Jiuxiang Dai

Nature Communications, Published online: 06 May 2024; doi:10.1038/s41467-024-48047-0

Previous measurements of FeSe0.45Te0.55 found one-dimensional (1D) defects that were interpretated as domain walls hosting propagating Majorana topological modes. Here, the authors reveal that these 1D defects correspond to sub-surface debris and show that the filling of the superconducting gap on these defects is topologically trivial.

The synthesis of environmentally stable and high Curie temperature 2D Fe₃O₄ nanosheets are reported that exhibit multi-level resistance under the stimulation of a microwave field at room temperature. Such behavior is ascribed to the emergence of bAPBs and sAPBs in the Fe₃O₄ structure and morphological evolution of spin texture. The findings pave a way for synthesizing environmentally stable 2D magnetic materials at room temperature, holding substantial potential for developing practical multistage memory devices upon 2D materials.

Abstract

2D magnetic materials hold substantial promise in information storage and neuromorphic device applications. However, achieving a 2D material with high Curie temperature (T _C), environmental stability, and multi-level magnetic states remains a challenge. This is particularly relevant for spintronic devices, which require multi-level resistance states to enhance memory density and fulfil low power consumption and multi-functionality. Here, the synthesis of 2D non-layered triangular and hexagonal magnetite (Fe₃O₄) nanosheets are proposed with high T _C and environmental stability, and demonstrate that the ultrathin triangular nanosheets show broad antiphase boundaries (bAPBs) and sharp antiphase boundaries (sAPBs), which induce multiple spin precession modes and multi-level resistance. Conversely, the hexagonal nanosheets display slip bands with sAPBs associated with pinning effects, resulting in magnetic-field-driven spin texture reversal reminiscent of “0” and “1” switching signals. In support of the micromagnetic simulation, direct explanation is offer to the variation in multi-level resistance under a microwave field, which is ascribed to the multi-spin texture magnetization structure and the randomly distributed APBs within the material. These novel 2D magnetite nanosheets with unique spin textures and spin dynamics provide an exciting platform for constructing real multi-level storage devices catering to emerging information storage and neuromorphic computing requirements.

Light: Science & Applications, Published online: 06 May 2024; doi:10.1038/s41377-024-01448-8

Light-deformable microrobots shape up for the biological obstacle course

Many applications require graphene as perfect as possible to achieve good performance. Synthesis of graphene films by chemical vapor deposition of carbon precursors on metal substrates, especially on Cu, remains the main way to produce high-quality graphene. This review focuses on how to produce super graphene, namely graphene with a perfect structure and free of contaminations.

Abstract

The quality requirements of graphene depend on the applications. Some have a high tolerance for graphene quality and even require some defects, while others require graphene as perfect as possible to achieve good performance. So far, synthesis of large-area graphene films by chemical vapor deposition of carbon precursors on metal substrates, especially on Cu, remains the main way to produce high-quality graphene, which has been significantly developed in the past 15 years. However, although many prototypes are demonstrated, their performance is still more or less far from the theoretical property limit of graphene. This review focuses on how to make super graphene, namely graphene with a perfect structure and free of contaminations. More specially, this study focuses on graphene synthesis on Cu substrates. Typical defects in graphene are first discussed together with the formation mechanisms and how they are characterized normally, followed with a brief review of graphene properties and the effects of defects. Then, the synthesis progress of super graphene from the aspects of substrate, grain size, wrinkles, contamination, adlayers, and point defects are reviewed. Graphene transfer is briefly discussed as well. Finally, the challenges to make super graphene are discussed and a strategy is proposed.

Nature, Published online: 07 May 2024; doi:10.1038/d41586-024-01339-3

Beware of graphene’s huge and hidden environmental costs

Femtosecond laser pulses are utilized to treat WS₂ flakes in acetonitrile solvent. Laser power is adjusted to remove sulfur atoms from WS₂ and replace them with carbon atoms, generated simultaneously by disintegrating acetonitrile molecules in the treatment process. The resultant W₂C shows significantly higher photothermal conversion efficiency compared to the parent WS₂.

Photothermal therapy is a new, promising approach for treating cancer in a noninvasive way. Herein, a photothermally efficient nanomaterial is synthesized by exposing bulk WS₂ powder, dispersed in carbon-rich acetonitrile, to high-intensity femtosecond laser pulses. The photothermal measurements show a significantly higher rise in temperature and enhance photothermal efficiency for the laser-treated material. The attribution is on the enhancement to the formation of tungsten semi-carbide (W₂C), which is verified by morphological and structural characterization. The tungsten semi-carbide is formed by laser-induced carburization; intense laser pulses knock S atoms out of the WS₂ lattice and dissociate the acetonitrile molecules, resulting in the formation of W₂C. Interestingly, the hexagonal W₂C has a two-dimensional (2D) layered morphology like that of the WS₂, suggesting its morphology is not random and might be determined by the morphology of the parent material. The study provides a simple route to transform a 2D transition-metal dichalcogenide into a 2D transition-metal carbide, which is useful for applications including photothermal therapy.

The ferromagnetic order at the single layer limit of two van der Waals materials based on transition metal dichlorides is probed. XMCD measurements demonstrate that the ferromagnetic order of FeCl₂ and NiCl₂ persist on top of a metallic substrate. LT-STS experiments, using functionalized tips with an organic magnetic sensor, confirm the magnetic order of the materials at the atomic scale even at zero applied magnetic fields.

Abstract

Magnetism in two dimensions is traditionally considered an exotic phase mediated by spin fluctuations, but far from collinearly ordered in the ground state. Recently, 2D magnetic states have been discovered in layered van der Waals compounds. Their robust and tunable magnetic state by material composition, combined with reduced dimensionality, foresee a strong potential as a key element in magnetic devices. Here, a class of 2D magnets based on metallic chlorides is presented. The magnetic order survives on top of a metallic substrate, even down to the monolayer limit, and can be switched from perpendicular to in-plane by substituting the metal ion from iron to nickel. Using functionalized STM tips as magnetic sensors, local exchange fields are identified, even in the absence of an external magnetic field. Since the compounds are processable by molecular beam epitaxy techniques, they provide a platform with large potential for incorporation into current device technologies.

Publication date: June 2024

Source: Materials Today, Volume 75

Author(s): Binod Paudel, Jeffrey A. Dhas, Yadong Zhou, Min-Ju Choi, David J. Senor, Chih-Hung Chang, Yingge Du, Zihua Zhu

Nature Electronics, Published online: 29 April 2024; doi:10.1038/s41928-024-01148-6

A non-volatile memory device that is based on an aluminium scandium nitride (Al0.68Sc0.32N) ferroelectric diode can operate at temperatures of up to 600 °C.

This paper presents the epitaxial growth of 8-in. wafer-scale highly oriented monolayer MoS₂ on sapphire with excellent spatial homogeneity, using a specially designed vertical chemical vapor deposition system. The as-grown 8-in. MoS₂ wafers are used for the batch fabrication of large-scale, high-performance devices, including field-effect transistors, logic circuits, and ring oscillators, showcasing the potential for practical industry-scale applications.

Abstract

Large-scale, high-quality, and uniform monolayer molybdenum disulfide (MoS₂) films are crucial for their applications in next-generation electronics and optoelectronics. Epitaxy is a mainstream technique for achieving high-quality MoS₂ films and is demonstrated at a wafer scale up to 4-in. In this study, the epitaxial growth of 8-in. wafer-scale highly oriented monolayer MoS₂ on sapphire is reported as with excellent spatial homogeneity, using a specially designed vertical chemical vapor deposition (VCVD) system. Field effect transistors (FETs) based on the as-grown 8-in. wafer-scale monolayer MoS₂ film are fabricated and exhibit high performances, with an average mobility and an on/off ratio of 53.5 cm² V⁻¹ s⁻¹ and 10⁷, respectively. In addition, batch fabrication of logic devices and 11-stage ring oscillators are also demonstrated, showcasing excellent electrical functions. This work may pave the way of MoS₂ in practical industry-scale applications.

Nature, Published online: 01 May 2024; doi:10.1038/s41586-024-07275-6

Using a cryogenic 300-mm wafer prober, a new approach for the testing of hundreds of industry-manufactured spin qubit devices at 1.6 K provides high-volume data on performance, allowing optimization of the complementary metal–oxide–semiconductor (CMOS)-compatible fabrication process.

Nature, Published online: 01 May 2024; doi:10.1038/s41586-024-07286-3

Centimetre-sized single-crystal rhombohedral boron nitride layers are achieved through bevel-edge epitaxy, and the resulting material exhibits robust, homogeneous and switchable ferroelectricity with a high Curie temperature.

Nature, Published online: 01 May 2024; doi:10.1038/d41586-024-01208-z

By adapting methods for fabricating and testing conventional computer chips, researchers have brought silicon-based quantum computers closer to reality — and to accessing the immense benefits of a mature chipmaking industry.

This article represents the first report on the low-temperature growth of ruby (chromium-doped Al₂O₃) crystals at 750 °C, which is one-third of the conventional required temperature (2050 °C). Supersaturation based on the de-composition of crystal–solvent intermediates enables crystal growth at low temperatures.

Abstract

Crystal growth methods that do not require high temperatures are highly needed for the facile growth of oxide single crystals with melting points of several thousand degrees Celsius. This paper represents the first report of a method for the low-temperature growth of ruby crystals (chromium-doped Al₂O₃) at 750 °C, which is one-third of the conventionally required temperature (2050 °C). In solution-based crystal growth, the target crystal is grown at a temperature considerably lower than its melting point. However, conventional crystal growth processes involving solvent evaporation and cooling require high temperatures to completely liquefy the material, with previously reported solution growth temperatures of ≈1100 °C. Supersaturation based on the decomposition of crystal–solvent intermediates eliminates the need to completely liquefy the material, enabling low-temperature crystal growth. The combination of computational and experimental investigations helps determine the optimum conditions for low-temperature crystal growth. The proposed method is a novel green process that breaks the conventional frontiers of crystal growth while ensuring eco-friendliness and low energy consumption. In addition, its scope can potentially be expanded to the synthesis of various crystals and direct growth on substrates with low melting points.

Freestanding palladium nanosheets with an unprecedented penta-twinned structure that can synergistically integrate the promotional effects of 2D structure and twin boundary on catalysis are realized through the precise control of reduction kinetics of metal precursors and structure-directing agents.

Abstract

Control over the morphology of nanomaterials to have a 2D structure and manipulating the surface strain of nanostructures through defect control have proved to be promising for developing efficient catalysts for sustainable chemical and energy conversion. Here a one-pot aqueous synthesis route of freestanding Pd nanosheets with a penta-twinned structure (Pd_PT NSs) is presented. The generation of the penta-twinned nanosheet structure can be succeeded by directing the anisotropic growth of Pd under the controlled reduction kinetics of Pd precursors. Experimental and computational investigations showed that the surface atoms of the Pd_PT NSs are effectively under a compressive environment due to the strain imposed by their twin boundary defects. Due to the twin boundary-induced surface strain as well as the 2D structure of the Pd_PT NSs, they exhibited highly enhanced electrocatalytic activity for oxygen reduction reaction compared to Pd nanosheets without a twin boundary, 3D Pd nanocrystals, and commercial Pd/C and Pt/C catalysts.

Learning from the principle of the absolute encoder in conventional automation systems, a novel photochromic rotary encoder based on NaNbO₃-based photochromic materials is designed that exhibited the following significant advantages over conventional encoders: sustainable utilization, higher-level data encryption, and unlimited storage capability.

Abstract

Photochromic materials are widely used in optical data storage, data encryption, and anti-counterfeiting because of their ability to be written, erased, and read repeatedly. However, conventional information encryption and storage capabilities based on a “matrix” pattern in a 2D plane are limited to fewer storage dimensions, making information less secure. Here, a new concept of expanding the storage dimensions is presented by manipulating the rotation angle in 2D photochromic encoding disks based on the principle of the absolute encoder. For this purpose, a novel photochromic material, NaNbO₃:xEu³⁺ is developed, with highly efficient red emission, a large luminescence switching contrast, and antithermal quenching behavior. The designed photochromic rotary encoder made with the NaNbO₃-based material exhibits higher-level data encryption than that of conventional encoders and unlimited storage capability. This study presents exciting opportunities for information storage and encryption using luminescent photochromic materials.

Vertical 2D heterostructures of tungsten disulfide (WS₂) and few-layer graphene (FLG) are studied by 4D scanning nanobeam diffraction (4D-SNBD). Grain boundaries (GBs) in the underlying 2D layer are shown to be essential for moiré formation in bottom-up metal–organic chemical vapor deposition (MOCVD) synthesis. Controlling the angle of the GB can allow twist-angle-controlled growth of moiré structures.

Abstract

2D materials have received a lot of interest over the past decade. Especially van der Waals (vdW) 2D materials, such as transition metal dichalcogenides (TMDCs), and their heterostructures exhibit semiconducting properties that make them highly suitable for novel device applications. Controllable mixing and matching of the 2D materials with different properties and precise control of the in-plane twist angle in these heterostructures are essential to achieve superior properties and need to be established through large-scale device fabrication. To gain fundamental insight into the potential control of these twist angles, 2D heterostructures of tungsten disulfide (WS₂) and graphene (Gr) grown by bottom-up synthesis via metal-organic chemical vapor deposition (MOCVD) are investigated using a scanning transmission electron microscope (STEM). Specifically, the combination of conventional high-resolution imaging with scanning nanobeam diffraction (SNBD) using advanced 4D STEM techniques is used to analyze moiré structures. The latter technique is used to reveal the epitaxial alignment within the WS₂/Gr heterostructure, showing a direct influence of the underlying Gr layers on the moiré structure in the subsequent WS₂ layers. In particular, the importance of grain boundaries (GBs) within the underlying WS₂ and Gr layers on the structure of moiré patterns with rotation angles below 2° is discussed.

Wet chemical strategies make it difficult to suppress capillary contraction and achieve orderly assembly of isotropic sheets. Cheng, Baughman, and co-workers have now used a nanoconfined water-induced alignment strategy to avoid capillary contraction, and obtained ultrastrong isotropic graphene sheets with a tensile strength of 1.87 GPa and porosity of 3.87 %.

Abstract

Highly ordered assembly of two-dimensional (2D) nanoplatelets plays a key role in enhancing the mechanical properties of layered nanocomposites. Layer-by-layer (LbL) assembly, vacuum-assisted filtration, and blade coating have been used to fabricate layered nanocomposites. However, the intrinsic wrinkles of 2D nanoplatelets and defects derived from assembling approaches make it difficult to align 2D nanoplatelets. Recently, the team of Prof. Qunfeng Cheng at Beihang University and their collaborator, Prof. Ray H. Baughman at the University of Texas at Dallas developed a novel approach for aligning graphene and Ti₃C₂T _x MXene nanoplatelets by nanoconfined assembly through continuous vacuum-assisted filtration. The resultant MXene-bridged sheet has ultrastrong mechanical properties and low porosity, providing a new concept for assembling 2D nanoplatelets into aligned and compact high-performance layered nanocomposites.