Jing Zhang

Nature Nanotechnology, Published online: 16 August 2021; doi:10.1038/s41565-021-00960-x

An optical analogue of magic-angle twisted graphene bilayer gives rise to rigorously stopped light, which coupled with gain allows for a new type of a nanolaser with remarkable figures of merit.

Nature Nanotechnology, Published online: 23 August 2021; doi:10.1038/s41565-021-00955-8

Measuring the gate capacitance serves as a probe of the correlated states in MoSe2/WS2 moiré superlattices, which can be further controlled via sample–gate coupling.

The field of two-dimensional (2D) materials has been developing at an impressive pace, with atomically thin crystals of an increasing number of different compounds that have become available, together with techniques enabling their assembly into functional heterostructures. The strategy to detect these atomically thin crystals—based on optical contrast enhanced by Fabry–Pérot interference—has however remained unchanged since the discovery of graphene. Such an absence of evolution is starting to pose problems, because for many of the 2D materials of current interest the optical contrast provided by the commonly used detection procedure is insufficient to identify the presence of individual monolayers, or to determine unambiguously the thickness of atomically thin multilayers. Here we explore an alternative detection strategy, in which the enhancement of optical contrast originates from the use of optically inhomogeneous substrates, leading to diffusively reflected light. Owing to...

Recent research on twisted bilayer graphene (TBG) uncovered that its twist-angle-dependent electronic structure leads to a host of unique properties, such as superconductivity, correlated insulating states, and magnetism. The flat bands that emerge at low twist angles in TBG result in sharp features in the electronic density of states, affecting transport. Here we show that they lead to superior and tuneable thermoelectric (TE) performance. Combining an iterative Boltzmann transport equation solver and electronic structure from an exact continuum model, we calculate TE transport properties of TBG at different twist angles, carrier densities, and temperatures. Our simulations show the room-temperature TE power factor (PF) in TBG reaches 40 mW m −1 K −2 , significantly higher than single-layer graphene and among the highest reported to date. The peak PF is observed near the magic angle, at a twist angle of ≈1.3 ∘ , and near complete band filling. We elu...

Strong interlayer transition in few layers of InSe/PdSe₂ heterojunction is presented. Interlayer transition at the interface is certified both theoretically (density functional theory) and experimentally (photoluminance spectroscopy and Kelvin probe force microscopy). Owing to the strong interlayer transition, the heterojunction shows a wide spectrum response (532 to 1650 nm) with ultra-high photoresponsivity 58.8 A W⁻¹ at 1650 nm.

Abstract

Near infrared (NIR) photodetectors based on 2D materials are widely studied for their potential application in next generation sensing, thermal imaging, and optical communication. Construction of van der Waals (vdWs) heterostructure provides a tremendous degree of freedom to combine and extend the features of 2D materials, opening up new functionalities on photonic and optoelectronic devices. Herein, a type-II InSe/PdSe₂ vdWs heterostructure with strong interlayer transition for NIR photodetection is demonstrated. Strong interlayer transition between InSe and PdSe₂ is predicted via density functional theory calculation and confirmed by photoluminance spectroscopy and Kelvin probe force microscopy. The heterostructure exhibits highly sensitive photodetection in NIR region up to 1650 nm. The photoresponsivity, detectivity, and external quantum efficiency at this wavelength respectively reaches up to 58.8 A W⁻¹, 1 × 10¹⁰ Jones, and 4660%. The results suggest that the construction of vdWs heterostructure with strong interlayer transition is a promising strategy for infrared photodetection, and this work paves the way to developing high-performance optoelectronic devices based on 2D vdWs heterostructures.

Rational band edge energy level control of quasi-2D metal halide perovskites by modifying the dipole moments of organic spacer cations is shown to improve the external quantum efficiency of blue perovskite light-emitting diodes by more than twofold.

Abstract

Perovskite light-emitting diodes (PeLEDs) have received great attention for their potential as next-generation display technology. While remarkable progress has been achieved in green, red, and near-infrared PeLEDs with external quantum efficiencies (EQEs) exceeding 20%, obtaining high performance blue PeLEDs remains a challenge. Poor charge balance due to large charge injection barriers in blue PeLEDs has been identified as one of the major roadblocks to achieve high efficiency. Here band edge control of perovskite emitting layers for blue PeLEDs with enhanced charge balance and device performance is reported. By using organic spacer cations with different dipole moments, that is, phenethyl ammonium (PEA), methoxy phenethyl ammonium (MePEA), and 4-fluoro phenethyl ammonium (4FPEA), the band edges of quasi-2D perovskites are tuned without affecting their band gaps. Detailed characterization and computational studies have confirmed the effect of dipole moment modification to be mostly electrostatic, resulting in changes in the ionization energies of ≈0.45 eV for MePEA and ≈ −0.65 eV for 4FPEA based thin films relative to PEA-based thin films. With improved charge balance, blue PeLEDs based on MePEA quasi-2D perovskites show twofold increase of the EQE as compared to the control PEA based devices.

A kind of solution-processed synaptic transistor is proposed with 2D material MXenes as the floating gate. The device shows biological synaptic behavior under electrical or optical stimuli. Thus, the applications related to conditional learning and image recognition are discussed, which suggest great potential for neuromorphic computing.

Abstract

The highly parallel artificial neural systems based on transistor-like devices have recently attracted widespread attention due to their high-efficiency computing potential and the ability to mimic biological neurobehavior. For the past decades, plenty of breakthroughs related to synaptic transistors have been investigated and reported. In this work, a kind of photoelectronic transistor that successfully mimics the behaviors of biological synapses has been proposed and systematically analyzed. For the individual device, MXenes and the self-assembled titanium dioxide on the nanosheet surface serve as floating gate and tunneling layers, respectively. As the unit electronics of the neural network, the typical synaptic behaviors and the reliable memory stability of the synaptic transistors have been demonstrated through the voltage test. Furthermore, for the first time, the UV-responsive synaptic properties of the MXenes floating gated transistor and its applications, including conditional reflex and supervised learning, have been measured and realized. These photoelectric synapse characteristics illustrate the great potential of the device in bio-imitation vision applications. Finally, through the simulation based on an artificial neural network algorithm, the device successfully realizes the recognition application of handwritten digital images. Thus, this article provides a highly feasible solution for applying artificial synaptic devices to hardware neuromorphic networks.

Assembling Ti₃C₂T _x nanosheets into films with well-defined microstructures and unique surface properties creates attractive physicochemical properties favorable for device design, which has emerged as a prevailing paradigm and expanded the scopes in functional film materials. This review provides a series of optimizing strategies about the competitive features of Ti₃C₂T _x -based films and its applications. In addition, prospects on the future development of Ti₃C₂T _x -based films are provided.

Abstract

2D titanium carbide (Ti₃C₂T _x ) MXene films, with their well-defined microstructures and chemical functionality, provide a macroscale use of nano-sized Ti₃C₂T _x flakes. Ti₃C₂T _x films have attractive physicochemical properties favorable for device design, such as high electrical conductivity (up to 20 000 S cm^–1), impressive volumetric capacitance (1500 F cm^–3), strong in-plane mechanical strength (up to 570 MPa), and a high degree of flexibility. Here, the appealing features of Ti₃C₂T _x -based films enabled by the layer-to-layer arrangement of nanosheets are reviewed. We devote attention to the key strategies for actualizing desirable characteristics in Ti₃C₂T _x -based functional films, such as high and tunable electrical conductivity, outstanding mechanical properties, enhanced oxidation-resistance and shelf life, hydrophilicity/hydrophobicity, adjustable porosity, and convenient processability. This review further discusses fundamental aspects and advances in the applications of Ti₃C₂T _x -based films with a focus on illuminating the relationship between the structural features and the resulting performances for target applications. Finally, the challenges and opportunities in terms of future research, development, and applications of Ti₃C₂T _x -based films are suggested. A comprehensive understanding of these competitive features and challenges shall provide guidelines and inspiration for the further development of Ti₃C₂T _x -based functional films, and contribute to the advances in MXene technology.

Based on organotrichlorosilane post-treatment, high-performance color-stable deep-blue perovskite light-emitting diodes that satisfy the latest Rec. 2020 standard are successfully demonstrated. The best deep-blue device shows a maximum external quantum efficiency of 1.1% with an emission peak of 458 nm, representing a state-of-the-art result for thin-film perovskite light-emitting diodes in this emission region.

Abstract

Recent studies of sky-blue perovskite light-emitting diodes (PeLEDs) have extensively promoted optimal device design to achieve an external quantum efficiency (EQE) above 12%. However, the development of thin-film deep-blue PeLEDs lags dramatically behind, especially with regards to meeting the latest Rec. 2020 standard. A trichloro(3,3,3-trifluoropropyl) silane post-treatment that drives the emission of perovskite into the deep-blue region, ranging from 440 to 460 nm, which meets the Rec. 2020 standard, is proposed. The chlorine ions released from the organotrichlorosilane molecules during their polycondensation reaction provide an addition halide source to fine tune the composition of the mixed halide perovskite films, leading to increase of bandgap and deep-blue emission. In addition, hydrogen bonds between the hydroxy groups of silane molecules and halide anions in perovskite can suppress ion migration for improving emission stability. As a result, an optimal PeLED is developed with deep-blue emission at 458 nm and excellent color stability, which yields an EQE and luminance of 1.1% and 130 cd m⁻², respectively, representing a state-of-the-art result for thin-film PeLEDs in this emission region. This work paves the way to achieve high-performance deep-blue PeLEDs with stable emissions to meet the demand for potential applications such as full-color display.

Lithium Metal Batteries

In article number 2101261, Seon Joon Kim, Young Soo Yun, and co-workers study lithiophilic surface-guided lithium metal nucleation and growth behaviors using a large-area Ti₃C₂T _x MXene electrode containing a large number of oxygen and fluorine dual heteroatoms. The lithiophilic MXene substrate significantly affects the surface tension of the lithium metal nuclei as well as the nucleation overpotential, forming highly reversible metal clusters composed of spherical lithium nanoparticles.

This work demonstrates the use of asymmetric nanochannel membranes composed of metal carbides/nitride and block copolymer layers for harvesting osmotic energy. This membrane with chemical, geometrical, and electrostatic heterostructures contributes to impairing the concentration polarization. In nanoconfinement, the membrane preserves the surface-charge-governed ion transport and exhibits excellent ion selectivity and flow, achieving a high-performance power density of 6.74 W m⁻².

Abstract

Membrane-based osmotic power harvesting is a strategy for sustainable power generation. 2D nanofluids with high ion conductivity and selectivity are emerging candidates for osmotic energy conversion. However, the ion diffusion under nanoconfinement is hindered by homogeneous 2D membranes with monotonic charge regulation and severe concentration polarization, which results in an undesirable power conversion performance. Here, an asymmetric nanochannel membrane with a two-layered structure is reported, in which the angstrom-scale channels of 2D transition metal carbides/nitrides (MXenes) act as a screening layer for controlling ion transport, and the nanoscale pores of the block copolymer (BCP) are the pH-responsive arrays with an ordered nanovoid structure. The heterogeneous nanofluidic device exhibits an asymmetric charge distribution and enlarged 1D BCP porosity under acidic and alkaline conditions, respectively; this improves the gradient-driven ion diffusion, allowing a high-performance osmotic energy conversion with a power density of up to 6.74 W m⁻² by mixing artificial river water and seawater. Experiments and theoretical simulations indicate that the tunable asymmetric heterostructure contributes to impairing the concentration polarization and enhancing the ion flux. This efficient osmotic energy generator can advance the fundamental understanding of the MXene-based heterogeneous nanofluidic devices as a paradigm for membrane-based energy conversion technologies.

In rare-earth permanent magnet SmCo₅ based magnetic multilayers featuring perpendicular magnetic anisotropy, efficient spin-orbit torque switching is demonstrated in the Pt/SmCo₅/Ta trilayer. Evidence of interfacial chiral magnetism, including isolated skyrmions and skyrmionium-like spin texture in these multilayers is also revealed.

Abstract

Interfacially asymmetric magnetic multilayers made of heavy metal/ferromagnet have attracted considerable attention in the spintronics community for accommodating spin-orbit torques (SOTs) and meanwhile for hosting chiral spin textures. In these multilayers, the accompanied interfacial Dzyaloshinskii–Moriya interaction (iDMI) permits the formation of Néel-type spin textures. While significant progresses have been made in Co, CoFeB, Co₂FeAl, CoFeGd based multilayers, it would be intriguing to identify new magnetic multilayers that could enable spin-torque controllability and meanwhile host nanoscale skyrmions. In this report, first, thin films made of permanent magnet SmCo₅ with perpendicular magnetic anisotropy are synthesized, in which the deterministic SOT switching, enabled by the spin Hall effect, in Pt/SmCo₅/Ta trilayer is demonstrated. Further, the stabilization of room-temperature skyrmions with diameters ≈100 nm in [Pt/SmCo₅/Ta]₁₅, together with a skyrmionium-like spin texture in [Pt/SmCo₅/Ir]₁₅ multilayers is shown. Based on the material specific parameters, micromagnetic simulations are also carried out. The results confirm the presence of chiral spin textures in this new material family. Through interfacial engineering, the results thus demonstrate that rare earth permanent magnets could be a new platform for studying interfacial chiral spintronics.

Interactions of cells through graphene nanomaterials is presented. The article concludes that graphene is a promising candidate for neural tissue engineering.

Abstract

Graphene unique physicochemical properties made it prominent among other allotropic forms of carbon, in many areas of research and technological applications. Interestingly, in recent years, many studies exploited the use of graphene family nanomaterials (GNMs) for biomedical applications such as drug delivery, diagnostics, bioimaging, and tissue engineering research. GNMs are successfully used for the design of scaffolds for controlled induction of cell differentiation and tissue regeneration. Critically, it is important to identify the more appropriate nano/bio material interface sustaining cells differentiation and tissue regeneration enhancement. Specifically, this review is focussed on graphene-based scaffolds that endow physiochemical and biological properties suitable for a specific tissue, the nervous system, that links tightly morphological and electrical properties. Different strategies are reviewed to exploit GNMs for neuronal engineering and regeneration, material toxicity, and biocompatibility. Specifically, the potentiality for neuronal stem cells differentiation and subsequent neuronal network growth as well as the impact of electrical stimulation through GNM on cells is presented. The use of field effect transistor (FET) based on graphene for neuronal regeneration is described. This review concludes the important aspects to be controlled to make graphene a promising candidate for further advanced application in neuronal tissue engineering and biomedical use.

In the WTe₂/CrCl₃ bilayer van der Waals heterostructure, a perfect Néel-type skyrmion–bimeron–ferromagnetic cyclic phase transition featured by the evolution of the topological number is achieved by resorting to a perpendicular magnetic field combined with heating and cooling, and the skyrmion can be written and erased by the ferroelectricity of the CuInP₂S₆ monolayer.

Abstract

As a promising candidate for the much-desired low power consumption spintronic devices, 2D magnetic van der Waals material also provides a versatile platform for the design and control of topological spin textures. In this work on WTe₂/CrCl₃ bilayer van der Waals heterostructures, a complete Néel-type skyrmion–bimeron–ferromagnet phase transition is demonstrated, accompanied by the evolution of the topological number. This cyclic transition, mediated by a perpendicular magnetic field, is largely driven by the competition between the out-of-plane magnetocrystalline anisotropy and magnetic dipole–dipole interaction. In the presence of a driving current, the Néel-type skyrmion gains a higher velocity yet larger skyrmion Hall angle, in comparison to the bimeron. By incorporating a ferroelectric CuInP₂S₆ monolayer as a substrate, writing and erasing of skyrmions may be regulated using a ferroelectric polarization. This work sheds light on a novel approach to the design and control of magnetic skyrmions on 2D van der Waals materials.

Platinum atomic layer deposition and thermal assisted conversion are combined to conformally and selectively coat structured substrates with layered platinum diselenide. The viability of the approach to controllably fabricate hybrid devices with a 2D material is demonstrated by the manufacture of a structured gas sensor and a fully integrated infrared photodetector on a Si-waveguide.

Abstract

2D materials display very promising intrinsic material properties, with multiple applications in electronics, photonics, and sensing. In particular layered platinum diselenide has shown high potential due to its layer-dependent tunable bandgap, low-temperature growth, and high environmental stability. Here, the conformal and area selective (AS) low-temperature growth of layered PtSe₂ is presented defining a new paradigm for 2D material integration. The thermally-assisted conversion of platinum which is deposited by AS atomic layer deposition to PtSe₂ is demonstrated on various substrates with a distinct 3D topography. Further the viability of the approach is presented by successful on-chip integration of hybrid semiconductor devices, namely by the manufacture of a highly sensitive ammonia sensors channel with 3D topography and fully integrated infrared-photodetectors on silicon photonics waveguides. The presented methodologies of conformal and AS growth therefore lay the foundation for new design routes for the synthesis of more complex hybrid structures with 2D materials.

Inspired by the ordered structure of muscles, MXene conductive hydrogels are anisotropic and can be used as wearable flexible sensors. The conductive hydrogels have the advantages of wide temperature tolerance range, high sensitivity and good stability. The flexible sensors can achieve motion detection through wireless technology and can be assembled into 3D arrays.

Abstract

Conductive hydrogels as flexible electronic devices, not only have unique attractions but also meet the basic need of mechanical flexibility and intelligent sensing. How to endow anisotropy and a wide application temperature range for traditional homogeneous conductive hydrogels and flexible sensors is still a challenge. Herein, a directional freezing method is used to prepare anisotropic MXene conductive hydrogels that are inspired by ordered structures of muscles. Due to the anisotropy of MXene conductive hydrogels, the mechanical properties and electrical conductivity are enhanced in specific directions. The hydrogels have a wide temperature resistance range of −36 to 25 °C through solvent substitution. Thus, the muscle-inspired MXene conductive hydrogels with anisotropy and low-temperature resistance can be used as wearable flexible sensors. The sensing signals are further displayed on the mobile phone as images through wireless technology, and images will change with the collected signals to achieve motion detection. Multiple flexible sensors are also assembled into a 3D sensor array for detecting the magnitude and spatial distribution of forces or strains. The MXene conductive hydrogels with ordered orientation and anisotropy are promising for flexible sensors, which have broad application prospects in human–machine interface compatibility and medical monitoring.

Mixed-dimensional (MD) heterostructures based on the combination of 2D and nD materials (n = 0, 1, 3) exhibit extraordinary optoelectrical properties, indicating a new way to develop optoelectronic synaptic devices. This review introduces the fabrication techniques of MD heterostructures, highlights the role of light in simulating synaptic functions, and discusses their potential applications in neuromorphic systems.

Abstract

Neuromorphic devices provide a hardware platform to implement synaptic functions into artificial electronic devices, which opens a new way to overcome the von Neumann bottleneck from the device level. Optoelectronic synaptic devices are expected to break the limitations of electrically stimulated synapses due to wider bandwidth, higher speed, and lower crosstalk. However, most optoelectronic synaptic devices are enabled by the defect-dominant photo-generated carrier trapping/de-trapping. Therefore, designable device structure and controllable synaptic functions are urgently desirable in optoelectronic synaptic devices. Among various functional materials, low-dimensional materials exhibit excellent optical and electrical properties and can be easily applied to build van der Waals (vdW) heterostructures with ideal surface characteristics. Herein, the basic morphology and characteristics of low-dimensional materials have been introduced and the typical constitution of mixed-dimensional (MD) vdW heterostructures has been reviewed to highlight their unique light-matter interaction. Then, optoelectronic synaptic devices are classified into three categories by the role of light as input, modulated and output signals based on different photoelectric conversion mechanisms. Furthermore, a bridge between neuromorphic devices and practical applications is established to illustrate their potential in neuromorphic systems. Finally, great challenges and possible study directions are presented to guide the development of MD vdW heterostructures in future neuromorphic systems.

Metallic/semimetallic transition metal dichalcogenides (m-TMDs) have grabbed widespread attention in recent years due to their exotic physical properties and potential applications in various fields. The state-of-the-art progress in m-TMDs is reviewed, including electronic and crystal structures, synthetic methods, physical properties, and practical applications. Moreover, views on development, challenges, and future prospects of m-TMDs are put forward.

Abstract

2D materials and the associated heterostructures define an ideal material platform for investigating physical and chemical properties, and exhibiting new functional applications in (opto)electronic devices, electrocatalysis, and energy storage. 2D transition metal dichalcogenides (2D TMDs), as a member of the 2D materials family including 2D semiconducting TMDs (s-TMDs) and 2D metallic/semimetallic TMDs (m-TMDs) have attracted considerable attention in the scientific community. Over the past decade, the 2D s-TMDs have been extensively researched and reviewed elsewhere. Because of their distinctive physical properties including intrinsic magnetism, charge-density-wave order and superconductivity, and potential applications, such as high-performance electronic devices, catalysis, and as metal electrode contacts, 2D m-TMDs have grabbed widespread attention in recent years. However, reviews demonstrating the m-TMDs systematically and comprehensively have been rarely reported. Here, the recent advances in 2D m-TMDs in the aspects of their unique structures, synthetic approaches, distinctive physical properties, and functional applications are highlighted. Finally, the current challenges and perspectives are discussed.

A self-interfacing flexible thermal device that can form robust mechanical and low-resistant thermal contact with planar and non-planar substrates without the need for external pressure or surface modification is presented. Based on a distinctive integration of a bioinspired adhesive architecture and percolating silver nanowire networks, the proposed device exhibits a strong attachment to target substrates while facilitating efficient thermal transport.

Abstract

Minimizing the thermal contact resistance (TCR) at the boundary between two bodies in contact is critical in diverse thermal transport devices. Conventional thermal contact methods have several limitations, such as high TCR, low interfacial adhesion, a requirement for high external pressure, and low optical transparency. Here, a self-interfacing flexible thermal device (STD) that can form robust van der Waals mechanical contact and low-resistant thermal contact to planar and non-planar substrates without the need for external pressure or surface modification is presented. The device is based on a distinctive integration of a bioinspired adhesive architecture and a thermal transport layer formed from percolating silver nanowire (AgNW) networks. The proposed device exhibits a strong attachment (maximum 538.9 kPa) to target substrates while facilitating thermal transport across the contact interface with low TCR (0.012 m² K kW⁻¹) without the use of external pressure, thermal interfacial materials, or surface chemistries.

A biomimetic neuro-receptor-mediated synapse device is developed by using crumpled MXene nanosheets. The biochemical reactions between acetylcholine receptor and acetylcholine triggered neuro-response, and the pathogenic autoantibody injured ones are both realized. The emulated neuro-responding is consistent with the disease of myasthenia gravis. The working mechanisms are expounded by the reversible valence changing theory of Ti in MXene.

Abstract

Artificial synapse devices can simulate the neuro-transmission in a completely electronic way, but the neuro-biochemical responses are still a challenge for them. Here, a novel three-terminal (3T) neuro-receptor-mediated (acetylcholine receptor (AChR) as a proof-of-concept) synapse device (NR-S) based on the solution–MXene interface is presented. It is demonstrated that the synaptic plasticity behavior triggered by neuro-transmitter (ACh) and the pathogenic autoantibody (AChR-ab) induced neuronal damage that can be detected and recorded. The improved sensitivities, including the linear responses to ACh in an extremely wide range (1 am to 1 µm) and ultra-low (1 am) limit of detection, are obtained using crumpled MXene. Furthermore, the ability of the proposed NR-S to determine the tiny neuronal injury caused by only 10 ng mL⁻¹ AChR-ab is conceptually proven. Collectively, the novel 3T NR-S has good application prospects in the field of the neuromorphic chip for not only realizing the bionic simulation of the chemically modulated or injured neuro-transmission but also offering an efficient experimental platform for neuro-biochemistry studies.

The observation of a new GaSe polymorph is reported, obtained by molecular beam epitaxy growth, with a cs-TL (D_3d). In particular, they show a comprehensive characterization of the atomic structure of this GaSe polymorph by aberration corrected scanning transmission electron microscopy. Moreover, the stability of this polymorph is verified by density functional theory calculations, from which the band structure and Raman intensities are predicted.

Abstract

2D GaSe is a semiconductor belonging to the group of post-transition metal chalcogenides with great potential for advanced optoelectronic applications. The weak interlayer interaction in multilayer 2D materials allows the formation of several polymorphs. Here, the first structural observation of a new GaSe polymorph is reported, characterized by a distinct atomic configuration with a centrosymmetric monolayer (D_3d point group). The atomic structure of this new GaSe polymorph is determined by aberration-corrected scanning transmission electron microscopy. Density-functional theory calculations verify the structural stability of this polymorph. Furthermore, the band structure and Raman intensities are calculated, predicting slight differences to the currently known polymorphs. In addition, the occurrence of layer rotations, interlayer relative orientations, as well as translation shear faults is discussed. The experimental confirmation of the new GaSe polymorph indicates the importance of investigating changes in the crystal structure, which can further impact the properties of this family of compounds.

A junction field-effect transistor (JFET) photodetector based on PdSe₂/MoS₂ is demonstrated with high responsivity (6 × 10² A W⁻¹) and high detectivity (10¹¹ Jones), which is realized by effective dual-gate modulation. The JFET photodetector provides a new approach to realize photodetectors with high responsivity and detectivity.

Abstract

2D materials have shown great promise for next-generation high-performance photodetectors. However, the performance of photodetectors based on 2D materials is generally limited by the tradeoff between photoresponsivity and photodetectivity. Here, a novel junction field-effect transistor (JFET) photodetector consisting of a PdSe₂ gate and MoS₂ channel is constructed to realize high responsivity and high detectivity through effective modulation of top junction gate and back gate. The JFET exhibits high carrier mobility of 213 cm² V⁻¹ s⁻¹. What is more, the high responsivity of 6 × 10² A W⁻¹, as well as the high detectivity of 10¹¹ Jones, are achieved simultaneously through the dual-gate modulation. The high performance is attributed to the modulation of the depletion region by the dual-gate, which can effectively suppress the dark current and enhance the photocurrent, thereby realizing high detectivity and responsivity. The JFET photodetector provides a new approach to realize photodetectors with high responsivity and detectivity.

The Ni/Ni₃N heterostructure electrocatalyst is successfully constructed. The inter-regulated d-band center of interfacial Ni and Ni₃N derived from electron transfer can weaken the hydrogen binding energy and strengthen hydroxyl binding energy, which can further decrease the formation energy of water species and change potential-determining step, resulting in enhanced alkaline hydrogen oxidation reaction performance.

Abstract

Developing highly efficient and stable non-precious metal-based electrocatalysts for alkaline hydrogen oxidation reaction (HOR) is essential for the commercialization of alkaline exchange membrane fuel cells but remains a big challenge. Here, a simple strategy for constructing the Ni/Ni₃N heterostructure electrocatalyst with remarkable catalytic performance toward HOR under alkaline electrolyte is reported. Density functional theory calculations and experimental results reveal that the inter-regulated d-band center of interfacial Ni and Ni₃N derived from electron transfer from Ni to Ni₃N across the interface can lead to the weakened hydrogen binding energy of Ni and strengthened hydroxyl binding energy of Ni₃N, which, together with the decreased formation energy of water species, contributes to the outstanding HOR performance.

The out-of-plane electrical properties of two-dimensional (2D) Bi₂O₂Se at the nanoscale is revealed via conductive atomic force microscopy. Nanoscale hillocks form on 2D Bi₂O₂Se under the vertical electric field to serve as conductive pathways, by which 2D Bi₂O₂Se exhibits a bipolar conduction window at the beginning of the forming process, and later transforms to stable unipolar resistance switching behavior.

Abstract

Two-dimensional (2D) bismuth oxyselenide (Bi₂O₂Se) with high electron mobility shows great potential for nanoelectronics. Although the in-plane properties of Bi₂O₂Se have been widely studied, its out-of-plane electrical transport behavior remains elusive, despite its importance in fabricating devices with new functionality and high integration density. Here, the out-of-plane electrical properties of 2D Bi₂O₂Se at nanoscale are revealed by conductive atomic force microscope. This work finds that hillocks with tunable heights and sizes are formed on Bi₂O₂Se after applying a vertical electric field. Intriguingly, such hillocks are conductive in the vertical direction, resulting in a previously unknown out-of-plane resistance switching in thick Bi₂O₂Se flakes while ohmic conductive characteristic in thin ones. Furthermore, the transformation is observed from bipolar to stable unipolar conduction in thick Bi₂O₂Se flake possessing such hillocks, suggesting its potential to function as a selector in vertical devices. This work reveals the unique out-of-plane transport behavior of 2D Bi₂O₂Se, providing the basis for fabricating vertical devices based on this emerging 2D material.

Nature Nanotechnology, Published online: 16 August 2021; doi:10.1038/s41565-021-00956-7

Twisted photonic graphene superlattices enable the realization of high-performance room-temperature magic-angle lasers.