Jing Zhang

Nature Communications, Published online: 15 February 2023; doi:10.1038/s41467-023-36512-1

Manipulating electrical and magnetic anisotropies will stimulate multi-terminal device applications. Here, the authors discover axis dependence of current rectifications, magnetic properties and magnon modes in van der Waals multiferroic CuCrP2S6.

Non-van-der-Waals 2D CuCrSe₂ nanosheets with thickness down to monolayer are produced using a redox-controlled exfoliation route. Differing from its bulk form with antiferromagnetic order, the CuCrSe₂ nanosheets exhibit intriguing even–odd-layer-dependent magnetism evolution, which can be attributed to the orbital shift of Cr layers due to Cu-induced centrosymmetry breaking.

Abstract

Van der Waals (vdW) layered materials with strong magnetocrystalline anisotropy have attracted significant interest as the long-range magnetic order in these systems can survive even when their thicknesses is reduced to the 2D limit. Even though the interlayer coupling between the neighboring magnetic layers is very weak, it has a determining effect on the magnetism of these atomic-thickness materials. Herein, a new 2D ferromagnetic material, namely, non-vdW CuCrSe₂ nanosheets with even–odd-layer-dependent ferromagnetism when laminated from an antiferromagnetic bulk is reported. Monolayer and even-layer CuCrSe₂ exhibit the anomalous Hall effect and a significantly enhanced magnetic ordering temperature of more than 125 K. In contrast, the linear Hall effect exists in the odd-layer samples. Theoretical calculations indicate that the layer-dependent magnetic coupling is attributable to the orbital shift of the Cr atoms in the CrSe₂ layers owing to the Cu-induced breaking of the centrosymmetry. Thus, this work sheds light on the exotic magnetic properties of layered materials that exhibit phenomena beyond weak interlayer interactions.

Nature Physics, Published online: 13 February 2023; doi:10.1038/s41567-023-01953-4

When electrons in a crystal interact with the surrounding lattice, they can form quasiparticles known as polarons. A computational approach to studying polarons in two-dimensional materials explains why they are rarely observed in these systems.

Nonvolatile and reversible multilevel switching of silicon photonic devices with Ge₂Sb₂Te₅ (GST) is demonstrated with In₂O₃ transparent microheaters that are compatible with diverse material platforms. The In₂O₃/GST segmented structure is proposed and demonstrated to enable multistage phase transitions with up to 64 switching levels, which is crucial for the development of nonvolatile reconfigurable devices in large-scale programmable optoelectronic systems.

Abstract

Reconfigurable silicon photonic devices are widely used in numerous emerging fields such as optical interconnects, photonic neural networks, quantum computing, and microwave photonics. Currently, phase change materials (PCMs) have been extensively investigated as promising candidates for building switching units due to their strong refractive index modulation. Here, nonvolatile multilevel switching of silicon photonic devices with Ge₂Sb₂Te₅ (GST) is demonstrated with In₂O₃ transparent microheaters that are compatible with diverse material platforms. With GST integrated on the silicon photonic waveguides and Mach-Zehnder interferometers (MZIs), repeatable and reversible multilevel modulation of GST is achieved by electro-thermally induced phase transitions. Particularly, the segmented switching unit of In₂O₃ and GST is proposed and demonstrated to be capable of producing about one order of magnitude larger temperature gradient than that of the nonsegmented unit, resulting in up to 64 distinguishable switching levels of 6-bit precision, and fine-tuning of the switching voltage pulses is promising to push the precision even further, to 7-bit, or 128 distinguishable switching levels. The capability of precise multilevel phase-change modulation is crucial to further facilitate the development of nonvolatile reconfigurable switches and variable attenuation devices as building blocks in large-scale programmable optoelectronic systems.

This study demonstrates the rational manipulation of lattice strain and crystal field energy splitting in Mg₃Sb₂ epitaxial films by the choice of substrates. Theoretical and experimental efforts unambiguously validate that valence band convergence is acquired in the Mg₃Sb₂ film with large in-plane compressive strain that is grown on InP substrate, leading to significantly improved carrier effective mass and Seebeck coefficients.

Abstract

Strain engineering is demonstrated to effectively regulate the functionality of materials, such as thermoelectric, ferroelectric, and photovoltaic properties. As the straightforward approach of strain engineering, epitaxial strain is usually proposed for rationally manipulating the electronic structure and performances of thermoelectric materials, but has rarely been verified experimentally. In this study, tunable and large epitaxial strains are demonstrated, as well as the resulting valence band convergence can be achieved in the Mg₃Sb₂ epi-films with the choice of substrates. The large epitaxial strains up to 8% in Mg₃Sb₂ films represent one of the most striking results in strain engineering. The angle-resolved photoemission spectroscopy measurements and the theoretical calculations reveal the vital role of epitaxial strain in tuning the crystal field splitting and the band structure of Mg₃Sb₂. Benefiting from the appropriate manipulation of the crystal field effect via in-plane compressive strain, the valence band convergence is unambiguously discovered in the strained Mg₃Sb₂ film grown on InP(111) substrate. As a result, a state-of-the-art thermoelectric power factor of 0.94 mWm⁻¹K⁻² is achieved in the strain-engineered Mg₃Sb₂ film, well exceeding that of the strain-relaxed Mg₃Sb₂. The work paves the way for effectively manipulating epitaxial strain and band convergence for Mg₃Sb₂ and other thermoelectric films.

An alternative strategy is developed for oxide dielectrics utilizing van der Waals epitaxy on ultrathin and flexible mica substrate, with a dielectric superlattice of Pb_0.92La_0.08(Zr_0.95Ti_0.05)O₃-SrTiO₃ carefully engineered to construct relaxor antiferroelectric films. An ultrathin flexible capacitor with a record high overall energy density of 12.19 J cm⁻³ and an efficiency of 90.98 % is achieved.

Abstract

Nanoengineered polar oxide films have attracted much attention for electric energy storage thanks to their high energy density, though they are all deposited on thick and rigid substrates, resulting in inferior overall energy density and poor manufacturability. Herein, an alternative strategy is developed for oxide dielectrics utilizing van der Waals epitaxy on ultrathin and flexible mica substrate, with a dielectric superlattice of Pb_0.92La_0.08(Zr_0.95Ti_0.05)O₃-SrTiO₃ carefully engineered to break its long-range antiferroelectric polar order. An ultrathin flexible capacitor is obtained as a result, with a record high overall energy density of 12.19 J cm⁻³ and an efficiency of 90.98%, and there is much room for further improvement since mica substrate can approach 2D limit. The superlattice can be easily rolled for large-scale manufacturing, and the energy storage performances are well maintained under large bending deformation as well as extended bending cycling. The study thus establishes a viable route for dielectric oxide films, paving way for their practical applications in high-energy density capacitors.

A van Hove singularity (VHS) possesses an infinite density of states at a certain energy point when it exists in two dimensions. By taking advantage of this two-dimensional VHS property, it is demonstrated that the type of charge carrier can be tuned in oxide ultrathin film systems, showing the potential for practical devices.

Abstract

Divergent density of states (DOS) can induce extraordinary phenomena such as significant enhancement of superconductivity and unexpected phase transitions. Moreover, van Hove singularities (VHSs) lead to divergent DOS in 2D systems. Despite recent interest in VHSs, only a few controllable cases have been reported to date. In this work, by utilizing an atomically ultra-thin SrRuO₃ film, the electronic structure of a 2D VHS is investigated with angle-resolved photoemission spectroscopy and transport properties are controlled. By applying electric fields with alkali metal deposition and ionic-liquid gating methods, the 2D VHS and the sign of the charge carrier are precisely controlled. Use of a tunable 2D VHS in an atomically flat oxide film could serve as a new strategy to realize infinite DOS near the Fermi level, thereby allowing efficient tuning of electric properties.

The band energy of the second phase is proposed to diminish the electric scattering caused by nanophase incorporation. It realizes similar electronic structures and different phonon structures between the Cu₂Se and dispersed nano-Cu₂Se_0.88S_0.06Te_0.06, which maintain a high power factor and suppress the thermal conductivity, resulting in a figure of merit (zT) value of 2.34 at 850 K.

Abstract

Hitherto, Cu₂Se incorporated with a dispersed second phase shows extremely low thermal conductivity and excellent thermoelectric properties. However, the significant mismatch in electronic band structure between the second phases and the matrix often causes a deterioration of carrier mobility. In this work, based on density functional theory (DFT) calculations, the electronic band structure of the second phase is adjusted through doping S and Te. It is found that Cu₂Se_0.88S_0.06Te_0.06 has a highly similar electronic band structure to the Cu₂Se matrix, which results in high carrier mobility and power factor in Cu₂Se-based composite materials. Additionally, the dispersed second-phase Cu₂Se_0.88S_0.06Te_0.06, dislocations, and nanograins are observed in the Cu₂Se/5 wt% Cu₂Se_0.88S_0.06Te_0.06 product, which leads to a substantial reduction in the thermal conductivity. Finally, high figure of merit (zT) values of 2.04 (by Dulong–Petit heat capacity) and 2.34 (by Differential Scanning Calorimetry (DSC) measured heat capacity) are achieved at 850 K, which are about 65% higher than that of Cu₂Se in this work and comparable to the recently reported p-type Cu₂Se with outstanding performance.

Nature Nanotechnology, Published online: 13 February 2023; doi:10.1038/s41565-023-01326-1

High-quality crystalline two-dimensional layers of metal halides can be on mica, MoS2 or WS2 at temperatures below 400 °C.

Nature Nanotechnology, Published online: 13 February 2023; doi:10.1038/s41565-023-01320-7

An ultralong-range coupling was demonstrated between photonic lattices in bilayer and multilayer moiré architectures, which is mediated by dark surface lattice resonances in the vertical direction.

High-quality Bi₂O₂Te nanosheets and continuous films are grown using low-pressure chemical vapor deposition method. The construction of top single-crystalline native oxide Bi₂TeO₆ and bottom high-mobility Bi₂O₂Te heterostructure is demonstrated via O intercalative oxidation at elevated temperatures in air, with an atomically sharp and low-stress interface, indicating the potential of Bi₂O₂Te with native oxide in planar integrated functional nanoelectronics.

Abstract

Following logic in the silicon semiconductor industry, the existence of native oxide and suitable fabrication technology is essential for 2D semiconductors in planar integronics, which are surface-sensitive to typical coating technologies. To date, very few types of integronics are found to possess this feature. Herein, the 2D Bi₂O₂Te developed recently is reported to possess large-area synthesis and controllable thermal oxidation behavior toward single-crystal native oxides. This shows that surface-adsorbed oxygen atoms are inclined to penetrate across [Bi₂O₂]_n ²ⁿ⁺ layers and bond with the underlying [Te]_n ²ⁿ⁻ at elevated temperatures, transforming directly into [TeO₄]_n ²ⁿ⁻ with the basic architecture remaining stable. The oxide can be adjusted to form in an accurate layer-by-layer manner with a low-stress sharp interface. The native oxide Bi₂TeO₆ layer (bandgap of ≈2.9 eV) exhibits visible-light transparency and is compatible with wet-chemical selective etching technology. These advances demonstrate the potential of Bi₂O₂Te in planar-integrated functional nanoelectronics such as tunnel junction devices, field-effect transistors, and memristors.

Nature Electronics, Published online: 06 February 2023; doi:10.1038/s41928-023-00917-z

Multilayer hexagonal boron nitride can be synthesized over large areas and used to enhance mobility in graphene heterostructures, illustrating the potential of the material as an insulator in commercial two-dimensional electronics.

The interface control of electronic structure in single-atomic-layer ruthenates is demonstrated. Combining thin film epitaxy and photoemission spectroscopy, a method to study interfacial electronic phases of ultrathin heterostructures is developed. Fermi liquid, Hund metal, and Mott insulator phases of SrRuO3 are discovered, exclusively in the single-atomic-layer limit. This work suggests a general way to investigate ultrathin interfacial electronic systems.

Abstract

Interfaces between dissimilar correlated oxides can offer devices with versatile functionalities, and great efforts have been made to manipulate interfacial electronic phases. However, realizing such phases is often hampered by the inability to directly access the electronic structure information; most correlated interfacial phenomena appear within a few atomic layers from the interface. Here, atomic-scale epitaxy and photoemission spectroscopy are utilized to realize the interface control of correlated electronic phases in atomic-scale ruthenate–titanate heterostructures. While bulk SrRuO₃ is a ferromagnetic metal, the heterointerfaces exclusively generate three distinct correlated phases in the single-atomic-layer limit. The theoretical analysis reveals that atomic-scale structural proximity effects yield Fermi liquid, Hund metal, and Mott insulator phases in the quantum-confined SrRuO₃. These results highlight the extensive interfacial tunability of electronic phases, hitherto hidden in the atomically thin correlated heterostructure. Moreover, this experimental platform suggests a way to control interfacial electronic phases of various correlated materials.

A straightforward strategy to rapidly and directly crystallize luminescent metal halide nanocrystals into close-packed face-centered-cubic (FCC) superlattices during the liquid-phase synthesis is demonstrated. The concomitant nanocrystal growth and superlattice formation process is governed by the interplay between the nanocrystal size and surface-coating ligand.

Abstract

The crystallization of nanocrystal building blocks into artificial superlattices has emerged as an efficient approach for tailoring the nanoscale properties and functionalities of novel devices. To date, ordered arrays of colloidal metal halide nanocrystals have mainly been achieved by using post-synthetic strategies. Here, a rapid and direct liquid-phase synthesis is presented to achieve a highly robust crystallization of luminescent metal halide nanocrystals into perfect face-centered-cubic (FCC) superlattices on the micrometer scale. The continuous growth of individual nanocrystals is observed within the superlattice, followed by the disassembly of the superlattices into individually dispersed nanocrystals owing to the highly repulsive interparticle interactions induced by large nanocrystals. Transmission electron microscopy characterization reveals that owing to an increase in solvent entropy, the structure of the superlattices transforms from FCC to hexagonal close-packed (HCP) and the nanocrystals disassemble. The FCC superlattice exhibits a single and slightly redshifted emission, due to the reabsorption-free property of the building block units. Compared to individual nanocrystals, the superlattices have three times higher quantum yield with improved environmental stability, making them ideal for use as ultrabright blue-light emitters. This study is expected to facilitate the creation of metamaterials with ordered nanocrystal structures and their practical applications.

The terminal atom-controlled etching of 2D-TMDs through defects introduced by laser irradiationand subsequent thermal etching in a controlled etching atmosphere are presented. The forward etched hole arrays are etched in Ar/H₂ atmosphere, the hexagonal hole arrays are etched in pure Ar atmosphere, and the reverse etched hole arrays are etched in Ar/sulfur vapor atmosphere.

Abstract

The controlled etching of 2D transition metal dichalcogenides (2D-TMDs) is critical to understanding the growth mechanisms of 2D materials and patterning 2D materials but remains a major comprehensive challenge. Here, a rational strategy to control the terminal atoms of 2D-TMDs etched holes is reported. Using laser irradiation combined with an improved anisotropic thermal etching process under a determined atmosphere, terminal atom-controlled etched hole arrays are created on 2D-TMDs. By adjusting the gas atmosphere during the thermal etching stage, triangular etched hole arrays terminated by the tungsten zigzag (W-ZZ) edge (in an Ar/H₂ atmosphere), hexagonal etched hole arrays terminated alternately by the W-ZZ edge and sulfur (selenium) zigzag (S-ZZ or Se-ZZ) edge (in a pure Ar atmosphere), and triangular etched hole arrays terminated by the S-ZZ (Se-ZZ) edge (in an Ar/sulfur [selenium] vapor atmosphere) can be obtained. Density functional theory reveals the forming energy of different edges and the different activities of metal atoms and chalcogenide atoms under different atmospheres, which determine the terminal atoms of the holes. This work may enhance the understanding of the etching and growth of 2D-TMDs. The 2D-TMDs hole arrays constructed by this work may have important applications in catalysis, nonlinear optics, spintronics, and large-scale integrated circuits.

Laser-induced forward transfer of polymers offers a high-resolution printing approach to generate fluorescent information labels on many different substrates without substrate pre-modification or ink optimization. In contrast to inkjet printing, the high deposition precision of this approach allows to store much more information per area and also enables simple and fast multi-channel readout using machine learning.

Abstract

Tagging, tracking, or validation of products are often facilitated by inkjet-printed optical information labels. However, this requires thorough substrate pretreatment, ink optimization, and often lacks in printing precision/resolution. Herein, a printing method based on laser-driven deposition of solid polymer ink that allows for printing on various substrates without pretreatment is demonstrated. Since the deposition process has a precision of <1 µm, it can introduce the concept of sub-positions with overlapping spots. This enables high-resolution fluorescent labels with comparable spot-to-spot distance of down to 15 µm (444,444 spots cm⁻²) and rapid machine learning-supported readout based on low-resolution fluorescence imaging. Furthermore, the defined thickness of the printed polymer ink spots can be used to fabricate multi-channel information labels. Additional information can be stored in different fluorescence channels or in a hidden topography channel of the label that is independent of the fluorescence.

A light-fuelled omnidirectional crawling soft robot is constructed by introducing twist-bend motion mode. Four less-interfered photo-thermal fillers are employed to realize visible/IR wavelength-selective controllability of the robot. Such twist-bend crawling robot exhibits in situ rotation, four-way turning, and four-way straight moving, and even proves to have the ability to avoid obstacles in complex geographical environments.

Abstract

In recent years, although light-driven soft actuators have attracted intense scientific attention and achieved remarkable progress, the design and construction of an intelligent robotic system with maneuverability, self-adaptability, untethered control, and greater freedom of action, in particular the omnidirectional motion capability on a plane, remains challenging. Herein, four types of photo-thermal fillers and an unprecedented twist-bend actuation mode is introduced into a liquid crystal elastomer-based soft robot. The obtained twist-bend crawling robot not only exhibits in situ rotation, four-way turning, and four-way linear motion under light irradiation with four wavelength bands (520, 655, 808, and 980 nm), but also demonstrates the ability to avoid obstacles in complex geographical environments. This work may bring a new perspective for fabrication and development of soft robots that can adapt to dynamic and complex environmental conditions.

Nature Communications, Published online: 03 February 2023; doi:10.1038/s41467-023-36286-6

Recent studies have reported the growth of 2D non-centrosymmetric single crystals on substrates with surface steps, but the mechanisms are still unclear. Here, the authors demonstrate a method to grow unidirectionally aligned transition metal dichalcogenide grains on various types of substrates, showing the importance of the simultaneous formation of grain nuclei and substrate steps.

Nature Electronics, Published online: 02 February 2023; doi:10.1038/s41928-023-00918-y

A thin and stretchable polymer layer can be fabricated over large areas with high uniformity using a vacuum-deposition method and used as the gate dielectric in stretchy carbon-nanotube-based transistors and circuits that can function at 40% strain.

Nature Electronics, Published online: 02 February 2023; doi:10.1038/s41928-022-00913-9

This Perspective explores the use of flexible electronics in the development of brain–computer interfaces, considering their potential impact on neuroscience, neuroprosthetic control, bioelectronic medicine, and brain and machine intelligence integration.