Jiuxiang Dai

Nature Nanotechnology, Published online: 15 May 2024; doi:10.1038/s41565-024-01675-5

Superionic fluoride dielectrics with a low ion migration barrier are capable of excellent capacitive coupling and are highly compatible with scalable device manufacturing processes for integrated electronics.

Large-area continuous thin films of metastable trigonal Cr₂S₃ are grown by ultralow gas-flow governed dynamic transport methods. The anisotropic SHG response of as-prepared Cr₂S₃ demonstrated effective second-order nonlinearity of 48.0 pm V⁻¹, in which sulfur vacancies and dangling bonds could break the surface central-symmetry and contribute nonlinear optical polarizabilities, providing a new understanding of SHG for nonlayered 2D materials.

Abstract

As for nonlayered 2D polymorphic materials, especially for Cr-based chalcogenides, large-area thin film growth with phase control is considered the most important synthesis challenge for magnetic, electronic, and optoelectronic devices. However, the synthesis methods of large continuous thin films for nonlayered 2D materials are still limited and rarely reported, also for the phase control growth, which is inhibited by isotropic 3D growth and similar Gibbs free energy for different phases. Herein, enhanced mass transport chemical vapor deposition is established to achieve the control synthesis of trigonal Cr₂S₃ thin films, in which the stable boundary layer supplies the continuous reaction species and tunes the reaction kinetics. The trigonal phase formation is confirmed by atomic structure characterization, optical absorption and piezoelectric measurements, demonstrating unique physical properties different from rhombohedral phase. The trigonal Cr₂S₃ thin films show obvious layer independent and dissimilar angle-resolved harmonic generation, indicating the surface broken symmetry that can be understood by the combination of negligible piezoelectric response for bulk. The work presents the large-area synthesized strategy by the modification of mass transport for nonlayered 2D materials with new phase formation and establishes the surface symmetry breaking dominated SHG mechanism for future nonlinear optical materials.

Abstract

Two-dimensional (2D) polarization materials have emerged as promising candidates for meeting the demands of device miniaturization, attributed to their unique electronic configurations and transport characteristics. Although the existing inherent and sliding mechanisms have been increasingly investigated in recent years, strategies for inducing 2D polarization with innovative mechanisms remain rare. In this study, we introduce a novel 2D Janus state by modulating the puckered structure. Combining scanning probe microscopy, transmission electron microscopy, and density functional theory calculations, we realized force-triggered out-of-plane and in-plane dipoles with distorted smaller warping in GeSe. The Janus state is preserved after removing the external mechanical perturbation, which could be switched by modulating the sliding direction. Our work offers a versatile method to break the space inversion symmetry in a 2D system to trigger polarization in the atomic scale, which may open an innovative insight into configuring novel 2D polarization materials.

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Electrochemiluminescence (ECL) is rapidly evolving from an analytical method into an optical microscopy. The orthogonality of the electrochemical trigger and the optical readout distinguishes it from classic microscopy and electrochemical techniques, owing to its near-zero background, remarkable sensitivity, and absence of photobleaching and phototoxicity. In this review, we summarize the recent advances in ECL imaging technology, emphasising original configurations which enable the imaging of biological entities and the improvement of the analytical properties by increasing the complexity and multiplexing of bioassays. Additionally, mapping the (electro)chemical reactivity in space provides valuable information on nanomaterials and facilitates deciphering ECL mechanisms for improving their performances in diagnostics and (electro)catalysis. Finally, we highlight the recent achievements in imaging at the ultimate limits of single molecules, single photons or single chemical reactions, and the current challenges to translate the ECL imaging advances to other fields such as material science, catalysis and biology.

Abstract

The quantity of sensor nodes within current computing systems is rapidly increasing in tandem with the sensing data. The presence of a bottleneck in data transmission between the sensors, computing, and memory units obstructs the system's efficiency and speed. To minimize the latency of data transmission between units, novel in-memory and in-sensor computing architectures are proposed as alternatives to the conventional von Neumann architecture, aiming for data-intensive sensing and computing applications. The integration of two-dimensional (2D) materials and 2D ferroelectric materials has been expected to build these novel sensing and computing architectures due to the dangling-bond-free surface, ultra-fast polarization flipping and ultra-low power consumption of the 2D ferroelectrics. Here, we review the recent progress of 2D ferroelectric devices for in-sensing and in-memory neuromorphic computing. Experimental and theoretical progresses on 2D ferroelectric devices, including passive ferroelectrics-integrated 2D devices and active ferroelectrics-integrated 2D devices, are reviewed followed by the integration of perception, memory, and computing application. Notably, 2D ferroelectric devices have been used to simulate synaptic weights, neuronal model functions, and neural networks for image processing. As an emerging device configuration, 2D ferroelectric devices have the potential to expand into the sensor-memory and computing integration application field, leading to new possibilities for modern electronics.

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Abstract

The disparity between growth substrates and application-specific substrates can be mediated by reliable graphene transfer, the lack of which currently strongly hinders the graphene applications. Conventionally, the removal of soft polymers, that support the graphene during the transfer, would contaminate graphene surface, produce cracks, and leave unprotected graphene surface sensitive to airborne contaminations. In this work, we found that polyacrylonitrile (PAN) can function as polymer medium for transferring wafer-size graphene, and encapsulating layer to deliver high-performance graphene devices. Therefore, PAN, that is compatible with device fabrication, does not need to be removed for subsequent applications. We achieved the crack-free transfer of 4-inch graphene onto SiO₂/Si wafers, and the wafer-scale fabrication of graphene-based field-effect transistor (FET) arrays with no observed clear doping, uniformly high carrier mobility (∼11,000 cm² V⁻¹ s⁻¹) and long-term stability at room temperature. Our work presents new concept for designing the transfer process of two-dimensional (2D) materials, in which multifunctional polymer can be retained, and offers a reliable method for fabricating wafer-scale devices of 2D materials with outstanding performance.

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This review provides an overview of hot-carrier engineered two-dimensional (2D) infrared optoelectronic devices. Basic principles of hot-carrier dynamics and photoelectric conversion are clarified in detail. The progresses of 2D infrared hot-carrier optoelectronic devices are summarized, with a specific emphasis on photodetectors, solar cells, light-emitting devices and novel functionalities. The challenges and prospects of hot-carrier device towards infrared applications are highlighted.

Abstract

Plasmonic hot carrier engineering holds great promise for advanced infrared optoelectronic devices. The process of hot carrier transfer has the potential to surpass the spectral limitations of semiconductors, enabling detection of sub-bandgap infrared photons. By harvesting hot carriers prior to thermalization, energy dissipation is minimized, leading to highly efficient photoelectric conversion. Distinguished from conventional band-edge carriers, the ultrafast interfacial transfer and ballistic transport of hot carriers present unprecedented opportunities for high-speed photoelectric conversion. However, a complete description on the underlying mechanism of hot-carrier infrared optoelectronic device is still lacking, and the utilization of this strategy for tailoring infrared response is in its early stages. This review aims to provide a comprehensive overview of the generation, transfer and transport dynamics of hot carriers. Basic principles of hot-carrier conversion in heterostructures are discussed in detail. In addition, progresses of two-dimensional (2D) infrared hot-carrier optoelectronic devices are summarized, with a specific emphasis on photodetectors, solar cells, light-emitting devices and novel functionalities through hot-carrier engineering. Furthermore, challenges and prospects of hot-carrier device towards infrared applications are highlighted.

Nature Materials, Published online: 13 May 2024; doi:10.1038/s41563-024-01894-0

Ultrathin and flat crystals of bismuth are grown between the atomically flat layers of a van der Waals material. These crystals exhibit outstanding electronic properties, including gate-tunable quantum oscillations of the magnetoresistance.

Ferroelectric polarization can be formed in some non-polar point groups when the edges break the crystal symmetry. Such polarizations can be maintained at macroscale and are switchable if the transition between multiple equivalent states with high symmetry can be realized via long ion displacements.

Abstract

In the classical model, ferroelectricity is associated with small ion displacements from paraelectric phases with high symmetry, and ferroelectric crystals must adopt one of the ten polar point groups with low symmetry according to Neumann's principle. In this work, it is proposed that this conclusion is based on perfect bulk crystals without taking the boundaries into account. First-principles evidence shows that ferroelectric polarizations may also be formed in some non-polar point groups as the edges generally break the crystal symmetry. Meanwhile, such polarizations can be maintained at macroscale and are switchable when the transition between multiple equivalent states with high symmetry can be realized via long ion displacements， essentially akin to ion conductors. For example, the switching barriers can be much reduced in sliding ferroelectric bilayer systems or ionic compounds with covalent-like directionality. Such unconventional ferroelectricity can be attributed to the boundaries and long ion displacements, and its existence in several systems like CuCrS₂ is supported by experimental observations. It may explain a series of unclarified phenomena reported previously as well as significantly expand the scope of ferroelectrics, especially those with high polarizations induced by long ion displacements.

A polarization-sensitive GeS₂/PbSe two-dimensional (2-D short-wave infrared (SWIR) spectral phototransistor, integrated with an artificial neural network (ANN) for deep-learning, reconstructs infrared spectra (900–1700 nm) with tunable V_G and δ. This detector achieves 96.7% accuracy in narrow-band selection, supports at least 8 multi-wavelengths’ channels, and narrow FWHM (15 nm), which is crucial for future hyperspectral detecting applications.

Abstract

Integrated computational spectrometers with gate-tunable nano heterostructures and reconstruction algorithms are attractive for on-chip gas-sensing spectrometers and have enabled versatile spectrum detectors. However, they require the selective and optical filtering capabilities of wavelengths, restricting their efficient implementation in narrow-band photodetection. In this study, a printable spectral phototransistor is developed with high dynamic detectivity (10¹² Jones and 10⁵ Hz at −3 db bandwidth) modulated by a GeS₂ nanosheet heterostructure at short-wave infrared (SWIR) regime. Using the transport mode switching of carriers in a heterostructure and the polarization-sensitivity of the GeS₂ two-dimension (2-D) nanosheet, this SWIR spectral phototransistor demonstrates an accurate narrow-band selective (96.7% accuracy) spectrum detector and performed a deep-learning analysis of an artificial neural network (ANN). Furthermore, this GeS₂ 2-D based spectral phototransistor, characterized by its high in-plane anisotropy and electrically reconfigurable properties, extends the applicability of narrow-band photodetection with 15 nm Full Width at Half Maximum (FWHM) to the recognition of trace-gases at the parts per billion (ppb) level.

Nature Communications, Published online: 14 May 2024; doi:10.1038/s41467-024-48431-w

Antiferromagnetic spintronics offer high speed operations, and reduced issues with stray fields compared to ferromagnetic systems, however, antiferromagnets are typically more challenging to manipulate electrically. Here, Yang, Kim, and coauthors demonstrate electrical control of magnon dispersion and frequency in an α-Fe2O3/Pt heterostructure.

Nature Photonics, Published online: 14 May 2024; doi:10.1038/s41566-024-01434-x

Exploiting the interactions between bright excitons and free carriers in an atomically thin semiconductor of trilayer tungsten diselenide WSe2 results in Fermi polarons that exhibit unusually large nonlinearity.

Nature Communications, Published online: 13 May 2024; doi:10.1038/s41467-024-48429-4

Here, the authors demonstrate a wafer-scale, low-temperature process using atomic layer deposition, for the synthesis of uniform, conformal amorphous boron nitride (aBN) thin films. They further fabricate aBN-encapsulated monolayer MoS2 field-effect transistors.

Abstract

Artificial heterostructures with structural advancements and customizable electronic interfaces are fundamental for achieving high-performance lithium-ion batteries (LIBs). Here, we propose a design idea for a covalently bonded lateral/vertical black phosphorus (BP)-graphdiyne oxide (GDYO) heterostructure achieved through a facile ball-milling approach. Lateral heterogeneity is realized by the sp²-hybridized mode P-C bonds, which connects the phosphorus atoms at the edges of BP with the carbon atoms of the terminal acetylene in GDYO. The vertical connection of the heterojunction of BP and GDYO is connected by P-O-C bond. Experimental and theoretical studies demonstrate that BP-GDYO incorporates interfacial and structural engineering features, including built-in electric fields, chemical bond interactions, and maximized nanospace confinement effects. Therefore, BP-GDYO exhibits improved electrochemical kinetics and enhanced structural stability. Moreover, through ex- and in-situ studies, we clarified the lithiation mechanism of BP-GDYO, highlighting that the introduction of GDYO inhibits the shuttle/dissolution effect of phosphorus intermediates, hinders volume expansion, provides more reactive sites, and ultimately promotes reversible lithium storage. The BP-GDYO anode exhibits lithium storage performance with high-rate capacity and long-cycle stability (602.6 mAh g⁻¹ after 1000 cycles at 2.0 A g⁻¹). The proposed interfacial and structural engineering are universal and represents a conceptual advance in building high-performance LIBs electrode.

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This study assesses the potential of layered 2D materials for gas sensing using second harmonic generation (SHG). It focuses on the adsorption behaviors of oxygen, ammonia, and water vapor on WS₂ surfaces. By applying the simplified bond hyperpolarizability model, it confirms physical adsorption and explores competitive interactions between gases, aligning with Langmuir's model and theoretical predictions from density functional theory.

Abstract

While understanding the competitive adsorption behavior of gas sensor is important, it is yet to be unraveled. Especially for the influence of water molecules to the gas adsorbed on 2D materials. This study explores the potential of layered 2D materials as a candidate material for gas sensing, employing non-destructive measurement, and second harmonic generation (SHG). The investigation focuses on analyzing oxygen, ammonia, and water vapor adsorbed on a WS₂ surface by studying the evolutions in electric dipole and electric field. Leveraging the simplified bond hyperpolarizability model (SBHM), a foundation is established for gas sensors utilizing high-quality 2D materials. This approach facilitates the detection of material modifications in response to environmental influences, including the inevitable water molecules. The obtained hyperpolarizability from SBHM exhibits remarkable consistency with Langmuir's adsorption model, confirming the physical adsorption in the system. In addition, the competitive effects between gases are explored by comparing experimental results with theoretical predictions based on Boltzmann distribution and density functional theory (DFT) calculations. This highlights the effectiveness of SHG and SBHM in studying gas adsorption on layered van der Waals materials.

2D flexible magnets hold great promise in flexible spintronics. By employing an aged precursor, 2D chromium selenide with internal voids can be synthesized. The unique structure induces ferrimagnetism and a small Young's modulus. This work offers avenues for obtaining 2D magnets with desired mechanical properties, paving the way for future flexible spintronics.

Abstract

2D magnetic materials with distinct mechanical properties are of great importance for flexible spintronics. However, synthesizing 2D magnets with atomic thickness is challenging and their mechanical properties remain largely unexplored. Here, the growth of a ferrimagnetic 2D Cr₉Se₁₃ with anomalous elasticity is reported by an aged-precursor-assisted method. The obtained 2D Cr₉Se₁₃ exhibits an out-of-plane ferrimagnetic order with a coercivity larger than those of conventional magnetic materials. Noteworthy, it presents decent breaking strength and a Young's modulus of 52 ± 8 GPa that is among the smallest of the 2D family. This exceptional elasticity is attributed to the unique internal voids in Cr₉Se₁₃, as evidenced by the formed edge dislocations under strain. This work not only offers a facile method to synthesize 2D magnets but also develops avenues for obtaining 2D materials with desired mechanical properties, paving the way for future flexible spintronics.

The strong electromagnetic loss capability of two-phase structural transitions of 1T and 2H-MS2 (M = Mo, V, W) can be achieved by molecular intercalation resulting in enhanced interfacial polarization and conduction losses.

Abstract

Polarization at interfaces is an important loss mechanism for electromagnetic wave (EMW) attenuation, though the motion behavior of carriers in interfaces composed of different types of conductors has yet to be investigated. Tuning the phase structure of transition metal dichalcogenides (TMDs) MS₂ (M = Mo, V, W) by organics small molecule intercalation to achieve the modulation of interfacial types is an effective strategy, where 1T-MS₂ exhibits metallic properties and 2H-MS₂ has semiconducting properties. To exclude the contribution of the intrinsic properties of TMDs materials, three TMDs (MoS₂, VS₂, and WS₂), which also possess phase transitions, are investigated. Among them, the 1T-MS₂ composite exhibits excellent EMW absorption performance under the synergistic effect of interfacial polarization and conduction loss. 1T-MoS₂/MOF-A exhibits the best EMW absorption performance with an RL_min of −61.07 dB at a thickness of 3.0 mm and an EAB of 7.2 GHz at 2.3 mm. The effectiveness of the modulation of the interfacial polarization using 1T-phase and 2H-phase MS₂ is demonstrated, which is important for the analysis of the carrier motion behavior during the interfacial loss.

Scanning tunneling microscopy results highlight the role of vertical charge orders in a layered charge density wave material. Spatially resolved spectroscopic measurements, along with density functional theory calculations, not only demonstrate the lateral coexistence of multiple insulating domains but also reveal the correlation between their electronic properties and stacking configurations.

Abstract

Vertical charge order shapes the electronic properties in layered charge density wave (CDW) materials. Various stacking orders inevitably create nanoscale domains with distinct electronic structures inaccessible to bulk probes. Here, the stacking characteristics of bulk 1T-TaS₂ are analyzed using scanning tunneling spectroscopy (STS) and density functional theory (DFT) calculations. It is observed that Mott-insulating domains undergo a transition to band-insulating domains restoring vertical dimerization of the CDWs. Furthermore, STS measurements covering a wide terrace reveal two distinct band insulating domains differentiated by band edge broadening. These DFT calculations reveal that the Mott insulating layers preferably reside on the subsurface, forming broader band edges in the neighboring band insulating layers. Ultimately, buried Mott insulating layers believed to harbor the quantum spin liquid phase are identified. These results resolve persistent issues regarding vertical charge order in 1T-TaS₂, providing a new perspective for investigating emergent quantum phenomena in layered CDW materials.

In₂O₃ nanocubes with high oxygen vacancy concentrations (H-In₂O₃) is designed and synthesized by a simple self-template method. Selective catalysis is realized in H-In₂O₃ for the consecutive solid–liquid–solid sulfur redox reactions.

Abstract

Lithium–sulfur (Li–S) battery is identified as an ideal candidate for next-generation energy storage systems in consideration of its high theoretical energy density and abundant sulfur resources. However, the shuttling behavior of soluble polysulfides (LiPSs) and their sluggish reaction kinetics severely limit the practical application of the current Li–S battery. In this work, a series of In₂O₃ nanocubes with different oxygen vacancy concentrations are designed and prepared via a facile self-template method. The introduced oxygen vacancy on In₂O₃ can effectively rearrange the charge distribution and enhance sulfiphilic property. Moreover, the In₂O₃ with high oxygen vacancy concentration (H-In₂O₃) can slightly slow down the solid–liquid conversion process and significantly accelerate the liquid–solid conversion process, thus reducing the accumulation of LiPSs in electrolyte and inhibiting the shuttle effect. Contributed by the unique selective catalytic capability, the prepared H-In₂O₃ exhibits excellent electrochemical performance when used as sulfur host. For instance, a high reversible capacity of 609 mAh g⁻¹ is obtained with only 0.044% capacity decay per cycle over 1000 cycles at 1.0 C. This work presents a typical example for designing advanced sulfur hosts, which is crucial for the commercialization of Li–S battery.

High-quality transition metal dichalcogenide (TMD) thin films can be synthesized using a two-step approach where a solid transition metal precursor layer is converted in a chalcogen-containing atmosphere to a TMD. Herein, a critical review of this method, demonstrating its versatility and outlining key features, applications, and outlook on this method's impact in the TMD synthesis community is given.

Abstract

The widely studied class of two-dimensional (2D) materials known as transition metal dichalcogenides (TMDs) are now well-poised to be employed in real-world applications ranging from electronic logic and memory devices to gas and biological sensors. Several scalable thin film synthesis techniques have demonstrated nanoscale control of TMD material thickness, morphology, structure, and chemistry and correlated these properties with high-performing, application-specific device metrics. In this review, the particularly versatile two-step conversion (2SC) method of TMD film synthesis is highlighted. The 2SC technique relies on deposition of a solid metal or metal oxide precursor material, followed by a reaction with a chalcogen vapor at an elevated temperature, converting the precursor film to a crystalline TMD. Herein, the variables at each step of the 2SC process including the impact of the precursor film material and deposition technique, the influence of gas composition and temperature during conversion, as well as other factors controlling high-quality 2D TMD synthesis are considered. The specific advantages of the 2SC approach including deposition on diverse substrates, low-temperature processing, orientation control, and heterostructure synthesis, among others, are featured. Finally, emergent opportunities that take advantage of the 2SC approach are discussed to include next-generation electronics, sensing, and optoelectronic devices, as well as catalysis for energy-related applications.

A-type antiferromagnetism is observed in a 2D van der Waals (vdW) metallic single-crystal (Fe_0.8Co_0.2)₃GaTe₂ with T_N≈203 K in bulk and ≈185 K in 9-nm nanosheets. A large and tunable tunneling magnetoresistance (TMR) ratio of 180% is realized in an all-vdW (Fe_0.8Co_0.2)₃GaTe₂/WSe₂/Fe₃GaTe₂ heterojunction. The TMR ratio sustains 0.4% at near-room temperature 280 K.

Abstract

Magnetic tunnel junctions (MTJs) are widely applied in spintronic devices for efficient spin detection through the imbalance of spin polarization at the Fermi level. The van der Waals (vdW) property of 2D magnets with atomically flat surfaces and negligible surface roughness greatly facilitates the development of MTJs, primarily in ferromagnets. Here, A-type antiferromagnetism in 2D vdW single-crystal (Fe_0.8Co_0.2)₃GaTe₂ is reported with T_N ≈ 203 K in bulk and ≈ 185 K in 9-nm nanosheets. The metallic nature and out-of-plane magnetic anisotropy make it a suitable candidate for MTJ electrodes. By constructing heterostructures based on (Fe_0.8Co_0.2)₃GaTe₂/WSe₂/Fe₃GaTe₂, a large tunneling magnetoresistance (TMR) ratio of 180% at low temperature is obtained, with the TMR signal persisting at near-room temperature 280 K. Furthermore, the TMR is tunable by the electric field, and the MTJ device operates stably with a low applied bias down to 1 mV (≈0.6 nA), highlighting its potential for energy-efficient spintronic devices. This work opens up new opportunities for 2D antiferromagnetic spintronics and quantum devices.

The study of MoS₂ crystal growth on distinct substrates by means of kinetic Monte Carlo simulations and varying key parameters such as adsorption rate, the energy barriers for adatom desorption, on-substrate adatom migration, or edge migration. Provides insights into the potential and limitations of these former processes, offering a theoretical framework for decision-making in the design and optimization of transition metal dichalcogenides synthesis.

Abstract

Controlling the morphology of 2D transition metal dichalcogenides (TMDs) plays a key role in their applications. Although chemical vapor deposition can achieve wafer-scale growth of 2D TMDs, a comprehensive theoretical framework for effective growth optimization is lacking. Atomistic modeling methods offer a promising approach to delve into the intricate dynamics underlying the growth. In this study, kinetic Monte Carlo (kMC) simulations are employed to identify crucial parameters that govern the morphology of MoS₂ flakes grown on diverse substrates. The simulations reveal that large adsorption rates significantly enhance growth speed, which however necessitates rapid edge migration to achieve compact triangles. Substrate etching can tune the adsorption–desorption process of adatoms and enable preferential growth within a specific substrate region, controlling the flake morphology. This study unravels the complex dynamics governing 2D TMD morphology, offering a theoretical framework for decision-making in the design and optimization of TMD synthesis processes.