Two‐Dimensional Mott Insulators in SrVO3 Ultrathin Films

Strongly correlated oxides that undergo a metal‐insulator transition (MIT) are a subject of great current interest for their potential application to future electronics as switches and sensors. Recent advances in thin film technology have opened up new avenues to tailor MIT for novel devices beyond conventional CMOS scaling. Here, dimensional‐crossover‐driven MITs are demonstrated in high‐quality epitaxial SrVO₃ (SVO) thin films grown by a pulsed electron‐beam deposition technique. Thick SVO films (∼25 nm) exhibit metallic behavior with the electrical resistivity following the T² law corresponding to a Fermi liquid system. A temperature driven MIT is induced in SVO ultrathin films with thicknesses below 6.5 nm. The transition temperature T_MIT is at 50 K for the 6.5 nm film, 120 K for the 5.7 nm film and 205 K for the 3 nm film. The emergence of the observed MIT can be attributed to the dimensional crossover from a three‐dimensional metal to a two‐dimensional Mott insulator, as the resulting reduction in the effective bandwidth W opens a band gap at the Fermi level. The magneto‐transport study of the SVO ultrathin films also confirm the observed MIT is due to the electron‐electron interactions other than disorder‐induced localization.     

Nanoscale Correlations between Metal–Insulator Transition and Resistive Switching Effect in Metallic Perovskite Oxides

Strongly correlated perovskite oxides are a class of materials with fascinating intrinsic physical functionalities due to the interplay of charge, spin, orbital ordering, and lattice degrees of freedom. Among the exotic phenomena arising from such an interplay, metal–insulator transitions (MITs) are fundamentally still not fully understood and are of large interest for novel nanoelectronics applications, such as resistive switching‐based memories and neuromorphic computing devices. In particular, rare‐earth nickelates and lanthanum strontium manganites are archetypical examples of bandwidth‐controlled and band‐filling‐controlled MIT, respectively, which are used in this work as a playground to correlate the switching characteristics of the oxides and their MIT properties by means of local probe techniques in a systematic manner. These findings suggest that an electric‐field‐induced MIT can be triggered in these strongly correlated systems upon generation of oxygen vacancies and establish that lower operational voltages and larger resistance ratios are obtained in those films where the MIT lies closer to room temperature. This work demonstrates the potential of using MITs in the next generation of nanoelectronics devices.     

Countering the Segregation of Transition‐Metal Ions in LiMn1/3Co1/3Ni1/3O2 Cathode for Ultralong Life and High‐Energy Li‐Ion Batteries

High‐voltage layered lithium transition‐metal oxides are very promising cathodes for high‐energy Li‐ion batteries. However, these materials often suffer from a fast degradation of cycling stability due to structural evolutions. It seriously impedes the large‐scale application of layered lithium transition‐metal oxides. In this work, an ultralong life LiMn_1/3Co_1/3Ni_1/3O₂ microspherical cathode is prepared by constructing an Mn‐rich surface. Its capacity retention ratio at 700 mA g^?1 is as large as 92.9% after 600 cycles. The energy dispersive X‐ray maps of electrodes after numerous cycles demonstrate that the ultralong life of the as‐prepared cathode is attributed to the mitigation of TM‐ions segregation. Additionally, it is discovered that layered lithium transition‐metal oxide cathodes with an Mn‐rich surface can mitigate the segregation of TM ions and the corrosion of active materials. This study provides a new strategy to counter the segregation of TM ions in layered lithium transition‐metal oxides and will help to the design and development of high‐energy cathodes with ultralong life.     

Large‐Area Atomic Layers of the Charge‐Density‐Wave Conductor TiSe2

Layered transition metal (Ti, Ta, Nb, etc.) dichalcogenides are important prototypes for the study of the collective charge density wave (CDW). Reducing the system dimensionality is expected to lead to novel properties, as exemplified by the discovery of enhanced CDW order in ultrathin TiSe₂. However, the syntheses of monolayer and large‐area 2D CDW conductors can currently only be achieved by molecular beam epitaxy under ultrahigh vacuum. This study reports the growth of monolayer crystals and up to 5 × 10⁵ µm² large films of the typical 2D CDW conductor—TiSe₂—by ambient‐pressure chemical vapor deposition. Atomic resolution scanning transmission electron microscopy indicates the as‐grown samples are highly crystalline 1T‐phase TiSe₂. Variable‐temperature Raman spectroscopy shows a CDW phase transition temperature of 212.5 K in few layer TiSe₂, indicative of high crystal quality. This work not only allows the exploration of many‐body state of TiSe₂ in 2D limit but also offers the possibility of utilizing large‐area TiSe₂ in ultrathin electronic devices.     

Topotactic Metal–Insulator Transition in Epitaxial SrFeOx Thin Films

Topotactic phase transformation enables structural transition without losing the crystalline symmetry of the parental phase and provides an effective platform for elucidating the redox reaction and oxygen diffusion within transition metal oxides. In addition, it enables tuning of the emergent physical properties of complex oxides, through strong interaction between the lattice and electronic degrees of freedom. In this communication, the electronic structure evolution of SrFeO_x epitaxial thin films is identified in real‐time, during the progress of reversible topotactic phase transformation. Using real‐time optical spectroscopy, the phase transition between the two structurally distinct phases (i.e., brownmillerite and perovskite) is quantitatively monitored, and a pressure–temperature phase diagram of the topotactic transformation is constructed for the first time. The transformation at relatively low temperatures is attributed to a markedly small difference in Gibbs free energy compared to the known similar class of materials to date. This study highlights the phase stability and reversibility of SrFeO_x thin films, which is highly relevant for energy and environmental applications exploiting the redox reactions.     

Hollow‐Structured Metal Oxides as Oxygen‐Related Catalysts

Metal oxide hollow structures with large surface area, low density, and high loading capacity have received great attention for energy‐related applications. Acting as oxygen‐related catalysts, hollow‐structured transition metal oxides offer low overpotential, fast reaction rate, and excellent stability. Herein, recent progress in the oxygen‐related catalysis (e.g., oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and metal–air batteries) of hollow‐structured transition metal oxides is discussed. Through a comprehensive outline of hollow‐structured spinels, perovskites, rutiles, etc., a rational design strategy is provided for an enhanced oxygen‐related catalysis performance from the viewpoint of crystal structures. Urgent challenges and further research directions are presented for hollow‐structured transition metal oxides toward excellent oxygen‐related catalysis.     

Structural Phase Transition Effect on Resistive Switching Behavior of MoS2‐Polyvinylpyrrolidone Nanocomposites Films for Flexible Memory Devices

The 2H phase and 1T phase coexisting in the same molybdenum disulfide (MoS₂) nanosheets can influence the electronic properties of the materials. The 1T phase of MoS₂ is introduced into the 2H‐MoS₂ nanosheets by two‐step hydrothermal synthetic methods. Two types of nonvolatile memory effects, namely write‐once read‐many times memory and rewritable memory effect, are observed in the flexible memory devices with the configuration of Al/1T@2H‐MoS₂‐polyvinylpyrrolidone (PVP)/indium tin oxide (ITO)/polyethylene terephthalate (PET) and Al/2H‐MoS₂‐PVP/ITO/PET, respectively. It is observed that structural phase transition in MoS₂ nanosheets plays an important role on the resistive switching behaviors of the MoS₂‐based device. It is hoped that our results can offer a general route for the preparation of various promising nanocomposites based on 2D nanosheets of layered transition metal dichalcogenides for fabricating the high performance and flexible nonvolatile memory devices through regulating the phase structure in the 2D nanosheets.     

A High‐On/Off‐Ratio Floating‐Gate Memristor Array on a Flexible Substrate via CVD‐Grown Large‐Area 2D Layer Stacking

Memristors such as phase‐change memory and resistive memory have been proposed to emulate the synaptic activities in neuromorphic systems. However, the low reliability of these types of memories is their biggest challenge for commercialization. Here, a highly reliable memristor array using floating‐gate memory operated by two terminals (source and drain) using van der Waals layered materials is demonstrated. Centimeter‐scale samples (1.5 cm × 1.5 cm) of MoS₂ as a channel and graphene as a trap layer grown by chemical vapor deposition (CVD) are used for array fabrication with Al₂O₃ as the tunneling barrier. With regard to the memory characteristics, 93% of the devices exhibit an on/off ratio of over 10³ with an average ratio of 10⁴. The high on/off ratio and reliable endurance in the devices allow stable 6‐level memory applications. The devices also exhibit excellent memory durability over 8000 cycles with a negligible shift in the threshold voltage and on‐current, which is a significant improvement over other types of memristors. In addition, the devices can be strained up to 1% by fabricating on a flexible substrate. This demonstration opens a practical route for next‐generation electronics with CVD‐grown van der Waals layered materials.     

2D Transition‐Metal‐Dichalcogenide‐Nanosheet‐Based Composites for Photocatalytic and Electrocatalytic Hydrogen Evolution Reactions

Hydrogen (H₂) is one of the most important clean and renewable energy sources for future energy sustainability. Nowadays, photocatalytic and electrocatalytic hydrogen evolution reactions (HERs) from water splitting are considered as two of the most efficient methods to convert sustainable energy to the clean energy carrier, H₂. Catalysts based on transition metal dichalcogenides (TMDs) are recognized as greatly promising substitutes for noble‐metal‐based catalysts for HER. The photocatalytic and electrocatalytic activities of TMD nanosheets for the HER can be further improved after hybridization with many kinds of nanomaterials, such as metals, oxides, sulfides, and carbon materials, through different methods including the in situ reduction method, the hot‐injection method, the heating‐up method, the hydro(solvo)thermal method, chemical vapor deposition (CVD), and thermal annealing. Here, recent progress in photocatalytic and electrocatalytic HERs using 2D TMD‐based composites as catalysts is discussed.     

Self‐Assembled Ruddlesden–Popper/Perovskite Hybrid with Lattice‐Oxygen Activation as a Superior Oxygen Evolution Electrocatalyst

The oxygen evolution reaction (OER) is pivotal in multiple gas‐involved energy conversion technologies, such as water splitting, rechargeable metal–air batteries, and CO₂/N₂ electrolysis. Emerging anion‐redox chemistry provides exciting opportunities for boosting catalytic activity, and thus mastering lattice‐oxygen activation of metal oxides and identifying the origins are crucial for the development of advanced catalysts. Here, a strategy to activate surface lattice‐oxygen sites for OER catalysis via constructing a Ruddlesden–Popper/perovskite hybrid, which is prepared by a facile one‐pot self‐assembly method, is developed. As a proof‐of‐concept, the unique hybrid catalyst (RP/P‐LSCF) consists of a dominated Ruddlesden–Popper phase LaSr₃Co_1.5Fe_1.5O_10‐δ (RP‐LSCF) and second perovskite phase La_0.25Sr_0.75Co_0.5Fe_0.5O_3‐δ (P‐LSCF), displaying exceptional OER activity. The RP/P‐LSCF achieves 10 mA cm^?2 at a low overpotential of only 324 mV in 0.1 m KOH, surpassing the benchmark RuO₂ and various state‐of‐the‐art metal oxides ever reported for OER, while showing significantly higher activity and stability than single RP‐LSCF oxide. The high catalytic performance for RP/P‐LSCF is attributed to the strong metal–oxygen covalency and high oxygen‐ion diffusion rate resulting from the phase mixture, which likely triggers the surface lattice‐oxygen activation to participate in OER. The success of Ruddlesden–Popper/perovskite hybrid construction creates a new direction to design advanced catalysts for various energy applications.     

In‐Plane Anisotropic Properties of 1T′‐MoS2 Layers

Crystal phases play a key role in determining the physicochemical properties of a material. To date, many phases of transition metal dichalcogenides have been discovered, such as octahedral (1T), distorted octahedral (1T′), and trigonal prismatic (2H) phases. Among these, the 1T′ phase offers unique properties and advantages in various applications. Moreover, the 1T′ phase consists of unique zigzag chains of the transition metals, giving rise to interesting in‐plane anisotropic properties. Herein, the in‐plane optical and electrical anisotropies of metastable 1T′‐MoS₂ layers are investigated by the angle‐resolved Raman spectroscopy and electrical measurements, respectively. The deconvolution of J₁ and J₂ peaks in the angle‐resolved Raman spectra is a key characteristic of high‐quality 1T′‐MoS₂ crystal. Moreover, it is found that its electrocatalytic performance may be affected by the crystal orientation of anisotropic material due to the anisotropic charge transport.     

Half‐Metallic Behavior in 2D Transition Metal Dichalcogenides Nanosheets by Dual‐Native‐Defects Engineering

Two‐dimensional transition metal dichalcogenides (TMDs) have been regarded as one of the best nonartificial low‐dimensional building blocks for developing spintronic nanodevices. However, the lack of spin polarization in the vicinity of the Fermi surface and local magnetic moment in pristine TMDs has greatly hampered the exploitation of magnetotransport properties. Herein, a half‐metallic structure of TMDs is successfully developed by a simple chemical defect‐engineering strategy. Dual native defects decorate titanium diselenides with the coexistence of metal‐Ti‐atom incorporation and Se‐anion defects, resulting in a high‐spin‐polarized current and local magnetic moment of 2D Ti‐based TMDs toward half‐metallic room‐temperature ferromagnetism character. Arising from spin‐polarization transport, the as‐obtained T‐TiSe_1.8 nanosheets exhibit a large negative magnetoresistance phenomenon with a value of ?40% (5T, 10 K), representing one of the highest negative magnetoresistance effects among TMDs. It is anticipated that this dual regulation strategy will be a powerful tool for optimizing the intrinsic physical properties of TMD systems.     

A General and Straightforward Route to Noble Metal‐Decorated Mesoporous Transition‐Metal Oxides with Enhanced Gas Sensing Performance

Controllable and efficient synthesis of noble metal/transition‐metal oxide (TMO) composites with tailored nanostructures and precise components is essential for their application. Herein, a general mercaptosilane‐assisted one‐pot coassembly approach is developed to synthesize ordered mesoporous TMOs with agglomerated‐free noble metal nanoparticles, including Au/WO₃, Au/TiO₂, Au/NbO_x, and Pt/WO₃. 3‐mercaptopropyl trimethoxysilane is applied as a bridge agent to cohydrolyze with metal oxide precursors by alkoxysilane moieties and interact with the noble metal source (e.g., HAuCl₄ and H₂PtCl₄) by mercapto (? SH) groups, resulting in coassembly with poly(ethylene oxide)‐b‐polystyrene. The noble metal decorated TMO materials exhibit highly ordered mesoporous structure, large pore size (≈14–20 nm), high specific surface area (61–138 m² g^?1), and highly dispersed noble metal (e.g., Au and Pt) nanoparticles. In the system of Au/WO₃, in situ generated SiO₂ incorporation not only enhances their thermal stability but also induces the formation of ε‐phase WO₃ promoting gas sensing performance. Owning to its specific compositions and structure, the gas sensor based on Au/WO₃ materials possess enhanced ethanol sensing performance with a good response (R_air/R_gas = 36–50 ppm of ethanol), high selectivity, and excellent low‐concentration detection capability (down to 50 ppb) at low working temperature (200 °C).     

Electron density changes at the crystal lattice sites upon superconducting phase transition

It is established that complex copper oxides exhibit a relationship between a change in the electron density at the metal crystal lattice site and the temperature (T _c) of a phase transition to the superconducting state. In crystals containing two structurally inequivalent positions of copper atoms, a change in the electron density upon the transition is different for these lattice sites. In particular, there is a limiting value of the electron density variation upon the superconducting transition, which is different for Cu(1) and Cu(2) lattice sites and corresponds to the two different minimum values of the correlation length. The electron density changes at the sites of a Kronig-Penney crystal lattice upon the superconducting transition are calculated. It is established that the transition from normal to superconducting phase is accompanied by an increase in the charge density at the center of the unit cell. This growth increases with T _c, which agrees with the Mössbauer spectroscopy data.     

Phase‐Change Hyperbolic Heterostructures for Nanopolaritonics: A Case Study of hBN/VO2

Unlike conventional plasmonic media, polaritonic van der Waals (vdW) materials hold promise for active control of light–matter interactions. The dispersion relations of elementary excitations such as phonons and plasmons can be tuned in layered vdW systems via stacking using functional substrates. In this work, infrared nanoimaging and nanospectroscopy of hyperbolic phonon polaritons are demonstrated in a novel vdW heterostructure combining hexagonal boron nitride (hBN) and vanadium dioxide (VO₂). It is observed that the insulator‐to‐metal transition in VO₂ has a profound impact on the polaritons in the proximal hBN layer. In effect, the real‐space propagation of hyperbolic polaritons and their spectroscopic resonances can be actively controlled by temperature. This tunability originates from the effective change in local dielectric properties of the VO₂ sublayer in the course of the temperature‐tuned insulator‐to‐metal phase transition. The high susceptibility of polaritons to electronic phase transitions opens new possibilities for applications of vdW materials in combination with strongly correlated quantum materials.     

Multishelled NixCo3–xO4 Hollow Microspheres Derived from Bimetal–Organic Frameworks as Anode Materials for High‐Performance Lithium‐Ion Batteries

Metal–organic frameworks (MOFs) featuring versatile topological architectures are considered to be efficient self‐sacrificial templates to achieve mesoporous nanostructured materials. A facile and cost‐efficient strategy is developed to scalably fabricate binary metal oxides with complex hollow interior structures and tunable compositions. Bimetal–organic frameworks of Ni‐Co‐BTC solid microspheres with diverse Ni/Co ratios are readily prepared by solvothermal method to induce the Ni _x Co_{3? x} O₄ multishelled hollow microspheres through a morphology‐inherited annealing treatment. The obtained mixed metal oxides are demonstrated to be composed of nanometer‐sized subunits in the shells and large void spaces left between adjacent shells. When evaluated as anode materials for lithium‐ion batteries, Ni _x Co_{3? x} O₄‐0.1 multishelled hollow microspheres deliver a high reversible capacity of 1109.8 mAh g^?1 after 100 cycles at a current density of 100 mA g^?1 with an excellent high‐rate capability. Appropriate capacities of 832 and 673 mAh g^?1 could also be retained after 300 cycles at large currents of 1 and 2 A g^?1, respectively. These prominent electrochemical properties raise a concept of synthesizing MOFs‐derived mixed metal oxides with multishelled hollow structures for progressive lithium‐ion batteries.     

Dark‐Exciton‐Mediated Fano Resonance from a Single Gold Nanostructure on Monolayer WS2 at Room Temperature

Strong spatial confinement and highly reduced dielectric screening provide monolayer transition metal dichalcogenides with strong many‐body effects, thereby possessing optically forbidden excitonic states (i.e., dark excitons) at room temperature. Herein, the interaction of surface plasmons with dark excitons in hybrid systems consisting of stacked gold nanotriangles and monolayer WS₂ is explored. A narrow Fano resonance is observed when the hybrid system is surrounded by water, and the narrowing of the spectral Fano linewidth is attributed to the plasmon‐enhanced decay of dark K‐K excitons. These results reveal that dark excitons in monolayer WS₂ can strongly modify Fano resonances in hybrid plasmon–exciton systems and can be harnessed for novel optical sensors and active nanophotonic devices.     

Flexible Nonvolatile Transistor Memory with Solution‐Processed Transition Metal Dichalcogenides

Nonvolatile field‐effect transistor (FET) memories containing transition metal dichalcogenide (TMD) nanosheets have been recently developed with great interest by utilizing some of the intriguing photoelectronic properties of TMDs. The TMD nanosheets are, however, employed as semiconducting channels in most of the memories, and only a few works address their function as floating gates. Here, a floating‐gate organic‐FET memory with an all‐in‐one floating‐gate/tunneling layer of the solution‐processed TMD nanosheets is demonstrated. Molybdenum disulfide (MoS₂) is efficiently liquid‐exfoliated by amine‐terminated polystyrene with a controlled amount of MoS₂ nanosheets in an all‐in‐one floating‐gate/tunneling layer, allowing for systematic investigation of concentration‐dependent charge‐trapping and detrapping properties of MoS₂ nanosheets. At an optimized condition, the nonvolatile memory exhibits memory performances with an ON/OFF ratio greater than 10⁴, a program/erase endurance cycle over 400 times, and data retention longer than 7 × 10³ s. All‐in‐one floating‐gate/tunneling layers containing molybdenum diselenide and tungsten disulfide are also developed. Furthermore, a mechanically‐flexible TMD memory on a plastic substrate shows a performance comparable with that on a hard substrate, and the memory properties are rarely altered after outer‐bending events over 500 times at the bending radius of 4.0 mm.     

Polymorphic Spin,Charge, and Lattice Waves in Vanadium Ditelluride

Lattice distortion, spin interaction, and dimensional crossover in transition metal dichalcogenides (TMDs) have led to intriguing quantum phases such as charge density waves (CDWs) and 2D magnetism. However, the combined effect of many factors in TMDs, such as spin–orbit, electron–phonon, and electron–electron interactions, stabilizes a single quantum phase at a given temperature and pressure, which restricts original device operations with various quantum phases. Here, nontrivial polymorphic quantum states, CDW phases, are reported in vanadium ditelluride (VTe₂) at room temperature, which is unique among various CDW systems; the doping concentration determines the formation of either of the two CDW phases in VTe₂ at ambient conditions. The two CDW polymorphs show different antiferromagnetic spin orderings in which the vanadium atoms create two different stripe-patterned spin waves. First-principles calculations demonstrate that the magnetic ordering is critically coupled with the corresponding CDW in VTe₂, which suggests a rich phase diagram with polymorphic spin, charge, and lattice waves all coexisting in a solid for new conceptual quantum state-switching device applications.     

Origin of Fracture‐Resistance to Large Volume Change in Cu‐Substituted Co3O4 Electrodes

The electrode materials conducive to conversion reactions undergo large volume change in cycles which restrict their further development. It has been demonstrated that incorporation of a third element into metal oxides can improve the cycling stability while the mechanism remains unknown. Here, an in situ and ex situ electron microscopy investigation of structural evolutions of Cu‐substituted Co₃O₄ supplemented by first‐principles calculations is reported to reveal the mechanism. An interconnected framework of ultrathin metallic copper formed provides a high conductivity backbone and cohesive support to accommodate the volume change and has a cube‐on‐cube orientation relationship with Li₂O. In charge, a portion of Cu metal is oxidized to CuO, which maintains a cube‐on‐cube orientation relationship with Cu. The Co metal and oxides remain as nanoclusters (less than 5 nm) thus active in subsequent cycles. This adaptive architecture accommodates the formation of Li₂O in the discharge cycle and underpins the catalytic activity of Li₂O decomposition in the charge cycle.