Jiuxiang Dai

Nature, Published online: 31 August 2022; doi:10.1038/s41586-022-04991-9

By combining large-scale first-principles GW-BSE calculations and micro-reflection spectroscopy, the nature of the exciton resonances in WSe2/WS2 moiré superlattices is identified, highlighting non-trivial exciton states and suggesting new ways of tuning many-body physics.

Nature, Published online: 31 August 2022; doi:10.1038/s41586-022-05031-2

A heterodimensional superlattice consisting of an alternating array of a two-dimensional material and a one-dimensional material shows unconventional octahedral stacking and an unexpected room-temperature anomalous Hall effect.

This review article gives a comprehensive overview of the unique properties and wide applications of liquid metal eutectic gallium–indium (EGaIn). The working principles for the EGaIn-based devices are illustrated and the developments of EGaIn-related techniques are summarized. The main challenges for the development of EGaIn-related techniques are analyzed and the potential applications of EGaIn in the new fields are prospected.

Abstract

Eutectic gallium–indium (EGaIn), a liquid metal with a melting point close to or below room temperature, has attracted extensive attention in recent years due to its excellent properties such as fluidity, high conductivity, thermal conductivity, stretchability, self-healing capability, biocompatibility, and recyclability. These features of EGaIn can be adjusted by changing the experimental condition, and various composite materials with extended properties can be further obtained by mixing EGaIn with other materials. In this review, not only the are unique properties of EGaIn introduced, but also the working principles for the EGaIn-based devices are illustrated and the developments of EGaIn-related techniques are summarized. The applications of EGaIn in various fields, such as flexible electronics (sensors, antennas, electronic circuits), molecular electronics (molecular memory, opto-electronic switches, or reconfigurable junctions), energy catalysis (heat management, motors, generators, batteries), biomedical science (drug delivery, tumor therapy, bioimaging and neural interfaces) are reviewed. Finally, a critical discussion of the main challenges for the development of EGaIn-based techniques are discussed, and the potential applications in new fields are prospected.

Piezoelectric resonators are ubiquitous elements for frequency filtering in telecom devices. A piezoelectric resonator consisting of a suspended van der Waals heterostructure of ultrathin single-crystal free-standing BaTiO₃ sandwiched between graphene sheets is reported. The heterostructure is piezoelectrically driven into a flexural motion and the resulting mechanics shows ferroelectric switching. The bulk-mode resonance at 233 GHz is the highest reported.

Abstract

Suspended piezoelectric thin films are key elements enabling high-frequency filtering in telecommunication devices. To meet the requirements of next-generation electronics, it is essential to reduce device thickness for reaching higher resonance frequencies. Here, the high-quality mechanical and electrical properties of graphene electrodes are combined with the strong piezoelectric performance of the free-standing complex oxide, BaTiO₃ (BTO), to create ultrathin piezoelectric resonators. It is demonstrated that the device can be brought into mechanical resonance by piezoelectric actuation. By sweeping the DC bias voltage on the top graphene electrode, the BTO membrane is switched between the two poled ferroelectric states. Remarkably, ferroelectric hysteresis is also observed in the resonance frequency, magnitude and Q-factor of the first membrane mode. In the bulk acoustic mode, the device vibrates at 233 GHz. This work demonstrates the potential of combining van der Waals materials with complex oxides for next-generation electronics, which not only opens up opportunities for increasing filter frequencies, but also enables reconfiguration by poling, via ferroelectric memory effect.

Single Au atoms in Au₁-S₂ low-coordination structure on ultrathin ZnIn₂S₄ nanosheets were prepared by a rational complex-exchange route. Endowed with much faster interfacial electron transfer from [Ru(bpy)₃]Cl₂ photosensitizer and more appropriate binding strength with *CO intermediates, low-coordination single Au atoms surpass Au nanoparticles counterpart in CO₂ photoreduction, leading to CH₄ product with both high activity and selectivity.

Abstract

Selective CO₂ photoreduction to hydrocarbon fuels such as CH₄ is promising and sustainable for carbon-neutral future. However, lack of proper binding strengths with reaction intermediates makes it still a challenge for photocatalytic CO₂ methanation with both high activity and selectivity. Here, low-coordination single Au atoms (Au₁-S₂) on ultrathin ZnIn₂S₄ nanosheets was synthesized by a complex-exchange route, enabling exceptional photocatalytic CO₂ reduction performance. Under visible light irradiation, Au₁/ZnIn₂S₄ catalyst exhibits a CH₄ yield of 275 μmol g⁻¹ h⁻¹ with a selectivity as high as 77 %. As revealed by detailed characterizations and density functional theory calculations, Au₁/ZnIn₂S₄ with Au₁-S₂ structure not only display fast carrier transfer to underpin its superior activity, but also greatly reduce the energy barrier for protonation of *CO and stabilize the *CH₃ intermediate, thereby leading to the selective CH₄ generation from CO₂ photoreduction.

Perovskite thin films with a 2D–3D gradient result in graded electronic bandgap structures that are ideal for photodiode applications. Low-dimensional perovskite phases at the interface with the electron blocking layer increase the activation energy for thermal charge generation and thereby effectively lower the dark current density to a record-value of 10^–9 mA cm^–2, resulting in maximized sensitivity.

Abstract

Low-dimensional perovskites attract increasing interest due to tunable optoelectronic properties and high stability. Here, it is shown that perovskite thin films with a vertical gradient in dimensionality result in graded electronic bandgap structures that are ideal for photodiode applications. Positioning low-dimensional, vertically-oriented perovskite phases at the interface with the electron blocking layer increases the activation energy for thermal charge generation and thereby effectively lowers the dark current density to a record-low value of 5 × 10⁻⁹ mA cm⁻² without compromising responsivity, resulting in a noise-current-based specific detectivity exceeding 7 × 10¹² Jones at 600 nm. These multidimensional perovskite photodiodes show promising air stability and a dynamic range over ten orders of magnitude, and thus represent a new generation of high-performance low-cost photodiodes.

Advanced Materials, Volume 34, Issue 34, August 25, 2022.

The magneto-transport properties of 2D CrSBr vertical van der Waals heterostructures are inspected, revealing a spontaneous spin alignment along the b-axis together with spin-reorientation and field-induced phases. In multilayers, a spin-valve behavior is observed with large negative magnetoresistance. This makes CrSBr of high interest not only as a new 2D magnetic model but also as a potential spintronic component.

Abstract

2D magnetic materials offer unprecedented opportunities for fundamental and applied research in spintronics and magnonics. Beyond the pioneering studies on 2D CrI₃ and Cr₂Ge₂Te₆, the field has expanded to 2D antiferromagnets exhibiting different spin anisotropies and textures. Of particular interest is the layered metamagnet CrSBr, a relatively air-stable semiconductor formed by antiferromagnetically-coupled ferromagnetic layers (T _c∼150 K) that can be exfoliated down to the single-layer. It presents a complex magnetic behavior with a dynamic magnetic crossover, exhibiting a low-temperature hidden-order below T*∼40 K. Here, the magneto-transport properties of CrSBr vertical heterostructures in the 2D limit are inspected. The results demonstrate the marked low-dimensional character of the ferromagnetic monolayer, with short-range correlations above T _c and an Ising-type in-plane anisotropy, being the spins spontaneously aligned along the easy axis b below T _c. By applying moderate magnetic fields along a and c axes, a spin-reorientation occurs, leading to a magnetoresistance enhancement below T*. In multilayers, a spin-valve behavior is observed, with negative magnetoresistance strongly enhanced along the three directions below T*. These results show that CrSBr monolayer/bilayer provides an ideal platform for studying and controlling field-induced phenomena in two-dimensions, offering new insights regarding 2D magnets and their integration into vertical spintronic devices.

Ferroelectric materials combine a voltage driven switching mechanism with nonvolatility making them promising for nonvolatile devices. Here the material basics and the three different memory devices are described as a starting point for an in-depth discussion of the current status of implementing ferroelectric devices for neuromorphic computing.

Abstract

Due to the voltage driven switching at low voltages combined with nonvolatility of the achieved polarization state, ferroelectric materials have a unique potential for low power nonvolatile electronic devices. The competitivity of such devices is hindered by compatibility issues of well-known ferroelectrics with established semiconductor technology. The discovery of ferroelectricity in hafnium oxide changed this situation. The natural application of nonvolatile devices is as a memory cell. Nonvolatile memory devices also built the basis for other applications like in-memory or neuromorphic computing. Three different basic ferroelectric devices can be constructed: ferroelectric capacitors, ferroelectric field effect transistors and ferroelectric tunneling junctions. In this article first the material science of the ferroelectricity in hafnium oxide will be summarized with a special focus on tailoring the switching characteristics towards different applications.The current status of nonvolatile ferroelectric memories then lays the ground for looking into applications like in-memory computing. Finally, a special focus will be given to showcase how the basic building blocks of spiking neural networks, the neuron and the synapse, can be realized and how they can be combined to realize neuromorphic computing systems. A summary, comparison with other technologies like resistive switching devices and an outlook completes the paper.

Bulk 2D van der Waals (vdW) single-crystalline SnSe₂ is reported as a novel inorganic plastic/ductile thermoelectric (TE) material. It exhibits good deformability and excellent power factor at room temperature. Combining the low cost and nontoxic elements, plastic bulk 2D vdW single-crystalline SnSe₂ is very promising for fabricating the high power density flexible TE device.

Abstract

The recently discovered ductile/plastic inorganic semiconductors pave a new avenue toward flexible thermoelectrics. However, the power factors of current ductile/plastic inorganic semiconductors are usually low (below 5 µW cm⁻¹ K⁻²) as compared with classic brittle inorganic thermoelectric materials, which greatly limit the electrical output power for flexible thermoelectrics. Here, large plasticity and high power factor in bulk two-dimensional van der Waals (2D vdW) single-crystalline SnSe₂ are reported. SnSe₂ crystals exhibit large plastic strains at room temperature and they can be morphed into various shapes without cracking, which is well captured by the inherent large deformability factor. As a semiconductor, the electrical transport properties of SnSe₂ can be readily tuned in a wide range by doping a tiny amount of halogen elements. A high power factor of 10.8 µW cm⁻¹ K⁻² at 375 K along the in-plane direction is achieved in plastic single-crystalline Br-doped SnSe₂, which is the highest value among the reported flexible inorganic and organic thermoelectric materials. Combining the good plasticity, excellent power factors, as well as low-cost and nontoxic elements, bulk 2D vdW single-crystalline SnSe₂ shows great promise to achieve high power density for flexible thermoelectrics.

A single-crystalline GaN epilayer is successfully grown on the WS₂-glass wafer. This study finds that the first layer of the nitrogen adatoms plays an important role in the interfacial lattice construction process, by forming a weak interaction with S atoms and covalent bonds with Ga atoms, simultaneously. This study may pave a way for heterogeneous integration by epitaxy.

Abstract

Use of 2D materials as buffer layers has prospects in nitride epitaxy on symmetry mismatched substrates. However, the control of lattice arrangement via 2D materials at the heterointerface presents certain challenges. In this study, the epitaxy of single-crystalline GaN film on WS₂-glass wafer is successfully performed by using the strong polarity of WS₂ buffer layer and its perfectly matching lattice geometry with GaN. Furthermore, this study reveals that the first interfacial nitrogen layer plays a crucial role in the well-constructed interface by sharing electrons with both Ga and S atoms, enabling the single-crystalline stress-free GaN, as well as a violet light-emitting diode. This study paves a way for the heterogeneous integration of semiconductors and creates opportunities to break through the design and performance limitations, which are induced by substrate restriction, of the devices.

The interlayer spacing of MoSe₂ is compressed by 0.3 Å through constructing CoSe₂/MoSe₂ heterostructures. Benefiting from the optimized electronic structure derived from interfacial charge transfer, and the rough morphology that fully exposes the active sites, the CoSe₂/MoSe₂ heterostructures reported here shows the best hydrogen evolution reaction performance among related powdered electrocatalysts.

Abstract

The construction of heterostructures is a versatile tactic to enhance catalytic activity. However, it is still elusive to realize the modulation of the interlayer spacing in this way to further improve the performance. Here, strong interfacial coupling between CoSe₂ and MoSe₂ by constructing CoSe₂/MoSe₂ heterostructures is achieved. The interlayer spacing of MoSe₂ is compressed by 0.3 Å. The enhanced charge transfer is validated by X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. Coupled with the morphology of hollow microtubes, which can facilitate the exposure of active sites, CoSe₂/MoSe₂ heterostructures reported here exhibit high activity (119 mV at 10 mA cm⁻²) and excellent stability with small degradation after 50 h operation, surpassing other analogous powdered electrocatalysts. This work sheds light on the importance of tuning the interlayer spacing to improve electrocatalytic activity.

Large-scale, low-cost, high-quality, and excellent-performance of raw graphene material are important factors that impact its industrialization development. Based on the excellent mechanical, electrical, thermo-acoustical, optical, and other physical and chemical properties of graphene, it has made a great progress in the development of mechanical sensors, microphone, sound source, electrophysiological detection for sports monitoring, health detection, voice recognition, etc.

Abstract

Graphene, as an emerging 2D material, has been playing an important role in flexible electronics since its discovery in 2004. The representative fabrication methods of graphene include mechanical exfoliation, liquid-phase exfoliation, chemical vapor deposition, redox reaction, etc. Based on its excellent mechanical, electrical, thermo-acoustical, optical, and other properties, graphene has made a great progress in the development of mechanical sensors, microphone, sound source, electrophysiological detection, solar cells, synaptic transistors, light-emitting devices, and so on. In different application fields, large-scale, low-cost, high-quality, and excellent performance are important factors that limit the industrialization development of graphene. Therefore, laser scribing technology, roll-to-roll technology is used to reduce the cost. High-quality graphene can be obtained through chemical vapor deposition processes. The performance can be improved through the design of structure of the devices, and the homogeneity and stability of devices can be achieved by mechanized machining means. In total, graphene devices show promising prospect for the practical fields of sports monitoring, health detection, voice recognition, energy, etc. There is a hot issue for industry to create and maintain the market competitiveness of graphene products through increasing its versatility and killer application fields.

Intergranular diffusion-assisted liquid-phase chemical vapor deposition using a sacrificial metal layer enables the synthesis of wafer-scale patternable 2D semiconductors with controllable thicknesses. The as-synthesized directly patterned transition metal dichalcogenides exhibit excellent charge-transport characteristics with stable photoswitching performance.

Abstract

2D semiconductors have attracted considerable interest in the quest to overcome some of the challenges associated with 3D bulk semiconductors. The application of 2D semiconductors in transistor-based electronic devices requires a reliable patterning technology with thickness controllability for continued transistor scaling. In this study, a facile synthesis approach is developed that allows direct patterning of transition metal dichalcogenides (TMDs) with thickness controllability at the wafer scale through intergranular diffusion-assisted liquid-phase chemical vapor deposition using a sacrificial metal layer. By depositing a liquid-phase transition metal precursor onto the pre-patterned polycrystalline Ni/SiO₂ substrate, a directly patterned transition metal layer can be formed on SiO₂ via intergranular diffusion through the Ni grain boundaries, enabling the growth of patternable TMDs with a controllable thickness. The as-synthesized directly patterned WS₂ transistor exhibits typical n-type transport behavior with a stable photoswitching performance. The proposed patterning technique can make the application of 2D semiconductors in advanced electronic devices more viable.

Significant electron-phonon coupling is discovered in a 2D semiconductor bilayer h-BP that even dominates over the intrinsic phonon-phonon scattering well above room-temperature, which drives phonon transport to an anomalous regime with temperature independent κ_l . This abnormal behavior is first observed in a semiconductor with minor electronic thermal conductivity, facilitating the experimental exploration of electron–phonon coupling impact on heat transport.

Abstract

The electron–phonon coupling (EPC) in semiconductors is typically much weaker than phonon–phonon scattering and its effect on lattice thermal conductivity κ _l has long been considered negligible. Herein, using first-principle calculations, it is discovered that the EPC can be significant or even dominant over the intrinsic phonon–phonon scattering via doping in 2D semiconductor hexagonal boron phosphorus (h-BP). Filling electron pocket till van Hove singularity gradually strengthens the EPC and consequently diminishes the room-temperature κ _l by 25% and 80% for monolayer and bilayer h-BP, respectively. Strikingly, at high doping levels, the EPC drives phonon transport in bilayer h-BP to an anomalous regime where κ _l becomes nearly temperature (T) independent deviated from the intrinsic 1/T trend. This distinctive behavior is governed by the joint effects of horizontal mirror symmetry breaking, and weak phonon–phonon scattering stemming from the predominance of normal processes. Further considering electronic contributions, the abnormal T-independent thermal conductivity is still reserved, thereby facilitating the experimental exploration of EPC effect on κ _l . This study unveils the exotic thermal transport phenomenon in one-atom-thick 2D semiconductors and offers a unique avenue to manipulate heat flow by externally controlling the EPC, which calls for future experimental verification.

A new method has been developed to fabricate low-cost Gallium Arsenide (GaAs) solar cells by multiple reuses of GaAs single crystal wafer using water-soluble, epitaxial, sacrificial, salt layer, and not requiring expensive chemical mechanical polishing techniques. Single junction GaAs solar cells have been fabricated, lifted off, and transferred to inexpensive substrates using just water as an etchant.

Abstract

This paper presents work on the heteroepitaxy of salts, specifically fluorides, on semiconductors and heteroepitaxy of semiconductors on salts. Fluorides layers are deposited on commercial Gallium Arsenide (GaAs) wafers followed by the heteroepitaxial growth of GaAs using metal-organic chemical vapor deposition (MOCVD). The fluoride layers consist of 2 lattice-engineered layers of alkaline-earth compounds to match with GaAs, and are used to sandwich another alkaline-earth compound with higher water-solubility as a sacrificial layer. The triple fluoride layers enable liftoff of free-standing semiconductor films which can be further transferred to desirable substrates. 2D-X-ray Diffraction (2D-XRD) measurements confirm epitaxial growth of both the fluorides and the subsequently grown GaAs films. Single junction (SJ) solar cell devices based on thus prepared films show a power conversion efficiency (PCE) of 10.3% under 1 sun illumination. After the completion of device fabrications, the GaAs film is lifted off from the substrate by a novel water-assisted epitaxial liftoff (H₂O-ELO) technique and transferred to a cheaper substrate. The original GaAs wafer is recycled and reused twice. Devices based on reused substrates show no significant degradation in performance. The semiconductor-salt-semiconductor scheme has great implications in high-performance, flexible, and large-area electronics.

The initiated chemical vapor deposition (iCVD) process which is used to prepare liquid crystalline polymer (LCP) coatings is illustrated. The director orientation of mesogenic units, and surface topology, of the as-deposited LCP coatings can be controlled and varied depending on the surface free energy of the substrates by the surface chemistry.

Abstract

Liquid crystalline polymer (LCP) coatings offer avenues for the fabrication of devices with smart surfaces incorporating responsive topography, reflectivity, or polarization from their unique anisotropic optical and mechanical properties. LCPs have most commonly been made by photopolymerization of reactive mesogens within confined cells. Here, an alternative method of preparing LCP coatings using initiated chemical vapor deposition (iCVD) is described. LCP-iCVD copolymer coatings comprised of an acrylic cyanobiphenyl mesogenic monomer copolymerized with vinyl ether crosslinkers are prepared. Systematic experiments examine the relationship between the monomer properties, reactor conditions, and polymer deposition rate. The alignment of the mesogenic phase in the as-synthesized coatings is also demonstrated to be controlled by the surface energy of the underlying substrate. Interestingly, the surface properties of LCP coatings measured by atomic force microscopy are shown to depend on the director orientation. The data presented in this work introduce mesogenic monomers into the library of vapor-deposited polymer coatings.

Nature Communications, Published online: 29 August 2022; doi:10.1038/s41467-022-32605-5

In isotropic two dimensional systems, long range ferromagnetic order is supressed by thermal fluctuations, and it is due to magnetic anisotropy that van der Waals magnetic materials can have ferromagnetic ordering at finite temperatures. Usually this magnetic anisotropy is relatively small, but in this manuscript Zhang et al make a two dimensional van der Waals material with exceptionally large perpendicular magnetic anisotropy and ferromagnetic ordering that exits up to 350 K.

An intense survey of novel reconfigurable devices based on 2D materials is presented with a focus on reconfigurable transistors that offer run-time control of charge carriers, threshold voltage, and subthreshold swing, and reconfigurable heterostructures manifested as multiple device configurations in one device. The working principles of these devices are discussed in detail and important figures of merit for improving performance are highlighted. In addition, the implementation status of circuits based on reconfigurable devices is reviewed, and a forward-looking view of their opportunities and challenges is provided.

Abstract

As the dimensions of the transistor, the key element of silicon technology, are approaching their physical limits, developing semiconductor technology with novel concepts and materials has been the main focus of scientific research and industry. In recent years, emerging reconfigurable technologies that offer device-level run-time reconfigurability have been explored and shown the potential to enhance device and circuit functions. Two-dimensional (2D) materials possess exquisite electronic properties and provide a suitable platform for reconfigurable technology owing to their atomic-thin thickness and high sensitivity to external electrical fields. In this review, we present an intensive survey of 2D-material-based devices with diverse reconfigurability, including carrier polarity, threshold voltage control, as well as multifunctional configurations enabled by 2D heterostructures. We discuss the working principles for these devices in detail and highlight the important figures of merit for performance improvement. We further provide a forward-looking perspective on the opportunities and challenges of these reconfigurable devices based on 2D materials in the field of computing technologies.

Nature Photonics, Published online: 29 August 2022; doi:10.1038/s41566-022-01060-5

Under near-infrared-light excitation, anti-Stokes-shift superfluorescence is observed near 590 nm at room temperature in a medium of lanthanide-doped upconversion nanoparticles. The spectral width and radiative decay lifetime are 2 nm and 46 ns, respectively, in the single-nanoparticle case.