Engineering the Band Topology in a Rhombohedral Trilayer Graphene Moiré Superlattice.

Moiré superlattices have become a fertile playground for topological Chern insulators, where the displacement field can tune the quantum geometry and Chern number of the topological band. However, in experiments, displacement field engineering of spontaneous symmetry-breaking Chern bands has not been demonstrated. Here in a rhombohedral trilayer graphene moiré superlattice, we use a thermodynamic probe and transport measurement to monitor the Chern number evolution as a function of the displacement field. At a quarter filling of the moiré band, a novel Chern number of three is unveiled to compete with the well-established number of two upon turning on the electric field and survives when the displacement field is sufficiently strong. The transition can be reconciled by a nematic instability on the Fermi surface due to the pseudomagnetic vector field potentials associated with moiré strain patterns. Our work opens more opportunities to active control of Chern numbers in van der Waals moiré systems.

Design and Properties of Novel Hydrophobic Natural Tea Saponin and Its Organogels.

It was first discovered that the excellent gelation ability of tea saponin can be obtained by introducing long-chain alkyl groups of dodecanoyl chloride into the glycosyl portion with direct esterification. The modified dodecanoyl chloride-tea saponin (DC-TS) was successfully synthesized and characterized with NMR, MS, and FT-IR. The tests showed that the long-chain alkyl group was successfully introduced. Combined with SEM and X-ray diffraction patterns, we found that the stable lamellar shape gels of DC-TS were formed in a variety of solvents. More interestingly, organogel was also obtained by adjusting good solvent and poor solvent as mixed solvent. It is worth noting that the driving force of organogels is the combination of hydrogen bonding and the hydrophobic interaction of the introduced alkyl chains with the rigid backbone of pentacyclic triterpenes. The modified tea saponin, a natural green surfactant, was discovered to have gelation properties, which has broadened tea saponin's scope of application and made it more promising.

A novel strategy by combining foam fractionation with high-speed countercurrent chromatography for the rapid and efficient isolation of antioxidants and cytostatics from Camellia oleifera cake.

Camellia oleifera cake is a by-product, which is rich in functional chemical components. However, it is typically used as animal feed with no commercial value. The purpose of this study was to isolate and identify compounds from Camellia oleifera cake using a combination of foam fractionation and high-speed countercurrent chromatography (HSCCC) and to investigate their biological activities. Foam fractionation with enhanced drainage through a hollow regular decahedron (HRD) was first established for simultaneously enriching flavonoid glycosides and saponins for further separation of target compounds. Under suitable operating conditions, the introduction of HRD resulted in a threefold increase in enrichment ratio with no negative effect on recovery. A novel elution-extrusion countercurrent chromatography (EECCC) coupled with the consecutive injection mode was established for the successful simultaneous isolation of flavonoid glycosides and saponins. As a result, 38.7 mg of kaemferol-3-O-[2-O-D-glucopyranosyl-6-O-α-L-rhamnopyranosyl]-ß-D-glucopyranoside (purity of 98.17%, FI), 70.8 mg of kaemferol-3-O-[2-O-ß-D-xylopyranosyl-6-O-α-L-rhamnopyranosyl]-ß-D-glucopyranoside (purity of 97.52%, FII), and 560 mg of an oleanane-type saponin (purity of 92.32%, FIII) were separated from the sample (900 mg). The present study clearly showed that FI and II were natural antioxidants (IC50 < 35 µg/mL) without hemolytic effect. FIII displayed the effect of inhibiting Hela cell proliferation (IC50 < 30 µg/mL). Further erythrocyte experiments showed that this correlated with the extremely strong hemolytic effect of FIII. Overall, this study offers a potential strategy for efficient and green isolation of natural products, and is beneficial to further expanding the application of by-products (Camellia oleifera cake) in food, cosmetics, and pharmacy.

Foam and fluid properties of purified saponins and non-purified water extracts from Camellia oleifera cake (by-product).

The physicochemical and foam properties of non-purified water extracts (WE) and purified tea saponins (TS) from Camellia oleifera cake (byproduct) were compared. WE showed different fluid properties at equal saponin concentrations (1.0 wt%) compared to TS. Particularly, it exhibited limited micelle size (average 434.1 nm), effective viscosity (0.15 Pa·s), and surface tension (43.9 mN/m) independently of pH. Moreover, the foam properties of WE were comparable to TS and better than sodium caseinate, especially foam stability. WE foam was more stable than TS foam under pH (3-7) and heating (40-80 °C). In the presence of NaCl, sucrose, and ethanol (5-20 wt%), WE and TS were effective and had similar foam behavior. Low concentrations of sucrose (<10 wt%)/ethanol (<20 wt%) significantly increased the foam capacity, while ethanol over 30 wt% was unfavorable. WE/TS foam contributes significantly to the desired physicochemical and sensory attributes (taste, texture, and appearance) of foods.

Chemical Potential Characterization of Symmetry-Breaking Phases in a Rhombohedral Trilayer Graphene.

Rhombohedral trilayer graphene has recently emerged as a natural flat-band platform for studying interaction-driven symmetry-breaking phases. The displacement field (D) can further flatten the band to enhance the density of states, thereby controlling the electronic correlation that tips the energy balance between spin and valley degrees of freedom. To characterize the energy competition, chemical potential measurement─a direct thermodynamic probe of Fermi surfaces─is highly demanding to be conducted under a constant D. In this work, we characterize D-dependent isospin flavor polarization, where electronic states with isospin degeneracies of one and two can be identified. We also developed a method to measure the chemical potential at a fixed D, allowing for the extraction of energy variation during phase transitions. Furthermore, symmetry breaking could also be invoked in Landau levels, manifesting as quantum Hall ferromagnetism. Our work opens more opportunities for the thermodynamic characterization of displacement-field tuned van der Waals heterostructures.

Design and Properties of Natural Rosin-Based Phosphoester Functional Surfactants.

As an important forestry biomass resource, rosin has a wide range of applications in medicine, adhesives, surfactants and other fields. Using natural dehydroabietic acid as a raw material, dehydroabietic acid-based phosphorus monoester (DPM) and diester (DPD) surfactants were designed and synthesized. The chemical structures and self-assembly properties were characterized by FT-IR, NMR and TEM, and the effects of pH on critical micelle concentration, γCMC, emulsifying properties, foam properties and micelle morphology were studied. The results showed that the CMC, γCMC value and aggregate morphology had certain pH responsiveness. The γCMC value under acidic conditions was smaller than γCMC under alkaline conditions, and the foaming performance and foam stability under acidic conditions were better than those under alkaline conditions. TEM micelle morphology studies have shown that DPM and DPD surfactants can self-assemble into rod-shaped and spherical micelle morphologies with a pH change in an aqueous solution. At the same pH, the foaming and emulsification properties of DPD were better than those of DPM. The best foaming and emulsification ability of DPD were 11.8 mL and 175 s, respectively. At the same time, the foaming ability of DPD is also affected by pH. DPD has excellent foaming properties in acidic conditions, but these disappeared in neutral conditions.

Giant ferroelectric polarization in a bilayer graphene heterostructure.

At the interface of van der Waals heterostructures, the crystal symmetry and the electronic structure can be reconstructed, giving rise to physical properties superior to or absent in parent materials. Here by studying a Bernal bilayer graphene moiré superlattice encapsulated by 30°-twisted boron nitride flakes, we report an unprecedented ferroelectric polarization with the areal charge density up to 1013 cm-2, which is far beyond the capacity of a moiré band. The translated polarization ~5 pC m-1 is among the highest interfacial ferroelectrics engineered by artificially stacking van der Waals crystals. The gate-specific ferroelectricity and co-occurring anomalous screening are further visualized via Landau levels, and remain robust for Fermi surfaces outside moiré bands, confirming their independence on correlated electrons. We also find that the gate-specific resistance hysteresis loops could be turned off by the other gate, providing an additional control knob. Furthermore, the ferroelectric switching can be applied to intrinsic properties such as topological valley current. Overall, the gate-specific ferroelectricity with strongly enhanced charge polarization may encourage more explorations to optimize and enrich this novel class of ferroelectricity, and promote device applications for ferroelectric switching of various quantum phenomena.

Multivalley Superconductivity in Monolayer Transition Metal Dichalcogenides.

In transition metal dichalcogenides (TMDs), Ising superconductivity with an antisymmetric spin texture on the Fermi surface has attracted wide interest due to the exotic pairing and topological properties. However, it is not clear whether the Q valley with a giant spin splitting is involved in the superconductivity of heavily doped semiconducting 2H-TMDs. Here by taking advantage of a high-quality monolayer WS2 on hexagonal boron nitride flakes, we report an ionic-gating induced superconducting dome with a record high critical temperature of ∼6 K, accompanied by an emergent nonlinear Hall effect. The nonlinearity indicates the development of an additional high-mobility channel, which (corroborated by first principle calculations) can be ascribed to the population of Q valleys. Thus, multivalley population at K and Q is suggested to be a prerequisite for developing superconductivity. The involvement of Q valleys also provides insights to the spin textured Fermi surface of Ising superconductivity in the large family of transition metal dichalcogenides.

Molecular Design, Supramolecular Assembly, and Excellent Dye Adsorption Capacity of Natural Rigid Dehydroabietic Acid-Tailored Amide Organogelators.

It is very appealing to synthesize functional soft materials from natural and abundant plant diterpenes because they have conformationally rigid and chiral properties. Herein, dehydroabietic-based monoamide (DA-1) and diamide (DA-2) were designed by introducing device interactions, π-π stacking and hydrogen bonding, with an aromatic group, C═O, and N-H. DA-1 and DA-2 can be gelled in a mixed solvent and a single solvent, respectively. Several novel supramolecular organic gels including highly entangled three-dimensional networks composed of rods or fibers were constructed. Interestingly, DA-2 forms a helical structure that is right-handed under the cooperative control of the solvent and the rigid structure of rosin. Gel formation was primarily driven by hydrogen bonding, π-π stacking, and van der Waals force. Combined with Gaussian calculation and X-ray diffraction (XRD), we established pack patterns for each system, revealing the roles played by rosin and amide groups. Moreover, the carbon tetrachloride gel of DA-2 can effectively remove Congo red in an aqueous solution, and the removal rate can reach 98.4%. This research explores an efficient organic gel for adsorbing Congo red dye with the secretions of pine trees.

Light Controllable Electronic Phase Transition in Ionic Liquid Gated Monolayer Transition Metal Dichalcogenides.

Ionic liquid gating has proved to be effective in inducing emergent quantum phenomena such as superconductivity, ferromagnetism, and topological states. The electrostatic doping at two-dimensional interfaces relies on ionic motion, which thus is operated at sufficiently high temperature. Here, we report the in situ tuning of quantum phases by shining light on an ionic liquid-gated interface at cryogenic temperatures. The light illumination enables flexible switching of the quantum transition in monolayer WS2 from an insulator to a superconductor. In contrast to the prevailing picture of photoinduced carriers, we find that in the presence of a strong interfacial electric field conducting electrons could escape from the surface confinement by absorbing photons, mimicking the field emission. Such an optical tuning tool in conjunction with ionic liquid gating greatly facilitates continuous modulation of carrier densities and hence electronic phases, which would help to unveil novel quantum phenomena and device functionality in various materials.

Strong anisotropic enhancement of photoluminescence in WS2 integrated with plasmonic nanowire array.

Layered transition metal dichalcogenides (TMDCs) have shown great potential for a wide range of applications in photonics and optoelectronics. Nevertheless, valley decoherence severely randomizes its polarization which is important to a light emitter. Plasmonic metasurface with a unique way to manipulate the light-matter interaction may provide an effective and practical solution. Here by integrating TMDCs with plasmonic nanowire arrays, we demonstrate strong anisotropic enhancement of the excitonic emission at different spectral positions. For the indirect bandgap transition in bilayer WS2, multifold enhancement can be achieved with the photoluminescence (PL) polarization either perpendicular or parallel to the long axis of nanowires, which arises from the coupling of WS2 with localized or guided plasmon modes, respectively. Moreover, PL of high linearity is obtained in the direct bandgap transition benefiting from, in addition to the plasmonic enhancement, the directional diffraction scattering of nanowire arrays. Our method with enhanced PL intensity contrasts to the conventional form-birefringence based on the aspect ratio of nanowire arrays where the intensity loss is remarkable. Our results provide a prototypical plasmon-exciton hybrid system for anisotropic enhancement of the PL at the nanoscale, enabling simultaneous control of the intensity, polarization and wavelength toward practical ultrathin photonic devices based on TMDCs.

Learning to Learn Adaptive Classifier-Predictor for Few-Shot Learning.

Few-shot learning aims to learn a well-performing model from a few labeled examples. Recently, quite a few works propose to learn a predictor to directly generate model parameter weights with episodic training strategy of meta-learning and achieve fairly promising performance. However, the predictor in these works is task-agnostic, which means that the predictor cannot adjust to novel tasks in the testing phase. In this article, we propose a novel meta-learning method to learn how to learn task-adaptive classifier-predictor to generate classifier weights for few-shot classification. Specifically, a meta classifier-predictor module, (MPM) is introduced to learn how to adaptively update a task-agnostic classifier-predictor to a task-specialized one on a novel task with a newly proposed center-uniqueness loss function. Compared with previous works, our task-adaptive classifier-predictor can better capture characteristics of each category in a novel task and thus generate a more accurate and effective classifier. Our method is evaluated on two commonly used benchmarks for few-shot classification, i.e., miniImageNet and tieredImageNet. Ablation study verifies the necessity of learning task-adaptive classifier-predictor and the effectiveness of our newly proposed center-uniqueness loss. Moreover, our method achieves the state-of-the-art performance on both benchmarks, thus demonstrating its superiority.

Corrections to "Learning to Learn Adaptive Classifier-Predictor for Few-Shot Learning".

In the above article [1], the results of "Fully-supervised (Upper bound)" in Tables III and IV were inadvertently set to intermediate records that were used as placeholders. This error has no effect on any of the interpretations and conclusions. Tables I and II of this amendment show the corrected results (highlighted in italics) of the original Tables III and IV.

Polarized resonant emission of monolayer WS2 coupled with plasmonic sawtooth nanoslit array.

Transition metal dichalcogenide (TMDC) monolayers have enabled important applications in light emitting devices and integrated nanophotonics because of the direct bandgap, spin-valley locking and highly tunable excitonic properties. Nevertheless, the photoluminescence polarization is almost random at room temperature due to the valley decoherence. Here, we show the room temperature control of the polarization states of the excitonic emission by integrating WS2 monolayers with a delicately designed metasurface, i.e. a silver sawtooth nanoslit array. The random polarization is transformed to linear when WS2 excitons couple with the anisotropic resonant transmission modes that arise from the surface plasmon resonance in the metallic nanostructure. The coupling is found to enhance the valley coherence that contributes to ~30% of the total linear dichroism. Further modulating the transmission modes by optimizing metasurfaces, the total linear dichroism of the plasmon-exciton hybrid system can approach 80%, which prompts the development of photonic devices based on TMDCs.

Inducing lasing in organic materials with low optical gain by three-dimensional plasmonic nanocavity arrays.

Lasing in organic media with very low gain has been pursued for a long time in optoelectronics. Here, we experimentally demonstrate that plasmonic lasing in the visible regime at room temperature can be achieved by hybridizing active media of very low optical gain such as ionic liquid and polymethylmethacrylate with three-dimensional (3D) plasmonic metamaterials. The 3D nanostructure consists of a double-layer N-shaped silver wire-hole array with strongly coupled multiple hot spots densely packed in each unit cell. These hot spots overlap perfectly with the gain media, allowing efficient gain-plasmon coupling in subwavelength volumes. The periodic arrangement of hot spots, as the metal and dielectric are distributed in an alternate manner along both transverse and vertical directions, results in ultrastrong suppression of scattering losses. In addition, the lasing characteristics, including threshold, intensity and polarization can be controlled by the lattice constant and geometry of metamaterials. Such a plasmonic nanolaser proves to be of low threshold and low gain requirement, providing an essential step towards easy-processing organic based optoelectronics.

Extraction and antioxidant activity of total triterpenoids in the mycelium of a medicinal fungus, Sanghuangporus sanghuang.

The scientific name of the traditional Chinese medicinal fungus, Sanghuang, has been clarified and confirmed that it is a new species -Sanghuangporus sanghuang in the recently discovered genus, Sanghuangporus. To maximize the yield of the active ingredients such as the triterpenoids from authentic Sanghuangporus sanghuang, four parameters of the extraction process, including the extraction time, solid-liquid ratio, extraction temperature, and ethanol concentration were determined. The Box-Behnken experimental design and the response surface method were used to optimize the triterpenoid extraction processes of Sanghuangporus sanghuang mycelium. The results showed that the parameters of the triterpenoid extraction processes were not simple linear relationships. Optimum conditions of ultrasonic extraction required an 80% ethanol concentration, a 1:20 solid-liquid ratio, a 20-min extraction time, and a 60 °C extraction temperature, to obtain a maximum triterpenoid extraction of 13.30 mg/g. Antioxidant capacity tests showed that the Sanghuangporus sanghuang triterpenoids had high clearance capabilities for hydroxyl free radicals, superoxide anions, 2,2-diphenyl-1-picrylhydrazyl free radicals, and 2,2'-azinobis-(3-ethylbenzthiazoline-6-sulfonate) radicals, indicating that the Sanghuangporus sanghuang triterpenoids had high antioxidant activities.

Mimicking Dynamic Adhesiveness and Strain-Stiffening Behavior of Biological Tissues in Tough and Self-Healable Cellulose Nanocomposite Hydrogels.

Although self-healing gels with structural resemblance to biological tissues attract great attention in biomedical fields, it remains a dilemma for combination between fast self-healing properties and high mechanical toughness. On the basis of the design of dynamic reversible cross-links, we incorporate rigid tannic acid-coated cellulose nanocrystal (TA@CNC) motifs into the poly(vinyl alcohol) (PVA)-borax dynamic networks for the fabrication of a high toughness and rapidly self-healing nanocomposite (NC) hydrogel, together with dynamically adhesive and strain-stiffening properties that are particularly indispensable for practical applications in soft tissue substitutes. The results demonstrate that the obtained NC gels present a highly interconnected network, where flexible PVA chains wrap onto the rigid TA@CNC motifs and form the dynamic TA@CNC-PVA clusters associated by hydrogen bonds, affording the critical mechanical toughness. The synergetic interactions between borate-diol bonds and hydrogen bonds impart a typical self-healing behavior into the NC gels, allowing the dynamic cross-linked networks to undergo fast rearrangement in the time scale of seconds. Moreover, the obtained NC hydrogels not only mimic the main feature of biological tissues with the unique strain-stiffening behavior but also display unique dynamic adhesiveness to nonporous and porous substrates. It is expected that this versatile approach opens up a new prospect for the rational design of multifunctional cellulosic hydrogels with remarkable performance to expand their applications.

Three dimensional chiral plasmon rulers based on silver nanorod trimers.

The symmetry dependences of plasmon excitation modes are studied in 3D silver nanorod trimers. The degenerate plasmon modes split into chiral modes by breaking the inversion and mirror symmetry of the nanorod trimer through translation and/or rotation of the middle rod. With a translation operation, successive evolution of the circular dichroism (CD) spectrum can be achieved through gradual breaking of the inversion symmetry. An additional rotation operation produces even dramatic spectral changes due to breaking a quasi-mirror symmetry resulted from the same angular distance of the middle rod to the top and bottom rods. Especially, pairs of new chiral modes can be excited due to the contact of the middle rod with the top-bottom rod pair. The spectral changes in the simulations, which are also demonstrated experimentally, envision the 3D chiral nanorod trimer system as plasmon ruler for spatial configuration retrieval and dynamic bio-process analysis at the single molecule level.

Design of diversified self-assembly systems based on a natural rosin-based tertiary amine for doxorubicin delivery and excellent emulsification.

A novel rosin-based ester tertiary amine (RETA) with three hydrophilic groups and a rigid hydrophobic group was synthesized from rosin by Diels-Alder addition, acylation and esterification reactions. RETA was characterized by infrared spectroscopy (FT-IR) and proton nuclear magnetic resonance spectroscopy (13C NMR). Results from testing surface tension, zeta potential, and transmission electron spectroscopy showed that RETA had unique pH responsiveness. RETA self-assembled into worm-like micelles, spherical micelles 130 nm in diameter and big spherical worm-like aggregates with diameter of 2 µm at pH = 5.76, 8.04 and 9.38, respectively. The critical micelle concentration (CMC) of RETA was 0.42 mmol/L, and the surface tension at CMC (γcmc) was 38.73 mN/m when pH was 8.04. The RETA had a potential application in delivering doxorubicin hydrochloride (DOX) due to the pH responsiveness. Self-assembly mixed systems of RETA and rosin-based phosphoric acid (DDPD) were designed to improve emulsification. The mixed systems had obvious synergistic effects and unexpected emulsification. The γcmc and CMC of mixtures were 41.74 mN/m and 0.20 mmol/L, the size of mixture micelles increased up to 300 nm in the optimum molar ratio of RETA/DDPD (7:3) by TEM and cryo-TEM. It was worth noting that the mixture system formed vesicles in the RETA/DDPD molar ratio of 5:5. The stability time of emulsion with RETA and DDPD as emulsifier were only 63 s and 52 s respectively, but the stability time increased to 234 s in the optimum molar ratio. In addition, the formation mechanisms of micelles at different pH and in various mixtures were discussed in detail. What's more, cytotoxicity results showed that the toxicity of RETA was lower significantly than that of lecithin, a food ingredient in egg yolk and soybean. The cell viability was more than 83% in the high concentration of RETA (4000 µg/ml).

Broadband modulation of terahertz waves through electrically driven hybrid bowtie antenna-VO2 devices.

Broadband modulation of terahertz (THz) light is experimentally realized through the electrically driven metal-insulator phase transition of vanadium dioxide (VO2) in hybrid metal antenna-VO2 devices. The devices consist of VO2 active layers and bowtie antenna arrays, such that the electrically driven phase transition can be realized by applying an external voltage between adjacent metal wires extended to a large area array. The modulation depth of the terahertz light can be initially enhanced by the metal wires on top of VO2 and then improved through the addition of specific bowties in between the wires. As a result, a terahertz wave with a large beam size (~10 mm) can be modulated within the measurable spectral range (0.3-2.5 THz) with a frequency independent modulation depth as high as 0.9, and the minimum amplitude transmission down to 0.06. Moreover, the electrical switch on/off phase transition depends very much on the size of the VO2 area, indicating that smaller VO2 regions lead to higher modulation speeds and lower phase transition voltages. With the capabilities in actively tuning the beam size, modulation depth, modulation bandwidth as well as the modulation speed of THz waves, our study paves the way in implementing multifunctional components for terahertz applications.