocrystal, and the results agree well with the experimental observation. Multiple techniques, including powder X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy, were performed to investigate the reaction process, and they presented supportive evidence. This work enables in-depth understanding of recognition and interaction of urea with nucleobases and comprehension of the denaturation related to RNA. We also demonstrate that terahertz spectroscopy is an effective and alternative tool for online measurement and quality control in pharmaceutical and chemical industries.

Combined Charge Extraction by Linearly Increasing Voltage and Time-Resolved Microwave Conductivity to Reveal the Dynamic Charge Carrier Mobilities in Thin-Film Organic Solar Cells.

This article reports a purely experiment-based method to evaluate the time-dependent charge carrier mobilities in thin-film organic solar cells (OSCs) using simultaneous charge extraction by linearly increasing the voltage (CELIV) and time-resolved microwave conductivity (TRMC) measurements. This method enables the separate measurement of electron mobility (µe) and hole mobility (µh) in a metal-insulator-semiconductor (MIS) device. A slope-injection-restoration voltage profile for MIS-CELIV is also proposed to accurately determine the charge densities. The dynamic behavior of µe and µh is examined in five bulk heterojunction (BHJ) OSCs of polymer:fullerene (P3HT:PCBM and PffBT4T:PCBM) and polymer:nonfullerene acceptor (PM6:ITIC, PM6:IT4F, and PM6:Y6). While the former exhibits fast decays of µh and µe, the latter, in particular, PM6:IT4F and PM6:Y6, exhibits slow decays. Notably, the high-performing PM6:Y6 demonstrates both a balanced mobility (µe/µh) of 1.0-1.1 within 30 µs and relatively large CELIV-TRMC mobility values among the five BHJs. The results exhibit reasonable consistency with a high fill factor. The proposed new CELIV-TRMC technique offers a path toward a comprehensive understanding of dynamic mobility and its correlation with the OSC performance.

Discovery of GPR84 Fluorogenic Probes Based on a Novel Antagonist for GPR84 Bioimaging.

GPR84 is a promising therapeutic target and biomarker for a range of diseases. In this study, we reported the discovery of BINOL phosphate (BINOP) derivatives as GPR84 antagonists. By investigating the structure-activity relationship, we identified 15S as a novel GPR84 antagonist. 15S exhibits low nanomolar potency and high selectivity for GPR84, while its enantiomer 15R is less active. Next, we rationally designed and synthesized a series of GPR84 fluorogenic probes by conjugating Nile red and compound 15S. The leading hybrid, probe F8, not only retained GPR84 activity but also exhibited low nonspecific binding and a turn-on fluorescent signal in an apolar environment. F8 enabled visualization and detection of GPR84 in GPR84-overexpressing HEK293 cells and lipopolysaccharide-stimulated neutrophils. Furthermore, we demonstrated that F8 can detect upregulated GPR84 protein levels in mice models of inflammatory bowel disease and acute lung injury. Thus, compound F8 represents a promising tool for studying GPR84 functions.

Tuning Pore Size in Graphene in the Angstrom Regime for Highly Selective Ion-Ion Separation.

Zero-dimensional pores spanning only a few angstroms in size in two-dimensional materials such as graphene are some of the most promising systems for designing ion-ion selective membranes. However, the key challenge in the field is that so far a crack-free macroscopic graphene membrane for ion-ion separation has not been realized. Further, methods to tune the pores in the Å-regime to achieve a large ion-ion selectivity from the graphene pore have not been realized. Herein, we report an Å-scale pore size tuning tool for single layer graphene, which incorporates a high density of ion-ion selective pores between 3.5 and 8.5 Å while minimizing the nonselective pores above 10 Å. These pores impose a strong confinement for ions, which results in extremely high selectivity from centimeter-scale porous graphene between monovalent and bivalent ions and near complete blockage of ions with the hydration diameter, DH, greater than 9.0 Å. The ion diffusion study reveals the presence of an energy barrier corresponding to partial dehydration of ions with the barrier increasing with DH. We observe a reversal of K+/Li+ selectivity at elevated temperature and attribute this to the relative size of the dehydrated ions. These results underscore the promise of porous two-dimensional materials for solute-solute separation when Å-scale pores can be incorporated in a precise manner.

Subcycle surface electron emission driven by strong-field terahertz waveforms.

The advent of intense terahertz (THz) sources opened a new era when the demonstration of the acceleration and manipulation of free electrons by THz pulses became within reach. THz-field-driven electron emission was predicted to be confined to a single burst due to the single-cycle waveform. Here we demonstrate the confinement of single-cycle THz-waveform-driven electron emission to one of the two half cycles from a solid surface emitter. Either the leading or the trailing half cycle was active, controlled by reversing the field polarity. THz-driven single-burst surface electron emission sources, which do not rely on field-enhancement structures, will impact the development of THz-powered electron acceleration and manipulation devices, all-THz compact electron sources, THz waveguides and telecommunication, THz-field-based measurement techniques and solid-state devices.

Mechanistic Insights on Functionalization of Graphene with Ozone.

The exposure of graphene to O3 results in functionalization of its lattice with epoxy, even at room temperature. This reaction is of fundamental interest for precise lattice patterning, however, is not well understood. Herein, using van der Waals density functional theory (vdW-DFT) incorporating spin-polarized calculations, we find that O3 strongly physisorbs on graphene with a binding energy of -0.46 eV. It configures in a tilted position with the two terminal O atoms centered above the neighboring graphene honeycombs. A dissociative chemisorption follows by surpassing an energy barrier of 0.75 eV and grafting an epoxy group on graphene reducing the energy of the system by 0.14 eV from the physisorbed state. Subsequent O3 chemisorption is preferred on the same honeycomb, yielding two epoxy groups separated by a single C-C bridge. We show that capturing the onset of spin in oxygen during chemisorption is crucial. We verify this finding with experiments where an exponential increase in the density of epoxy groups as a function of reaction temperature yields an energy barrier of 0.66 eV, in agreement with the DFT prediction. These insights will help efforts to obtain precise patterning of the graphene lattice.

Selective Photonic Gasification of Strained Oxygen Clusters on Graphene for Tuning Pore Size in the Å Regime.

Controlling the size of single-digit pores, such as those in graphene, with an Å resolution has been challenging due to the limited understanding of pore evolution at the atomic scale. The controlled oxidation of graphene has led to Å-scale pores; however, obtaining a fine control over pore evolution from the pore precursor (i.e., the oxygen cluster) is very attractive. Herein, we introduce a novel "control knob" for gasifying clusters to form pores. We show that the cluster evolves into a core/shell structure composed of an epoxy group surrounding an ether core in a bid to reduce the lattice strain at the cluster core. We then selectively gasified the strained core by exposing it to 3.2 eV of light at room temperature. This allowed for pore formation with improved control compared to thermal gasification. This is because, for the latter, cluster-cluster coalescence via thermally promoted epoxy diffusion cannot be ruled out. Using the oxidation temperature as a control knob, we were able to systematically increase the pore density while maintaining a narrow size distribution. This allowed us to increase H2 permeance as well as H2 selectivity. We further show that these pores could differentiate CH4 from N2, which is considered to be a challenging separation. Dedicated molecular dynamics simulations and potential of mean force calculations revealed that the free energy barrier for CH4 translocation through the pores was lower than that for N2. Overall, this study will inspire research on the controlled manipulation of clusters for improved precision in incorporating Å-scale pores in graphene.

Unsymmetrical Phosphodiesters as GPR84 Antagonists with High Blood Exposure for the Treatment of Lung Inflammation.

GPR84 is a proinflammatory G protein-coupled receptor that mediates myeloid immune cell functions. Blocking GPR84 with antagonists is a promising approach for treating inflammatory and fibrotic diseases. Previously, a GPR84 antagonist 604c, with a symmetrical phosphodiester structure, has displayed promising efficacy in a mouse model of ulcerative colitis. However, the low blood exposure resulting from physicochemical properties prevented its uses in other inflammatory diseases. In this study, a series of unsymmetrical phosphodiesters with lower lipophilicity were designed and tested. The representative compound 37 exhibited a 100-fold increase in mouse blood exposure compared to 604c while maintaining in vitro activity. In a mouse model of acute lung injury, 37 (30 mg/kg, po) significantly reduced the infiltration of proinflammatory cells and the release of inflammatory cytokines and ameliorated pathological changes equally or more effectively than N-acetylcysteine (100 mg/kg, po). These findings suggest that 37 is a promising candidate for treating lung inflammation.

Structure Evolution of Graphitic Surface upon Oxidation: Insights by Scanning Tunneling Microscopy.

Oxidation of graphitic materials has been studied for more than a century to synthesize materials such as graphene oxide, nanoporous graphene, and to cut or unzip carbon nanotubes. However, the understanding of the early stages of oxidation is limited to theoretical studies, and experimental validation has been elusive. This is due to (i) challenging sample preparation for characterization because of the presence of highly mobile and reactive epoxy groups formed during oxidation, and (ii) gasification of the functional groups during imaging with atomic resolution, e.g., by transmission electron microscopy. Herein, we utilize a low-temperature scanning tunneling microscope (LT-STM) operating at 4 K to solve the structure of epoxy clusters form upon oxidation. Three distinct nanostructures corresponding to three stages of evolution of vacancy defects are found by quantitatively verifying the experimental data by the van der Waals density functional theory. The smallest cluster is a cyclic epoxy trimer. Their observation validates the theoretical prediction that epoxy trimers minimize the energy in the cyclic structure. The trimers grow into honeycomb superstructures to form larger clusters (1-3 nm). Vacancy defects evolve only in the larger clusters (2-3 nm) in the middle of the cluster, highlighting the role of lattice strain in the generation of vacancies. Semiquinone groups are also present and are assigned at the carbon edge in the vacancy defects. Upon heating to 800 °C, we observe cluster-free vacancy defects resulting from the loss of the entire epoxy population, indicating a reversible functionalization of epoxy groups.