Jiuxiang Dai

Light: Science & Applications, Published online: 09 May 2024; doi:10.1038/s41377-024-01443-z

Illustration of entangled photon pair generation in an integrated silicon carbide (SiC) platform. This achievement, coupled with SiC’s potential to integrate various electrical, photonic and quantum technologies in the same device platform, offers promises for the scalable implementation of quantum information processing.

Twisted Bilayer WS₂

In article number 2313638, Xuetao Gan, Xuewen Wang, Wei Huang, and co-workers demonstrate an improved CVD setup for the direct synthesis of high-quality twisted bilayer WS₂ with a wide twist angle range from 0 to 120° by utilizing tilted SiO₂/Si substrates. The strong correlation between the second harmonic generation (SHG) response and the twist angle suggests a tunable SHG performance with twist angles.

Black phosphorus (BP)/graphdiyne oxide (GDYO) lateral/vertical heterostructures are prepared via solid-state mechanochemistry. The sp²-hybridized mode P-C and out-of-plane P-O-C bonds realize the lateral and vertical connection, respectively. GDYO regulates the volume expansion of BP, provides active sites, and restrains the shuttle effect of Li_xP_y. The heterostructures combine interface and structural engineering, demonstrating high-rate performance and long-term stability in lithium storage.

Abstract

Artificial heterostructures with structural advancements and customizable electronic interfaces are fundamental for achieving high-performance lithium-ion batteries (LIBs). Here, a design idea for a covalently bonded lateral/vertical black phosphorus (BP)-graphdiyne oxide (GDYO) heterostructure achieved through a facile ball-milling approach, is designed. Lateral heterogeneity is realized by the sp²-hybridized mode P-C bonds, which connect 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, 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, is clarified. The BP-GDYO anode exhibits lithium storage performance with high-rate capacity and long-cycle stability (602.6 mAh g⁻¹ after 1 000 cycles at 2.0 A g⁻¹). The proposed interfacial and structural engineering is universal and represents a conceptual advance in building high-performance LIBs electrode.

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.

“Graphene's Wetting Transparency: A Liquid Interface Study – This compelling digital visualization showcases a single-layer graphene sheet's interaction with a submerged organic droplet. The graphene, almost optically transparent, conforms to the liquid surface, offering a vivid display of surface tension and intermolecular forces at play between the droplet, graphene, and the liquid substrate.”

Abstract

Graphene's wetting transparency offers promising avenues for creating multifunctional devices by allowing real-time wettability control on liquid substrates via the flow of different liquids beneath graphene. Despite its potential, direct measurement of floating graphene's wettability remains a challenge, hindering the exploration of these applications. The current study develops an experimental methodology to assess the wetting transparency of single-layer graphene (SLG) on liquid substrates. By employing contact angle measurements and Neumann's Triangle model, the challenge of evaluating the wettability of floating free-suspended single-layer graphene is addressed. The research reveals that for successful contact angle measurements, the testing and substrate liquids must be immiscible. Using diiodomethane as the testing liquid and ammonium persulfate solution as liquid substrate, the study demonstrates the near-complete wetting transparency of graphene. Furthermore, it successfully showcases the feasibility of real-time wettability control using graphene on liquid substrates. This work not only advances the understanding of graphene's interaction with liquid interfaces but also suggests a new avenue for the development of multifunctional materials and devices by exploiting the unique wetting transparency of graphene.

A triboelectric potential powered vertical field-effect transistor (tribotronic vertical field-effect transistor or graphene barristor) is demonstrated, offering an effective way to modulate the Schottky barrier of vertical van der Waals heterostructure by mechanical displacement.

Abstract

Graphene has attracted considerable interest for next-generation electronics. However, the absence of natural bandgap has limited the current on/off ratio of graphene-based transistors. Vertical integration of 2D heterostructures offers a promising approach to address this challenge, enabling high-current-density vertical field-effect transistor (VFET) with large on/off ratio. Here, a triboelectric potential-powered VFET with a vertical stacked graphene/MoS₂ heterostructure and a sliding-mode triboelectric nanogenerator (TENG) coupled with gate dielectrics are proposed. The tribotronic VFET has an ultrashort channel length in vertical direction, exhibiting excellent current driving capability with an ultrahigh on-state current density of 950 A cm⁻² and a good current on/off ratio of 630. It also demonstrates reconfigurable diode behavior with a rectification ratio over 10². Temperature-dependent studies are applied to tribotronic devices for the first time, indicating an effective modulation on the Schottky barrier height of 150 meV by the triboelectric potential. A green LED pixel is driven by the tribotronic VFET as a demonstration to work as a tactile interactive light-emitting device. The demonstrated tribotronic vertical device offers a promising strategy for integrating various 2D layered materials with TENG in vertical direction, enabling 3D integration of low-power and interactive devices for next-generation electronics.

Non-metallic substrates offer a promising platform for direct growth of 2D materials, reducing transfer complexities and enabling wafer-scale fabrication. The substrates and growth methods play critical roles in controlling crystal nucleation and growth, with potential for preserving material properties and scaling up production.

Abstract

The advent of 2D materials has ushered in the exploration of their synthesis, characterization and application. While plenty of 2D materials have been synthesized on various metallic substrates, interfacial interaction significantly affects their intrinsic electronic properties. Additionally, the complex transfer process presents further challenges. In this context, experimental efforts are devoted to the direct growth on technologically important semiconductor/insulator substrates. This review aims to uncover the effects of substrate on the growth of 2D materials. The focus is on non-metallic substrate used for epitaxial growth and how this highlights the necessity for phase engineering and advanced characterization at atomic scale. Special attention is paid to monoelemental 2D structures with topological properties. The conclusion is drawn through a discussion of the requirements for integrating 2D materials with current semiconductor-based technology and the unique properties of heterostructures based on 2D materials. Overall, this review describes how 2D materials can be fabricated directly on non-metallic substrates and the exploration of growth mechanism at atomic scale.

In this study, a penetrated nucleation mechanism is introduced to explain the 2D non-metal material's growth on metal substrates. By considering the bonding strength of the metal substrate atoms, two types of surface morphological evolution (type A and B) are identified, supported by the detailed first-principles simulations of boron on metal substrates and existing experimental evidence.

Abstract

2D materials have attracted considerable attention in the past decades for their unique properties, making the understanding of their nucleation process key to effective synthesis. Traditional explanations of thin-film growth, focusing on the competition between atom interactions at the interface and within layers, often fall short of explaining real experimental results. Herein, a penetrated nucleation mechanism is proposed for 2D materials growth on metal substrates, taking into account the role of metal substrate atoms. This approach leads to a better understanding of how the surface shape evolves in two specific ways during growth in real experimental findings. Supported by detailed first-principles simulations of boron on metal substrates and thermodynamic analyses of other studies involving metals and nonmetals, the above-proposed mechanism is validated. Moreover, a broad strategy for growing large-scale 2D materials on metal surfaces without creating undesired alloy layers is also presented, by adjusting the interfacial interactions by surface passivation, validated by existing experiments.

Thermoelectric technology enables direct energy conversion between heat and electricity. Topological semimetals, with unique band structures, hold great potential for transverse thermoelectrics owing to co-existence of high mobility electrons and holes. Herein, large transverse thermoelectric power factor is observed in NbAs₂ topological semimetal at relatively low magnetic field, demonstrating it as a promising transverse thermoelectric material.

Abstract

The distinctive properties of topological semimetals, including linear band dispersion and compensated electron-hole carriers, have positioned these materials at the forefront of research in power generation and solid-state cooling due to their remarkable magneto-thermoelectric performance. In this work, the transverse thermoelectric characteristics of the topological semimetal NbAs₂ are studied. Specifically, under a magnetic field of 9 T, the Nernst coefficient displays a linear and unsaturated trend, reaching a peak of 600 µV K⁻¹ at 35 K. Consequently, this engenders a substantial transverse power factor (tPF) of 850 µW cm⁻¹ K⁻² under a 5 T magnetic field. The exceptional attributes of NbAs₂ driven by its remarkably high carrier mobility and compensated electron-hole concentration near the Fermi level are revealed by band structure analysis based on theoretical calculations and quantum oscillations. This work not only underscores the immense potential of topological semimetals as transverse thermoelectric materials for niche applications where magnetic fields exist but also provides valuable guidance for the discovery and optimization of topological materials for promising thermoelectric performance.

Nature Photonics, Published online: 08 May 2024; doi:10.1038/s41566-024-01437-8

Super-stealth laser cutting with nanometre precision and aspect ratios of the order of 1,000 is demonstrated. The technique is applicable to a broad variety of transparent solids, including silica, lithium tantalate, lithium niobate, YAG, Ce:YAG, Ti:sapphire and β-Ga2O3.

Nature, Published online: 08 May 2024; doi:10.1038/s41586-024-07362-8

The introduction of chemical short-range disorder substantially affects the crystal structure of layered lithium oxide cathodes, leading to improved charge transfer and structural stability.

Nature, Published online: 08 May 2024; doi:10.1038/s41586-024-07369-1

Electro-optical photonic integrated circuits based on lithium tantalate perform as well as current state-of-the-art ones using lithium niobate but the material has the advantage of existing commercial uses in consumer electronics, easing the problem of scalability.

Nature Reviews Materials, Published online: 08 May 2024; doi:10.1038/s41578-024-00689-8

Developing circuits for flexible and stretchable devices demands not only high performance but also reliable and predictable components, such as organic thin-film transistors. High-throughput characterization is required to build reliable structure–property relationships, which are critical for the commercialization of new materials.