Local gate control of Mott metal-insulator transition in a 2D metal-organic framework.

Electron-electron interactions in materials lead to exotic many-body quantum phenomena, including Mott metal-insulator transitions (MITs), magnetism, quantum spin liquids, and superconductivity. These phases depend on electronic band occupation and can be controlled via the chemical potential. Flat bands in two-dimensional (2D) and layered materials with a kagome lattice enhance electronic correlations. Although theoretically predicted, correlated-electron Mott insulating phases in monolayer 2D metal-organic frameworks (MOFs) with a kagome structure have not yet been realised experimentally. Here, we synthesise a 2D kagome MOF on a 2D insulator. Scanning tunnelling microscopy (STM) and spectroscopy reveal a MOF electronic energy gap of ∼200 meV, consistent with dynamical mean-field theory predictions of a Mott insulator. Combining template-induced (via work function variations of the substrate) and STM probe-induced gating, we locally tune the electron population of the MOF kagome bands and induce Mott MITs. These findings enable technologies based on electrostatic control of many-body quantum phases in 2D MOFs.

Gigantic Anisotropy of Self-Induced Spin-Orbit Torque in Weyl Ferromagnet Co2MnGa.

Spin-orbit torque (SOT) is receiving tremendous attention from both fundamental and application-oriented aspects. Co2MnGa, a Weyl ferromagnet that is in a class of topological quantum materials, possesses cubic-based high structural symmetry, the L21 crystal ordering, which should be incapable of hosting anisotropic SOT in conventional understanding. Here we show the discovery of a gigantic anisotropy of self-induced SOT in Co2MnGa. The magnitude of the SOT is comparable to that of heavy metal/ferromagnet bilayer systems, despite the high inversion symmetry of the Co2MnGa structure. More surprisingly, a sign inversion of the self-induced SOT is observed for different crystal axes. This finding stems from the interplay of the topological nature of the electronic states and their strong modulation by external strain. Our research enriches the understanding of the physics of self-induced SOT and demonstrates a versatile method for tuning SOT efficiencies in a wide range of materials for topological and spintronic devices.

Giant piezoresistivity in a van der Waals material induced by intralayer atomic motions.

The presence of the van der Waals gap in layered materials creates a wealth of intriguing phenomena different to their counterparts in conventional materials. For example, pressurization can generate a large anisotropic lattice shrinkage along the stacking orientation and/or a significant interlayer sliding, and many of the exotic pressure-dependent properties derive from these mechanisms. Here we report a giant piezoresistivity in pressurized ß'-In2Se3. Upon compression, a six-orders-of-magnitude drop of electrical resistivity is obtained below 1.2 GPa in ß'-In2Se3 flakes, yielding a giant piezoresistive gauge πp of -5.33 GPa-1. Simultaneously, the sample undergoes a semiconductor-to-semimetal transition without a structural phase transition. Surprisingly, linear dichroism study and theoretical first principles modelling show that these phenomena arise not due to shrinkage or sliding at the van der Waals gap, but rather are dominated by the layer-dependent atomic motions inside the quintuple layer, mainly from the shifting of middle Se atoms to their high-symmetric location. The atomic motions link to both the band structure modulation and the in-plane ferroelectric dipoles. Our work not only provides a prominent piezoresistive material but also points out the importance of intralayer atomic motions beyond van der Waals gap.

Molecularly Controlled Quantum Well Width Distribution and Optoelectronic Properties in Quasi-2D Perovskite Light-Emitting Diodes.

Owing to their excellent optoelectronic properties, quasi-2D perovskites with self-assembled multiple quantum well (MQW) structures have shown great potential in light-emitting diode (LED) applications. Understanding the correlation between the bulky cation, quantum well assembly, and optoelectronic properties of a quasi-2D perovskite is important. Here, we demonstrate that the dipole moment of the bulky cation can be one of the fundamental factors that controls the distribution and crystallinity of different quantum wells. We find that the bulky cation with a moderate dipole moment leads to moderately distributed well-width MQWs, resulting in a superior device efficiency due to the simultaneous achievement of favorable optical and electronic properties. The peak external quantum efficiency and the maximum luminance of the champion device are 10.8% and 19082 cd m-2, respectively, positioning it among the best-performing quasi-2D green perovskite LEDs without further surface passivation or additive doping. This work provides a perspective on the rational design of bulky cations in quasi-2D perovskite LEDs, which is also essential for the development of other mixed-dimensional perovskite optoelectronic devices.

Polarity-Tunable Photocurrent through Band Alignment Engineering in a High-Speed WSe2/SnSe2 Diode with Large Negative Responsivity.

Excellent light-matter interaction and a wide range of thickness-tunable bandgaps in layered vdW materials coupled by the facile fabrication of heterostructures have enabled several avenues for optoelectronic applications. Realization of high photoresponsivity at fast switching speeds is a critical challenge for 2D optoelectronics to enable high-performance photodetection for optical communication. Moving away from conventional type-II heterostructure pn junctions towards a WSe2/SnSe2 type-III configuration, we leverage the steep change in tunneling current along with a light-induced heterointerface band shift to achieve high negative photoresponsivity, while the fast carrier transport under tunneling results in high speed. In addition, the photocurrent can be controllably switched from positive to negative values, with ∼104× enhancement in responsivity, by engineering the band alignment from type-II to type-III using either the drain or the gate bias. This is further reinforced by electric-field dependent interlayer band structure calculations using density functional theory. The high negative responsivity of 2 × 104 A/W and fast response time of ∼1 µs coupled with a polarity-tunable photocurrent can lead to the development of next-generation multifunctional optoelectronic devices.

A saccharide-based binder for efficient polysulfide regulations in Li-S batteries.

The viability of lithium-sulfur batteries as an energy storage technology depends on unlocking long-term cycle stability. Most instability stems from the release and transport of polysulfides from the cathode, which causes mossy growth on the lithium anode, leading to continuous consumption of electrolyte. Therefore, development of a durable cathode with minimal polysulfide escape is critical. Here, we present a saccharide-based binder system that has a capacity for the regulation of polysulfides due to its reducing properties. Furthermore, the binder promotes the formation of viscoelastic filaments during casting which endows the sulfur cathode with a desirable web-like microstructure. Taken together this leads to 97% sulfur utilisation with a cycle life of 1000 cycles (9 months) and capacity retention (around 700 mAh g-1 after 1000 cycles). A pouch cell prototype with a specific energy of up to 206 Wh kg-1 is produced, demonstrating the promising potential for practical applications.

Crossover from 2D Ferromagnetic Insulator to Wide Band Gap Quantum Anomalous Hall Insulator in Ultrathin MnBi2Te4.

Intrinsic magnetic topological insulators offer low disorder and large magnetic band gaps for robust magnetic topological phases operating at higher temperatures. By controlling the layer thickness, emergent phenomena such as the quantum anomalous Hall (QAH) effect and axion insulator phases have been realized. These observations occur at temperatures significantly lower than the Néel temperature of bulk MnBi2Te4, and measurement of the magnetic energy gap at the Dirac point in ultrathin MnBi2Te4 has yet to be achieved. Critical to achieving the promise of this system is a direct measurement of the layer-dependent energy gap and verification of a temperature-dependent topological phase transition from a large band gap QAH insulator to a gapless TI paramagnetic phase. Here we utilize temperature-dependent angle-resolved photoemission spectroscopy to study epitaxial ultrathin MnBi2Te4. We directly observe a layer-dependent crossover from a 2D ferromagnetic insulator with a band gap greater than 780 meV in one septuple layer (1 SL) to a QAH insulator with a large energy gap (>70 meV) at 8 K in 3 and 5 SL MnBi2Te4. The QAH gap is confirmed to be magnetic in origin, as it becomes gapless with increasing temperature above 8 K.

Spatial calcium kinetics after a traumatic brain injury.

Accurate modelling of intracellular calcium ion ([Formula: see text]) concentration evolution is valuable as it is known to rapidly increase during a Traumatic Brain Injury. In the work presented here, our older non-spatial model dealing with the effect of mechanical stress upon the [Formula: see text] transportation in a neuron is spatialized by considering the brain tissue as a solid continuum with the [Formula: see text] activity occurring at every material point. Starting with one-dimensional representation, the brain tissue geometry is progressively made realistic and under the action of pressure or kinematic impulses, the effect of dimensionality and material behaviour on the correlation between the stress and concomitant [Formula: see text] concentration is investigated. The spatial calcium kinetics model faithfully captures the experimental observations concerning the [Formula: see text] concentration, load rate, magnitude and duration and most importantly shows that the critical location for primary injury may not be the most important location as far as secondary injury is concerned.

Detection of Halomethanes Using Cesium Lead Halide Perovskite Nanocrystals.

The extensive use of halomethanes (CH3X, X = F, Cl, Br, I) as refrigerants, propellants, and pesticides has drawn serious concern due to their adverse biological and atmospheric impact. However, there are currently no portable rapid and accurate monitoring systems for their detection. This work introduces an approach for the selective and sensitive detection of halomethanes using photoluminescence spectral shifts in cesium lead halide perovskite nanocrystals. Focusing on iodomethane (CH3I) as a model system, it is shown that cesium lead bromide (CsPbBr3) nanocrystals can undergo rapid (<5 s) halide exchange, but only after exposure to oleylamine to induce nucleophilic substitution of the CH3I and release the iodide species. The extent of the halide exchange is directly dependent on the CH3I concentration, with the photoluminescence emission of the CsPbBr3 nanocrystals exhibiting a redshift of more than 150 nm upon the addition of 10 ppmv of CH3I. This represents the widest detection range and the highest sensitivity to the detection of halomethanes using a low-cost and portable approach reported to date. Furthermore, inherent selectivity for halomethanes compared to other organohalide analogues is achieved through the dramatic differences in their alkylation reactivity.

Transforming solid-state precipitates via excess vacancies.

Many phase transformations associated with solid-state precipitation look structurally simple, yet, inexplicably, take place with great difficulty. A classic case of difficult phase transformations is the nucleation of strengthening precipitates in high-strength lightweight aluminium alloys. Here, using a combination of atomic-scale imaging, simulations and classical nucleation theory calculations, we investigate the nucleation of the strengthening phase θ' onto a template structure in the aluminium-copper alloy system. We show that this transformation can be promoted in samples exhibiting at least one nanoscale dimension, with extremely high nucleation rates for the strengthening phase as well as for an unexpected phase. This template-directed solid-state nucleation pathway is enabled by the large influx of surface vacancies that results from heating a nanoscale solid. Template-directed nucleation is replicated in a bulk alloy as well as under electron irradiation, implying that this difficult transformation can be facilitated under the general condition of sustained excess vacancy concentrations.

Near-Direct Bandgap WSe2/ReS2 Type-II pn Heterojunction for Enhanced Ultrafast Photodetection and High-Performance Photovoltaics.

Pn heterojunctions comprising layered van der Waals (vdW) semiconductors have been used to demonstrate current-rectifiers, photodetectors, and photovoltaic devices. However, a direct or near-direct heterointerface bandgap for enhanced photogeneration in high light-absorbing few-layer vdW materials remains unexplored. In this work, for the first time, density functional theory calculations show that the heterointerface of few-layer group-6 transition metal dichalcogenide (TMD) WSe2 with group-7 ReS2 results in a sizable (0.7 eV) near-direct type-II bandgap. The interlayer IR bandgap is confirmed through IR photodetection, and microphotoluminescence measurements demonstrate type-II alignment. Few-layer flakes exhibit ultrafast response time (5 µs), high responsivity (3 A/W), and large photocurrent-generation and responsivity-enhancement at the hetero-overlap region (10-100×). Large open-circuit voltage of 0.64 V and short-circuit current of 2.6 µA enable high output electrical power. Finally, long-term air-stability and facile single contact metal fabrication process make the multifunctional few-layer WSe2/ReS2 heterostructure diode technologically promising for next-generation optoelectronics.

Ion Agglomeration and Transport in MgCl2-Based Electrolytes for Rechargeable Magnesium Batteries.

Magnesium halide salts are an exciting prospect as stable and high-performance electrolytes for rechargeable Mg batteries (RMBs). By nature of their complex equilibria, these salts exist in solution as a variety of electroactive species (EAS) in equilibrium with counterions such as AlCl4-. Here we investigated ion agglomeration and transport of several such EAS in MgCl2 salts dissolved in ethereal solvents under both equilibrium and operating conditions using large-scale atomistic simulations. We found that the solute morphology is strongly characterized by the presence of clusters and is governed by the solvation structures of EAS. Specifically, the isotropic solvation of Mg2+ results in the slow formation of a bulky cluster, compared with chainlike analogues observed in the Cl-containing EAS such as Mg2Cl3+, MgCl+, and Mg2Cl22+. We further illustrate these clusters can reduce the diffusivity of charge-carrying species in the MgCl2-based electrolyte by at least an order of magnitude. Our findings for cluster formation, morphology, and kinetics can provide useful insight into the electrochemical reactions at the anode-electrolyte interface in RMBs.

Electric Field Control of Molecular Charge State in a Single-Component 2D Organic Nanoarray.

Quantum dots (QD) with electric-field-controlled charge state are promising for electronics applications, e.g., digital information storage, single-electron transistors, and quantum computing. Inorganic QDs consisting of semiconductor nanostructures or heterostructures often offer limited control on size and composition distribution as well as low potential for scalability and/or nanoscale miniaturization. Owing to their tunability and self-assembly capability, using organic molecules as building nanounits can allow for bottom-up synthesis of two-dimensional (2D) nanoarrays of QDs. However, 2D molecular self-assembly protocols are often applicable on metals surfaces, where electronic hybridization and Fermi level pinning can hinder electric-field control of the QD charge state. Here, we demonstrate the synthesis of a single-component self-assembled 2D array of molecules [9,10-dicyanoanthracene (DCA)] that exhibit electric-field-controlled spatially periodic charging on a noble metal surface, Ag(111). The charge state of DCA can be altered (between neutral and negative), depending on its adsorption site, by the local electric field induced by a scanning tunneling microscope tip. Limited metal-molecule interactions result in an effective tunneling barrier between DCA and Ag(111) that enables electric-field-induced electron population of the lowest unoccupied molecular orbital (LUMO) and, hence, charging of the molecule. Subtle site-dependent variation of the molecular adsorption height translates into a significant spatial modulation of the molecular polarizability, dielectric constant, and LUMO energy level alignment, giving rise to a spatially dependent effective molecule-surface tunneling barrier and likelihood of charging. This work offers potential for high-density 2D self-assembled nanoarrays of identical QDs whose charge states can be addressed individually with an electric field.

Atomistic Mechanisms of Mg Insertion Reactions in Group XIV Anodes for Mg-Ion Batteries.

Magnesium (Mg) metal has been widely explored as an anode material for Mg-ion batteries (MIBs) owing to its large specific capacity and dendrite-free operation. However, critical challenges, such as the formation of passivation layers during battery operation and anode-electrolyte-cathode incompatibilities, limit the practical application of Mg-metal anodes for MIBs. Motivated by the promise of group XIV elements (namely, Si, Ge, and Sn) as anodes for lithium- and sodium-ion batteries, here, we conduct systematic first-principles calculations to explore the thermodynamics and kinetics of group XIV anodes for MIBs and to identify the atomistic mechanisms of the electrochemical insertion reactions of Mg ions. We confirm the formation of amorphous Mg xX phases (where X = Si, Ge, and Sn) in anodes via the breaking of the stronger X-X bonding network replaced by weaker Mg-X bonding. Mg ions have higher diffusivities in Ge and Sn anodes than in Si, resulting from weaker Ge-Ge and Sn-Sn bonding networks. In addition, we identify thermodynamic instabilities of Mg xX that require a small overpotential to avoid aggregation (plating) of Mg at anode/electrolyte interfaces. Such comprehensive first-principles calculations demonstrate that amorphous Ge and crystalline Sn can be potentially effective anodes for practical applications in MIBs.

Atomistic insights into the adsorption and stimuli-responsive behavior of poly(N-isopropylacrylamide)-graphene hybrid systems.

Non-covalent functionalization of graphene materials with responsive polymers is a promising approach for synthesizing new, hybrid composites with improved dispersibility and functional properties. However, the interplay between various components of the hybrid systems, their structural configurations, and stimuli-responsive behavior are not yet well understood at the atomic level. Here, we investigate the temperature-responsive behavior of physisorbed poly(N-isopropylacrylamide) (PNIPAM) on to graphene (G) and graphene oxide (GO) sheets in aqueous solution using large scale molecular dynamics simulations. It was observed that PNIPAM can be spontaneously anchored to the surfaces of both G and GO at 290 K with a macromolecular coil shape. However, the configuration of PNIPAM on G is markedly different in comparison with that on GO, leading to its distinct thermoresponsive behavior. Specifically, the adsorption on G gives rise to an increase in the temperature of the coil-to-globule transition when compared to the native polymer, the origin of which can be interpreted in terms of the interactions and the solvation behavior. The results obtained here are of significance to the design and manipulation of graphene-based stimuli-responsive hybrid systems with optimal functional properties.

Designing Optoelectronic Properties by On-Surface Synthesis: Formation and Electronic Structure of an Iron-Terpyridine Macromolecular Complex.

Supramolecular chemistry protocols applied on surfaces offer compelling avenues for atomic-scale control over organic-inorganic interface structures. In this approach, adsorbate-surface interactions and two-dimensional confinement can lead to morphologies and properties that differ dramatically from those achieved via conventional synthetic approaches. Here, we describe the bottom-up, on-surface synthesis of one-dimensional coordination nanostructures based on an iron (Fe)-terpyridine (tpy) interaction borrowed from functional metal-organic complexes used in photovoltaic and catalytic applications. Thermally activated diffusion of sequentially deposited ligands and metal atoms and intraligand conformational changes lead to Fe-tpy coordination and formation of these nanochains. We used low-temperature scanning tunneling microscopy and density functional theory to elucidate the atomic-scale morphology of the system, suggesting a linear tri-Fe linkage between facing, coplanar tpy groups. Scanning tunneling spectroscopy reveals the highest occupied orbitals, with dominant contributions from states located at the Fe node, and ligand states that mostly contribute to the lowest unoccupied orbitals. This electronic structure yields potential for hosting photoinduced metal-to-ligand charge transfer in the visible/near-infrared. The formation of this unusual tpy/tri-Fe/tpy coordination motif has not been observed for wet chemistry synthetic methods and is mediated by the bottom-up on-surface approach used here, offering pathways to engineer the optoelectronic properties and reactivity of metal-organic nanostructures.

Strong Depletion in Hybrid Perovskite p-n Junctions Induced by Local Electronic Doping.

A semiconductor p-n junction typically has a doping-induced carrier depletion region, where the doping level positively correlates with the built-in potential and negatively correlates with the depletion layer width. In conventional bulk and atomically thin junctions, this correlation challenges the synergy of the internal field and its spatial extent in carrier generation/transport. Organic-inorganic hybrid perovskites, a class of crystalline ionic semiconductors, are promising alternatives because of their direct badgap, long diffusion length, and large dielectric constant. Here, strong depletion in a lateral p-n junction induced by local electronic doping at the surface of individual CH3 NH3 PbI3 perovskite nanosheets is reported. Unlike conventional surface doping with a weak van der Waals adsorption, covalent bonding and hydrogen bonding between a MoO3 dopant and the perovskite are theoretically predicted and experimentally verified. The strong hybridization-induced electronic coupling leads to an enhanced built-in electric field. The large electric permittivity arising from the ionic polarizability further contributes to the formation of an unusually broad depletion region up to 10 µm in the junction. Under visible optical excitation without electrical bias, the lateral diode demonstrates unprecedented photovoltaic conversion with an external quantum efficiency of 3.93% and a photodetection responsivity of 1.42 A W-1 .