Depth‐Resolved Modulation of Metal–Oxygen Hybridization and Orbital Polarization across Correlated Oxide Interfaces

Interface‐induced modifications of the electronic, magnetic, and lattice degrees of freedom drive an array of novel physical properties in oxide heterostructures. Here, large changes in metal–oxygen band hybridization, as measured in the oxygen ligand hole density, are induced as a result of interfacing two isovalent correlated oxides. Using resonant X‐ray reflectivity, a superlattice of SrFeO₃ and CaFeO₃ is shown to exhibit an electronic character that spatially evolves from strongly O‐like in SrFeO₃ to strongly Fe‐like in CaFeO₃. This alternating degree of Fe electronic character is correlated with a modulation of an Fe 3d orbital polarization, giving rise to an orbital superstructure. At the SrFeO₃/CaFeO₃ interfaces, the ligand hole density and orbital polarization reconstruct in a single unit cell of CaFeO₃, demonstrating how the mismatch in these electronic parameters is accommodated at the interface. These results provide new insight into how the orbital character of electrons is altered by correlated oxide interfaces and lays out a broadly applicable approach for depth‐resolving band hybridization.     

Visualizing the Anomalous Charge Density Wave States in Graphene/NbSe2 Heterostructures

Metallic layered transition metal dichalcogenides (TMDs) host collective many-body interactions, including the competing superconducting and charge density wave (CDW) states. Graphene is widely employed as a heteroepitaxial substrate for the growth of TMD layers and as an ohmic contact, where the graphene/TMD heterostructure is naturally formed. The presence of graphene can unpredictably influence the CDW order in 2D CDW conductors. This work reports the CDW transitions of 2H-NbSe₂ layers in graphene/NbSe₂ heterostructures. The evolution of Raman spectra demonstrates that the CDW phase transition temperatures (T_CDW) of NbSe₂ are dramatically decreased when capped by graphene. The induced anomalous short-range CDW state is confirmed by scanning tunneling microscopy measurements. The findings propose a new criterion to determine the T_CDW through monitoring the line shape of the A_1g mode. Meanwhile, the 2D band is also discovered as an indicator to observe the CDW transitions. First-principles calculations imply that interfacial electron doping suppresses the CDW states by impeding the lattice distortion of 2H-NbSe₂. The extraordinary random CDW lattice suggests deep insight into the formation mechanism of many collective electronic states and possesses great potential in modulating multifunctional devices.     

The Role of Interfaces in Polyethylene/Metal‐Oxide Nanocomposites for Ultrahigh‐Voltage Insulating Materials

Recent progress in the development of polyethylene/metal‐oxide nanocomposites for extruded high‐voltage direct‐current (HVDC) cables with ultrahigh electric insulation properties is presented. This is a promising technology with the potential of raising the upper voltage limit in today's underground/submarine cables, based on pristine polyethylene, to levels where the loss of energy during electric power transmission becomes low enough to ensure intercontinental electric power transmission. The development of HVDC insulating materials together with the impact of the interface between the particles and the polymer on the nanocomposites electric properties are shown. Important parameters from the atomic to the microlevel, such as interfacial chemistry, interfacial area, and degree of particle dispersion/aggregation, are discussed. This work is placed in perspective with important work by others, and suggested mechanisms for improved insulation using nanoparticles, such as increased charge trap density, adsorption of impurities/ions, and induced particle dipole moments are considered. The effects of the nanoparticles and of their interfacial structures on the mechanical properties and the implications of cavitation on the electric properties are also discussed. Although the main interest in improving the properties of insulating polymers has been on the use of nanoparticles, leading to nanodielectrics, it is pointed out here that larger microscopic hierarchical metal‐oxide particles with high surface porosity also impart good insulation properties. The impact of the type of particle and its inherent properties (purity and conductivity) on the nanocomposite dielectric and insulating properties are also discussed based on data obtained by a newly developed technique to directly observe the charge distribution on a nanometer scale in the nanocomposite.     

ZnO Nanorods on a LaAlO3–SrTiO3 Interface: Hybrid 1D–2D Diodes with Engineered Electronic Properties

Integrating nanomaterials with different dimensionalities and properties is a versatile approach toward realizing new functionalities in advanced devices. Here, a novel diode‐type heterostructure is reported consisting of 1D semiconducting ZnO nanorods and 2D metallic LaAlO₃–SrTiO₃ interface. Tunable insulator‐to‐metal transitions, absent in the individual components, are observed as a result of the competing temperature‐dependent conduction mechanisms. Detailed transport analysis reveals direct tunneling at low bias, Fowler–Nordheim tunneling at high forward bias, and Zener breakdown at high reverse bias. Our results highlight the rich electronic properties of such artificial diodes with hybrid dimensionalities, and the design principle may be generalized to other nanomaterials.     

Generalized Synthesis of Metal Oxide Nanosheets and Their Application as Li‐Ion Battery Anodes

Metal oxide nanosheets have attracted great attention in various fields, such as energy storage, catalysis, and sensors. Current synthesis methods of metal oxide nanosheets are laborious and not scalable. Herein, a facile and scalable method for the synthesis of metal oxide nanosheets is presented, which requires neither hydro‐/solvothermal conditions nor postsynthesis template removal. The synthesis is versatile, as evidenced by the wide variety of metal oxide nanosheets derived. Nanosheet properties such as crystallinity, crystallite size, and carbon content can be controlled by tuning the synthesis conditions. The metal oxide nanosheets demonstrate promising performance as Li‐ion battery anodes.     

Nitrogen Boosts Defective Vanadium Oxide from Semiconducting to Metallic Merit

2D metal oxide nanosheets have attracted substantial attention for various applications owing to their appealing advantages. Yet, the exploration of effective methodology for fabrication of metallic 2D metal oxides with a high concentration of N dopants in a scalable manner remains challenging. Herein, a topochemical strategy is demonstrated on vanadium oxide nanosheets by combining 2D nanostructuring, heteroatom‐doping, and defect engineering for modulating their intrinsic electronic structure and greatly enhancing their electrochemical property. O vacancies and N dopants (V? O? N and V? N bonds) are in situ formed in vanadium oxide via nitridation and lead to semiconductive‐to‐metallic phase transformation evidenced by experimental results and theoretical calculation. Overall, the N‐VO_0.9 nanosheets exhibit a metallic electron transportation behavior and excellent electrochemical performance. These findings shed light on the rational design and electron structure tuning of 2D nanostructures for energy and electronics applications.     

Efficient Spin-Orbit Torque Switching of the Semiconducting Van Der Waals Ferromagnet Cr2Ge2Te6

Being able to electrically manipulate the magnetic properties in recently discovered van der Waals ferromagnets is essential for their integration in future spintronics devices. Here, the magnetization of a semiconducting 2D ferromagnet, i.e., Cr₂Ge₂Te₆, is studied using the anomalous Hall effect in Cr₂Ge₂Te₆/tantalum heterostructures. The thinner the flakes, hysteresis and remanence in the magnetization loop with out-of-plane magnetic fields become more prominent. In order to manipulate the magnetization in such thin flakes, a combination of an in-plane magnetic field and a charge current flowing through Ta—a heavy metal exhibiting giant spin Hall effect—is used. In the presence of in-plane fields of 20 mT, charge current densities as low as 5 × 10⁵ A cm^–2 are sufficient to switch the out-of-plane magnetization of Cr₂Ge₂Te₆. This finding highlights that current densities required for spin-orbit torque switching of Cr₂Ge₂Te₆ are about two orders of magnitude lower than those required for switching nonlayered metallic ferromagnets such as CoFeB. The results presented here show the potential of 2D ferromagnets for low-power memory and logic applications.     

Selective High‐Frequency Mechanical Actuation Driven by the VO2 Electronic Instability

Relaxation oscillators consist of periodic variations of a physical quantity triggered by a static excitation. They are a typical consequence of nonlinear dynamics and can be observed in a variety of systems. VO₂ is a correlated oxide with a solid‐state phase transition above room temperature, where both electrical resistance and lattice parameters undergo a drastic change in a narrow temperature range. This strong nonlinear response allows to realize spontaneous electrical oscillations in the megahertz range under a DC voltage bias. These electrical oscillations are employed to set into mechanical resonance a microstructure without the need of any active electronics, with small power consumption and with the possibility to selectively excite specific flexural modes by tuning the value of the DC electrical bias in a range of few hundreds of millivolts. This actuation method is robust and flexible and can be implemented in a variety of autonomous DC‐powered devices.     

Solution Processed Metal Oxide High‐κ Dielectrics for Emerging Transistors and Circuits

The electronic functionalities of metal oxides comprise conductors, semiconductors, and insulators. Metal oxides have attracted great interest for construction of large‐area electronics, particularly thin‐film transistors (TFTs), for their high optical transparency, excellent chemical and thermal stability, and mechanical tolerance. High‐permittivity (κ) oxide dielectrics are a key component for achieving low‐voltage and high‐performance TFTs. With the expanding integration of complementary metal oxide semiconductor transistors, the replacement of SiO₂ with high‐κ oxide dielectrics has become urgently required, because their provided thicker layers suppress quantum mechanical tunneling. Toward low‐cost devices, tremendous efforts have been devoted to vacuum‐free, solution processable fabrication, such as spin coating, spray pyrolysis, and printing techniques. This review focuses on recent progress in solution processed high‐κ oxide dielectrics and their applications to emerging TFTs. First, the history, basics, theories, and leakage current mechanisms of high‐κ oxide dielectrics are presented, and the underlying mechanism for mobility enhancement over conventional SiO₂ is outlined. Recent achievements of solution‐processed high‐κ oxide materials and their applications in TFTs are summarized and traditional coating methods and emerging printing techniques are introduced. Finally, low temperature approaches, e.g., ecofriendly water‐induced, self‐combustion reaction, and energy‐assisted post treatments, for the realization of flexible electronics and circuits are discussed.     

Confining Tiny MoO2 Clusters into Reduced Graphene Oxide for Highly Efficient Low Frequency Microwave Absorption

Herein, a supermolecular‐scale cage‐confinement pyrolysis strategy is proposed to build two dielectric electromagnetic wave absorbents, in which MoO₂ nanoparticles are sandwiched uniformly between porous carbon shells and reduced graphene oxide (RGO). Both sandwich structures are derived from hybrid hydrogels doped by two different crosslinkers (with/without oxygen bridge), which can precisely confine Mo source (e.g., PMo₁₂). Without adding magnetic components, both absorbents exhibit excellent low frequency absorption performance in combination with electrically tunable ability and enhanced reflection loss value, which is superior over other relative 2D dielectric absorbers and satisfies the requirements of portable electronics. Notably, introducing oxygen bridges in the crosslinker generates a more stable confining configuration, which in turn renders its corresponding derivative exhibiting an extra multifrequency electromagnetic wave absorption trait. The intrinsic electromagnetic wave adjustment mechanism of the ternary hybrid absorbent is also explored. The result reveals that the elevated electromagnetic wave absorbing property is attributed to moderate attenuation constant and glorious impendence matching. The cage‐confinement pyrolysis route to fabricate 2D MoO₂‐based dielectric electromagnetic wave absorbents opens a new path for the design of electromagnetic wave absorbents used in multi/low frequency.