Jing Zhang

Nature Materials, Published online: 26 June 2023; doi:10.1038/s41563-023-01595-0

Polar domains have been observed in twist-stacked van der Waals layers, but their dynamics are unexplored. Here, using operando electron microscopy, it is found that polar domains in an antiferroelectric arrangement cannot transition to a ferroelectric state due to topological protection of the domain wall network.

Nature Materials, Published online: 06 July 2023; doi:10.1038/s41563-023-01603-3

The authors introduce a spin-optical laser based on a monolayer transition metal dichalcogenide coupled to a heterostructure microcavity supporting high-Q spin-valley resonances originating from photonic Rashba-type spin splitting of a bound state in the continuum.

Nature Materials, Published online: 06 July 2023; doi:10.1038/s41563-023-01600-6

The authors report the emergence of a transient hexatic state during laser-induced transformation between two charge-density wave (CDW) phases in a thin film of the CDW material 1T-TaS2.

The up-to-date growth strategies for the controlled synthesis of wafer-scale 2D semiconducting TMDCs polycrystalline and single-crystal films are systematically summarized. The large-area accurate doping of 2D semiconducting TMDCs and its effect on the device performances are discussed. The challenges regarding the improvement of electronic device performances of 2D semiconducting TMDCs are highlighted, and the further research directions are proposed.

Abstract

2D semiconducting transition metal dichalcogenide (TMDCs) possess atomically thin thickness, a dangling-bond-free surface, flexible band structure, and silicon-compatible feature, making them one of the most promising channels for constructing state-of-the-art field-effect transistors in the post-Moore's era. However, the existing 2D semiconducting TMDCs fall short of meeting the industry criteria for practical applications in electronics due to their small domain size and the lack of an effective approach to modulate intrinsic physical properties. Therefore, it is crucial to prepare and dope 2D semiconducting TMDCs single crystals with wafer size. In this review, the up-to-date progress regarding the wafer-scale growth of 2D semiconducting TMDC polycrystalline and single-crystal films is systematically summarized. The domain orientation control of 2D TMDCs and the seamless stitching of unidirectionally aligned 2D islands by means of substrate design are proposed. In addition, the accurate and uniform doping of 2D semiconducting TMDCs and the effect on electronic device performances are also discussed. Finally, the dominating challenges pertaining to the enhancement of the electronic device performances of TMDCs are emphasized, and further development directions are put forward. This review provides a systematic and in-depth summary of high-performance device applications of 2D semiconducting TMDCs.

Nature Materials, Published online: 29 June 2023; doi:10.1038/s41563-023-01567-4

A two-dimensional atomically flat insulator with large dielectric constant and high breakdown field strength has been successfully grown. This material could serve as the dielectric and encapsulation layers for two-dimensional materials for studying their emergent physics, as well as for next-generation electronics.

Nature, Published online: 29 June 2023; doi:10.1038/s41586-023-06363-3

Quantum metric-induced nonlinear transport in a topological antiferromagnet

Nature Materials, Published online: 26 June 2023; doi:10.1038/s41563-023-01574-5

High-quality aluminium nitride (AlN) heteroepitaxial films are obtained by the controlled discretization and coalescence of columns using nanopatterned AlN/sapphire templates with regular hexagonal holes. The density of dislocation etch pits in the AlN heteroepitaxial films is reduced to approximately 104 cm–2, approaching the value of that in AlN bulk single crystals.

2D vdW ferroelectric materials reveal great potential applications in electronics, optoelectronics, and spintronics, owing to robust spontaneous polarization, widespread bandgap tunability, inert surfaces, and silicon-based technology compatibility. This review covers their recent advances in ferroelectricity origin and practical applications, especially in artificial intelligence. The hot topic of sliding ferroelectrics in physical origins and novel properties is also focused on.

Abstract

In recent years, an increasing number of 2D van der Waals (vdW) materials are theory-predicted or laboratory-validated to possess in-plane (IP) and/or out-of-plane (OOP) spontaneous ferroelectric polarization. Due to their dangling-bond-free surfaces, interlayer charge coupling, robust polarization, tunable energy band structures, and compatibility with silicon-based technologies, vdW ferroelectric materials exhibit great promise in ferroelectric memories, neuromorphic computing, nanogenerators, photovoltaic devices, spintronic devices, and so on. Here, the very recent advances in the field of vdW ferroelectrics (FEs) are reviewed. First, theories of ferroelectricity are briefly discussed. Then, a comprehensive summary of the non-stacking vdW ferroelectric materials is provided based on their crystal structures and the emerging sliding ferroelectrics. In addition, their potential applications in various branches/frontier fields are enumerated, with a focus on artificial intelligence. Finally, the challenges and development prospects of vdW ferroelectrics are discussed.

Van der Waals heterojunctions as photocatalysts by traditional wet chemical methods encounter the challengeof charge transfer, because of steric hindrance and barriers caused by a large interfacial spacing. Introducing phosphoric acid molecules at the heterojunction interface acts as an electron "bridge" to eliminate the interfacial potential difference and create a photoelectron transport channel to promote the photocatalytic reaction.

Abstract

Constructing Van der Waals heterojunction is a crucial strategy to achieve excellent photocatalytic activity. However, in most Van der Waals heterojunctions synthesized by ex situ assembly, electron transfer encounters huge hindrances at the interface between the two components due to the large spacing and potential barrier. Herein, a phosphate-bridged Van der Waals heterojunction of cobalt phthalocyanine (CoPc)/tungsten disulfide (WS₂) bridged by phosphate (xCoPc-nPO₄ ⁻-WS₂) is designed and prepared by the traditional wet chemistry method. By introducing a small phosphate molecule into the interface of CoPc and WS₂, creates an electron “bridge”, resulting in a compact combination and eliminating the space barrier. Therefore, the phosphate (PO₄ ⁻) bridge can serve as an efficient electron transfer channel in heterojunction and can efficiently transmit photoelectrons from WS₂ to CoPc under excited states. These excited photoelectrons are captured by the catalytic central Co²⁺ in CoPc and subsequently convert CO₂ molecules into CO and CH₄ products, achieving 17-fold enhancement on the 3CoPc-0.6PO₄ ⁻-WS₂ sample compared to that of pure WS₂. Introducing a small molecule “bridge” to create an electron transfer channel provides a new perspective in designing efficient photocatalysts for photocatalytic CO₂ reduction into valuable products.

A series of multimodal emission lanthanide-based metal-organic frameworks emit red and green light originating from Eu³⁺ and Tb³⁺ under ultraviolet light irradiation. Meanwhile, under 980 nm laser irradiation, they show cyan upconversion cooperative luminescence derived from Yb³⁺ and characteristic upconversion luminescence from lanthanide activators (Eu³⁺, Tb³⁺, or Ho³⁺), respectively. These metal-organic frameworks are successfully used for optical information encryption.

Abstract

Although remarkable progress on luminescent materials is made in advanced optical information storage and anti-counterfeiting applications, many challenges still remain in these fields. Currently, most luminescent materials are based on a single photoluminescent model that can be easily imitated by substitutes. In this work, a series of multimodal emission lanthanide-based metal–organic frameworks (MOFs) are developed, where they emit red and green light originating from Eu³⁺ and Tb³⁺ under ultraviolet light irradiation. Meanwhile, under 980 nm near-infrared laser irradiation, these MOFs show cyan upconversion cooperative luminescence derived from Yb³⁺ and characteristic upconversion luminescence from lanthanide activators (Eu³⁺, Tb³⁺, or Ho³⁺), respectively. Based on the integrated optical functionality, the functional information storage applications are successfully designed, which indicates that multimodal emission features can be easily detected under ultraviolet lamps (254 or 393 nm) or 980 nm near-infrared laser. And, the unique optical features show a high level of security in the advanced information storage application, which would be sufficiently complex to be forged.

2D PdTe₂/WSe₂ van der Waals (vdWs) field effect transistor (FET) device with broadband photo-response is constructed by using the ideal bandgap and excellent optoelectronic properties of PdTe₂ nanoflakes and few-layer p-type WSe₂. Benefiting from strong Schottky barrier and carrier-depletion of WSe₂ layer modulated by gate voltage, an extremely low dark current of ≈1.2 pA and high light on/off ratio of ≈10⁶ is achieved.

Abstract

Due to its unique band structure and topological properties, the 2D topological semimetal exhibits potential applications in photoelectric detection, polarization sensitive imaging, and Schottky barrier diodes. However, its inherent large dark current hinders the further improvement of the performance of the semimetal-based photodetectors. In this study, a van der Waals (vdWs) field effect transistor (FET) composed of semimetal PdTe₂ and transition metal dichalcogenides (TMDs) WSe₂ is fabricated, which exhibits high sensitivity photoelectric detection performance in a wide band from visible light (405 nm) to mid-infrared (5 µm). The dark current and the noise level in the device are greatly suppressed by the effective control of the gate. Benefiting from the extremely low dark current (1.2 pA), the device achieves an optical on/off ratio up to 10⁶, a high detectivity of 9.79 × 10¹³ Jones and a rapid response speed (219 and 45 µs). This research demonstrates the latent capacity of the 2D topological semimetal/TMDs vdWs FET for broadband, high-performance, and miniaturized photodetection.

The precise modulation of diffusion of metal ion/atoms and their reduction/oxidation probability holds promise to overcome the speed, size, and energy issues of present-day computer. Here, this work shows that the diffusion metal ion can be modulated by the defects inside the switching medium and confines metal filaments in a precise, 1D channel.

Abstract

The neuromorphic and in-memory computing using memristors are promising for the building of the next generation computing systems. However, the diffusion dynamics of metal ions/atoms inside the switching medium impose variability in conducting filament (CF) formation, thus limiting their use in von-Neumann architecture. The precise modulation on the diffusion of metal ions/atoms and their reduction/oxidation probability holds promise to overcome the speed, size, and energy issues of present-day computers. Here, this study shows that the diffusion of metal ions can be modulated by defects inside the switching medium and confines metal filaments in a precise 1D channel. This filament confinement by the defect engineering leads to an anomalous switching mechanism with two interchangeable modes: unipolar threshold and bipolar modes. The variation between two modes can be modulated by controlling defects in the structures, leading to a uniform switching with low SET/RESET voltage variations of 17.3% and −17.6%, respectively. Moreover, the convolutional neural network is implemented to emulate synaptic plasticity and image recognition to achieve recognition accuracy of 87% due to a highly linear weight update, demonstrating its potential for in-memory computing.

Based on a concentration modulation strategy, the aggregation state of ZnIn₂S₄ (ZIS) atomic layers is reversibly regulated to achieve the desired band structure and photo-electrochemical properties for efficient photocatalytic H₂ evolution.

Abstract

It is technically challenging to reversibly tune the layer number of 2D materials in the solution. Herein, a facile concentration modulation strategy is demonstrated to reversibly tailor the aggregation state of 2D ZnIn₂S₄ (ZIS) atomic layers, and they are implemented for effective photocatalytic hydrogen (H₂) evolution. By adjusting the colloidal concentration of ZIS (ZIS-X, X = 0.09, 0.25, or 3.0 mg mL⁻¹), ZIS atomic layers exhibit the significant aggregation of (006) facet stacking in the solution, leading to the bandgap shift from 3.21 to 2.66 eV. The colloidal stacked layers are further assembled into hollow microsphere after freeze-drying the solution into solid powders, which can be redispersed into colloidal solution with reversibility. The photocatalytic hydrogen evolution of ZIS-X colloids is evaluated, and the slightly aggregated ZIS-0.25 displays the enhanced photocatalytic H₂ evolution rates (1.11 µmol m⁻² h⁻¹). The charge-transfer/recombination dynamics are characterized by time-resolved photoluminescence (TRPL) spectroscopy, and ZIS-0.25 displays the longest lifetime (5.55 µs), consistent with the best photocatalytic performance. This work provides a facile, consecutive, and reversible strategy for regulating the photo-electrochemical properties of 2D ZIS, which is beneficial for efficient solar energy conversion.

Micropatterning

In article number 2211478, Christian Doonan, Paolo Falcaro, and co-workers describe the fabrication of oriented metal–organic framework (MOF) micropatterns with anisotropic optical properties. A mixed-linker strategy is used to impart X-ray sensitivity, while heteroepitaxial growth affords the orientation of MOF crystals. The 3D-oriented MOF films are patterned by resist-free X-ray photolithography. The MOF micropatterns with immobilized guest-molecules showcase fluorescence anisotropy.

The unexpected discovery of ferroelectricity in functional oxides such as HfO₂- and AlN- has aroused significant interest in computing device applications. Herein, the fundamental science on the manner in which the “hidden” ferroelectricity can be achieved in the ferroelectric (Hf,Zr)O₂ and (Al,Sc)N and their practical applications are thoroughly discussed.

Abstract

Ferroelectric materials are considered ideal for emerging memory devices owing to their characteristic remanent polarization, which can be switched by applying a sufficient electric field. However, even several decades after the initial conceptualization of ferroelectric memory, its applications are limited to a niche market. The slow advancement of ferroelectric memories can be attributed to several extant issues, such as the absence of ferroelectric materials with complementary metal–oxide–semiconductor (CMOS) compatibility and scalability. Since the 2010s, ferroelectric memories have attracted increasing interest because of newly discovered ferroelectricity in well-established CMOS-compatible materials, which are previously known to be non-ferroelectric, such as fluorite-structured (Hf,Zr)O₂ and wurtzite-structured (Al,Sc)N. With advancing material fabrication technologies, for example, accurate chemical doping and atomic-level thickness control, a metastable polar phase, and switchable polarization with a reasonable electric field can be induced in (Hf,Zr)O₂ and (Al,Sc)N. Nonetheless, various issues still exist that urgently require solutions to facilitate the use of the ferroelectric (Hf,Zr)O₂ and (Al,Sc)N in emerging memory devices. Thus, ferroelectric (Hf,Zr)O₂ and (Al,Sc)N are comprehensively reviewed herein, including their fundamental science and practical applications.

Hybrid metasurfaces of plasmonic lattices and 2D materials provide a versatile platform for both fundamental and practical studies because of their unprecedented ability for precise manipulation of light at the nanoscale. This review summarizes how the structure design and nanofabrication led to application advances of enhanced photoluminescence, quantum emission, optoelectronic detection, nonlinear process, and valleytronics in hybrid metasurfaces.

Abstract

Plasmonic metasurfaces can significantly enhance the interaction between light and 2D materials. Hybrid structures of plasmonic lattices and 2D materials show great promise for both fundamental and practical studies because of their unprecedented ability for precise manipulation of light at the nanoscale. This review starts with an overview of the basic concepts of plasmonic lattices and optical properties of 2D materials, as well as fabrication strategies for hybrid metasurfaces. Then, the enhanced photoluminescence, quantum emission, optoelectronic detection, nonlinear process, and valleytronics in hybrid metasurfaces are summarized, and their development for nanophotonic functional devices are reviewed. Further, several compelling topics are also outlined that provide outlooks for future directions of hybrid metasurfaces such as novel structural design and high-quality fabrication, all-dielectric metasurfaces, dynamic metasurfaces, and plasmonic mediation of chemical reactions and physical processes. It is believed that hybrid metasurfaces of plasmonic lattices and 2D materials can open prospects for versatile platforms for light-matter interactions and contribute to the revolutions on nanophotonic devices.

Nature Electronics, Published online: 19 June 2023; doi:10.1038/s41928-023-00982-4

A magnetic random-access memory device that has an antiferromagnetic material as its storage element can be electrically read using ferromagnetic tunnelling.

The study explores chalcogen-rich 2D transition-metal (TM)-chalcogenides' potential for phase-change memory. The focus is on the layered TM-chalcogenide NbTe₄, showing a lower melting point and higher crystallization temperature than traditional phase-change materials (PCMs) such as Ge₂Sb₂Te₅. Further device evaluation demonstrates that NbTe₄ is an excellent solution for current PCM issueslike poor thermal stability and the high Reset energy.

Abstract

2D van der Waals (vdW) transition metal di-chalcogenides (TMDs) have garnered significant attention in the nonvolatile memory field for their tunable electrical properties, scalability, and potential for phase engineering. However, their complex switching mechanism and complicated fabrication methods pose challenges for mass production. Sputtering is a promising technique for large-area 2D vdW TMD fabrication, but the high melting point (typically T _m > 1000 °C) of TMDs requires elevated temperatures for good crystallinity. This study focuses on the low-T _m 2D vdW TM tetra-chalcogenides and identifies NbTe₄ as a promising candidate with an ultra-low T _m of around 447 °C (onset temperature). As-grown NbTe₄ forms an amorphous phase upon deposition that can be crystallized by annealing at temperatures above 272 °C. The simultaneous presence of a low T _m and a high crystallization temperature T _c can resolve important issues facing current phase-change memory compounds, such as high Reset energies and poor thermal stability of the amorphous phase. Therefore, NbTe₄ holds great promise as a potential solution to these issues.

The crystal structure of EuCd₂Sb₂ changes from trigonal to tetragonal symmetry after being treated under high pressure and high temperature. The antiferromagnetic transition and insulating electric behaviors are observed in the tetragonal EuCd₂Sb₂. Charge self-consistent simulations predict that the Kondo scale is dramatically suppressed by Hund's coupling.

Abstract

Magnetic and electronic properties of quantum materials heavily rely on the crystal structure even in the same chemical compositions. In this study, it is demonstrated that a layered tetragonal EuCd₂Sb₂ structure can be obtained by treating bulk trigonal EuCd₂Sb₂ under high pressure (6 GPa) and high temperature (600 °C). Magnetization measurements of the newly formed layered tetragonal EuCd₂Sb₂ confirm an antiferromagnetic ordering with Neel temperature (T _N) around 16 K, which is significantly higher than that (T _N ≈ 7 K) of trigonal EuCd₂Sb₂, consistent with heat capacity measurements. Moreover, bad metal behavior is observed in the temperature dependence of the electrical resistivity and the resistivity shows a dramatic increase around the Neel temperature. Electronic structure calculations with local density approximation dynamic mean–field theory (LDA+DMFT) show that this material is strongly correlated with well-formed large magnetic moments, due to Hund's coupling, which is known to dramatically suppress the Kondo scale.

Neuromorphic vision sensors are realized using Y6/Al₂O₃/In₂O₃ phototransistors featuring dual photo-gates. The Y6/Al₂O₃ interface and In₂O₃ layer function as the negative and positive photogates, respectively, resulting in wavelength-dependent bidirectional photoresponses. The phototransistors can perform image sensing, pre-and post-processing tasks by opto-synaptic characteristics, promising as a fundamental unit to construct an efficient bio-inspired neuromorphic vision system.

Abstract

Bio-inspired neuromorphic vision sensors, integrating optical sensing, and processing functions have attracted significant attention for developing future low-power and high-efficiency imaging systems. However, the compulsory electrical signal modulation to achieve inhibitory behaviors in most reported neuromorphic vision sensors results in additional hardware and computational latency. Herein, bidirectional photoresponsive optoelectronic synapses based on In₂O₃/Al₂O₃/Y6 phototransistors are achieved, realizing all-optical-configured synaptic weight updates enabled by dual photogates. The inhibitory and excitatory photoresponses originate from the photogating effects provided by trapped photogenerated electrons in Al₂O₃ under near-infrared light and the ionized oxygen vacancies in In₂O₃ under ultra-violet light, respectively. The bidirectional phototransistor illustrates outstanding optoelectronic synaptic characteristics with low nonlinearity and asymmetry, demonstrating high efficiencies in both preprocessing and postprocessing tasks, such as noise reduction, contrast enhancement, and pattern recognition. The proposed dual-photogate optoelectronic synapses provide effective strategies to construct high-efficiency neuromorphic vision sensors and in-sensor computing systems.

Nature Nanotechnology, Published online: 15 June 2023; doi:10.1038/s41565-023-01354-x

This Review highlights the role of transition metal dichalcogenides, hexagonal boron nitride and stacked heterostructures in applications in quantum communication, computation, sensing and single-photon detection.

Nature Nanotechnology, Published online: 15 June 2023; doi:10.1038/s41565-023-01425-z

Making versatile electron microscope tools

Nature Nanotechnology, Published online: 15 June 2023; doi:10.1038/s41565-023-01426-y

Premelting occurs at 2D faults

Nature Communications, Published online: 15 June 2023; doi:10.1038/s41467-023-39302-x

Interplay between structure and composition of grain boundaries remains elusive, particularly at the atomic level. Here, the authors discover the atomic motifs, which is the smallest structural unit, control the most important chemical properties of grain boundaries.

Carboxylic acid-containing ammonium ligands separate nucleation and crystal growth in in-situ-formed perovskite nanocrystals, enabling quantum-confined nanocrystals with narrow size distributions. Deprotonated phosphonates further enhance photoluminescence quantum yields to unity through defect passivation. These allow for perovskite light-emitting diodes with a maximum current efficiency of 109 cd A⁻¹ and a 45.6 h operating half-time with an initial brightness of 100 cd m⁻².

Abstract

Colloidal perovskite nanocrystals (PNCs) display bright luminescence for light-emitting diode (LED) applications; however, they require post-synthesis ligand exchange that may cause surface degradation and defect formation. In situ-formed PNCs achieve improved surface passivation using a straightforward synthetic approach, but their LED performance at the green wavelength is not yet comparable with that of colloidal PNC devices. Here, it is found that the limitations of in situ-formed PNCs stem from uncontrolled formation kinetics: conventional surface ligands confine perovskite nuclei but fail to delay crystal growth. A bifunctional carboxylic-acid-containing ammonium hydrobromide ligand that separates crystal growth from nucleation is introduced, leading to the formation of quantum-confined PNC solids exhibiting a narrow size distribution. Controlled crystallization is further coupled with defect passivation using deprotonated phosphinates, enabling improvements in photoluminescence quantum yield to near unity. Green LEDs are fabricated with a maximum current efficiency of 109 cd A⁻¹ and an average external quantum efficiency of 22.5% across 25 devices, exceeding the performance of their colloidal PNC-based counterparts. A 45.6 h operating half-time is further documented for an unencapsulated device in N₂ with an initial brightness of 100 cd m⁻².

A highly efficient ferroelectric transistor with exceptional memory window efficiency is demonstrated, offering a wide sensing margin for neuromorphic computing hardware . The device exhibits remarkable memory window tunability, achieving an impressive 94.4% accuracy in classifying the MNIST dataset. Integration with 2D materials reduces the depolarization field in the gate stack, leading to enhanced retention and endurance. This research highlights the potential for ferroelectric transistors to enable high-density memory and energy-efficient synapses in neuromorphic computing.

Abstract

In-memory computing, particularly neuromorphic computing, has emerged as a promising solution to overcome the energy and time-consuming challenges associated with the von Neumann architecture. The ferroelectric field-effect transistor (FeFET) technology, with its fast and energy-efficient switching and nonvolatile memory, is a potential candidate for enabling both computing and memory within a single transistor. In this study,  the capabilities of an integrated ferroelectric HfO₂ and 2D MoS₂ channel FeFET in achieving high-performance 4-bit per cell memory with low variation and power consumption synapses, while retaining the ability to implement diverse learning rules, are demonstrated. Notably, this device accurately recognizes MNIST handwritten digits with over 94% accuracy using online training mode. These results highlight the potential of FeFET-based in-memory computing for future neuromorphic computing applications.

Subtle modulation of ion-intercalation structures has been realized by controllable and selective etching of B atoms from B-doped Ti₃AlC₂ precursors, which generates boron-vacancy doped MXene nanosheets that are able to achieve high electrochemical activity, fast ion transport, and facilitated electron transfer, simultaneously, for high-rate and high volumetric pseudo-intercalation charge storage.

Abstract

The design of pseudocapacitive electrodes that exhibit high-rate and high volumetric capacitances is a big challenge, since it requires subtle modulation of ion-intercalation structures that are able to achieve high electrochemical activity, fast ion transport, and facilitated electron transfer, simultaneously. Herein, controllable and selective etching of B atoms from B-doped Ti₃AlC₂ precursors is reported, which generates boron-vacancy doped MXene (B-V-MXene) nanosheets with finely-regulated, ion-intercalation structures. Electrochemical studies and density-functional-theory calculations demonstrate that Ti around vacancies possess higher surface-redox activity with protons than those on pristine MXenes for the improvement of capacitances. In addition, interlayer spacing can be optimized on B-V-MXenes in promoting proton intercalation. More importantly, the dopant B atoms can increase the electron density on Ti, facilitating the adsorption of the intercalated protons; and further, B 2p-Ti 3d hybridized band sits closer to the Fermi energy than that of C 2p bands, which bridges the energy gap for electron transfer in the pseudo-capacitive reaction. With synergy of all these effects, the novel B-V-MXene compact electrodes can deliver the previously unmatched high volumetric capacitances of 807 F cm⁻³ at 1,000 mV s⁻¹ and 1,815 F cm⁻³ at 5 mV s⁻¹, with excellent cycle stability over 10,000 cycles.