Linear Electro-Optic Effect in 2D Ferroelectric for Electrically Tunable Metalens.

The advent of 2D ferroelectrics, characterized by their spontaneous polarization states in layer-by-layer domains without the limitation of a finite size effect, brings enormous promise for applications in integrated optoelectronic devices. Comparing with semiconductor/insulator devices, ferroelectric devices show natural advantages such as non-volatility, low energy consumption and high response speed. Several 2D ferroelectric materials have been reported, however, the device implementation particularly for optoelectronic application remains largely hypothetical. Here, the linear electro-optic effect in 2D ferroelectrics is discovered and electrically tunable 2D ferroelectric metalens is demonstrated. The linear electric-field modulation of light is verified in 2D ferroelectric CuInP2S6. The in-plane phase retardation can be continuously tuned by a transverse DC electric field, yielding an effective electro-optic coefficient rc of 20.28 pm V-1. The CuInP2S6 crystal exhibits birefringence with the fast axis oriented along its (010) plane. The 2D ferroelectric Fresnel metalens shows efficacious focusing ability with an electrical modulation efficiency of the focusing exceeding 34%. The theoretical analysis uncovers the origin of the birefringence and unveil its ultralow light absorption across a wide wavelength range in this non-excitonic system. The van der Waals ferroelectrics enable room-temperature electrical modulation of light and offer the freedom of heterogeneous integration with silicon and another material system for highly compact and tunable photonics and metaoptics.

All-electrical skyrmionic magnetic tunnel junction.

Topological whirls or 'textures' of spins such as magnetic skyrmions represent the smallest realizable emergent magnetic entities1-5. They hold considerable promise as robust, nanometre-scale, mobile bits for sustainable computing6-8. A longstanding roadblock to unleashing their potential is the absence of a device enabling deterministic electrical readout of individual spin textures9,10. Here we present the wafer-scale realization of a nanoscale chiral magnetic tunnel junction (MTJ) hosting a single, ambient skyrmion. Using a suite of electrical and multimodal imaging techniques, we show that the MTJ nucleates skyrmions of fixed polarity, whose large readout signal-20-70% relative to uniformly magnetized states-corresponds directly to skyrmion size. The MTJ exploits complementary nucleation mechanisms to stabilize distinctly sized skyrmions at zero field, thereby realizing three non-volatile electrical states. Crucially, it can electrically write and delete skyrmions to both uniform states with switching energies 1,000 times lower than the state of the art. Here, the applied voltage emulates a magnetic field and, in contrast to conventional MTJs, it reshapes both the energetics and kinetics of the switching transition, enabling deterministic bidirectional switching. Our stack platform enables large readout and efficient switching, and is compatible with lateral manipulation of skyrmionic bits, providing the much-anticipated backbone for all-electrical skyrmionic device architectures9,10. Its wafer-scale realizability provides a springboard to harness chiral spin textures for multibit memory and unconventional computing8,11.

Multistate Compound Magnetic Tunnel Junction Synapses for Digital Recognition.

The quest to mimic the multistate synapses for bioinspired computing has triggered nascent research that leverages the well-established magnetic tunnel junction (MTJ) technology. Early works on the spin transfer torque MTJ-based artificial neural network (ANN) are susceptible to poor thermal reliability, high latency, and high critical current densities. Meanwhile, work on spin-orbit torque (SOT) MTJ-based ANN mainly utilized domain wall motion, which yields negligibly small readout signals differentiating consecutive states and has designs that are incompatible with technological scale-up. Here, we propose a multistate device concept built upon a compound MTJ consisting of multiple SOT-MTJs (number of MTJs, n = 1-4) on a shared write channel, mimicking the spin-based ANN. The n + 1 resistance states representing varying synaptic weights can be tuned by varying the voltage pulses (±1.5-1.8 V), pulse duration (100-300 ns), and applied in-plane fields (5.5-10.5 mT). A large TMR difference of more than 13.6% is observed between two consecutive states for the 4-cell compound MTJ, a 4-fold improvement from reported state-of-the-art spin-based synaptic devices. The ANN built upon the compound MTJ shows high learning accuracy for digital recognition tasks with incremental states and retraining, achieving test accuracy as high as 95.75% in the 4-cell compound MTJ. These results provide an industry-compatible platform to integrate these multistate SOT-MTJ synapses directly into neuromorphic architecture for in-memory and unconventional computing applications.

Spin-related Cu-Co pair to increase electrochemical ammonia generation on high-entropy oxides.

The electrochemical conversion of nitrate to ammonia is a way to eliminate nitrate pollutant in water. Cu-Co synergistic effect was found to produce excellent performance in ammonia generation. However, few studies have focused on this effect in high-entropy oxides. Here, we report the spin-related Cu-Co synergistic effect on electrochemical nitrate-to-ammonia conversion using high-entropy oxide Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O. In contrast, the Li-incorporated MgCoNiCuZnO exhibits inferior performance. By correlating the electronic structure, we found that the Co spin states are crucial for the Cu-Co synergistic effect for ammonia generation. The Cu-Co pair with a high spin Co in Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O can facilitate ammonia generation, while a low spin Co in Li-incorporated MgCoNiCuZnO decreases the Cu-Co synergistic effect on ammonia generation. These findings offer important insights in employing the synergistic effect and spin states inside for selective catalysis. It also indicates the generality of the magnetic effect in ammonia synthesis between electrocatalysis and thermal catalysis.

Field-free switching of perpendicular magnetization by two-dimensional PtTe2/WTe2 van der Waals heterostructures with high spin Hall conductivity.

The key challenge of spin-orbit torque applications lies in exploring an excellent spin source capable of generating out-of-plane spins while exhibiting high spin Hall conductivity. Here we combine PtTe2 for high spin conductivity and WTe2 for low crystal symmetry to satisfy the above requirements. The PtTe2/WTe2 bilayers exhibit a high in-plane spin Hall conductivity σs,y ≈ 2.32 × 105 × h/2e Ω-1 m-1 and out-of-plane spin Hall conductivity σs,z ≈ 0.25 × 105 × h/2e Ω-1 m-1, where h is the reduced Planck's constant and e is the value of the elementary charge. The out-of-plane spins in PtTe2/WTe2 bilayers enable the deterministic switching of perpendicular magnetization at room temperature without magnetic fields, and the power consumption is 67 times smaller than that of the Pt control case. The high out-of-plane spin Hall conductivity is attributed to the conversion from in-plane spin to out-of-plane spin, induced by the crystal asymmetry of WTe2. Our work establishes a low-power perpendicular magnetization manipulation based on wafer-scale two-dimensional van der Waals heterostructures.

Oxygen Plasma Induced Nanochannels for Creating Bimetallic Hollow Nanocrystals.

Platinum-based metal catalysts are considered excellent converters in various catalytic reactions, particularly in fuel cell applications. The atomic structure at the nanocrystal surface and the metal interface both influence the catalytic performance, controlling the efficiency of the electrochemical reactions. Here we report the synthesis of Ag/Pt and Ag/Pd core/shell nanocrystals and insight into the formation mechanism of these bimetallic core/shell nanocrystals when undergoing oxygen plasma treatment. We carefully designed the oxidation treatment that determines the structural and compositional evolution. The accelerated oxidation-triggered diffusion of Ag toward the outer metal shell leads to the Kirkendall effect. After prolonged oxygen plasma treatment, most core/shell nanocrystals evolve into hollow spheres. At the same time, a minor fraction of the metal remains unchanged with a well-protected Ag core and a monocrystalline Pt or Pd shell. We hypothesize that the O2 plasma disturbs the Pt or Pd shell surface and introduces active O species that react with the diffused Ag from the inside out. Based on EDX elemental mapping, combined with several electron microscopic techniques, we deduced the formation mechanism of the hollow structures to be as follows: (I) the oxidation of Ag within the Pt or Pd lattice causes a disrupted crystal lattice of Pt or Pd; (II) nanochannels arise at the defect locations on the Pt or Pd shell; (III) the remaining Ag atoms pass through these nanochannels and leave a hollow crystal behind. Our findings deepen the understanding of interface dynamics of bimetallic nanostructured catalysts under an oxidative environment and unveil an alternative approach for catalyst pretreatment.

Atmospheric solids analysis probe-mass spectrometry (ASAP-MS) as a rapid fingerprinting technique to differentiate the harvest seasons of Tieguanyin oolong teas.

Atmospheric solids analysis probe-mass spectrometry (ASAP-MS), an ambient mass spectrometry technique, was used to differentiate spring and autumn Tieguanyin teas. Two configurations were used to obtain their chemical fingerprints - ASAP attached to a high-resolution quadrupole time-of-flight mass spectrometer (i.e., ASAP-QTOF) and to a single-quadrupole mass spectrometer (i.e., Radian™ ASAP™ mass spectrometer). Then, orthogonal projections to latent structures-discriminant analysis was conducted to identify features that held promise in differentiating harvest seasons. Four machine learning models - decision tree, linear discriminant analysis, support vector machine, and k-nearest neighbour - were built using these features, and high classification accuracy of up to 100% was achieved. The markers were putatively identified using their accurate masses and MS/MS fragmentation patterns from ASAP-QTOF. This approach was successfully transferred to the Radian ASAP MS, which is more deployable in the field. Overall, this study demonstrated the potential of ASAP-MS as a rapid fingerprinting tool for differentiating spring and autumn Tieguanyin.

Patterning at the Resolution Limit of Commercial Electron Beam Lithography.

It has been long known that low molecular weight resists can achieve a very high resolution, theoretically close to the probe diameter of the electron beam lithography (EBL) system. Despite technological improvements in EBL systems, the advances in resists have lagged behind. Here we demonstrate that a low-molecular-mass single-source precursor resist (based on cadmium(II) ethylxanthate complexed with pyridine) is capable of a achieving resolution (4 nm) that closely matches the measured probe diameter (∼3.8 nm). Energetic electrons enable the top-down radiolysis of the resist, while they provide the energy to construct the functional material from the bottom-up─unit cell by unit cell. Since this occurs only within the volume of resist exposed to primary electrons, the minimum size of the patterned features is close to the beam diameter. We speculate that angstrom-scale patterning of functional materials is possible with single-source precursor resists using an aberration-corrected electron beam writer with a spot size of ∼1 Å.

Production of C3 -C6 Acetate Esters via CO Electroreduction in a Membrane Electrode Assembly Cell.

Electrocatalytic carbon monoxide reduction has been previously reported to yield a range of carbonaceous products including alcohols, hydrocarbons and carboxylic acids. However, esters, an important family of organic compounds, have not been formed. Herein, we report the electrosynthesis of C3 to C6 acetate esters (H3 C-(C=O)-O-R) from carbon monoxide using copper catalysts in a membrane electrode assembly cell. Ethyl acetate and propyl acetate could be produced with an unprecedented total Faradaic efficiency (FE) of ∼22 % and with a current density of up to -55 mA cm-2 , alongside minor quantities of methyl acetate and butyl acetate. The esters are produced via the addition reaction of ethenone (H2 C=C=O) and alcohols produced during CO reduction. We show that the near water-free reaction conditions and the high local pH play key roles in the formation of the esters.

2D-Oriented Attachment of 1D Colloidal Semiconductor Nanocrystals via an Etchant.

The oriented attachment (OA) of 0D semiconductor nanocrystals into 1D and 2D nanostructures with unique properties is useful for the fabrication of quantum confined nanomaterials that are otherwise difficult to produce by direct synthesis. Given that the OA of 1D nanocrystals such as nanorods generally produces linear chains, rod-couple structures, or clustered columns, linking them in a facet-specific manner to produce 2D structures is challenging. Here, we report that 1D Cu2-xS nanorods undergo etching on exposure to hexylphosphonic acid under mild heating, which results in an increased curvature and a reduction in surface ligands at those sites. This causes the nanorods to fuse via their basal tip facets into chains and then cojoin through diametrically opposed side facets, resulting in atomically coupled, 2D raftlike structures. The stepwise OA of 1D nanocrystals into 2D nanostructures illustrated here expands the range of nanoarchitectures that can be produced via solution-processed methods.

Unveiling the Emergent Traits of Chiral Spin Textures in Magnetic Multilayers.

Magnetic skyrmions are topologically wound nanoscale textures of spins whose ambient stability and electrical manipulation in multilayer films have led to an explosion of research activities. While past efforts focused predominantly on isolated skyrmions, recently ensembles of chiral spin textures, consisting of skyrmions and magnetic stripes, are shown to possess rich interactions with potential for device applications. However, several fundamental aspects of chiral spin texture phenomenology remain to be elucidated, including their domain wall (DW) structure, thermodynamic stability, and morphological transitions. Here the evolution of these textural characteristics are unveiled on a tunable multilayer platform-wherein chiral interactions governing spin texture energetics can be widely varied-using a combination of full-field electron and soft X-ray microscopies with numerical simulations. With increasing chiral interactions, the emergence of Néel helicity, followed by a marked reduction in domain compressibility, and finally a transformation in the skyrmion formation mechanism are demonstrated. Together with an analytical model, these experiments establish a comprehensive microscopic framework for investigating and tailoring chiral spin texture character in multilayer films.

Block copolymer self-assembly assisted fabrication of laterally organized- and stacked- nanoarrays.

Block copolymer (BCP) self-assembly processes are often seen as reliable techniques for advanced nanopatterning to achieve functional surfaces and create templates for nanofabrication. By taking advantage of the tunability in pitch, diameter and feature-to-feature separation of the self-assembled BCP features, complex, laterally organized- and stacked- multicomponent nanoarrays comprising of gold and polymer have been fabricated. The approaches not only demonstrate nanopatterning of up to two levels of hierarchy but also investigate how a variation in the feature-to-feature gap at the first hierarchy affects the self-assembly of polymer features at the second. Such BCP self-assembly enabled multicomponent nanoarray configurations are rarely achieved by other nanofabrication approaches and are particularly promising for pushing the boundaries of block copolymer lithography and in creating unique surface architectures and complex morphologies at the nanoscale.

Nosocomial infections among COVID-19 patients: an analysis of intensive care unit surveillance data.

Surveillance of nosocomial infections, like catheter-associated urinary tract infection (CAUTI), central line-associated bloodstream infection, possible ventilator-associated pneumonia and secondary bloodstream infections were observed to study the impact of COVID-19 outbreak in ICUs from Tan Tock Seng Hospital and National Centre for Infectious Diseases, Singapore between February and June 2020. Higher nosocomial infection rates were observed in COVID-19 patients, although it was not statistically significant. Moreover, COVID-19 patients seem to be more predisposed to CAUTI despite a higher proportion of non-COVID-19 patients having urinary catheters. Thus, continued vigilance to ensure adherence to IPC measures is needed.

Room-Temperature Patterning of Nanoscale MoS2 under an Electron Beam.

Molybdenum disulfide (MoS2) is traditionally grown at a high temperature and subsequently patterned to study its electronic properties or make devices. This method imposes severe limitations on the shape and size of MoS2 crystals that can be patterned precisely at required positions. Here, we describe a method of direct nanoscale patterning of MoS2 at room temperature by exposing a molybdenum thiocubane single-source precursor to a beam of electrons. Molybdenum thiocubanes with various alkylxanthate moieties [Mo4S4(ROCS2)6, where R = alkyl] were prepared using a "self-assembly" approach. Micro-Raman and micro-FTIR spectroscopic studies suggest that exposure to a relatively smaller dose of electrons results in the breakdown of xanthate moieties, leading to the formation of MoS2. High-resolution transmission electron micrographs suggest that the growth of MoS2 most likely happens along (100) planes. An electron-beam-induced chemical transformation of a molybdenum thiocubane resist was exploited to fabricate sub-10 nm MoS2 lines and dense dots as small as 13 nm with a pitch of 33 nm. Since this technique does not require the liftoff and etching steps, patterning of MoS2 with interesting shapes, sizes, and thicknesses potentially leading to tunable band gap is possible.

New gold standard: weakly capped infant Au nanoclusters with record high catalytic activity for 4-nitrophenol reduction and hydrogen generation from an ammonia borane-sodium borohydride mixture.

Increasing the surface area-to-volume ratio of materials through size reduction is a desired approach to access maximum possible surface sites in applications such as catalysis. However, increase in the surface energy with the decrease in dimension warrants strong ligands to stabilize nanosystems, which mask the accessibility of the active surface sites. Owing to this, the realization of the true potential of a catalyst's surface remains challenging. Here, we employed a rationally designed strategy to synthesize infant Au nanoclusters-that alleviates the requirement of any separate ligand removal step-to unleash their actual potential to register a record high maximum turn-over frequency (TOFmax) of 72 900 h-1 and 65 500 h-1 in the benchmark catalytic reduction of 4-nitrophenol and catalytic H2 generation from an ammonia borane-sodium borohydride mixture, respectively. Such a phenomenal catalytic activity has been realized via the synthesis and stabilization of Au nanoclusters using solid citric acid and a super-concentrated aqueous AuCl3 solution, a pathway entirely different from the conventional modifications of the Turkevich and Brust methods. The crux of the synthetic strategy lies in precise control of the water content and thereby ensuring that the final Au nanoclusters remain in the solid state. During the synthesis, citric acid not only acts as a reducing agent to yield 'infant' Au nanoclusters but also provides a barrier matrix to arrest their growth. In solution, its weak capping ability and rapid dissolution allows the reactants to easily access the active sites of Au nanoclusters, thus leading to faster catalysis. Our study reveals that the true potential of metal nanoclusters as catalysts is actually far higher than what has been reported in the literature.

Inverse Stellation of CuAu-ZnO Multimetallic-Semiconductor Nanostartube for Plasmon-Enhanced Photocatalysis.

One-dimensional (1D) metallic nanocrystals constitute an important class of plasmonic materials for localization of light into subwavelength dimensions. Coupled with their intrinsic conductive properties and extended optical paths for light absorption, metallic nanowires are prevalent in light-harnessing applications. However, the transverse surface plasmon resonance (SPR) mode of traditional multiply twinned nanowires often suffers from weaker electric field enhancement due to its low degree of morphological curvature in comparison to other complex anisotropic nanocrystals. Herein, simultaneous anisotropic stellation and excavation of multiply twinned nanowires are demonstrated through a site-selective galvanic reaction for a pronounced manipulation of light-matter interaction. The introduction of longitudinal extrusions and cavitation along the nanowires leads to a significant enhancement in plasmon field with reduced quenching of localized surface plasmon resonance (LSPR). The as-synthesized multimetallic nanostartubes serve as a panchromatic plasmonic framework for incorporation of photocatalytic materials for plasmon-assisted solar fuel production.

Flexible Modulation of CO-Release Using Various Nuclearity of Metal Carbonyl Clusters on Graphene Oxide for Stroke Remediation.

Utilizing the size-dependent adsorption properties of ruthenium carbonyl clusters (Ru-carbon monoxide (CO)) onto graphene oxide (GO), a facile CO-release platform for in situ vasodilation as a treatment for stroke-related vascular diseases is developed. The rate and amount of formation of the CO-release-active RuII (CO)2 species can be modulated by a simple mixing procedure at room temperature. The subsequent thermally induced oxidation of RuII (CO)2 to RuO2 on the GO surface results in the release of CO. Further modulation of thermal and CO-release properties can be achieved via a hybridization of medium- and high-nuclearity of Ru-CO clusters that produces a RuO2 /RuII (CO)2 /6 Ru-CO-GO composite, where 6 Ru-CO-GO provides a photothermally activated reservoir of RuII (CO)2 species and the combined infrared absorption properties of GO and RuO2 provides photothermal response for in situ CO-release. The RuO2 /RuII (CO)2 /6 Ru-CO-GO composite does not produce any cytotoxicity and the efficacy of the composite is further demonstrated in a cortical photothrombotic ischemia rat model.

Direct Patterning of Zinc Sulfide on a Sub-10 Nanometer Scale via Electron Beam Lithography.

Nanostructures of metal sulfides are conventionally prepared via chemical techniques and patterned using self-assembly. This poses a considerable amount of challenge when arbitrary shapes and sizes of nanostructures are desired to be placed at precise locations. Here, we describe an alternative approach of nanoscale patterning of zinc sulfide (ZnS) directly using a spin-coatable and electron beam sensitive zinc butylxanthate resist without the lift-off or etching step. Time-resolved electron beam damage studies using micro-Raman and micro-FTIR spectroscopies suggest that exposure to a beam of electrons leads to quick disappearance of xanthate moieties most likely via the Chugaev elimination, and further increase of electron dose results in the appearance of ZnS, thereby making the exposed resist insoluble in organic solvents. Formation of ZnS nanocrystals was confirmed by high-resolution transmission electron microscopy and selected area electron diffraction. This property was exploited for the fabrication of ZnS lines as small as 6 nm and also enabled patterning of 10 nm dots with pitches as close as 22 nm. The ZnS patterns fabricated by this technique showed defect-induced photoluminescence related to sub-band-gap optical transitions. This method offers an easy way to generate an ensemble of functional ZnS nanostructures that can be arbitrarily patterned and placed in a precise way. Such an approach may enable programmable design of functional chalcogenide nanostructures.

Carbon nanoparticle-modified multi-wall carbon nanotubes with fast adsorption kinetics for water treatment.

Carbon nanoparticle-modified multi-wall carbon nanotubes were prepared using a dehydration of carbohydrate compound method. The structural change was characterized by transmission electron microscopy, Raman spectroscopy, and Brunauer, Emmett and Teller measurement. Fast adsorption kinetics was observed for multi-wall carbon nanotubes with modification, as demonstrated by the adsorption of the model compound methylene blue. This work provides a novel facile engineering strategy to equip multi-wall carbon nanotubes with fast adsorption kinetics, which is promising for efficient water purification.

Fluorescence Concentric Triangles: A Case of Chemical Heterogeneity in WS2 Atomic Monolayer.

We report a novel optical property in WS2 monolayer. The monolayer naturally exhibits beautiful in-plane periodical and lateral homojunctions by way of alternate dark and bright band in the fluorescence images of these monolayers. The interface between different fluorescence species within the sample is distinct and sharp. This gives rise to intriguing concentric triangular fluorescence patterns in the monolayer. The novel optical property of this special WS2 monolayer is facilitated by chemical heterogeneity. The photoluminescence of the bright band is dominated by emissions from trion and biexciton while the emission from defect-bound exciton dominates the photoluminescence at the dark band. The discovery of such concentric fluorescence patterns represents a potentially new form of optoelectronic or photonic functionality.