Jiuxiang Dai

Nature Electronics, Published online: 28 September 2023; doi:10.1038/s41928-023-01034-7

Nature Materials, Published online: 28 September 2023; doi:10.1038/s41563-023-01676-0

Nature Materials, Published online: 28 September 2023; doi:10.1038/s41563-023-01671-5

1D magnetic van der Waals materials with composition MX₃ (M = Cr, V, and X = Cl, Br, I) confined in carbon nanotubes are investigated. Atomic-resolution scanning transmission electron microscopy identifies unique 1D face-sharing octahedron MX₃ structure. Charge transfer from carbon nanotube to MX₃ single-chain plays a critical role in stabilizing the chain structures.

Abstract

Magnetic materials in reduced dimensions are not only excellent platforms for fundamental studies of magnetism, but they play crucial roles in technological advances. The discovery of intrinsic magnetism in monolayer 2D van der Waals systems has sparked enormous interest, but the single-chain limit of 1D magnetic van der Waals materials has been largely unexplored. This paper reports on a family of 1D magnetic van der Waals materials with composition MX₃ (M = Cr, V, and X = Cl, Br, I), prepared in fully-isolated fashion within the protective cores of carbon nanotubes. Atomic-resolution scanning transmission electron microscopy identifies unique structures that differ from the well-known 2D honeycomb lattice MX₃ structure. Density functional theory calculations reveal charge-driven reversible magnetic phase transitions.

Combining magnetic Kerr microscopy and density functional theory calculations, a successful imaging of antiferromagnetic-ferromagnetic (AFM-FM) phase transition in van der Waals (vdW) AFM CrSBr based on its unique magneto-optic property is demonstrated. These findings reveal enormous prospects for magnetism imaging and control in 2D spintronics with vdW AFM.

Abstract

The advent of van der Waals (vdW) ferromagnetic (FM) and antiferromagnetic (AFM) materials offers unprecedented opportunities for spintronics and magneto-optic devices. Combining magnetic Kerr microscopy and density functional theory calculations, the AFM-FM transition is investigated and a surprising abnormal magneto-optic anisotropy in vdW CrSBr associated with different magnetic phases (FM, AFM, or paramagnetic state) is discovered. This unique magneto-optic property leads to different anisotropic optical reflectivity from different magnetic states, permitting direct imaging of the AFM Néel vector orientation and the dynamic process of the AFM-FM transition within a magnetic field. Using Kerr microscopy, not only the domain nucleation and propagation process is imaged but also the intermediate spin-flop state in the AFM-FM transition is identified. The unique magneto-optic property and clear identification of the dynamics process of the AFM-FM phase transition in CrSBr demonstrate the promise of vdW magnetic materials for future spintronic technology.

A tunable approach is reported first to manipulate the amorphous native GaO _x shells of GaSb NWs via a simple in situ annealing process. Furthermore, these native GaO _x shells are studied systematically for improving the bias-stress stability of the GaSb NWs FETs and broadening the application in chargeable-dielectric free synaptic transistors with typical synaptic behaviors and reliable learning stability.

Abstract

The inhomogeneous native oxide shells on the surfaces of III–V group semiconductors typically yield inferior and unstable electrical properties metrics, challenging the development of next-generation integrated circuits. Herein, the native GaO _x shells are profitably utilized by a simple in-situ thermal annealing process to achieve high-performance GaSb nanowires (NWs) field-effect-transistors (FETs) with excellent bias-stress stability and synaptic behaviors. By an optimal annealing time of 5 min, the as-constructed GaSb NW FET demonstrates excellent stability with a minimal shift of transfer curve (ΔV _th ≈ 0.54 V) under a 60 min gate bias, which is far more stable than that of pristine GaSb NW FET (ΔV _th ≈ 8.2 V). When the high bias-stress stability NW FET is used as the chargeable-dielectric free synaptic transistor, the typical synaptic behaviors, such as short-term plasticity, long-term plasticity, spike-time-dependent plasticity, and reliable learning stability are demonstrated successfully through the voltage tests. The mobile oxygen ion in the native GaO _x shell strongly offsets the trapping states and leads to enhanced bias-stress stability and charge retention capability for synaptic behaviors. This work provides a new way of utilizing the native oxide shell to realize stable FET for chargeable-dielectric free neuromorphic computing systems.

Nature Communications, Published online: 29 September 2023; doi:10.1038/s41467-023-41643-6

Nature Synthesis, Published online: 29 September 2023; doi:10.1038/s44160-023-00419-y

A two-terminal near-infrared (NIR) synaptic device based on multilayer MoSe₂ moiré superlattice can effectively achieve NIR light response and highly parallel sensing and memory functions. The 10 × 10 integrated retinal morphological device array reflects the ability of high-fidelity image under NIR illumination and demonstrates great potential in the field of NIR bionic eye application.

Abstract

Near-infrared (NIR) synaptic devices have attracted great attention in the field of NIR vision sensors due to their highly parallel sensing and memory functions, which emulate the basic biomimetic behaviors of the human visual system. However, it is a great challenge for the 2D material to achieve NIR light response and integration, which obstructs the progress of NIR synaptic devices. Hence, a two-terminal NIR synaptic device based on a multilayer MoSe₂ moiré superlattice is reported. The moiré structure dominated by interlayer excitons significantly enhances the interlayer coupling of multilayer MoSe₂, resulting in a narrower band gap nearly to that of bulk to achieve NIR light response and broadband absorption from 240 nm to 1700 nm. The existence of Se vacancies in MoSe₂ moiré superlattice makes the device show the fundamental synaptic performance. Furthermore, a 10×10 integrated retinal morphologic device array is constructed. Under the 100 s light pulse stimulation of 1060 nm, the pattern obtained after attenuation of 50 s still has a memory level of 14.84%, reflecting the excellent storage function under NIR illumination. This research opens up a new avenue for the realization of NIR artificial retina and bionic eye based on 2D materials.

To build artificial synapses is critical for constructing artificial neural network. Here, the synaptic behaviors can be mimicked by our device which is made of ion-irradiated GeSe nanosheets FET. The mechanism derives from interfacial potential barrier modulation of GeSe and electrodes. It may provide a promising opportunity for artificial neuromorphic system applications based on 2D layered materials.

Abstract

Two-dimensional (2D) layered materials have many potential applications in memristors owing to their unique atomic structures and electronic properties. Memristors can overcome the in-memory bottleneck for use in brain-like neuromorphic computing. However, exploiting additional lateral memtransistors based on 2D layered materials remains challenging. There are few studies on p-type semiconductors that have not been theoretically analyzed. In this study, a lateral memtransistor based on p-type GeSe nanosheets is investigated. A three-terminal GeSe memtransistor that modulated the interfacial barrier height was fabricated using low-energy ion irradiation; the memtransistor exhibited a low operating voltage. The memtransistor successfully mimics biological synapse, including neuroplasticity functions, such as short-term plasticity, long-term plasticity, paired-pulse facilitation, and spike-timing-dependent plasticity. The mechanism of interfacial modulation was verified by experimental results and theoretical calculations. The results show that it is feasible to modulate the interface of 2D GeSe nanosheets using low-energy ion irradiation to realize a lateral memtransistor. This may provide promising opportunities for artificial neuromorphic system applications based on 2D layered materials.

The controllable synthesis of transition metal phosphorous chalcogenides (TMPCs) based on subtracting elements mechanism is reported. Among them, the SnPS₃ exhibits effective nonlinear susceptibility χ ⁽²⁾ of 8.41 × 10⁻¹¹ m V⁻¹. The CdPSe₃ photodetector displays high responsivity (582 mA W⁻¹) and detectivity (3.19 × 10¹¹ Jones). This research will pave the way for exploration of ternary TMPCs in optical and photoelectric detection.

Abstract

The 2D ternary transition metal phosphorous chalcogenides (TMPCs) have attracted extensive research interest due to their widely tunable band gap, rich electronic properties, inherent magnetic and ferroelectric properties. However, the synthesis of TMPCs via chemical vapor deposition (CVD) is still challenging since it is difficult to control reactions among multi-precursors. Here, a subtractive element growth mechanism is proposed to controllably synthesize the TMPCs. Based on the growth mechanism, the TMPCs including FePS₃, FePSe₃, MnPS₃, MnPSe₃, CdPS₃, CdPSe₃, In₂P₃S₉, and SnPS₃ are achieved successfully and further confirmed by Raman, second-harmonic generation (SHG), and scanning transmission electron microscopy (STEM). The typical TMPCs–SnPS₃ shows a strong SHG signal at 1064 nm, with an effective nonlinear susceptibility χ ⁽²⁾ of 8.41 × 10⁻¹¹ m V⁻¹, which is about 8 times of that in MoS₂. And the photodetector based on CdPSe₃ exhibits superior detection performances with responsivity of 582 mA W⁻¹, high detectivity of 3.19 × 10¹¹ Jones, and fast rise time of 611 µs, which is better than most previously reported TMPCs-based photodetectors. These results demonstrate the high quality of TMPCs and promote the exploration of the optical properties of 2D TMPCs for their applications in optoelectronics.

This review highlights three emerging applications of 2D materials in the post-artificial intelligence (AI) era. 2D-based smart fibers demonstrate diverse functionalities, ranging from healthcare monitoring to antipathogenic protection in a wearable manner. Soft robotics addresses the limitations of conventional robotics by introducing 2D-hybridized soft actuators and sensors. Single atom catalysts supported on 2D materials contribute sustainable energy storage and conversion.

Abstract

Recent consecutive discoveries of various 2D materials have triggered significant scientific and technological interests owing to their exceptional material properties, originally stemming from 2D confined geometry. Ever-expanding library of 2D materials can provide ideal solutions to critical challenges facing in current technological trend of the fourth industrial revolution. Moreover, chemical modification of 2D materials to customize their physical/chemical properties can satisfy the broad spectrum of different specific requirements across diverse application areas. This review focuses on three particular emerging application areas of 2D materials: smart fibers, soft robotics, and single atom catalysts (SACs), which hold immense potentials for academic and technological advancements in the post-artificial intelligence (AI) era. Smart fibers showcase unconventional functionalities including healthcare/environmental monitoring, energy storage/harvesting, and antipathogenic protection in the forms of wearable fibers and textiles. Soft robotics aligns with future trend to overcome longstanding limitations of hard-material based mechanics by introducing soft actuators and sensors. SACs are widely useful in energy storage/conversion and environmental management, principally contributing to low carbon footprint for sustainable post-AI era. Significance and unique values of 2D materials in these emerging applications are highlighted, where the research group has devoted research efforts for more than a decade.

Nature Materials, Published online: 02 October 2023; doi:10.1038/s41563-023-01674-2

Abstract

Monolayer molybdenum disulfide (MoS₂) has emerged as one of the most promising channel materials for next-generation nanoelectronics and optoelectronics owing to its atomic thickness, dangling-bond-free flat surface, and high electrical quality. Currently, high-quality monolayer MoS₂ wafers are primarily grown on sapphire substrates incompatible with conventional device fabrication, and thus transfer processes to a suitable substrate are typically required before the device can be processed. Here, we demonstrate the batch production of transfer-free MoS₂ top-gate devices directly on sapphire growth substrates via step engineering. By introducing substrate steps on growth substrate sapphire, high-κ dielectric layers with superior quality and uniform can be directly deposited on the epitaxially grown monolayer MoS₂. For the substrate with a maximum step density of 100 µm⁻¹, the gate capacitance can reach ∼ 1.87 µF·cm⁻², while the interface trap state density (D_it) can be as low as ∼ 7.6 × 10¹⁰ cm⁻²·eV⁻¹. The direct deposition of high-quality dielectric layers on grown monolayer MoS₂ enables the batch fabrication of top-gate devices devoid of transfer and thus excellent device yield of > 96%, holding great promise for large-scale two-dimensional (2D) integrated circuits.

Abstract

As a very promising epitaxy technology, the remote epitaxy has attracted extensive attention in recent years, in which graphene is the most used interlayer material. As an isomorphic of graphene, two-dimensional (2D) hexagonal boron nitride (h-BN), is another promising interlayer for the remote epitaxy. However, there is a current debate on the feasibility of using h-BN as interlayer in the remote epitaxy. Herein, we demonstrate that the potential field of sapphire can completely penetrate monolayer h-BN, and hence the remote epitaxy of ZrS₂ layers can be realized on sapphire substrates through monolayer h-BN. The field of sapphire can only partially penetrate the bilayer h-BN and result in the mixing of remote epitaxy and van der Waals (vdWs) epitaxy. Due to the weak interfacial scattering and high crystalline quality of ZrS₂ epilayer, the ZrS₂ photodetector with monolayer h-BN shows the best performance, with an on/off ratio of more than 2 × 10⁵ and a responsivity up to 379 mA·W⁻¹. This work provides an efficient approach to prepare single-crystal transition metal dichalcogenides and their heterojunctions with h-BN, which have great potential in developing large-area 2D electronic devices.

Nature Physics, Published online: 02 October 2023; doi:10.1038/s41567-023-02229-7

Electron microscopy has found extensive application in elucidating the structure and properties of 2D transition metal dichalcogenides (TMDs), a highly promising material family employed across various fields. This review provides an overview of the static and in situ investigation of functional structures, defects, and interfaces in TMDs, primarily utilizing scanning transmission electron microscopy and electron energy loss spectroscopy.

Abstract

2D transition metal dichalcogenides (TMDs) exhibit remarkable properties that are strongly influenced by their atomic structures, as well as by various types of defects and interfaces that can be precisely engineered and controlled. These features make 2D TMDs and TMD-based materials highly promising for a wide range of applications in electronics, optoelectronics, magnetism, spintronics, catalysis, energy, etc. By providing a comprehensive approach to understand the structure–property–functionality relationship in materials at the atomic scale, electron microscopy, and spectroscopy techniques have emerged as invaluable tools for studying both the static characteristics and dynamic evolutions of 2D TMDs. In this review, the primary focus lies in exploring intrinsic and artificial structures in TMDs and their heterostructures, along with their corresponding properties, through cutting-edge aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). Additionally, recent advancements in the field of in situ visualization and manipulation of 2D TMDs using electron beams are highlighted. It is anticipated that the up-to-date overview provided, along with a glimpse into the future development of STEM-based techniques, will make a substantial contribution to advancing research on 2D materials.

Schematic of the CoFe₂O₄ (CFO) thin film transfer process from Au/CFO/α-MoO₃/STO onto flexible (polyethylene terephthalate) substrate via mechanical exfoliation by breaking weak van der Waals force between MoO₃ sheets. A cross-sectional transmission electron microscopy image of the detached α-MoO₃ on CFO and supporting Au.

Abstract

Integration of functional thin films onto flexible substrates is driven by the need to improve the performance and durability of flexible electronic devices. A van der Waals epitaxy technology that accomplishes the transfer of oxide or metal thin films via exfoliation or dissolution of sacrificial α-MoO₃ layers produced by sputtering is presented. The α-MoO₃ thin films, consisting of weakly bonded 2D layers, grow epitaxially on SrTiO₃ (001) substrates, exhibiting mosaic domains rotated by 90°. Metallic Au films grown on the α-MoO₃ are transferred by mechanical exfoliation or by dissolving the α-MoO₃ in water at 45 °C. Spinel-structured CoFe₂O₄ thin films grown on α-MoO₃ layers are easily transferred to flexible substrates via mechanical exfoliation, and the magnetic anisotropy of the transferred CoFe₂O₄ films is modulated by bending.

Nature Communications, Published online: 05 October 2023; doi:10.1038/s41467-023-41816-3

Acoustic devices play an increasingly important role in modern society for information technology and intelligent systems. In this review, the recent progress of acoustic devices based on suspended 2D materials and their composites, especially applications in the audio frequency, static pressure, and ultrasonic frequency range, is briefly summarized, emphasizing the advantageous properties of suspended 2D materials and related outstanding device performance.

Abstract

Acoustic devices play an increasingly important role in modern society for information technology and intelligent systems, and recently significant progress has been made in the development of communication, sensing, and energy transduction applications. However, conventional material systems, such as polymers, metals and silicon, show limitations to fulfill the evolving requirements for high-performance acoustic devices of small size, low power consumption, and multifunctional capabilities. 2D materials hold the promise in overcoming the development bottleneck of acoustic devices aforementioned, given their atomic-thin thickness, extensive surface area, superior physical properties, and remarkable layer-stacking tunability. By suspending the 2D materials, mechanical and thermal disruption from substrate will be eliminated, which will enable the development of new classes of acoustic devices with unprecedented sensitivity and accuracy. In this review, the recent progress of acoustic devices based on suspended 2D materials and their composites, especially applications in the audio frequency, static pressure, and ultrasonic frequency range, is briefly summarized, emphasizing the advantageous properties of suspended 2D materials and related outstanding device performance. Together with the development of 2D membrane synthesis, transfer, as well as microelectromechanical fabrication process, suspended 2D materials will shed light on the next-generation high-performance acoustic devices.

A bespoke facility for the epitaxial growth and in-situ characterization of 2D semiconductors is used for wafer scale growth of a novel γ'-GaSe polymorph. Theory and experiment verify the optical anisotropy and resonant UV absorption of the layers. These properties are exploited for photon sensing in the UV-C spectral range, offering a scalable route to deep-UV optoelectronics.

Abstract

2D semiconductors (2SEM) can transform many sectors, from information and communication technology to healthcare. To date, top-down approaches to their fabrication, such as exfoliation of bulk crystals by “scotch-tape,” are widely used, but have limited prospects for precise engineering of functionalities and scalability. Here, a bottom-up technique based on epitaxy is used to demonstrate high-quality, wafer-scale 2SEM based on the wide band gap gallium selenide (GaSe) compound. GaSe layers of well-defined thickness are developed using a bespoke facility for the epitaxial growth and in situ studies of 2SEM. The dominant centrosymmetry and stacking of the individual van der Waals layers are verified by theory and experiment; their optical anisotropy and resonant absorption in the UV spectrum are exploited for photon sensing in the technological UV-C spectral range, offering a scalable route to deep-UV optoelectronics.

In this work, back focal plane (BFP) technique is used to recognize the second harmonic generation (SHG) emission dipole orientation, confirming the in-plane emission dipole of NbOI₂. Furthermore, with increasing hydrostatic pressure, NbOI₂ undergoes a process of increasing, decreasing, and disappearing SHG intensity, corresponding to structural distortion and ferroelectric-paraelectric phase transition, respectively.

Abstract

2D in-plane ferroelectric NbOI₂ exhibits strong second harmonic generation (SHG) and ultrahigh effective susceptibility. To push forward their applications in nonlinear photonics and optoelectronics, it is highly desirable to understand the emission dipole orientation and tunability of SHG, which is not achieved. Here, by integrating tight focusing from parabolic mirror with back focal plane (BFP) imaging technique, for the first time it is demonstrated that SHG emission of NbOI₂ presents purely in-plane dipole orientation in consistent with numerical simulations, suggesting the in-plane components of the SHG susceptibility tensor in NbOI₂ dominate the emission. Moreover, with the aid of ab-initio calculations, it is found that the hydrostatic pressure can dramatically change the structure and resultant SHG intensity of NbOI₂. Explicitly, SHG intensity endures a slight increase due to the distortion of octahedral at low pressure pressure, and then monotonously decreases due to the improvement of structural symmetry with further increasing pressure, and drastically quenching resulting from the ferroelectric to paraelectric phase transition. This work unambiguously demonstrates the dipole emission behavior of SHG and the relationship between structural evolution and SHG intensity, which provides an avenue for tunable nonlinear optics and optoelectronics.