Jiuxiang Dai

Abstract

The formation of moiré superlattices in twisted van der Waals (vdW) homostructures provides a versatile platform for designing the electronic band structure of two-dimensional (2D) materials. In graphene and transition metal dichalcogenides (TMDs) moiré systems, twist angle has been shown to be a key parameter for regulating the moiré superlattice. However, the effect of the modulation of the twist angle on moiré potential and interlayer coupling has not been the subject of experimental investigation. Here, we report the observation of the modulation of moiré potential and intralayer excitons in the WS₂/WS₂ homostructure. By accurately adjusting the torsion angle of the homobilayers, the depth of the moiré potential can be modulated. The confinement effect of the moiré potential on the intralayer excitons was further demonstrated by the changing of temperature and valley polarization. Furthermore, we show that a detection of atomic reconstructions by the low-frequency Raman mapping to map out inhomogeneities in moiré lattices on a large scale, which endows the uniformity of interlayer coupling. Our results provide insights for an in-depth understanding of the behaviors of moiré excitons in the twisted van der Waals homostructure, and promote the study of electrical engineering and topological photonics.

Nature Electronics, Published online: 29 September 2022; doi:10.1038/s41928-022-00836-5

An artificial synaptic transistor that uses a stretchable bilayer semiconductor as the channel and an encapsulating elastomer as the dielectric can exhibit both excitatory and inhibitory synaptic behaviour, even when under 50% strain.

Nature Synthesis, Published online: 29 September 2022; doi:10.1038/s44160-022-00165-7

Two-dimensional materials have many desirable properties but controllable synthesis is difficult. Now, a flux-assisted growth approach has been designed to reproducibly prepare high-quality, atomically thin materials. Eighty atomically thin composite flakes have been prepared by this approach.

A synaptic memtransistor with highly linear and symmetric synaptic weight update characteristics is achieved, based on the polarization switching in two-dimensional ferroelectric semiconductor of α-In₂Se₃. The α-In₂Se₃ memtransistor-type synapse shows high accuracy of 97.76% for digit patterns recognition task in simulated artificial neural networks, promising for high-performance neuromorphic computing.

Abstract

Brain-inspired neuromorphic computing hardware based on artificial synapses offers efficient solutions to perform computational tasks. However, the nonlinearity and asymmetry of synaptic weight updates in reported artificial synapses have impeded achieving high accuracy in neural networks. Here, this work develops a synaptic memtransistor based on polarization switching in a two-dimensional (2D) ferroelectric semiconductor (FES) of α-In₂Se₃ for neuromorphic computing. The α-In₂Se₃ memtransistor exhibits outstanding synaptic characteristics, including near-ideal linearity and symmetry and a large number of programmable conductance states, by taking the advantages of both memtransistor configuration and electrically configurable polarization states in the FES channel. As a result, the α-In₂Se₃ memtransistor-type synapse reaches high accuracy of 97.76% for digit patterns recognition task in simulated artificial neural networks. This work opens new opportunities for using multiterminal FES memtransistors in advanced neuromorphic electronics.

Abstract

Substitutional atomic doping of transition metal dichalcogenides (TMDs) in the chemical vapor deposition (CVD) process is a promising and effective strategy for modifying their physicochemical properties. However, the conventional CVD method only allows narrow-range modulation of the dopant concentration owing to the low reactivity of the precursors. Moreover, the growth of wafer-scale monolayer TMD films with high dopant concentrations is much more challenging. Herein, we report a facile doping approach based on liquid precursor-mediated CVD process for achieving high vanadium (V) doping in the MoS₂ lattice with excellent doping uniformity and stability. The lateral growth of the host MoS₂ lattice and the reactivity of the V precursor were simultaneously improved by introducing an alkali metal halide as a reaction promoter. The metal halide promoter enabled the wafer-scale synthesis of V-incorporated MoS₂ monolayer film with excessively high doping concentrations. The excellent wafer-scale uniformity of the highly V-doped MoS₂ film was confirmed through a series of microscopic, spectroscopic, and electrical analyses.

Intrinsic threshold breakdown voltage with an ultrahigh gain is observed in an avalanche photodetector (APD) based on a graphite/InSe Schottky junction, which is attributed to the high ionization rate due to the reduced dimensionality of electron–phonon scattering in layered InSe. This work opens up a new avenue for future APDs with both low energy consumption and high sensitivity.

Abstract

Realizing both ultralow breakdown voltage and ultrahigh gain is one of the major challenges in the development of high-performance avalanche photodetector. Here, it is reported that an ultrahigh avalanche gain of 3 × 10⁵ can be realized in the graphite/InSe Schottky photodetector at a breakdown voltage down to 5.5 V. Remarkably, the threshold breakdown voltage can be further reduced down to 1.8 V by raising the operating temperature, approaching the theoretical limit of 1.5 Eg\[{{\cal E}_{\bf g}}\]/e, with Eg${{\cal E}_{\bf g}}$ the bandgap of semiconductor. A 2D impact ionization model is developed and it is uncovered that observation of high gain at low breakdown voltage arises from reduced dimensionality of electron–phonon scattering in the layered InSe flake. These findings open up a promising avenue for developing novel weak-light detectors with low energy consumption and high sensitivity.

Given their low-temperature solution processability and mechanical flexibility, organic field-effect transistors are promising candidates for developing Internet-of-Things technologies and realizing next-generation wearable applications. Here, the authors compare the performance of flexible transistors with the emerging vertical-transistor technology, outline the critical device fabrication and material-design strategies, and discuss their role and function in a wide range of flexible electronics applications.

Abstract

The development of flexible and conformable devices, whose performance can be maintained while being continuously deformed, provides a significant step toward the realization of next-generation wearable and e-textile applications. Organic field-effect transistors (OFETs) are particularly interesting for flexible and lightweight products, because of their low-temperature solution processability, and the mechanical flexibility of organic materials that endows OFETs the natural compatibility with plastic and biodegradable substrates. Here, an in-depth review of two competing flexible OFET technologies, planar and vertical OFETs (POFETs and VOFETs, respectively) is provided. The electrical, mechanical, and physical properties of POFETs and VOFETs are critically discussed, with a focus on four pivotal applications (integrated logic circuits, light-emitting devices, memories, and sensors). It is pointed out that the flexible function of the relatively newer VOFET technology, along with its perspective on advancing the applicability of flexible POFETs, has not been reviewed so far, and the direct comparison regarding the performance of POFET- and VOFET-based flexible applications is most likely absent. With discussions spanning printed and wearable electronics, materials science, biotechnology, and environmental monitoring, this contribution is a clear stimulus to researchers working in these fields to engage toward the plentiful possibilities that POFETs and VOFETs offer to flexible electronics.

Wafer-scale continuous hexagonal boron nitride thick films are prepared by magnetron sputtering, and the two-inch complete films are peeled and transferred. The integrity and continuity of the transferred films are verified by measuring the resistance switching behavior. This work systematically studies the stripping process, characterizes the transferred films, and explores the application in the field of resistance switching.

Abstract

A film stripping method that allows for liquid phase exfoliation assisted by spin coating polymethyl methacrylate has been investigated, resulting in a two-inch hexagonal boron nitride (hBN) film to be fully stripped and then transferred. A number of key factors that can influence the stripping and the transferring process of the films grown by sputtering have been systematically analyzed, including different solutions, different concentration of solution and different thickness of films. The morphology and properties of the hBN films before and after stripping have been characterized. The band edge absorption peak of the transferred film is 229 nm and the corresponding optical band gap is 5.50 eV. Such transferred hBN films have been fabricated into transparent resistive switching devices on indium-tin-oxide glass, demonstrating a constant resistance window of ≈10² even under different applied voltages. This work systematically studies the stripping process, characterizes the transferred films, and explores the application in the field of resistance switching, which lay a foundation for the further application of hBN materials in optoelectronic devices.

The van der Waals epitaxial growth of CrS₂ nanoflakes with thicknesses down to 1.5 nm are realized via a Te-assisted chemical vapor deposition routine. Structure and phase characterization confirm the Cr and S stoichiometric ratio of ≈1:2 with a 1T phase layered structure, and a room-temperature ferromagnetic signal is also revealed in the superconducting quantum interference device test.

Abstract

2D magnetic materials have been attracting enormous interest for both fundamental research and potential spintronic applications. Here, this work demonstrates the van der Waals epitaxial growth of air-stable 2D magnetic CrS₂ on mica substrate by chemical vapor deposition (CVD). The layered nature, high crystallinity, stoichiometry, and high-purity 1T phase of the individual CrS₂ crystallites are verified by a series of complementary characterization techniques including atomic force microscope, scanning transmission electron microscope and energy dispersive X-ray spectroscopy. The single-crystalline 1T-phase CrS₂ crystallites exhibit a metallic nature with a moderate conductivity of 1.78 × 10² S m⁻¹ and a room-temperature ferromagnetism with a coercive field of 28.7 Oe. The CVD growth of the air-stable CrS₂ crystallites with 1T phase and high crystallinity offer a new platform for the further physics research and spintronic device applications.

Recent advances in etching of 2D materials are comprehensively presented and typical etching modes of chemical vapor deposition (CVD) etching are shown. Linear etching usually results in etched lines along specific crystal lattice. Anisotropic etching tends to form etched holes with Euclidean geometric patterns. Fractal etching typically leads to self-similar symmetric patterns, such as snow-like patterns.

Abstract

Etching, considered as the reverse process of growth, has drawn intensive interests in the past decades. Rather from offering building blocks for formation of materials, etching is served as removing basic units from the matrix. Generally, etching plays a critical role in three aspects: first, it can serve as direct top-down method to precisely make designed patterns for electronic devices. Second, it can offer an indirect way to probe the detailed growth mechanism of 2D materials, enhancing understanding of growth process. Finally, it can be an efficient and facile way to visualize grain boundaries. Herein, several commonly used etching techniques for 2D materials are presented of which chemical vapor deposition etching has attracted the most intensive attentions. Thereafter, the typical etching modes of 2D materials are demonstrated, wherein linear etching, anisotropic etching, and fractal etching are comprehensively exhibited, respectively. Furthermore, the etching mechanism of 2D materials is elucidated, thereby offering a guideline for probing their in-depth etching dynamics and kinetics. Finally, relevant concerns regarding uniformity and reproducibility within etching process are discussed and the expected future is envisaged.

Here, the authors discover a universal Ag-assisted exfoliation method, which can be used to directly integrate large-scale 2D crystals with plasmonic nanostructures. The long propagation of surface plasmonic polariton in the hybrid structures results in extraordinarily photoluminescence enhancement for monolayer MoS₂ and MoSe₂. This technique allows to direct preparation and exploration of optical properties of emergent 2D crystals, which paves a new way for preparing large-scale 2D materials and their future applications.

Abstract

Advanced exfoliation techniques are crucial for exploring the intrinsic properties and applications of 2D materials. Though the recently discovered Au-enhanced exfoliation technique provides an effective strategy for the preparation of large-scale 2D crystals, the high cost of gold hinders this method from being widely adopted in industrial applications. In addition, direct Au contact could significantly quench photoluminescence (PL) emission in 2D semiconductors. It is therefore crucial to find alternative metals that can replace gold to achieve efficient exfoliation of 2D materials. Here, the authors present a one-step Ag-assisted method that can efficiently exfoliate many large-area 2D monolayers, where the yield ratio is comparable to Au-enhanced exfoliation method. Differing from Au film, however, the surface roughness of as-prepared Ag films on SiO₂/Si substrate is much higher, which facilitates the generation of surface plasmons resulting from the nanostructures formed on the rough Ag surface. More interestingly, the strong coupling between 2D semiconductor crystals (e.g., MoS₂, MoSe₂) and Ag film leads to a unique PL enhancement that has not been observed in other mechanical exfoliation techniques, which can be mainly attributed to enhanced light-matter interaction as a result of extended propagation of surface plasmonic polariton (SPP). This work provides a lower-cost and universal Ag-assisted exfoliation method, while at the same time offering enhanced SPP-matter interactions.

A facile process for the growth of large-area transition metal dichalcogenide heterostructures is demonstrated. Spectroscopic mapping confirms the uniform characteristics down to the micrometer scale. Strong photo-response is observed on lateral MoS₂–MoS₂/WS₂ heterostructures.

Abstract

Despite the plethora of intriguing phenomena observed in heterostructure stacks of 2D transition metal dichalcogenide (2D-TMDC) flakes, their application in functional devices is still hampered due to the lack of reliable growth methodologies for large-area heterostructures. Here, a scalable process for obtaining as-grown transition metal di-chalcogenide heterostructures by a combination of atomic layer deposition of monolayer MoS₂ and solution-based processing of ultrathin WS₂ is presented. Spatially uniform optical and electrical characteristics of the individual TMDC layers and heterostructures are demonstrated down to micrometer length scales using Raman and photoluminescence spectroscopy, and light-beam-induced current measurements. An enhanced photogenerated current is observed for lateral MoS₂–MoS₂/WS₂ heterostructures demonstrating the suitability of this approach for the preparation of functional devices.

Narrow bandgap inorganic ferroelectric thin film materials have become a research trend in recent years. Narrow bandgap ferroelectric materials play an important role in the fields of solar cell, photodetector, and photocatalysis. The materials, fabrication methods, properties and applications of narrow bandgap ferroelectric thin films are summarized in detail.

Abstract

Harvesting solar energy to solve the energy crisis and environmental pollution is a research hotspot in the photovoltaic industry today. Conventional ferroelectric materials have a bandgap between 3.0 and 3.8 eV, and their low absorption coefficients and photocurrent densities limit the photovoltaic applications of ferroelectric materials. Inorganic ferroelectric thin film materials with narrow bandgap (1.1–2.0 eV) have high optical absorption coefficient and broad spectral response, wide bandgap tuning range (1.2–3.0 eV), as well as high photocurrent density and switchable ferroelectric photoelectric effect, which have great potential for photovoltaic and optoelectronic applications in solar cells, photodetection, photocatalysis. This review summarizes the recent developments of narrow bandgap inorganic ferroelectric thin-film materials and aims to stimulate research interest and efforts in ferroelectric photovoltaic materials.

The dielectric constant modified by the interfacial geometric factor (interfacial distance and van der Waals gap) can accurately describe the electrostatic screening effect in van der Waals 2D metal/semiconductor junctions. A field-effect transistor with a high on/off ratio can be designed by screening out the 2D metal/semiconductor junction with high electrostatic controllability.

Abstract

The prediction of the dielectric response in 2D metal–semiconductor junctions (MSJs) is challenging since the screening is inhomogeneous. Herein, a generalized model is proposed to uncover the relationship between interfacial electrostatic screening and the modulation of Schottky barrier height (Φ_SB) in 2D MSJs. This model is developed based on the effective dielectric constant, interfacial distance, and van der Waals gap that sheds light on the profound difference between dielectric screening in 2D and conventional MSJs. This model can predict the variation of Φ_SB for a series of 2D MSJs. Combining thermionic field emission theory with the model, several compatible 2D metals are provided for low-dimensional electronics, among which the α-graphyne-based junction exhibits the largest on/off ratio.

A facile and scalable 2D semiconductor printing strategy utilizing the interface capture effect and hyperdispersed 2D nanosheet ink to print arrays of atomic-thick semiconducting thin films is demonstrated, which enables printed devices with micrometer-scale resolution, high density, and attractive performance, thus paving the way for practical large-area application of 2D crystals in electronic devices at an affordable cost.

Abstract

2D semiconductor crystals offer the opportunity to further extend Moore's law to the atomic scale. For practical and low-cost electronic applications, directly printing devices on substrates is advantageous compared to conventional microfabrication techniques that utilize expensive photolithography, etching, and vacuum-metallization processes. However, the currently printed 2D transistors are plagued by unsatisfactory electrical performance, thick semiconductor layers, and low device density. Herein, a facile and scalable 2D semiconductor printing strategy is demonstrated utilizing the interface capture effect and hyperdispersed 2D nanosheet ink to fabricate high-quality and atomic-thick semiconductor thin-film arrays without additional surfactants. Printed robust thin-film transistors using 2D semiconductors (e.g., MoS₂) and 2D conductive electrodes (e.g., graphene) exhibit high electrical performance, including a carrier mobility of up to 6.7 cm² V⁻¹ s⁻¹ and an on/off ratio of 2 × 10⁶ at 25 °C. As a proof of concept, 2D transistors are printed with a density of ≈47 000 devices per square centimeter. In addition, this method can be applied to many other 2D materials, such as NbSe₂, Bi₂Se₃, and black phosphorus, for printing diverse high-quality thin films. Thus, the strategy of printable 2D thin-film transistors provides a scalable pathway for the facile manufacturing of high-performance electronics at an affordable cost.

Publication date: September 2022

Source: Materials Today, Volume 58

Author(s): Cordelia Sealy

Publication date: September 2022

Source: Materials Today, Volume 58

Author(s): Laurie Donaldson

Abstract

Research on two-dimensional materials in the past decades has brought many insights of low-dimensional science on a wide range of related topics. As a novel two-dimensional structure, the atomic-scale capillaries which can conceptually be seen as the empty space left by removing few layers of two-dimensional materials from their bulk van der Waals crystals offer a unique platform of investigating physical and chemical processes of ions, molecules, and atoms under two-dimensional confinements. Investigation of many important problems, such as capillary condensation and water network structure that are difficult to be explored experimentally in other confinement structures, has now been accessible; two-dimensional migration of ions, water, and gases shows abnormal transport properties beyond conventional theory prediction; influence of quantum effect to molecule permeation is observable even at room temperature. All these discoveries greatly extend our fundamental understandings of nano-science, and stimulate the development of potential applications. We review the fabrication of these two-dimensional capillaries which are created by the assembly of van der Waals heterostructures, and discuss the ultimate steric effects in the smallest possible confinements. Exotic interactions between capillary interior and confined particles are also summarized. When coupled with external stimuli, these channels exhibit tunable mass transport behaviors, which not only gives feedback to the mechanism understanding but in turn guides the channel structure optimization.

Two-dimensional van der Waals topological materials (2D vdW TMs) are an important and quickly growing field. This review systematically summarizes and discusses the preparations, properties, and device applications of 2D vdW TMs over the past decade. Some significant challenges and opportunities for the practical application of 2D vdW TMs in 2D topological electronics are briefly addressed.

Abstract

Over the past decade, 2D van der Waals (vdW) topological materials (TMs), including topological insulators and topological semimetals, which combine atomically flat 2D layers and topologically nontrivial band structures, have attracted increasing attention in condensed-matter physics and materials science. These easily cleavable and integrated TMs provide the ideal platform for exploring topological physics in the 2D limit, where new physical phenomena may emerge, and represent a potential to control and investigate exotic properties and device applications in nanoscale topological phases. However, multifaced efforts are still necessary, which is the prerequisite for the practical application of 2D vdW TMs. Herein, this review focuses on the preparation, properties, and device applications of 2D vdW TMs. First, three common preparation strategies for 2D vdW TMs are summarized, including single crystal exfoliation, chemical vapor deposition, and molecular beam epitaxy. Second, the origin and regulation of various properties of 2D vdW TMs are introduced, involving electronic properties, transport properties, optoelectronic properties, thermoelectricity, ferroelectricity, and magnetism. Third, some device applications of 2D vdW TMs are presented, including field-effect transistors, memories, spintronic devices, and photodetectors. Finally, some significant challenges and opportunities for the practical application of 2D vdW TMs in 2D topological electronics are briefly addressed.

Nature Electronics, Published online: 22 September 2022; doi:10.1038/s41928-022-00828-5

This Review examines the development of micro light-emitting diodes, exploring key performance characteristics, leading manufacturing approaches and current system demonstrations, as well considering the potential future applications of the technology.

Nature Nanotechnology, Published online: 22 September 2022; doi:10.1038/s41565-022-01214-0

Secondary-ion mass spectrometry shows the presence of oxygen in the carbon sublattice of MXene, demonstrating the existence of oxycarbide MXenes.

Nature Nanotechnology, Published online: 22 September 2022; doi:10.1038/s41565-022-01200-6

Epitaxy on nanopatterned graphene enables the realization of a broad spectrum of freestanding single-crystalline membranes with substantially reduced defects.

Nature Electronics, Published online: 23 September 2022; doi:10.1038/s41928-022-00835-6

A monocrystalline native oxide dielectric, β-Bi2SeO5, with a high dielectric constant has been synthesized by oxidizing a two-dimensional (2D) semiconductor, Bi2O2Se. In 2D transistors, the ultrathin β-Bi2SeO5 dielectric demonstrates sub-0.5-nm equivalent oxide thickness and leakage current below the low-power limit, meeting the requirements of the International Roadmap for Devices and Systems.

This review summarizes the theoretical and experimental progress of Ge-based monoelemental and binary two-dimensional (2D) materials, with an emphasis on their crystal structures and electronic, mechanical, thermal, optical, and photoelectric properties. The application prospects of these materials in field effect transistors, photodetectors, optical devices, catalysts, energy storage devices, solar cells, thermoelectric devices, sensors, biomedical materials, and spintronic devices are discussed in detail.

Abstract

Two-dimensional (2D) materials based on group IVA elements have attracted extensive attention owing to their rich chemical structures and novel properties. This comprehensive review focuses on the phases of Ge monoelemental and binary 2D materials including germanene and its derivatives, Ge-IVA binary compounds, Ge-VA binary compounds, and Ge-VIA binary compounds. The latest progress in predictive modeling, fabrication, and fundamental and physical property modulation of their stable 2D configurations are presented. Accordingly, various interesting applications of these Ge-based 2D materials are discussed, particularly field effect transistors, photodetectors, optical devices, catalysts, energy storage devices, solar cells, thermoelectric devices, sensors, biomedical materials, and spintronic devices. Finally, this review concludes with a few perspectives and an outlook for quickly expanding the application scope Ge-based 2D materials based on recent developments.

Publication date: February 2023

Source: Progress in Materials Science, Volume 132

Author(s): Rajalakshmi Sakthivel, Murugan Keerthi, Ren-Jei Chung, Jr-Hau He