zemin zheng

The 2D transition metal dichalcogenide alloy, Mo_{1− x} W _x S₂, is synthesized to investigate the influence of configurational entropy on the piezoelectrical property. Mo_0.46W_0.54S₂, in which two cations have similar concentrations and the maximum configurational entropy is attained, exhibits the best piezoelectric properties. Combined with excellent mechanical durability, a mechanical sensor based on the Mo_0.46W_0.54S₂ alloy is demonstrated for real-time health monitoring.

Abstract

Piezoelectricity in 2D transition metal dichalcogenides (TMDs) has attracted considerable interest because of their excellent flexibility and high piezoelectric coefficient compared to conventional piezoelectric bulk materials. However, the ability to regulate the piezoelectric properties is limited because the entropy is constant for certain binary TMDs other than multielement ones. Herein, in order to increase the entropy, a ternary TMDs alloy, Mo_{1− x} W _x S₂, with different W concentrations, is synthesized. The W concentration in the Mo_{1− x} W _x S₂ alloy can be controlled precisely in the low-supersaturation synthesis and the entropy can be tuned accordingly. The Mo_0.46W_0.54S₂ alloy (x = 0.54) has the highest configurational entropy and best piezoelectric properties, such as a piezoelectric coefficient of 4.22 pm V⁻¹ and a piezoelectric output current of 150 pA at 0.24% strain. More importantly, it can be combined into a larger package to increase the output current to 600 pA to cater to self-powered applications. Combining with excellent mechanical durability, a mechanical sensor based on the Mo_0.46W_0.54S₂ alloy is demonstrated for real-time health monitoring.

A giant magnetoresistance is induced by self-introducing interstitial Cr atoms in the van der Waals gaps of CrTe₂ layers. A large negative magnetoresistance (10%) with a plateau-like feature is revealed, resulting from the antiferromagnetic interlayer coupling between the adjacent CrTe₂ layers below the Néel temperature. These findings offer a new horizon in engineering functional structures for 2D magnet-based spintronics.

Abstract

One of the most promising avenues in 2D materials research is the synthesis of antiferromagnets employing 2D van der Waals (vdW) magnets. However, it has proven challenging, due in part to the complicated fabrication process and undesired adsorbates as well as the significantly deteriorated ferromagnetism at atomic layers. Here, the engineering of the antiferromagnetic (AFM) interlayer exchange coupling between atomically thin yet ferromagnetic CrTe₂ layers in an ultra-high vacuum-free 2D magnetic crystal, Cr₅Te₈ is reported. By self-introducing interstitial Cr atoms in the vdW gaps, the emergent AFM ordering and the resultant giant magnetoresistance effect are induced. A large negative magnetoresistance (10%) with a plateau-like feature is revealed, which is consistent with the AFM interlayer coupling between the adjacent CrTe₂ main layers in a temperature window of 30 K below the Néel temperature. Notably, the AFM state has a relatively weak interlayer exchange coupling, allowing a switching between the interlayer AFM and ferromagnetic states at moderate magnetic fields. This work represents a new route to engineering low-power devices that underpin the emerging spintronic technologies, and an ideal laboratory to study 2D magnetism.

Recent advances in emerging 2D-material-based integrated memory devices are reviewed in terms of working principles, device architectures, array integration, and specific brain-inspired applications. Future challenges and promising research lines toward reliable, practical neuromorphic computing chips are highlighted.

Abstract

With the advent of the Internet of Things and big data, massive data must be rapidly processed and stored within a short timeframe. This imposes stringent requirements on memory hardware implementation in terms of operation speed, energy consumption, and integration density. To fulfill these demands, 2D materials, which are excellent electronic building blocks, provide numerous possibilities for developing advanced memory device arrays with high performance, smart computing architectures, and desirable downscaling. Over the past few years, 2D-material-based memory-device arrays with different working mechanisms, including defects, filaments, charges, ferroelectricity, and spins, have been increasingly developed. These arrays can be used to implement brain-inspired computing or sensing with extraordinary performance, architectures, and functionalities. Here, recent research into integrated, state-of-the-art memory devices made from 2D materials, as well as their implications for brain-inspired computing are surveyed. The existing challenges at the array level are discussed, and the scope for future research is presented.

Graphene, owing to its excellent physical properties, has attracted widespread attention in the electron microscopy (EM) field to achieve high-resolution multi-dimensional imaging. In this review, the focus is on the synthesis of high-quality graphene, various approaches to producing suspended graphene membranes, and killer applications in multi-dimensional EM characterization. Prospects of graphene membranes for more cutting-edge applications are also proposed.

Abstract

Technological breakthrough in electron microscopy (EM) has started a resolution revolution in EM imaging. Nowadays, the promotion of resolution demands a robust background-noise-free EM support for specimen preparation, which is a primary bottleneck of high-resolution EM imaging. Owing to the atomic thickness and excellent physical properties, graphene has attracted widespread attention in the EM field to achieve high-resolution multi-dimensional imaging. However, it is still challenging to prepare high-quality suspended graphene membranes. Problems like breakage, contamination and wrinkling reduce the quality of suspended graphene membranes, which inhibit their wide and killer applications in EM imaging. In this review, suspended graphene membranes are looked deeply into for multi-dimensional EM imaging. This study begins with a brief introduction to EM development, followed by a discussion of the synthesis of high-quality graphene. Then it summarizes various approaches to produce suspended graphene membranes and their killer applications in multi-dimensional EM characterization, including high-resolution 2D imaging, cryo-EM 3D reconstruction, and 4D in situ liquid EM. Based on current achievements, the prospects of graphene membranes for more cutting-edge applications are finally proposed.

A new family of 2D transition metal carbo-chalcogenides (TMCCs) can be considered an atomic-level combination of TM carbides (MXenes) and TM dichalcogenides (TMDCs). Realized by electrochemical-assisted delamination, Nb₂S₂C and Ta₂S₂C are the first examples of TMCCs to be delaminated into single layers. These newly developed 2D TMCCs are 50% stronger than TMDC counterparts and have a wide range of applications.

Abstract

Here, a new family of 2D transition metal carbo-chalcogenides (TMCCs) is reported, which can be considered a combination of two well-known families, TM carbides (MXenes) and TM dichalcogenides (TMDCs), at the atomic level. Single sheets are successfully obtained from multilayered Nb₂S₂C and Ta₂S₂C using electrochemical lithiation followed by sonication in water. The parent multilayered TMCCs are synthesized using a simple, scalable solid-state synthesis followed by a topochemical reaction. Superconductivity transition is observed at 7.55 K for Nb₂S₂C. The delaminated Nb₂S₂C outperforms both multilayered Nb₂S₂C and delaminated NbS₂ as an electrode material for Li-ion batteries. Ab initio calculations predict the elastic constant of TMCC to be over 50% higher than that of TMDC.

Recent research progress on the use of liquid-phase exfoliation beyond layered van der Waals (vdW) crystals is summarized. Questions remain unanswered on how these 3D strongly bonded nonlayered crystals exfoliate into nanoplatelets, and on the insight of the exfoliation mechanism. The answer to the questions helps in revealing the future direction of this newly grown sub-family of 2D-class of materials.

Abstract

For nearly 15 years, researchers have been using liquid-phase exfoliation (LPE) to produce 2D nanosheets from layered crystals. This has yielded multiple 2D materials in a solution-processable form whose utility has been demonstrated in multiple applications. It was believed that the exfoliation of such materials is enabled by the very large bonding anisotropy of layered materials where the strength of intralayer chemical bonds is very much larger than that of interlayer van der Waals bonds. However, over the last five years, a number of papers have raised questions about our understanding of exfoliation by describing the LPE of nonlayered materials. These results are extremely surprising because, as no van der Waals gap is present to provide an easily cleaved direction, the exfoliation of such compounds requires the breaking of only chemical bonds. Here the progress in this unexpected new research area is examined. The structure and properties of nanoplatelets produced by LPE of nonlayered materials are reviewed. A number of unexplained trends are found, not least the preponderance of isotropic materials that have been exfoliated to give high-aspect-ratio nanoplatelets. Finally, the applications potential of this new class of 2D materials are considered.

Nanopatterning bridges the microstructure of 2D materials and integrated chip devices, essentially enabling and prompting their successful application in industry. A critical summary on the recent development of key nanopatterning technologies of 2D materials, with the aim of realizing large-scale device integration, is provided. This contribution offers a pioneering reference and guidelines to promote 2D materials from laboratory research to practical use.

Abstract

With the reduction of feature size and increase of integration density, traditional 3D semiconductors are unable to meet the future requirements of chip integration. The current semiconductor fabrication technologies are approaching their physical limits based on Moore's law. 2D materials such as graphene, transitional metal dichalcogenides, etc., are of great promise for future memory, logic, and photonic devices due to their unique and excellent properties. To prompt 2D materials and devices from the laboratory research stage to the industrial integrated circuit-level, it is necessary to develop advanced nanopatterning methods to obtain high-quality, wafer-scale, and patterned 2D products. Herein, the recent development of nanopatterning technologies, particularly toward realizing large-scale practical application of 2D materials is reviewed. Based on the technological progress, the unique requirement and advances of the 2D integration process for logic, memory, and optoelectronic devices are further summarized. Finally, the opportunities and challenges of nanopatterning technologies of 2D materials for future integrated chip devices are prospected.

CrSBr is a layered material possessing intralayer ferromagnetic and interlayer antiferromagnetic (AFM) ordering at low temperatures (T < 132 K). At higher temperatures or in the presence of modest magnetic fields, AFM correlations are suppressed, yielding a magnetic phase with a comparatively high sheet susceptibility (χ^2D). Magnetic force microscopy is sensitive to χ^2D and identifies local nanoscale magnetic ordering.

Abstract

2D materials can host long-range magnetic order in the presence of underlying magnetic anisotropy. The ability to realize the full potential of 2D magnets necessitates systematic investigation of the role of individual atomic layers and nanoscale inhomogeneity (i.e., strain) on the emergence of stable magnetic phases. Here, spatially dependent magnetism in few-layer CrSBr is revealed using magnetic force microscopy (MFM) and Monte Carlo-based simulations. Nanoscale visualization of the magnetic sheet susceptibility is extracted from MFM data and force–distance curves, revealing a characteristic onset of both intra- and interlayer magnetic correlations as a function of temperature and layer-thickness. These results demonstrate that the presence of a single uncompensated layer in odd-layer terraces significantly reduces the stability of the low-temperature antiferromagnetic (AFM) phase and gives rise to multiple coexisting magnetic ground states at temperatures close to the bulk Néel temperature (T _N). Furthermore, the AFM phase can be reliably suppressed using modest fields (≈16 mT) from the MFM probe, behaving as a nanoscale magnetic switch. This prototypical study of few-layer CrSBr demonstrates the critical role of layer parity on field-tunable 2D magnetism and validates MFM for use in nanomagnetometry of 2D materials (despite the ubiquitous absence of bulk zero-field magnetism in magnetized sheets).

2D MBenes, early transition metal borides, have recently gained tremendous attention due to their unique properties. This Perspective sheds light on the material–structure–property relationship of MBenes and elucidates the most prospective applications in various fields, thus opening a promising paradigm for designing new functional systems and high-performance devices with multipurpose functionalities.

Abstract

2D MBenes, early transition metal borides, are a very recent derivative of ternary or quaternary transition metal boride (MAB) phases and represent a new member in the flatland. Although holding great potential toward various applications, mainly theoretical knowledge about their potential properties is available. Theoretical calculations and preliminary experimental attempts demonstrate their rich chemistry, excellent reactivity, mechanical strength/stability, electrical conductivity, transition properties, and energy harvesting possibility. Compared to MXenes, MBenes’ structure appears to be more complex due to multiple crystallographic arrangements, polymorphism, and structural transformations. This makes their synthesis and subsequent delamination into single flakes challenging. Overcoming this bottleneck will enable a rational control over MBenes’ material–structure–property relationship. Innovations in MBenes’ postprocessing approaches will allow for the design of new functional systems and devices with multipurpose functionalities thus opening a promising paradigm for the conscious design of high-performance 2D materials.

2D TaSe₂-MoSe₂ metal–semiconductor heterostructures are successfully achieved usin an edge-induced epitaxial growth mode. The unique contact potential and strong current rectification behavior will facilitate the development high-performance transition metal dichalcogenide-based electronic devices.

Abstract

Van der Waals (vdWs) heterostructures based on 2D metals and semiconductors have attracted considerable attention due to their excellent properties and great application potential in next-generation electronic and optoelectronic devices. To obtain such vdWs heterostructures, the conventional approach with artificial exfoliation and stacking of 2D metals onto 2D semiconductors in the vertical direction is still far from satisfactory, because of the low yield and impurity-involved transfer process. Here, two-step vapor deposition growth of 2D TaSe₂-MoSe₂ metal–semiconductor heterostructures is reported. Raman maps confirm the precise spatial modulation of the as-grown 2D TaSe₂-MoSe₂ heterostructures. Structural analysis reveals that the upper 1T-TaSe₂ is formed heteroepitaxially on/around the presynthesized 2H-MoSe₂ monolayers with an epitaxial relationship of (10-10)_TaSe2//(10-10)_MoSe2 and [0001]_TaSe2//[0001]_MoSe2. Based on the detailed characterizations of morphology, structure, and composition, an edge-induced growth mechanism is proposed to illustrate the formation process of the 2D heterostructures, confirmed by first-principle calculations. In addition, Kelvin probe force microscope characterizations and electrical transport measurements confirm that the 2D metal–semiconductor heterostructures exhibit typical rectification characteristics with a contact potential height of ≈431 mV. The direct growth of high-quality 2D metal–semiconductor heterostructures marks an important step toward high-performance integrated optoelectronic devices.

Nature, Published online: 20 April 2022; doi:10.1038/s41586-022-04472-z

A study reveals light as a new dynamic knob to control ferromagnetic order in moiré superlattices.

Nature Nanotechnology, Published online: 02 May 2022; doi:10.1038/s41565-022-01115-2

A ferromagnetic tunnelling contact enables electrically controlled valley polarization in monolayer WSe2.

Nature Communications, Published online: 22 April 2022; doi:10.1038/s41467-022-29929-7

While material defect sites are active for chemical reactions, it is important to understand how different defect types impact reactivity. Here, authors prepare Frenkel-defected MoS2 monolayers and demonstrate improved performances for H2 evolution electrocatalysis than pristine or doped MoS2.