Jiuxiang Dai

A fully light-controlled artificial synapse is fabricated based on the nonvolatile bidirectional photoresponse of SnSe films by PLD. The origin of negative persistent photoconductivity behavior with wavelength selectivity is systematically investigated by experiments and DFT calculations. The device is capable of mimicking various synaptic functions, recognizing handwritten digital and face images and simulating some biological behaviors of humans and anemonefish.

Abstract

2D-layered materials are recognized as up-and-coming candidates to overcome the intrinsic physical limitation of silicon-based devices. Herein, the coexistence of positive persistent photoconductivity (PPPC) and negative persistent photoconductivity (NPPC) in SnSe thin films prepared by pulsed laser deposition provides an excellent avenue for engineering novel devices. It is determined that surface oxygen is co-regulated by physisorption and chemisorption, and the NPPC is attributed to the photo-controllable oxygen desorption behavior. The dominant behavior of chemisorption induces high stability, while physisorption provides room for adjusting NPPC. A simple fully light-modulated artificial synaptic device based on SnSe film is constructed to operate various synaptic plasticity and reversible modulation of conductance by applying 430 and 255 nm illuminations. A three-layer artificial neural network structure with a high accuracy of 95.33% to recognize handwritten digital images is implemented based on the device. Furthermore, the pressure-related cognition response of humans while climbing and the foraging and recognition behaviors of anemonefish are mimicked. This work demonstrates the potential of 2D-layered materials for developing neuromorphic computing and simulating biological behaviors without additional treatment. Furthermore, the one-step method for preparation is highly adaptable and expected to realize large-area growth and integration of SnSe-based devices.

Periodic patterns based on Si trenchs. Such structures direct control over the ferroelectric polarization to induce patterned micro-scale ferroelectric domains with alternating polarization. Flat regions of the α‑In₂Se₃ flakes maintain the original polarization direction, while suspended regions at the edge of the Si trenches have reversed polarization.

Abstract

2D) Van der Waals ferroelectrics offer the opportunity for developing novel nanoelectronics devices. For device applications, it is necessary to generate controllable ferroelectric polarization domains and achieve non-destructive polarization switching. However, it is very challenging to use the electric field to manipulate the domain state of ultra-thin ferroelectric film due to the large leakage current and even electric breakdown. Here, the flexoelectric effect on the manipulation of polarization states at bending α-In₂Se₃ flakes is explored via piezoresponse force microscopy (PFM). By introducing patterned Si trench substrates, the stripe micron-scale ferroelectric domains with alternating arrangements of the out of-plane polarization in the curved α-In₂Se₃ are observed. It is found that the polarization at the bending region of α-In₂Se₃ can be directly reversed by the large flexoelectric field. The controllable mechanical modulation of α-In₂Se₃ ferroelectric domains opens up potential applications of ferroelectrics in strain engineering functional devices.

Nature Electronics, Published online: 09 December 2024; doi:10.1038/s41928-024-01320-y

Two nanometre CMOS technology

Highly anisotropic 2D insulator CrOCl nanosheets are synthesized via space-confined chemical vapor deposition growth. A MoS₂/CrOCl heterostructure with single-mirror symmetry stacking and ultrastrong interfacial coupling is built to realize interfacial symmetry breaking, which endows its device with polarization-sensitive photodetection and bulk photovoltaic effect. This study opens new prospects for exploring CrOCl-based physics and electronics via symmetry engineering.

Abstract

Chromium oxychloride (CrOCl), a van der Waals antiferromagnetic insulator, has attracted significant interest in 2D optoelectronic, ferromagnetic, and quantum devices. However, the bottom–up preparation of 2D CrOCl remains challenging, limiting its property exploration and device application. Herein, the controllable synthesis of 2D CrOCl crystals by chemical vapor deposition is demonstrated. The combination reaction of precursors together with the space-confined growth strategy, providing stable and stoichiometric growth conditions, enable a robust synthesis of high-crystallinity CrOCl nanosheets with regular rhombus-like morphology and uniform thickness. By tuning the growth temperature from 675 to 800 °C, the thickness of CrOCl nanosheets can be continuously modulated from 10.2 to 30.8 nm, with the domain size increasing from 16.9 to 25.5 µm. The as-grown CrOCl nanosheets exhibit significant structural/optical anisotropy, ultrahigh insulativity, and superior air stability. Furthermore, a MoS₂/CrOCl heterostructure with single-mirror symmetry stacking and ultrastrong interfacial coupling is built to realize interfacial symmetry breaking, a novel interface phenomenon that converts MoS₂ from isotropy to anisotropy. Consequently, the MoS₂/CrOCl heterostructure device achieves polarization-sensitive photodetection and bulk photovoltaic effect, which are nonexistent in high-symmetry 2D materials. This work paves the way for the future exploration of CrOCl-based 2D physics and devices via symmetry engineering.

This study employed a simple approach metal film to apply strain to 2D materials. The high surface energy of metals helps to provide higher interaction forces, thereby improving strain transfer efficiency. Biaxial compressive and uniaxial tensile strain can be applied to monolayer MoS₂, with the highest modulation rate of 542 and 161.7 meV/%, respectively.

Abstract

2D materials, especially their monolayers, have garnered significant attention due to their unique electrical, optical and mechanical properties. Strain engineering is an effective way to modulate these properties. However, challenges remain in preventing slip or decoupling between the 2D material and the substrate due to the inherent weak van der Waals interactions. In this study, metal films are employed to apply strain to 2D materials. The high surface energy of metals helps to provide higher interaction forces, thereby improving strain transfer efficiency. Biaxial compressive and uniaxial tensile strain can be applied to monolayer MoS₂, with the highest modulation rate of 542 and 161.7 meV/%, respectively, as characterized by photoluminescence (PL) spectra. Furthermore, this new approach can be broader to other 2D materials, such as WS₂ or WSe₂, allowing for precise control over strain manipulation. The work introduces a promising new approach for efficient and controllable strain engineering of 2D materials.

Topological spin textures in two-dimensional (2D) magnetic materials hold great potential for chiral spin-based information storage and processing. In this work, room-temperature topological magnetism is predicted to emerge in 2D Janus chromium chalcohalide monolayers, benefiting from the dual-ligand enhanced high Curie temperatures and significant Dzyaloshinsky–Moriya interactions, offering insights for the design of next-generation 2D room-temperature topological magnets.

Abstract

Strong Dzyaloshinskii–Moriya interaction (DMI) and topological spin textures in two-dimensional (2D) magnetic materials hold great potential for novel spin-based information storage and processing. The current research efforts on topological magnetism are highly limited by insufficient magnetic strength, resulting in a narrow temperature-external magnetic field (T-B) phase diagram window. In this study, 2D Janus Cr₂X₃Y₃ (X = Cl, Br, I; Y = S, Se, Te) monolayers are investigated by employing first-principles calculations and atomic spin simulations, demonstrating desirable characteristics such as high Curie temperatures, significant DMI, and chiral spin textures benefited from the ligand substitution. Notably, stable field-free room-temperature magnetic skyrmions are observed in the Cr₂Br₃S₃ monolayer and can persist under long-range magnetic and temperature fields. Additionally, meron chains and skyrmion chains are preserved in the Cr₂I₃S₃ monolayer under appropriate magnetic field and temperature. Based on the tight-binding approximation, a ligand-resolved six-electron model is developed to distinguish superexchange interactions through X- and Y-ligand hopping channels. This model elucidates the interplay between electronegativity and orbital degeneracy, shedding light on their influence on magnetic strength. This discovery highlights and expands the potential of ligand substitution for achieving high-temperature topological magnetism in 2D magnetic materials.

Electrically reconfigurable 2D semiconductor transistors are achieved using a facile in situ dielectric engineering method. Based on the integration of the permanent charge trapping effect and the transient voltage modulation capability, such transistors exhibit run-time electrically configurable functionalities including non-volatile memory, threshold voltage tunable logic switch, non-volatile TCAM, and artificial synapse.

Abstract

The co-integration of logic, memory, synapse, and other essential functionalities into one single element with run-time reconfigurability is explored as a promising approach for an efficient and flexible in-memory computing platform. However, despite ample research focused on such reconfigurable semiconductor technology, it remains challenging to achieve a CMOS-compatible device concept that is with simplified device structure, versatile functionalities, and efficient operation schemes. Here, a new type of run-time electrically reconfigurable device is demonstrated based on dielectric-engineered 2D semiconductor transistors. With an engineered semiconductor/charge-trap layer/dielectric film heterostructure, the 2D charge-trap transistor (CTT) resembles a simplified metal-oxide-semiconductor field-effect transistor (MOSFET) structure. Both multilevel permanent charge trapping and transient voltage-modulating capabilities can be realized in the 2D CTTs, giving rise to various switchable device function modes including non-volatile memory, threshold voltage-variable logic switch, and artificial synapse. Leveraging the monolithic integration of multiple 2D CTTs and time-sequential reconfigurable operation strategy, high-performance logic inverter and non-volatile ternary content-addressable memory (TCAM) with compact architecture can be created. The performance of the 2D CTTs in synapse mode is evaluated with the simulation of the convolutional neural network, showing great potential for future neuromorphic computing hardware.

Utilizing monomorphic ferrodistortion with a single tetragonal polar nanoregions and oxygen octahedral tilt to simultaneously boost EC value and temperature stability in NaNbO₃-based ferrodistortive relaxor. A high ΔT of 0.96 K and an ultrawide temperature span ΔT _span of 110 K with a record-high figure of merit of 4.74 is achieved in Ta-doped NaNbO₃-BaTiO₃ ceramics.

Abstract

A synergistic realization of high electrocaloric effect (ECE) and excellent temperature stability in ferroelectrics are foundation for practical applications, which is, however, a major challenge in ferroelectric cooling community thus far. In confront with this long-standing issue, an emergent monomorphic ferrodistortion strategy in NaNbO₃-based relaxor is proposed with flexible tetragonal polar nanoregions immersing in short-range oxygen octahedral tilt. This not only contributes to large quantities of polar entities to increase entropy change but also produces highly robust oxygen octahedral tilt to persist ferroelectricity and obstruct thermal agitations. Therefore, a high ΔT of 0.96 K and an ultrawide temperature span ΔT _span of 110 K with a record-high figure of merit of 4.74 is achieved in Ta-doped NaNbO₃-BaTiO₃ ceramics and these superior EC performances present a remarkable breakthrough in ferroelectric bulks cooling. This work thus provides an innovative way of utilizing ferrodistortive relaxor feature with polar nanoregions immersing in oxygen octahedral tilt to simultaneously boost EC value and temperature stability and thus monomorphic ferrodistortion is proposed as an effective strategy to develop high-performance entropy-change materials.

Nature, Published online: 11 December 2024; doi:10.1038/s41586-024-08227-w

Angle-resolved photoemission spectroscopy of superconducting magic-angle twisted bilayer graphene reveals flat-band replicas that are indicative of strong electron–phonon coupling; these replicas are absent in non-superconducting twisted bilayer graphene.

Nature, Published online: 11 December 2024; doi:10.1038/s41586-024-08234-x

Nanoscale imaging and control of altermagnetism in manganese telluride is achieved, paving the way for the experimental realization of the theoretically predicted field of altermagnetism.

Nature, Published online: 11 December 2024; doi:10.1038/d41586-024-04030-9

Using X-ray microscopy, the properties of a new class of magnetism — altermagnetism — have been explored in prototypical crystal material, manganese telluride. A range of magnetic textures, including exotic vortices, have been imaged with nanoscale resolution, and controlled with magnetic fields in microstructures.

Controlling exciton resonances in twodimensional materials can create dynamic flat optics

A grossite-type fast ionic conductor CaGa₄O₇, with a high defect tolerance structure, is used to develop single-component multimodal luminescence materials. Almost all luminescent modes and full-spectrum emissions are realized by doping this single host. A series of dynamic anti-counterfeiting devices and digital information encryption technology with temporal and spatial resolution are proposed to show the visualized optical anti-counterfeiting application.

Abstract

With the development of optical anti-counterfeiting and the increasing demand for high-level information encryption, multimodal luminescence (MML) materials attract much attention. However, the discovery of these multifunctional materials is very accidental, and the versatile host suitable for developing such materials remains unclear. Here, a grossite-type fast ionic conductor CaGa₄O₇, characterized by layered and tunnel structure with excellent defect tolerance, is found to meet the needs of various luminescent processes. Almost all luminescent modes, including down/up-conversion luminescence (DCL/UCL), long persistent luminescence (LPL), mechanoluminescence (ML), and X-ray excited optical luminescence (XEOL), are realized in this single host. Full-spectrum (from violet to near-infrared) photoluminescence and ML as well as multicolor XEOL are achieved by simply changing the doped luminescent center. A series of anti-counterfeiting devices, including the quasi-dynamic display of famous paintings, digital information encryption, and multi-color handwritten signatures, are designed to show the encryption of information in temporal and spatial dimensions. This study clarifies the importance of defect tolerance of the host for the development of MML materials, and provides a unique insight into the cross-field applications of special functional materials, which is a new strategy to accelerate the development of novel MML materials.

This study introduces AZBX (AZn₂BO₃X₂) as a quasi-vdW layered dielectric for 2D semiconductor devices, offering a wide bandgap (≈5.6 eV) and high dielectric constant (κ = 13.5). KZBB/MoS₂ FETs demonstrate excellent performance, including steep subthreshold swing, high on/off ratio, minimal hysteresis, and low leakage, highlighting AZBX's potential for advanced 2D electronics.

Abstract

The development of dielectrics with atomic planes and van der Waals (vdW) interfaces is essential for enhancing the performance of 2D devices. However, vdW dielectrics often have smaller bandgaps compared to traditional 3D dielectrics, limiting their options. This study introduces AZBX (AZn₂BO₃X₂, where A = K or Rb, X = Cl or Br), a nonlinear deep-ultraviolet optical crystal, as a quasi-vdW layered dielectric ideal for 2D electronic devices. Focusing on KZBB, it's excellent dielectric properties, including a wide bandgap, high dielectric constant (high-κ), and smooth interfaces are demonstrated. When used as the top gate dielectric in a KZBB/MoS₂ field-effect transistor (FET) with MoS₂ channels and graphene contacts, the device exhibits outstanding performance, with a steep subthreshold swing (≈ 73 mV dec⁻¹), high on/off ratio (≈ 10⁷), negligible hysteresis (0–8 mV), and stable, low leakage current (≈10⁻⁷ A cm^{− 2}) before breakdown. This work expands the 2D material and dielectric landscape and highlights the strong potential of AZBX as high-performance dielectrics.

Post-exfoliation annealing (PEA) is proven to reduce surface screening charges in the van der Waals ferroelectric semiconductor α-In₂Se₃. PEA simultaneously enhances the ferroelectric properties and n-type channel performance. The In₂Se_3−3xO_3x passivation layer formed through PEA is carefully controlled, and its mechanism is specifically addressed. Furthermore, the enhanced electrical polarization is confirmed to be attributed to improved artificial synaptic characteristics.

Abstract

A van der Waals (vdW) α-In₂Se₃ ferroelectric semiconductor channel–based field-effect transistor (FeS-FET) has emerged as a next-generation electronic device owing to its versatility in various fields, including neuromorphic computing, nonvolatile memory, and optoelectronics. However, screening charges cause by the imperfect surface morphology of vdW α-In₂Se₃ inhibiting electrical polarization remain an unresolved issue. In this study, for the first time, a method is elucidated to recover the inherent electric polarization in both in- and out-of-plane directions of the α-In₂Se₃ channel based on post-exfoliation annealing (PEA) and improve the electrical performance of vdW FeS-FETs. Following PEA, an ultra-thin In₂Se_3−3xO_3x layer formed on the top surface of the α-In₂Se₃ channel is demonstrated to passivate surface defects and enhance the electrical performance of FeS-FETs. The on/off current ratio of the α-In₂Se₃ FeS-FET has increased from 5.99 to 1.84 × 10⁶, and the magnitude of ferroelectric resistance switching has increased from 1.20 to 26.01. Moreover, the gate-modulated artificial synaptic operation of the α-In₂Se₃ FeS-FET is demonstrated and illustrate the significance of the engineered interface in the vdW FeS-FET for its application to multifunctional devices. The proposed α-In₂Se₃ FeS-FET is expected to provide a significant breakthrough for advanced memory devices and neuromorphic computing.

A unique mechanism is identified where molecular oxygen, after adsorbing on VSe₂ at room temperature, undergoes a spin state transition from a triplet (S = 1) to a doublet (S = 1/2). Driven by charge transfer from VSe₂, this non-radical pathway activates oxygen, potentially advancing catalytic and biomedical applications by avoiding the need for UV irradiation or radical centers.

Abstract

Oxygen in the excited state is essential for organic synthesis and medical treatment. Herein, a novel phenomenon is reported in which the magnetic ground state of molecular oxygen undergoes a transition at room temperature from S = 1 to S = 1/2, corresponding to the transition of O₂ from a triplet to a doublet state after stable physical adsorption on the defect-free surface of bulk VSe₂. This density functional theory (DFT) calculations demonstrate the stable physical adsorption of O₂ on both 1T- and 2H-VSe₂ surfaces without further decomposition. Electron spin resonance (ESR) measurements confirm the spin state transition. Theoretical simulations reveal the charge transfer from entangled V-3d and Se-4p bands to oxygen as the leading cause of the spin state transition. This mechanism has not been previously proposed and offers multiple potential applications, from organic synthesis to medicine. Moreover, this approach can be extended to reveal new aspects of known catalytic materials and to design novel catalysts.

A single-layer MoS₂ self-assembled 3D nanostructure as a novel metal-free SERS substrate enables effectively improved SERS performance both experimentally and theoretically. The EF and LOD of the substrate reaches 1.68 × 10⁶ and 10⁻⁸ M, respectively. The distinct SERS properties are attributed to the enhanced charge transfer between the substrate and probe molecules brought by the 3D structure.

Abstract

Surface-enhanced Raman spectroscopy (SERS) technology boasts merits of fingerprint recognition, a low detection limit, high sensitivity, and straightforward operation, and holds a significant position in the realm of molecular detection (even at the single-molecule level). Recently, molybdenum disulfide (MoS₂), as a special SERS substrate, has demonstrated various advantages like high molecular compatibility and an anti-fluorescence background, thus emerging as a promising non-metal substrate. Nevertheless, so far, how to improve and achieve SERS effects comparable to metal substrates remains a challenge for MoS₂ based substrates. Therefore, this work presents and acquires a 3D hollow structured MoS₂, which can be achieved through a simple hydrothermal method. Fortunately, the substrate achieves a detection limit of 10⁻⁸ M and an enhancement factor of 10⁶ for rhodamine 6G (R6G) molecules, significantly improving the performance of the non-noble-metal MoS₂ SERS. Theoretical analysis suggests that this should be attributed to the enhanced charge transfer between the substrate and probe molecules brought by the distinct monolayer self-assembly and oxygen substitution in the 3D MoS₂ architecture. The work provides a novel method to enhance the SERS performance of 2D materials, which is readily achievable and is expected to become a key cornerstone for the development of composite substrates.

This work reports giant ultrabroadband two-photon absorption (TPA) driven bulk photovoltaic effect (BPVE) in ferroelectric α-In₂Se₃ utilizing wavelength-tunable terahertz emission spectroscopy. Ferroelectric α-In₂Se₃ outperforms standard terahertz emitters like p-InAs, enabling efficient chiral terahertz wave emission with tunable orientation and ellipticity. The work highlights TPA-induced BPVE in narrow-bandgap ferroelectrics supports high-performance BPVE devices and guides chiral THz wave design.

Abstract

Bulk photovoltaic effect (BPVE) can break the Shockley–Queisser limit by leveraging the inherent asymmetry of crystal lattice without a junction. However, this effect is mainly confined to UV–vis spectrum due to the wide-bandgap nature of traditional ferroelectric materials, thereby limiting the exploration of the infrared light-driven efficient BPVE. Herein, giant two-photon absorption (TPA) driven BPVE is uncovered from visible to infrared in ferroelectric α-In₂Se₃ utilizing wavelength-tunable terahertz (THz) emission spectroscopy. Remarkably, α-In₂Se₃ exhibits exceptional THz emission efficiency in the infrared region, surpassing renowned THz emitters like p-InAs and achieving an efficiency approximately eight times the magnitude of standard ZnTe. The power exponent-type pump fluence and quadruple polarization features reveal a unique TPA-driven BPVE, corroborated by a fourth-order nonlinear oscillator model. Notably, TPA-engendered BPVE efficiency approaches 68% of that observed in the single-photon absorption process. Moreover, the TPA responses display clear polarization anisotropy, with considerably relative phase and amplitude driven by synchronous in-plane and out-of-plane polarization, leading to chiral THz waves with high efficiency, tunable orientation, and controllable ellipticity. This work highlights the advantages of TPA-induced BPVE responses in narrow-bandgap ferroelectric semiconductors, enhancing spectral utilization efficiency, aiding high-performance devices based on BPVE, and guiding chiral THz wave design.

Near-field thermophotovoltaics promise high output power from low temperature thermal sources, but demonstrating large area devices has proven challenging. Here a novel epitaxial co-designed emitter-cell device fabricated with scalable processes demonstrates 1.2 mW from a 460 °C heat source, a greater than 20-fold enhancement over the far-field. Modeling of various cell changes offers a pathway to even higher power.

Abstract

Thermophotovoltaics, devices that convert thermal infrared photons to electricity, offer a key pathway for a variety of critical renewable energy technologies including thermal energy storage, waste heat recovery, and direct solar-thermal power generation. However, conventional far-field devices struggle to generate reasonable powers at lower temperatures. Near-field thermophotovoltaics provide a pathway to substantially higher powers by leveraging photon tunneling effects. Here a large area near-field thermophotovoltaic device is presented, created with an epitaxial co-fabrication approach, that consists of a self-supported 0.28 cm² emitter-cell pair with a 150 nm gap. The device generates 1.22 mW at 460 °C, a 25-fold increase over the same cell measured in a far-field configuration. Furthermore, the near-field device demonstrates short circuit current densities greater than the far-field photocurrent limit at all the temperatures tested, confirming the role of photon tunneling effects in the performance enhancement. Modeling suggests several practical directions for cell improvements and further increases in power density. These results highlight the promise of near-field thermophotovoltaics, especially for low temperature applications.

Two-dimensional nonstoichiometric AgCr_1-xS₂ nanoflakes with Cr vacancy defects have been successfully synthesized by CVD epitaxial growth method. SHG and PFM signal obtained at room temperature confirm the robust ferroelectric properties of the material, offering a novel approach for constructing ternary ultrathin 2D TMDs ferroelectric materials and presenting a potential pathway for the development of exceptional multifunctional materials.

Abstract

Ferroelectricity in two-dimensional (2D) materials at room temperature has attracted significant interest due to their substantial potential for applications in non-volatile memory, nanoelectronics, and optoelectronics. The intrinsic tendency of 2D materials toward nonstoichiometry results in atomic configurations that differ from those of their stoichiometric counterparts, thereby giving rise to potential ferroelectric polarization properties. However, reports on the emergence of room temperature ferroelectric effects in nonstoichiometric 2D materials remain limited. This study reports the observation of room temperature ferroelectricity in nonstoichiometric AgCr_1-xS₂ ternary 2D transition metal dichalcogenides synthesized via chemical vapor deposition. The noncentrosymmetric crystal structure and switchable ferroelectric polarization are confirmed through second harmonic generation (SHG) and piezoresponse force microscopy (PFM) measurements. It is determined that the primary cause of ferroelectric polarization is the interlayer movement of ordered asymmetric Ag atoms under the influence of numerous chromium (Cr) vacancies along with interlayer atom displacement. Furthermore, two types of electrical devices based on in-plane (IP) and out-of-plane (OOP) polarization are demonstrated. This work offers a new perspective for fabricating ternary ultrathin 2D transition metal dichalcogenides ferroelectric materials and presents a potential pathway for creating exceptional multifunctional materials.