Jiuxiang Dai

The deterministic positioning of single-crystal plasmonic nanostructures into organized configurations represents a foundational capability for the advancement of wafer-based technologies. In this work, a benchtop nanofabrication route is presented for the formation of large-area arrays of gold nanotriangles. The route is unique in that it avoids colloid-to-substrate transfers, instead opting for the direct synthesis of nanotriangles on substrate surfaces.

Abstract

The advancement of nanoenabled wafer-based devices requires the establishment of core competencies related to the deterministic positioning of nanometric building blocks over large areas. Within this realm, plasmonic single-crystal gold nanotriangles represent one of the most attractive nanoscale components but where the formation of addressable arrays at scale has heretofore proven impracticable. Herein, a benchtop process is presented for the formation of large-area periodic arrays of gold nanotriangles. The devised growth pathway sees the formation of an array of defect-laden seeds using lithographic and vapor-phase assembly processes followed by their placement in a growth solution promoting planar growth and threefold symmetric side-faceting. The nanotriangles formed in this high-yield synthesis distinguish themselves in that they are epitaxially aligned with the underlying substrate, grown to thicknesses that are not readily obtainable in colloidal syntheses, and present atomically flat pristine surfaces exhibiting gold atoms with a close-packed structure. As such, they express crisp and unambiguous plasmonic modes and form photoactive surfaces with highly tunable and readily modeled plasmon resonances. The devised methods, hence, advance the integration of single-crystal gold nanotriangles into device platforms and provide an overall fabrication strategy that is adaptable to other nanomaterials.

Nature Communications, Published online: 08 November 2022; doi:10.1038/s41467-022-34621-x

Transition-metal carbides/nitrides (MXene)-organic single crystal vertical transistor exhibits a fast switching characteristic under an ultralow operating voltage of −1 mV, meanwhile consumes only 8.7 aJ per spike when working as an electrical synaptic transistor.

Abstract

Organic field-effect transistors with parallel transmission and learning functions are of interest in the development of brain-inspired neuromorphic computing. However, the poor performance and high power consumption are the two main issues limiting their practical applications. Herein, an ultralow-power vertical transistor is demonstrated based on transition-metal carbides/nitrides (MXene) and organic single crystal. The transistor exhibits a high J _ON of 16.6 mA cm⁻² and a high J _ON/J _OFF ratio of 9.12 × 10⁵ under an ultralow working voltage of −1 mV. Furthermore, it can successfully simulate the functions of biological synapse under electrical modulation along with consuming only 8.7 aJ of power per spike. It also permits multilevel information decoding modes with a significant gap between the readable time of professionals and nonprofessionals, producing a high signal-to-noise ratio up to 114.15 dB. This work encourages the use of vertical transistors and organic single crystal in decoding information and advances the development of low-power neuromorphic systems.

Interfaced with an n-type semiconductor (Nb:SrTiO₃) having a higher Fermi level, the 2D metallic Ti₃C₂T_x MXene forms a Schottky diode. Advantageously, it does not suffer from Fermi-level pinning, unlike conventional metals. The number of intercalated water molecules in the MXene interlayers appears to depend on the synthesis approach. Removing the interfacial water controls the diode properties.

Abstract

Striving for the sixth-generation communication technology discovery, semiconductors beyond Si with wider bandgaps as well as non-conventional metals are actively being sought to achieve high speeds whilst maintaining devices miniaturization. 2D materials may provide the potential for downsizing, but their functional advantage over existing counterparts still longs to be discovered. Along that path, surface-adsorbed or bulk-intercalated water molecules remaining after wet-chemical synthesis of 2D materials are generally seen as obstacles to high-performance achievement. Herein, the control of such water within the interlayers of solution-processed metallic 2D titanium carbide (MXene) by vacuum annealing duration is demonstrated. Moreover, the impact of water removal on work function (WF) and functional terminations is unveiled for the first time. Furthermore, the usefulness of such water for controlling a novel Schottky diode in contact with an n-type oxide semiconductor, niobium-doped strontium titanate (Nb:SrTiO₃) is observed. The advantage of MXene compared to conventional gold as facile processing, WF tunability, and lower turn-on voltage in the Schottky anode application is highlighted. This fundamental study shows the way for a novel Schottky diode preparation in atmospheric conditions and provides implications for further research directions aiming at commercialization.

npj 2D Materials and Applications, Published online: 07 November 2022; doi:10.1038/s41699-022-00355-z

Nature Electronics, Published online: 07 November 2022; doi:10.1038/s41928-022-00872-1

npj 2D Materials and Applications, Published online: 04 November 2022; doi:10.1038/s41699-022-00357-x

Room-temperature ferromagnetism and strong spin–orbit coupling are realized at a novel topological surface state of the nonmagnetic insulator FeSi by exploiting the proximity effect from a wide-bandgap insulator. A consequent spintronic functionality, that is, current-induced magnetization switching at room temperature, shows great potential for the application of proximity-controlled light-element topological materials as noble-metal-free devices.

Abstract

Strongly spin–orbit coupled states at metal interfaces, topological insulators, and 2D materials enable efficient electric control of spin states, offering great potential for spintronics. However, there are still materials challenges to overcome, including the integration into advanced silicon electronics and the scarce resources of constituent heavy elements of those materials. Through magneto-transport measurements and first-principles calculations, here robust spin–orbit coupling (SOC)-induced properties of a ferromagnetic topological surface state in FeSi and their controllability via hybridization with adjacent materials are demonstrated. In comparison to the case of its naturally oxidized surface, the ferromagnetic transition temperature is greatly increased beyond room temperature and the effective SOC strength is almost doubled at the surface in proximity to a wide-bandgap fluoride insulator. Those enhanced magnetic properties enable room-temperature magnetization switching, being applicable to spin–orbit torque based spintronic devices. Realization of strong SOC in the noble-metal-free silicon-based compound will accelerate spintronic applications.

Flexocatalysis in 2D centrosymmetric semiconductors is demonstrated for the first time via dynamic flexoelectric polarization, largely expanding the polarization-based mechanocatalysis from non-centrosymmetric materials into 2D centrosymmetric semiconductors. The flexocatalysis shows the distinguished performance comparable to the state-of-the-art piezocatalysis, with excellent stability and reproducibility. It opens the field of flexoelectric effect-based mechanochemistry in 2D centrosymmetric semiconductors.

Abstract

Catalysis is vitally important for chemical engineering, energy, and environment. It is critical to discover new mechanisms for efficient catalysis. For piezoelectric/pyroelectric/ferroelectric materials that have a non-centrosymmetric structure, interfacial polarization-induced redox reactions at surfaces leads to advanced mechanocatalysis. Here, the first flexocatalysis for 2D centrosymmetric semiconductors, such as MnO₂ nanosheets, is demonstrated largely expanding the polarization-based-mechanocatalysis to 2D centrosymmetric materials. Under ultrasonic excitation, the reactive species are created due to the strain-gradient-induced flexoelectric polarization in MnO₂ nanosheets composed nanoflowers. The organic pollutants (Methylene Blue et al.) can be effectively degraded within 5 min; the performance of the flexocatalysis is comparable to that of state-of-the-art piezocatalysis, with excellent stability and reproducibility. Moreover, the factors related to flexocatalysis such as material morphology, adsorption, mechanical vibration intensity, and temperature are explored, which give deep insights into the mechanocatalysis. This study opens the field of flexoelectric effect-based mechanochemistry in 2D centrosymmetric semiconductors.

Emerging 2D metal oxides exhibit intriguing electronic and optical properties, which have great potential in emerging functional devices. Diverse structures and various synthesis methods open a new field of vision for device-fabrication-based 2D metal oxides. Their unique properties can bring new vigor and vitality into 2D material family.

Abstract

2D metal oxides have aroused increasing attention in the field of electronics and optoelectronics due to their intriguing physical properties. In this review, an overview of recent advances on synthesis of 2D metal oxides and their electronic applications is presented. First, the tunable physical properties of 2D metal oxides that relate to the structure (various oxidation-state forms, polymorphism, etc.), crystallinity and defects (anisotropy, point defects, and grain boundary), and thickness (quantum confinement effect, interfacial effect, etc.) are discussed. Then, advanced synthesis methods for 2D metal oxides besides mechanical exfoliation are introduced and classified into solution process, vapor-phase deposition, and native oxidation on a metal source. Later, the various roles of 2D metal oxides in widespread applications, i.e., transistors, inverters, photodetectors, piezotronics, memristors, and potential applications (solar cell, spintronics, and superconducting devices) are discussed. Finally, an outlook of existing challenges and future opportunities in 2D metal oxides is proposed.

Nature Communications, Published online: 05 November 2022; doi:10.1038/s41467-022-34373-8

Light-Emitting Devices

A color-tunable light-emitting diode is realized by Jiang Pu, Yasumitsu Miyata, Taishi Takenobu, and co-workers in article number 2203250 using compositionally graded monolayer transition metal dichalcogenide alloys. By controlling the light-emitting positions in the alloys, the composition gradient of the bandgap enables continuous and reversible light emission with energies ranging from 2.1 to 1.7 eV. The results provide a new approach for exploring monolayer semiconductor alloy based broadband optoelectronic device applications.

Scalable approaches of high-quality mono/multilayer hexagonal boron nitride (h-BN) synthesis are reviewed, the challenges and opportunities for each method are discussed, and their relevance to emerging applications is contextualized. Maintaining stoichiometric balance B:N = 1 in the growing crystal and enabling stacking order between layers emerge as the main challenges. Advances in these aspects will inform/guide the synthesis of other 2D materials with >1 constituent element.

Abstract

Hexagonal boron nitride (h-BN) is a layered inorganic synthetic crystal exhibiting high temperature stability and high thermal conductivity. As a ceramic material it has been widely used for thermal management, heat shielding, lubrication, and as a filler material for structural composites. Recent scientific advances in isolating atomically thin monolayers from layered van der Waals crystals to study their unique properties has propelled research interest in mono/few layered h-BN as a wide bandgap insulating support for nanoscale electronics, tunnel barriers, communications, neutron detectors, optics, sensing, novel separations, quantum emission from defects, among others. Realizing these futuristic applications hinges on scalable cost-effective high-quality h-BN synthesis. Here, the authors review scalable approaches of high-quality mono/multilayer h-BN synthesis, discuss the challenges and opportunities for each method, and contextualize their relevance to emerging applications. Maintaining a stoichiometric balance B:N = 1 as the atoms incorporate into the growing layered crystal and maintaining stacking order between layers during multi-layer synthesis emerge as some of the main challenges for h-BN synthesis and the development of processes to address these aspects can inform and guide the synthesis of other layered materials with more than one constituent element. Finally, the authors contextualize h-BN synthesis efforts along with quality requirements for emerging applications via a technological roadmap.

Abstract

Two-dimensional (2D) nanomaterials have aroused immense attention in extensive applications due to their intriguing physical and chemical properties. However, there is a formidable challenge to prepare few-layered and functionalized 2D nanomaterials in an effective and universal way. Herein, we developed an integrated strategy of glucose-assisted mechanochemical exfoliation and cosolvent-intensified sonication exfoliation to effectively exfoliate and functionalize 2D materials. Taking exfoliation of boron nitride (BN) as an example, the production yield and functionalization ratio of BN nanosheets (BNNSs) reached 47.5% and 25.8 wt.%, 188% and 16% higher than that of BNNSs without sonication exfoliation, respectively. The introduction of glucose not only augmented the friction force between adjacent BN layers to promote the efficiency of ball-milling-driven exfoliation supported by density functional theory calculation, but also reacted with active edges of BNNSs for functionalization. Afterwards, cosolvent-intensified sonication exfoliation strongly stabilized exfoliated BNNSs, obviously boosting the exfoliation yield. This proposed method is universal for preparing various 2D nanomaterials like molybdenum disulfide, tungsten disulfide, and graphene nanosheets. The thin plate structure and high functionalization ratio enabled the release of property superiorities of 2D nanomaterials. Our work offers a promising prototype to realize mass production of functionalized 2D nanomaterials.

Abstract

Infrared photodetectors have attracted much attention considering their wide civil and military applications. Two-dimensional (2D) materials offer new opportunities for the development of costless, high-level integration and high-performance infrared photodetectors. With the advent of a broad investigation of infrared photodetectors based on graphene and transition metal chalcogenides (TMDs) exhibiting unique properties in recent decades, research on the better performance of 2D-based infrared photodetectors has been extended to a larger scale, including explorations of new materials and artificial structure designs. In this review, after a brief background introduction, some major working mechanisms, including the photovoltaic effect, photoconductive effect, photogating effect, photothermoelectric effect and bolometric effect, are briefly offered. Then, the discussion mainly focuses on the recent progress of three categories of 2D materials beyond graphene and TMDs. Noble transition metal dichalcogenides, black phosphorus and arsenic black phosphorous and 2D ternary compounds are great examples of explorations of mid-wavelength or even long-wavelength 2D infrared photodetectors. Then, four types of rational structure designs, including type-II band alignments, photogating-enhanced designs, surface plasmon designs and ferroelectric-enhanced designs, are discussed to further enhance the performance via diverse mechanisms, which involve the narrower-bandgap-induced interlayer exciton transition, gate modulation by trapped carriers, surface plasmon polaritons and ferroelectric polarization in sequence. Furthermore, applications including imaging, flexible devices and on-chip integration for 2D-based infrared photodetectors are introduced. Finally, a summary of the state-of-the-art research status and personal discussion on the challenges are delivered.

Nature Communications, Published online: 03 November 2022; doi:10.1038/s41467-022-34158-z

A quasi-one-dimensional van der Waals metallic nanowire Nb₂PdS₆ is synthesized, and its electrical characteristics are analyzed. The 4.64 nm-thick Nb₂PdS₆ shows a breakdown current density (J _BD) of 52 MA cm⁻² when a high electrical field is delivered.

Abstract

A quasi-one-dimensional van der Waals metallic nanowire Nb₂PdS₆ is synthesized, and its electrical characteristics are analyzed. The chemical vapor transport method is applied to produce centimeter-scale Nb₂PdS₆ crystals with needle-like structures and X-ray diffraction (XRD) confirms their high crystallinity. Scanning transmission electron microscopy reveals the crystal orientation and atomic arrangement of the specific region with atomic resolution. The electrical properties are examined by delaminating bulk Nb₂PdS₆ crystals into a few nanometer-scale wires onto 100 nm-SiO₂/Si substrates using a mechanical exfoliation process. Ohmic behavior is confirmed at the low-field measurements regardless of their thickness variation, and 4.64 nm-thick Nb₂PdS₆ shows a breakdown current density (J_BD) of 52 MA cm⁻² when the high electrical field is delivered. Moreover, with further exfoliation down to a single atomic chain, the J_BD of Nb₂PdS₆ is predicted to have a value of 527 MA cm⁻². The breakdown of Nb₂PdS₆ proceeds due to the Joule heating mechanism, and the Nb₂PdS₆ nanowires are well fitted to the 1D thermal dissipating model.

This review focuses on FET-based tactile pressure sensors. The working principles of this kind of tactile sensors are discussed in detail, the state-of-the-art protocols for high-performance tactile sensing are highlighted, and the major advances in large-scale tactile sensor arrays and their applications in robotics, health care, and smart manufacturing in terms of transistor matrix are also introduced.

Abstract

The past several decades have witnessed great progress in high-performance field effect transistors (FET) as one of the most important electronic components. At the same time, due to their intrinsic advantages, such as multiparameter accessibility, excellent electric signal amplification function, and ease of large-scale manufacturing, FET as tactile sensors for flexible wearable devices, artificial intelligence, Internet of Things, and other fields to perceive external stimuli has also attracted great attention and become a significant field of general concern. More importantly, FET has a unique three-terminal structure, which enables its different components to detect external mechanics through different sensing mechanisms. On one hand, it provides an important platform to shed deep insights into the underlying mechanisms of the tactile sensors. On the other hand, these properties could in turn endow excellent components for the construction of tactile matrix sensor arrays with high quality. With special emphasis on the configuration of FETs, this review classified and summarized structure-optimized FET tactile sensors with gate, dielectric layer, semiconductor layer, and source/drain electrodes as sensing active components, respectively. The working principles and the state-of-the-art protocols in terms of high-performance tactile sensors are detail discussed and highlighted, the innovative pixel distribution and integration analysis of the transistor sensor matrix array concerning flexible electronics are also introduced. We hope that the introduction of this review can provide some inspiration for future researchers to design and fabricate high-performance FET-based tactile sensor chips for flexible electronics and other fields.

Structure–processing–property–performance paradigm of materials science provides key guidance for obtaining unprecedented nonplanar van der Waals (vdW) nanoarchitectures. In this review, critical summary and deep insights are provided on the recent development of these emerging nonplanar vdW nanoarchitectures, specifically, nanoscrolls, nanotubes, nanospirals, nanoshells, etc., with the aim to realize the practical optoelectronic applications at the nanoscale.

Abstract

The unique atomic thickness and mechanical flexibility of 2D van der Waals (vdW) materials endow them with spatial designability and constructability. It is easy to break the inherent planar construction through various spatial manipulations, thus creating vdW nanoarchitectures with nonplanar topologies. The basic properties before evolution are retained and tunable by architecture-related feature sizes, and other newly generated properties are inspiring as they are beyond the reach of 2D allotropes, bringing great competitiveness for their encouraging applications in optoelectronics. Here, these representative nonplanar vdW nanoarchitectures (i.e., nanoscrolls, nanotubes, spiral nanopyramids, spiral nanowires, nanoshells, etc.) are summarized and their structural evolution processes are elucidated. Their fascinating nascent properties based on their distinctive structural features, focusing on generally enhanced light–matter interactions and device physics, are further introduced. Finally, their opportunities and challenges for in-depth experimental exploration are prospected. It is a brand-new idea to modify the properties of 2D vdW materials from micro- and nanostructural design and evolution, offering a solid platform for twistronics, valleytronics, and integrated nanophotonics.