Jing Zhang

Nature Photonics, Published online: 07 July 2022; doi:10.1038/s41566-022-01020-z

Researchers demonstrate a self-calibrating programmable photonic integrated circuit. The findings may be useful for the accurate control of large-scale photonic integrated circuits in applications such as light-based machine learning.

Nature Materials, Published online: 07 July 2022; doi:10.1038/s41563-022-01287-1

Superconductivity is reported in magic-angle twisted four-layer and five-layer graphene systems. While they find that all magic-angle graphene systems fit into a unified hierarchy of systems that share a set of flat bands in their electronic band structures, they also report that there is a key distinction between magic-angle twisted bilayer graphene and the other family members, related to the difference in the way the electrons move between the layers in a magnetic field.

Nature Materials, Published online: 07 July 2022; doi:10.1038/s41563-022-01300-7

A rectified Josephson supercurrent is realized in lateral junctions using a proximitized ferromagnetic Pt barrier, with important implications for practical magnetic field free-superconducting spintronics.

Nature Materials, Published online: 07 July 2022; doi:10.1038/s41563-022-01286-2

The twist angle dependence of correlations in alternating twist quadrilayer graphene is reported.

Excimer formation is unambiguously demonstrated in non-van der Waals 2D semiconductor Bi₂O₂Se via transient absorption spectroscopy. The excimer in Bi₂O₂Se nanosheets is diffusive and its formation can be described as excitons relax to an excimer geometry driven by the ultrafast photoscreening of the intrinsic built-in dipolar electric field.

Abstract

The layered semiconductor Bi₂O₂Se is a promising new-type 2D material that holds layered structure via electrostatic forces instead of van der Waals (vdW) attractions. Aside from the huge success in device performance, the non-vdW nature in Bi₂O₂Se with a built-in interlayer electric field has also provided an appealing platform for investigating unique photoexcited carrier dynamics. Here, experimental evidence for the observation of excimers in multilayer Bi₂O₂Se nanosheets via transient absorption spectroscopy is presented. It is found that the excimer formation is the primary decay pathway of photoexcited excitons and three-stage excimer dynamics with corresponding time scales are established. Excitation-fluence-dependent excimer dynamics further suggest that the excimer is diffusive and its formation can be simply described as excitons relaxed to an excimer geometry. This work indicates the outstanding promise of unique excitonic processes in Bi₂O₂Se, which may motivate novel device designs.

Nature Electronics, Published online: 04 July 2022; doi:10.1038/s41928-022-00787-x

A pair of rows of field-effect transistors fabricated perpendicular to the growth direction on an aligned carbon nanotube array can create twinned physically unclonable functions for use in secure communication.

Transition Metal Carbo-Chalcogenides

In article number 2200574, Michael Naguib and co-workers report on the synthesis of 2D transition metal carbo-chalcogenides (TMCCs) by exfoliating their bulk layered counterparts (e.g., Nb₂S₂C and Ta₂S₂C) through electrochemical Li-ion intercalation followed by agitation in water. 2D TMCCs combine the surface of 2D TM dichalcogenides (TMDCs) and the core of 2D TM carbides (MXenes) offering unique characteristics of both TMDCs and MXenes.

Transition Metal Dichalcogenides

In article number 2200492, Miguel M. Ugeda and co-workers demonstrate the feasibility of synthesis and stability of aliovalent alloys of transition metal dichalcogenide materials in the single-layer limit and report the evolution of the electronic ground state (both the electronic structure and behavior of the collective electronic phases) of a 2D Ising superconductor with structural disorder.

Flexible Sensors

In article number 2201663, Kohei Nakajima, Kuniharu Takei, and co-workers propose a multitasking flexible sensor realized by using reservoir computing analysis to estimate rain-droplet volume and wind flow for future weathercasting. Surface morphology and conditions of the sensor are investigated to detect continuous water-droplet conductivity precisely, and an optimized reservoir computing algorithm is applied for water-volume and wind-flow estimations using a single sensor.

Grain boundary engineering in 2D electronic materials for emergent properties is reviewed. Reported structural engineering methods and emergent properties are introduced for various stacking and stitching boundaries such as twist boundary, tilt boundary, stacking fault, and dislocation arrays. Also, remaining challenges and outlook for the future research directions are discussed.

Abstract

Engineering the boundary structures in 2D materials provides an unprecedented opportunity to program the physical properties of the materials with extensive tunability and realize innovative devices with advanced functionalities. However, structural engineering technology is still in its infancy, and creating artificial boundary structures with high reproducibility remains difficult. In this review, various emergent properties of 2D materials with different grain boundaries, and the current techniques to control the structures, are introduced. The remaining challenges for scalable and reproducible structure control and the outlook on the future directions of the related techniques are also discussed.

The magnon current excited in an insulating antiferromagnetic layer by an electronic spin current in an epitaxial all-oxide heterostructure is demonstrated to be effective for manipulating the perpendicular magnetization in a ferromagnetic layer. Furthermore, the critical current density to switch the magnetization is about one order of magnitude smaller than in conventional metallic systems.

Abstract

The search for efficient approaches to realize local switching of magnetic moments in spintronic devices has attracted extensive attention. One of the most promising approaches is the electrical manipulation of magnetization through electron-mediated spin torque. However, the Joule heat generated via electron motion unavoidably causes substantial energy dissipation and potential damage to spintronic devices. Here, all-oxide heterostructures of SrRuO₃/NiO/SrIrO₃ are epitaxially grown on SrTiO₃ single-crystal substrates following the order of the ferromagnetic transition metal oxide SrRuO₃ with perpendicular magnetic anisotropy, insulating and antiferromagnetic NiO, and metallic transition metal oxide SrIrO₃ with strong spin–orbit coupling. It is demonstrated that instead of the electron spin torques, the magnon torques present in the antiferromagnetic NiO layer can directly manipulate the perpendicular magnetization of the ferromagnetic layer. This magnon mechanism may significantly reduce the electron motion-related energy dissipation from electron-mediated spin currents. Interestingly, the threshold current density to generate a sufficient magnon current to manipulate the magnetization is one order of magnitude smaller than that in conventional metallic systems. These findings suggest a route for developing highly efficient all-oxide spintronic devices operated by magnon current.

Nature Nanotechnology, Published online: 04 July 2022; doi:10.1038/s41565-022-01153-w

A non-volatile silicon photonics switch based on phase-change materials actuated by graphene heaters shows a switching energy density that is within an order of magnitude of the fundamental thermodynamic limit.

Nature, Published online: 29 June 2022; doi:10.1038/d41586-022-01732-w

A simple method for incorporating molecules into the gaps of stacked semimetallic materials through immersion offers an efficient way of filtering electrons, which could be useful for information-storage technologies.

Nature, Published online: 29 June 2022; doi:10.1038/s41586-022-04846-3

By intercalating layered 2D atomic crystals with selected chiral molecules, a new class of chiral molecular intercalation superlattices is reported, demonstrating highly ordered structures and achieving high tunnelling magnetoresistance and spin polarization ratios.

Mixed-dimensional van der Waals (vdW) heterostructures based on molybdenum disulfide and lanthanum aluminate are used to realize an optoelectronic memory boasting a distinctive persistent photocurrent phenomenon. These findings provide useful guidelines how to configure next-generation optoelectronic devices, and how to analyze extraordinary optoelectronic properties of the two-dimensional vdW materials and functional oxides, not only separately, but also synergistically.

Abstract

The role of interface in atomically thin two-dimensional (2D) van der Waals materials is crucial in their novel optoelectronic properties. This study reports a mixed-dimensional optoelectronic memory device based on a heterostructure comprising 2D monolayer molybdenum disulfide and bulk lanthanum aluminate. The reversible photo-induced doping process accompanying persistent photocurrent phenomena is controlled using a gate voltage which is applied across the lanthanum aluminate dielectric substrate under light illumination. The extremely low gate electric field (<2 × 10³ V cm⁻¹) and the opposite gate voltage polarity compared with the general photo-doping cases indicate that the conventional band bending mechanism cannot be applied to this optoelectronic device. This distinctive photo-induced memory concept is validated in lanthanum aluminate-based heterostructures with other 2D materials such as graphene and tungsten diselenide. It is postulated that the heterostructure of atomically thin van der Waals materials that are in contact with various functional oxides provides a novel platform for next-generation optoelectronic devices.

Transient photoconductivity and terahertz spectroscopy are used to determine the long- and short-range sum mobility of PEA₂PbI₄ thin films in this study. For both ranges, a sum mobility of 8 cm² (V s)^–1 is found. This previously unreported mobility independent of the probed length scale indicates “single-crystal”-like behavior in a thin film, which can be advantageous in device fabrication.

Abstract

The use of layered, 2D perovskites can improve the stability of metal halide perovskite thin films and devices. However, the charge carrier transport properties in layered perovskites are still not fully understood. Here, the sum of the electron and hole mobilities (Σμ) in thin films of the 2D perovskite PEA₂PbI₄, through transient electronically contacted nanosecond-to-millisecond photoconductivity measurements, which are sensitive to long-time, long-range (micrometer length scale) transport processes is investigated. After careful analysis, accounting for both early-time recombination and the evolution of the exciton-to-free-carrier population, a long-range mobility of 8.0 +/− 0.6 cm² (V s)^–1, which is ten times greater than the long-range mobility of a comparable 3D material FA_0.9Cs_0.1PbI₃ is determined. These values are compared to ultra-fast transient time-resolved THz photoconductivity measurements, which are sensitive to early-time, shorter-range (tens of nm length scale) mobilities. Mobilities of 8 and 45 cm² (V s)^–1 in the case of the PEA₂PbI₄ and FA_0.9Cs_0.1PbI₃, respectively, are obtained. This previously unreported concurrence between the long-range and short-range mobility in a 2D material indicates that the polycrystalline thin films already have single-crystal-like qualities. Hence, their fundamental charge carrier transport properties should aid device performance.

The metallic-phase (1T) transition metal dichalcogenides monolayers are widely used in various fields, whereas their synthetic process relies on the empirical attempts under stringent conditions. The coherent gas-phase electron injection and solid-state alkali intercalation strategy to develop 1T-MoX₂ (X = S, Se) monolayers comprehensively investigates the underlying cation selection rules for the spatial compatibility optimization and 1T-phase formation energy reduction.

Abstract

The technological barriers of the dimensional engineering and interfacial instability seriously hinder the scalable production of metallic (1T) transition metal dichalcogenides (TMD) monolayers. In this article, a facile and fast electron injection strategy is developed to modulate the d orbits of Mo center in trigonal prismatic 2H phases (MoS₂ and MoSe₂); meanwhile various cations (Li⁺, Na⁺, and K⁺) reinforce the in-plane 1T-atomic arrangement and expand the out-of-plane spacing for easy exfoliation. Theoretical and experimental evaluations further elucidate the decisive electron-donating capability and suitable ionic radius in stabilizing 1T coordination. The as-tailored 1T-MoS₂/MoSe₂ anodes can achieve the robust Na⁺ storage in the half cells (5000 cycles at 5 A g^–1) and extreme power output of 3134.9 W kg^–1 in the full cell. This phase-engineering approach enables the precise dimensional manipulation of the 1T TMDs, which further extends their application horizons as the cation host for the power-oriented battery systems.

Nature Materials, Published online: 27 June 2022; doi:10.1038/s41563-022-01282-6

Hafnium dioxide is of technological interest as it is compatible with silicon; however, previous work indicates that a nanometre grain size is required to generate ferroelectricity. Here ferroelectric Y-doped HfO2 thin films with high crystallinity are grown with large crystal grain sizes, indicating that ferroelectricity is intrinsic.

Nature Electronics, Published online: 27 June 2022; doi:10.1038/s41928-022-00776-0

This Review examines the development of micro-thermoelectric devices, exploring progress in device design, integration and performance, and the potential applications of the technology in cooling, power generation and sensing.

The amorphous Ce-doped NiFe-based catalysts are developed by using a facile electrodeposition approach, which shows superior activity and excellent durability for water oxidation in alkaline media.

Abstract

NiFe-based hydroxides are considered as promising nonprecious catalysts for water oxidation due to their low cost and easy preparation. However, the rational design of NiFe-based electrocatalysts remains a great challenge to address the sluggish reaction kinetics and severe deactivation problems for oxygen evolution reaction (OER). Here, the authors report a facile approach to fabricate an amorphous Ce-doped NiFe hydroxide catalyst, which enables high activity and outstanding stability toward OER in alkaline media. The overpotential of electrodeposited amorphous Ce-doped NiFe is only 195 mV at 10 mA cm⁻². Meanwhile, the durability of the amorphous Ce-doped NiFe is maintained for 300 h at 100 mA cm⁻². The comprehensive characterization results reveal that the improved electrochemical performance of the amorphous Ce-doped NiFe catalyst is originated from the favorable oxidation transition of active sites enabled by Ce-doping. It is a very good strategy to introduce highly oxidized state ions to regulate the NiFe-based catalyst to improve the catalytic activity and stability.

Here, using four metal halides and two chalcogenides as prototype material systems, their growth process is investigated and it is found that the growth temperature decreases by maximum 35% on van der Waals (vdW) templates compared with non-vdW substrates. This work provides a universal vdW template-assisted method for the low-temperature synthesis of high-crystallinity 2D materials toward applications in flexible electronics and carbon neutralization.

Abstract

To date, the synthesis of high-quality 2D crystals using vapor deposition methods usually requires high temperature, hindering the integration of 2D materials with Si circuits and exacerbating energy consumption. Exploring low-temperature growth strategies and understanding synthesis mechanism are critical for the practical application of 2D materials. Herein, a van der Waals (vdW) template-assisted growth of 2D crystals (including PbI₂, CdI₂, BiI₃, CuI, Sb₂Te_3, and Bi₂Se₃) is reported, the growth temperature decreases by maximum 35% compared with traditional vapor deposition. The low-temperature 2D growth process resulting from the low surface diffusion barrier of precursors on vdW surfaces is proposed, confirmed by the density functional theory and molecular dynamics calculations. Particularly, the grown 2D crystals can be peeled off from vdW templates easily and transferred to arbitrary substrates for functional applications and the exfoliated vdW templates can be reused for another round of growth. Although the growth temperature is reduced greatly, the excellent photoelectric performance of grown 2D crystals is demonstrated, benefitting from high crystalline quality. These findings provide a universal method for the low-temperature synthesis of high-crystallinity 2D materials toward applications in flexible electronics.

The synthesis of α-GeTe nanoplates on different substrates is reported via the chemical vapor deposition process and the systematical investigation of their structure and electrical properties. 2D α-GeTe nanoplates with excellent conductivity and an extraordinary breakdown current density provide an accessible strategy to improve the performance of 2D electronic devices.

Abstract

Two-dimensional (2D) materials have attracted extensive attention due to their important prospects in electronics and optoelectronics. Synthesizing new 2D materials, characterizing their properties, and developing their applications are still important topics. Herein, the synthesis of α-GeTe nanoplates on different substrates via the chemical vapor deposition process and the systematical investigation of their structure and electrical properties, is reported. By controlling the synthesis temperature and carrier gas, α-GeTe nanoplates, with a lateral dimension up to 30 µm and a thickness down to 1.2 nm, which corresponds to the thickness of one unit cell, can be obtained on 2D WSe₂ substrate. Electrical transport studies show 2D α-GeTe nanoplates have an excellent conductivity (9.33 × 10⁵ S m⁻¹) and an extraordinary breakdown current density (6.1× 10⁷ A cm⁻²). Compared with traditional WSe₂ transistors with deposited metal electrodes, the WSe₂ transistors with the metallic α-GeTe nanoplates as van der Waals metal electrodes achieved much better performance, such as higher on-state current (from 7.83 to 23.23 µA µm⁻¹) and electron mobility (from 16.5 to 75.0 cm² V¹ S¹). This study demonstrates an effective pathway to achieve ultrathin 2D materials and provides an accessible strategy to improve the performance of 2D electronic devices.

This study demonstrates the high doping efficiency of halide perovskites using a simple molecular charge transfer approach and provides a new opportunity for employing 2D perovskites in high-efficiency optoelectronic devices. A thin p-type dopant layer, tetrafluoro-tetracyanoquinodimethane and molybdenum trioxide, deposited using thermal evaporation improves the control of damage-free electronic doping. The efficient charge transfer without deterioration of the perovskite microstructure improves the Hall mobility up to 100 cm² V⁻¹ s⁻¹.

Abstract

2D metal halide perovskites are attracting great interest for their diverse applications owing to their intrinsic superior stability compared to their 3D counterparts; however, their device performance is limited by insufficient charge transport because of the insulating bulky organic ligands. Electrical doping is a direct and efficient method for improving the electrical properties of emerging semiconductors; however, its feasibility and mechanism remain elusive in metal halide perovskites. To clarify this issue, in this study, diverse organic/inorganic molecules are deposited on a typical phenylethyl ammonium tin iodide ((PEA)₂SnI₄) perovskite by constructing a heterojunction. In addition, the variations in the electrical performance of the perovskite semiconductor are monitored. The low work function of the dopant molecules enables the spontaneous electron transfer from the perovskite, resulting in the p-doping effect on the perovskite host, which is verified by a series of characterization methods. The efficient charge transfer without deterioration of the perovskite microstructure improves the Hall mobility up to 100 cm² V⁻¹ s⁻¹. Therefore, this work demonstrates the high doping efficiency of halide perovskites using a simple molecular charge transfer approach and provides a new opportunity for employing 2D perovskites in high-efficiency optoelectronic devices.

Field-Free Switching

In article number 2112561, Jung-Il Hong, Chanyong Hwang, and co-workers demonstrate field-free spin-orbit torque switching of perpendicular magnetization in amorphous and ferrimagnetic Gd/Co multilayers accompanied by a tilted magnetic anisotropy axis. This tilted anisotropy could facilitate the development of magnetic memory and logic devices without the need for external application of a global magnetic field or manufacturing complex magnetic structures to induce symmetry breaking.

Single crystals of van der Waals layered antiferroelectric and magnetic CuCrP₂S₆ show a spontaneous macroscopic polarization mediated by the defect-dipole polarization at the quasi-antipolar state. The highly tunable local ferroelectric state and the defect-dipole polarization can be achieved by a temperature specific poling procedure and survive even without the external electric field. The defect-dipole is likely related to a metastable Cu site within the van der Waals gap and is a smoking gun of a uniaxial quadruple potential well.

Abstract

Recent success in experimental and theoretical works on metal thiophosphates (MTPs) paved the way to add multiple functionalities of complex oxides, such as ferroelectricity, in 2D materials. To realize multiferroicity and magnetoelectric coupling on layered van der Waals materials, incorporating magnetic ions in the ferroelectric framework is desirable. Unfortunately, replacing the metal ion with a magnetic one in MTPs results in antiferroelectricity in which spontaneous macroscopic polarization is absent. Herein, the emergence of a tunable local ferroelectric state in antiferroelectric CuCrP₂S₆ possessing magnetic Cr³⁺ ion is reported. The spontaneous macroscopic polarization is observed, which is switchable by an external poling field through controlling a defect-dipole polarization in the quasi-antipolar state. The observations suggest that the formation of defect dipoles, which is facilitated by an order-disorder-type structural transition, is likely related to a metastable Cu site within the van der Waals gap and therefore is a smoking gun of the existence of a uniaxial quadruple potential well. The interaction between the defect-dipole polarization and dipoles in the antipolar matrix may lead to the emerging local ferroelectricity in antiferroelectric CuCrP₂S₆. The findings suggest a possibility of utilizing the local ferroelectricity of multiferroic MTPs for novel 2D applications.