Jiuxiang Dai

The concept of graphenization – stabilization of layered materials at the monolayer limit – is employed to design a new 2D honeycomb magnet, GdAlSi. The monolayer films of layered GdAlSi synthesized on Si manifest an easy-plane ferromagnetic order controlled by low magnetic fields – a fingerprint of 2D ferromagnetism. Remarkably, the emerging ferromagnetism exhibits a non-monotonic evolution with the number of monolayers.

Abstract

2D magnets are expected to give new insights into the fundamentals of magnetism, host novel quantum phases, and foster development of ultra-compact spintronics. However, the scarcity of 2D magnets often makes a bottleneck in the research efforts, prompting the search for new magnetic systems and synthetic routes. Here, an unconventional approach is adopted to the problem, graphenization – stabilization of layered honeycomb materials in the 2D limit. Tetragonal GdAlSi, stable in the bulk, in ultrathin films gives way to its layered counterpart – graphene-like anionic AlSi layers coupled to Gd cations. A series of inch-scale films of layered GdAlSi on silicon is synthesized, down to a single monolayer, by molecular beam epitaxy. Graphenization induces an easy-plane ferromagnetic order in GdAlSi. The magnetism is controlled by low magnetic fields, revealing its 2D nature. Remarkably, it exhibits a non-monotonic evolution with the number of monolayers. The results provide a fresh platform for research on 2D magnets by design.

Abstract

Recently, the coexistence of topology and superconductivity has garnered considerable attention. Specifically, the dimensionality of these materials is crucial for the realization of topological quantum computation. However, the naturally grown materials, especially with one-dimensional feature that exhibits the coexistence of topology and superconductivity, still face challenges in terms of experimental realization and scalability, which hinders the fundamental research development and the potential to revolutionize quantum computing. Here, we report the first experimental synthesis of quasi-one-dimensional InNbS₂ nanoribbons that exhibit the coexistence of topological order and superconductivity via a chemical vapor transport method. Especially, the in-plane upper critical field of InNbS₂ nanoribbons exceeds the Pauli paramagnetic limit by more than 2.2 times, which can be attributed to the enhanced spin-orbit coupling and the weakened interlayer interaction between the NbS₂ layers induced by the insertion of In atoms, making InNbS₂ exhibit spin-momentum locking similar to that of monolayer NbS₂. Moreover, for the first time, we report the superconducting diode effect in a quasi-one-dimensional superconductor system without any inherent geometric imperfections. The measured maximum efficiency is manifested as 14%, observed at μ₀H ≈ ±60 mT, and we propose that the superconducting diode effect can potentially be attributed to the presence of the nontrivial topological band. Our work provides a platform for studying exotic phenomena in condensed matter physics and potential applications in quantum computing and quantum information processing.

Abstract

Metal halide perovskites (MHPs) have emerged as highly promising candidates for the next generation of photonics and optoelectronic devices, owing to their prominent optical and excitonic properties, as well as the convenience of fabrication. Particularly, ultrathin two-dimensional (2D) MHPs, which are generally prepared by exfoliation, solution growth, and chemical vapor deposition method, have attracted dramatically increasing attentions owing to their combined features of ultrathin 2D morphology and superior performance of MHPs. Despite the growing interest in ultrathin 2D MHPs, there is currently a lack of a comprehensive and systematic overview of the distinct advantages offered by each growth method for producing these materials. This review critically assesses the preliminary studies on the materials design and preparation of ultrathin MHPs. Furthermore, it explores heterostructures based on ultrathin MHPs and offers insights into the challenges and opportunities that lie ahead for this enticing class of 2D materials.

Abstract

Owing to the atomically thin nature, two-dimensional (2D) oxide materials have been widely reported to exhibit exciting transport and dielectric properties, such as fine gate controllability and ultrahigh carrier mobility, that outperform their bulk counterpart. However, unlike the successful synthesis of bulk oxide single crystals, reliable methods for synthesizing large-area single crystal of 2D oxide, that would suppress the negative influence from defective grain boundaries, remain unavailable, especially for nonlayered oxide. Herein, we report that the lattice symmetry between the substrate and cerium dioxide (CeO₂) would allow for the aligned nucleation and epitaxial growth of CeO₂ on sapphire substrates, enabling the wafer-sized growth of CeO₂ single crystal. The careful tuning of the growth temperature and oxygen flow rate contributed to the harvesting of CeO₂ wafer with reduced thickness and enhanced growth rates. The removal of grain boundaries improved the dielectric performance in terms of high dielectric strength (E_bd ≈ 8.8 MV·cm⁻¹), suppressed leakage current, along with high dielectric constants (ε_r ≈ 24). Our work demonstrates that with fine dielectric performance and ease of synthesizing wafer-sized single crystals, CeO₂ can function as potential candidate as gate insulator for 2D-materials based nanoelectronics, and we believe the reported protocol of aligned nucleation can be extended to other 2D oxides.

Nature Reviews Materials, Published online: 08 July 2024; doi:10.1038/s41578-024-00698-7

Thanks to improved control of device fabrication and an expanding characterization toolbox, moiré materials stay in the spotlight as we discover more about the unique phenomena they realize.

A surface-permeation driven lithium intercalation pathway was proposed to finely control the fabrication of phase engineered edge contact to 2D vdW heterostructure compromising MoS₂/hBN/graphene, enabling the development of an ultrafast and multi-bit memory device, offering promising potential as energy-efficient artificial synapses for neuromorphic computing applications.

Abstract

Local phase transition in transition metal dichalcogenides (TMDCs) by lithium intercalation enables the fabrication of high-quality contact interfaces in two-dimensional (2D) electronic devices. However, controlling the intercalation of lithium is hitherto challenging in vertically stacked van der Waals heterostructures (vdWHs) due to the random diffusion of lithium ions in the hetero-interface, which hinders their application for contact engineering of 2D vdWHs devices. Herein, a strategy to restrict the lithium intercalation pathway in vdWHs is developed by using surface-permeation assisted intercalation while sealing all edges, based on which a high-performance edge-contact MoS₂ vdWHs floating-gate transistor is demonstrated. Our method avoids intercalation from edges that are prone to be random but intentionally promotes lithium intercalation from the top surface. The derived MoS₂ floating-gate transistor exhibits improved interface quality and significantly reduced subthreshold swing (SS) from >600 to 100 mV dec^–1. In addition, ultrafast program/erase performance together with well-distinguished 32 memory states are demonstrated, making it a promising candidate for low-power artificial synapses. The study on controlling the lithium intercalation pathways in 2D vdWHs offers a viable route toward high-performance 2D electronics for memory and neuromorphic computing purposes.

Light: Science & Applications, Published online: 29 June 2024; doi:10.1038/s41377-024-01486-2

Emerging probing perspective of two-dimensional materials physics: terahertz emission spectroscopy

Molybdenum carbide thin films are deposited by ALD with MoCl₅ and 1,4-bis(trimethylgermyl)-1,4-dihydropyrazine [(Me₃Ge)₂DHP] at 200–300 °C. The films are very smooth and superconductive up to 4.4 K, which makes them good candidates for quantum computing applications. Crystalline phase analysis includes also calculating Raman modes for various molybdenum carbides and nitrides.

Abstract

The development of deposition processes for metal carbide thin films is rapidly advancing, driven by their potential for applications including catalysis, batteries, and semiconductor devices. Within this landscape, atomic layer deposition (ALD) offers exceptional conformality, uniformity, and thickness control on spatially complex structures. This paper presents a comprehensive study on the thermal ALD of MoC_x with MoCl₅ and 1,4-bis(trimethylgermyl)-1,4-dihydropyrazine [(Me₃Ge)₂DHP] as precursors, focusing on the functional properties and characterization of the films. The depositions are conducted at 200–300 °C and very smooth films with RMS Rq ≈0.3–0.6 nm on Si, TiN, and HfO₂ substrates are obtained. The process has a high growth rate of 1.5 Å cycle⁻¹ and the films appear to be continuous already after 5 cycles. The films are conductive even at thicknesses below 5 nm, and films above 18 nm exhibit superconductivity up to 4.4 K. In lieu of suitable references, Raman modes for molybdenum carbides and nitrides are calculated and X-ray diffraction and X-ray photoelectron spectroscopy are used for phase analysis.

Nature Communications, Published online: 02 July 2024; doi:10.1038/s41467-024-49783-z

C–H activation in long-chain organic molecules remains largely unexplored. Here, the authors report light-driven C–H activation mediated by 2D TMDCs and the resultant synthesis of luminescent carbon dots.

Nature Nanotechnology, Published online: 01 July 2024; doi:10.1038/s41565-024-01676-4

Here the authors present a pH-sensitive DNA origami nanoswitch that hides ligands for death receptors and displays them as a cytotoxic hexagonal pattern in acidic tumour microenvironments. This reduces tumour growth in a murine model of breast cancer with minimal on-target, off-tumour toxicity.

Nature Nanotechnology, Published online: 01 July 2024; doi:10.1038/s41565-024-01695-1

This Review explores adopting 2D semiconductors to overcome the scaling bottleneck of Si-based electronics. Recent trends and potential approaches for the development of 2D materials as a channel are discussed. Following this, the prerequisites, obstacles and feasible technologies for integrating contacts and gate dielectrics with 2D semiconductor-based channels are examined. The Review also provides an industrial perspective towards facilitating monolithic 3D integration.

Nature Nanotechnology, Published online: 02 July 2024; doi:10.1038/s41565-024-01717-y

A highly tunable Nernst effect has been demonstrated in graphene/indium selenide devices, achieving a record Nernst coefficient at ultralow temperatures, highlighting its potential for quantum technologies and low-temperature applications.

Proceedings of the National Academy of Sciences, Volume 121, Issue 28, July 2024.

A novel microwave-assisted intercalation (MAI) strategy is proposed for efficient intercalation of layered MXene to prepare large-sized single-layer MXene. After intercalation of multi-layer Ti₃C₂T _x (M-T) with Li₃AlF₆ as an intercalator and ethylene glycol as a solvent under microwave irradiation for 5 min, high-quality large-sized single-layer Ti₃C₂T _x nanosheets (S-T) are achieved finally by a series simple treatments.

Abstract

A novel microwave-assisted intercalation (MAI) strategy is proposed for fast and efficient intercalation of layered MXene to prepare large-size single-layer MXene. After LiF-HCl etching of Ti₃AlC₂, the as-prepared multi-layer Ti₃C₂T _x (M-T) are intercalated with Li₃AlF₆ as an intercalator and ethylene glycol (EG) as a solvent under microwave irradiation for 5 min. Furthermore, the dispersed high-quality large-sized single-layer Ti₃C₂T _x (S-T) nanosheets with a thickness of 1.66 nm and a large lateral size over 20 µm are achieved with a yield of over 60% after a further ultrasonic delamination followed by electrostatic precipitation, acid washing, and calcination. In addition, Pd/S-T composite catalyst, which is constructed with Pd nanoparticles supported on the as-prepared S-T nanosheets, exhibits an excellent performance for rapid and efficient selective hydrogenation of nitroarenes with H₂ under a mild condition. At room temperature, full conversion of nitrobenzene and 100% aniline selectivity are achieved over Pd/S-T catalyst in 20 min with 0.5 MPa of H₂ pressure. This work provides a novel method for facile, fast, and large-scale preparation of single-layer MXene and develops a new approach for constructing efficient nanocatalytic systems.

By subjecting 2D cubic In₂O₃ crystals to significant strain gradients using an atomic force microscope (AFM) tip, the crystal symmetry is broken, resulting in the separation of positive and negative charge centers. Generated polarization voltage is successfully achieved 20 nN sensitivity of nano-stress sensing. Also, the flexoelectric coefficient is calculated.

Abstract

The phenomenon of flexoelectricity, wherein mechanical deformation induces alterations in the electron configuration of metal oxides, has emerged as a promising avenue for regulating electron transport. Leveraging this mechanism, stress sensing can be optimized through precise modulation of electron transport. In this study, the electron transport in 2D ultra-smooth In₂O₃ crystals is modulated via flexoelectricity. By subjecting cubic In₂O₃ (c-In₂O₃) crystals to significant strain gradients using an atomic force microscope (AFM) tip, the crystal symmetry is broken, resulting in the separation of positive and negative charge centers. Upon applying nano-scale stress up to 100 nN, the output voltage and power values reach their maximum, e.g. 2.2 mV and 0.2 pW, respectively. The flexoelectric coefficient and flexocoupling coefficient of c-In₂O₃ are determined as ≈0.49 nC m⁻¹ and 0.4 V, respectively. More importantly, the sensitivity of the nano-stress sensor upon c-In₂O₃ flexoelectric effect reaches 20 nN, which is four to six orders smaller than that fabricated with other low dimensional materials based on the piezoresistive, capacitive, and piezoelectric effect. Such a deformation-induced polarization modulates the band structure of c-In₂O₃, significantly reducing the Schottky barrier height (SBH), thereby regulating its electron transport. This finding highlights the potential of flexoelectricity in enabling high-performance nano-stress sensing through precise control of electron transport.

This review describes recent (mostly last five years) progress in the synthesis, structural characterization, and properties of nanotubes from layered transition metal dichalcogenides, including in-silico investigations. In the picture, ultra long multiwall nanotubes (up to 0.5 mm) nanotubes reported in Ref. 27 are shown via SEM (bottom) TEM (right) and electron diffraction (top).

Abstract

Inorganic layered compounds (2D-materials), particularly transition metal dichalcogenide (TMDC), are the focus of intensive research in recent years. Shortly after the discovery of carbon nanotubes (CNTs) in 1991, it was hypothesized that nanostructures of 2D-materials can also fold and seam forming, thereby nanotubes (NTs). Indeed, nanotubes (and fullerene-like nanoparticles) of WS₂ and subsequently from MoS₂ were reported shortly after CNT. However, TMDC nanotubes received much less attention than CNT until recently, likely because they cannot be easily produced as single wall nanotubes with well-defined chiral angles. Nonetheless, NTs from inorganic layered compounds have become a fertile field of research in recent years. Much progress has been achieved in the high-temperature synthesis of TMDC nanotubes of different kinds, as well as their characterization and the study of their properties and potential applications. Their multiwall structure is found to be a blessing rather than a curse, leading to intriguing observations. This concise minireview is dedicated to the recent progress in the research of TMDC nanotubes. After reviewing the progress in their synthesis and structural characterization, their contributions to the research fields of energy conversion and storage, polymer nanocomposites, andunique optoelectronic devices are being reviewed. These studies suggest numerous potential applications for TMDC nanotubes in various technologies, which are briefly discussed.

Nature Nanotechnology, Published online: 03 July 2024; doi:10.1038/s41565-024-01706-1

Mirror twin boundaries in monolayer MoS2—line defects with reflection-mirroring symmetry—are one-dimensionally metallic. In this work, the authors fabricate these mirror twin boundary networks by epitaxity and incorporate them into ultrascaled 2D transistor circuits as gate electrodes.

Publication date: August 2024

Source: Materials Today, Volume 77

Author(s): Iqra Shahbaz, Muhammad Tahir, Lihong Li, Yanlin Song

Abstract

Hot spot engineering in plasmonic nanostructures plays a significant role in surface-enhanced Raman scattering (SERS) for bioanalysis and cell imaging. However, creating stable, reproducible, and strong SERS signals remains challenging due to the potential interference from surrounding chemicals and locating SERS-active analytes into hot-spot regions. Herein, we developed a straightforward approach to synthesize intra-gap nanoparticles encapsulating 4-nitrobenzenethiol (4-NBT) as a reporter molecule within these gaps to avoid outside interference. We made three kinds of intra-gap nanoparticles using nanorods, bipyramids, and nanospheres as cores, in which the nanorods based intra-gap nanoparticles exhibit the highest SERS activity. The advantage of our method is the ease of preparation of high-yield and stable intra-gap nanoparticles characterized by a short incubation time (10 min) with 4-NBT and quick synthesis without requiring an additional step to centrifuge for the purification of core nanoparticles. The intense localized field in the synthesized hot spots of these plasmonic gap nanostructures holds great promise as a SERS substrate for a broad range of quantitative optical applications.

A novel spin-coating method creates light-driven microrobots (LMNRs) using bulk heterojunction organic semiconductor solar cells (OSSC). This technique uniformly coats various structures with a near-infrared (NIR)-responsive OSSC, enabling microrobots to move under NIR light. Applications include microplastic removal, cargo transport, and precise, light-guided motion.

Abstract

Emerging light-driven micro/nanorobots (LMNRs) showcase profound potential for sophisticated manipulation and various applications. However, the realization of a versatile and straightforward fabrication technique remains a challenging pursuit. This study introduces an innovative bulk heterojunction organic semiconductor solar cell (OSC)-based spin-coating approach, aiming to facilitate the arbitrary construction of LMNRs. Leveraging the distinctive properties of a near-infrared (NIR)-responsive organic semiconductor heterojunction solution, this technique enables uniform coating across various dimensional structures (0D, 1D, 2D, 3D) to be LMNRs, denoted as “motorization.” The film, with a slender profile measuring ≈140 nm in thickness, effectively preserves the original morphology of objects while imparting actuation capabilities exceeding hundreds of times their own weight. The propelled motion of these microrobots is realized through NIR-driven photoelectrochemical reaction-induced self-diffusiophoresis, showcasing a versatile array of controllable motion profiles. The strategic customization of arbitrary microrobot construction addresses specific applications, ranging from 0D microrobots inducing living crystal formation to intricate, multidimensional structures designed for tasks such as microplastic extraction, cargo delivery, and phototactic precise maneuvers. This study advances user-friendly and versatile LMNR technologies, unlocking new possibilities for various applications, signaling a transformative era in multifunctional micro/nanorobot technologies.