High-throughput Synthesis of Solution-Processable van der Waals Heterostructures through Electrochemistry

Two-dimensional van der Waals heterostructures (2D vdWHs) have recently gained widespread attention because of their abundant and exotic properties, which open up many new possibilities for next-generation nanoelectronics. However, practical applications remain challenging due to the lack of high-throughput techniques for fabricating high-quality vdWHs. Here, we demonstrate a general electrochemical strategy to prepare solution-processable high-quality vdWHs, in which electrostatic forces drive the stacking of electrochemically exfoliated individual assemblies with intact structures and clean interfaces into vdWHs with strong interlayer interactions. Thanks to the excellent combination of strong light absorption, interfacial charge transfer, and decent charge transport properties in individual layers, thin-film photodetectors based on graphene/In₂Se₃ vdWHs exhibit great promise for near-infrared (NIR) photodetection, owing to a high responsivity (267 mA W⁻¹), fast rise (72 ms) and decay (426 ms) times under NIR illumination. This approach enables various hybrid systems, including graphene/In₂Se₃, graphene/MoS₂ and graphene/MoSe₂ vdWHs, providing a broad avenue for exploring emerging electronic, photonic, and exotic quantum phenomena.     

3D-Graphene/Boron Nitride-stacking Material: a Fundamental van der Waals Heterostructure

The 3D periodic graphene/h-BN(G/BN) heterostuctures were studied. The stacking forms between the graphene and h-BN layers were discussed. The varieties of the geometric and electronic configurations at the interface between graphene and h-BN layers were also reported. The metal-semiconductor transform of the G/BN material can be achieved by adjusting the stacking form of the h-BN layers or changing the proportion of graphene layers in the unit cell. An electrostatic potential well was found at the interface. Due to the potential well and the only dispersion correlation at the interface, the dielectric constant ε_zz in vertical direction was independent on the variety of the thickness or the proportion of the compositions in an unit cell.     

基于二维材料及其范德瓦尔斯异质结的光电探测器

近些年来,石墨烯、黑磷和过渡金属二硫化物以及其他二维材料受到了越来越多的关注。凭借其独特的结构和优异的电学、光学特性,这些二维材料在光电器件中得到了广泛应用,具有良好的发展潜力。本文概述了二维材料在光电探测器领域的最新研究进展,介绍了一些常见的二维材料及其制备方法,阐述了光电探测器件的基本原理和评价参数,以及回顾了二维材料及其异质结构在光电探测器中的应用,最后总结了该领域仍然面临的挑战并对其未来的发展方向进行了展望。     

二维材料范德华间隙的利用

Since their discovery, two-dimensional (2D) materials have attracted significant research attention owing to their excellent and controllable physical and chemical properties. These materials have emerged rapidly as important material system owing to their unique properties such as electricity, optics, quantum properties, and catalytic properties. 2D materials are mostly bonded by strong ionic or covalent bonds within the layers, and the layers are stacked together by van der Waals forces, thereby making it possible to peel off 2D materials with few or single layers. The weak interaction between the layers of 2D materials also enables the use of van der Waals gaps for regulating the electronic structure of the system and further optimizing the material properties. The introduction of guest atoms can significantly change the interlayer spacing of the original material and coupling strength between the layers. Also, interaction between the guest and host atom also has the potential to change the electronic structure of the original material, thereby affecting the material properties. For example, the electron structure of a host can be modified by interlayer guest atoms, and characteristics such as carrier concentration, optical transmittance, conductivity, and band gap can be tuned. Organic cations intercalated between the layers of 2D materials can produce stable superlattices, which have great potential for developing new electronic and optoelectronic devices. This method enables the modulation of the electrical, magnetic, and optical properties of the original materials, thereby establishing a family of 2D materials with widely adjustable electrical and optical properties. It is also possible to introduce some new properties to the 2D materials, such as magnetic properties and catalytic properties, by the intercalation of guest atoms. Interlayer storage, represented by lithium-ion batteries, is also an important application of 2D van der Waals gap utilization in energy storage, which has also attracted significant research attention. Herein, we review the studies conducted in recent years from the following aspects: (1) changing the layer spacing to change the interlayer coupling; (2) introducing the interaction between guest and host atoms to change the physico-chemical properties of raw materials; (3) introducing the guest substances to obtain new properties; and (4) interlayer energy storage. We systematically describe various interlayer optimization methods of 2D van der Waals gaps and their effects on the physical and chemical properties of synthetic materials, and suggest the direction of further development and utilization of 2D van der Waals gaps.     

Progress of transition metal chalcogenides as efficient electrocatalysts for energy conversion

The continuous excessive usage of fossil fuels has resulted in its fast depletion, leading to an escalating energy crisis as well as several environmental issues leading to increased research towards sustainable energy conversion. Electrocatalysts play crucial role in the development of numerous novel energy conversion devices, including fuel cells and solar fuel generators. In particular, high-efficiency and cost-effective catalysts are required for large-scale implementation of these new devices. Over the last few years, transition metal chalcogenides have emerged as highly efficient electrocatalysts for several electrochemical devices such as water splitting, carbon dioxide electroreduction, and, solar energy converters. These transition metal chalcogenides exhibit high electrochemical tunability, abundant active sites, and superior electrical conductivity. Hence, they have been actively explored for various electrocatalytic activities. Herein, we have provided comprehensive review of transition-metal chalcogenide electrocatalysts for hydrogen evolution, oxygen evolution, and carbon dioxide reduction and illustrated structure–property correlation that increases their catalytic activity.     

Electrochemical interfaces on chalcogenides: Some structural perspectives and synergistic effects of single-surface active sites

The use of single-atom metals (SAM) as catalysts of energy conversion reactions is a recent topic, which has gained popularity in the last two decades. Transition metal dichalcogenides emerged as important electrocatalysts since it was discovered that their chalcogenide edge sites are active towards the electrocatalytic hydrogen evolution reaction (HER) and could also serve as supports for other metals within the same applications. Currently, several groups have reported a novel metal?chalcogenide arrangement, with the possibility of isolating metals at specific sites on chalcogenides to enhance their properties resulting in a synergistic effect in which both chalcogenide and single-atom metal features are exploited, either as promoters or active sites. Theoretical studies have been the basis of these reports.     

Disorder engineering in transition metal dichalcogenides toward efficient high current density reduction electrocatalysts

In this perspective, we highlight the importance of nanoscale disorder and mesoscale morphology to enhance the activity and tune the selectivity of group VI transition metal dichalcogenide electrocatalysts toward two paramount reductions reactions as H₂ evolution reaction and CO₂ reduction. The strategy we propose takes advantage of the metastable nanoscale atomic arrangement of highly disordered and amorphous materials, to overcome the limits of the typical transition metal dichalcogenide crystalline catalysts. For the H₂ evolution reaction, going beyond the creation of point defects in crystalline structures in favor of fully amorphous organizations not only increases the per-site activity and active surface area but also improves the conductivity and the reaction kinetics. In addition, the incorporation of nanoscale disorder promotes the formation of complex products in CO₂ reduction through reaction pathways inaccessible on other sites. On the other hand, the mesoscale architecture of the catalyst controls mass transport in both the liquid and gas phase, as well as determines the real-world performance of catalysts. We suggest that by exploiting disordered nanoscale organization and controlled mesoscale features, the performances can be drastically improved to reach the state-of-art metallic electrocatalysts.     

TMDs beyond MoS2 for Electrochemical Energy Storage

Atomically thin sheets of two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted interest as high capacity electrode materials for electrochemical energy storage devices owing to their unique properties (high surface area, high strength and modulus, faster ion diffusion, and so on), which arise from their layered morphology and diversified chemistry. Nevertheless, low electronic conductivity, poor cycling stability, large structural changes during metal-ion insertion/extraction along with high cost of manufacture are challenges that require further research in order for TMDs to find use in commercial batteries and supercapacitors. Here, a systematic review of cutting-edge research focused on TMD materials beyond the widely studied molybdenum disulfide or MoS₂ electrode is reported. Accordingly, a critical overview of the recent progress concerning synthesis methods, physicochemical and electrochemical properties is given. Trends and opportunities that may contribute to state-of-the-art research are also discussed.     

Toward sustainable and effective HER electrocatalysts: Strategies for the basal plane site activation of transition metal dichalcogenides

Due to their low cost and overall sustainability, transition metal dichalcogenides (TMDCs) are potential alternatives to noble metals as catalysts to produce green hydrogen. A promising route to improve their performances consists of activating their basal plane, both increasing the number of active sites or their specific activity. This can be accomplished by exploiting point defects, in-plane boundaries and strain. In particular, single atom adsorbed or incorporated into TMDCs have shown remarkable results in electrochemical half-cell tests. Topological curvature or grain boundaries (and related defects) can also be used to further boost the performances. A crucial point for the application of such strategies is related to the development of cost effective and sustainable methods for the scale-up of synthetic protocols.     

Electrochemiluminescence bioassays based on carbon nitride nanomaterials and 2D transition metal carbides

Electrochemiluminescence (ECL) is a powerful technique for bioassays. To meet the growing demand for bioassays, it is necessary to develop new ECL emitters and co-reaction acceleration strategies to improve detection sensitivity and expand the application scope. Carbon nitride nanomaterials and 2D transition metal carbides, as newly emerging carbon-based nanomaterials, have been increasingly used for ECL bioassays due to their attractive optical and electrochemical properties as well as diversity. In this minireview, we summarized the latest advances in ECL bioassays using carbon nitride nanomaterials and 2D transition metal carbides in the past two years. Finally, we briefly discuss the future trends and challenges of carbon-based nanomaterials for ECL bioanalysis.     

Functionalization of Two‐Dimensional MoS2: On the Reaction Between MoS2 and Organic Thiols

Two‐dimensional layered transition metal dichalcogenides (TMDs) have attracted great interest owing to their unique properties and a wide array of potential applications. However, due to their inert nature, pristine TMDs are very challenging to functionalize. We demonstrate a general route to functionalize exfoliated 2H‐MoS₂ with cysteine. Critically, MoS₂ was found to be facilitating the oxidation of the thiol cysteine to the disulfide cystine during functionalization. The resulting cystine was physisorbed on MoS₂ rather than coordinated as a thiol (cysteine) filling S‐vacancies in the 2H‐MoS₂ surface, as originally conceived. These observations were found to be true for other organic thiols and indeed other TMDs. Our findings suggest that functionalization of two‐dimensional MoS₂ using organic thiols may not yield covalently or datively tethered functionalities, rather, in this instance, they yield physisorbed disulfides that are easily removed.     

Phase Instability in van der Waals In2Se3 Determined by Surface Coordination

van der Waals In₂Se₃ has attracted significant attention for its room-temperature 2D ferroelectricity/antiferroelectricity down to monolayer thickness. However, instability and potential degradation pathway in 2D In₂Se₃ have not yet been adequately addressed. Using a combination of experimental and theoretical approaches, we here unravel the phase instability in both α- and β′-In₂Se₃ originating from the relatively unstable octahedral coordination. Together with the broken bonds at the edge steps, it leads to moisture-facilitated oxidation of In₂Se₃ in air to form amorphous In₂Se_3−3xO_3x layers and Se hemisphere particles. Both O₂ and H₂O are required for such surface oxidation, which can be further promoted by light illumination. In addition, the self-passivation effect from the In₂Se_3−3xO_3x layer can effectively limit such oxidation to only a few nanometer thickness. The achieved insight paves way for better understanding and optimizing 2D In₂Se₃ performance for device applications.     

Chemical and Electrochemical Lithiation of van der Waals Tetrel-Arsenides

Lithiation of van der Waals tetrel-arsenides, GeAs and SiAs, has been investigated. Electrochemical lithiation demonstrated large initial capacities of over 950 mAh g⁻¹ accompanied by rapid fading over successive cycling in the voltage range 0.01–2 V. Limiting the voltage range to 0.5–2 V achieved more stable cycling, which was attributed to the intercalation process with lower capacities. Ex situ powder X-ray diffraction confirmed complete amorphization of the samples after lithiation, as well as recrystallization of the binary tetrel-arsenide phases after full delithiation in the voltage range 0.5–2 V. Solid-state synthetic methods produce layered phases, in which Si-As or Ge-As layers are separated by Li cations. The first layered compounds in the corresponding ternary systems were discovered, Li_0.9Ge_2.9As_3.1 and Li₃Si₇As₈, which crystallize in the Pbam (No. 55) and P2/m (No. 10) space groups, respectively. Semiconducting layered GeAs and SiAs accommodate the extra charge from Li cations through structural rearrangement in the Si-As or Ge-As layers and eventually by replacement of the tetrel dumbbells with sets of Li atoms. Ge and Si monoarsenides demonstrated high structural flexibility and a mild ability for reversible lithiation.     

Transition metals on the (0 0 0 1) surface of graphite: Fundamental aspects of adsorption,diffusion, and morphology

In this article, we review basic information about the interaction of transition metal atoms with the (0 0 0 1) surface of graphite, especially fundamental phenomena related to growth. Those phenomena involve adatom-surface bonding, diffusion, morphology of metal clusters, interactions with steps and sputter-induced defects, condensation, and desorption. General traits emerge which have not been summarized previously. Some of these features are rather surprising when compared with metal-on-metal adsorption and growth. Opportunities for future work are pointed out.     

Preparation and Crystal Growth of Transition Metal Dichalcogenides

Dichalcogenides are known from almost all transition metals. The representatives of this class of compounds show a number of interesting physical and chemical properties depending on their constituent transition metal and crystal structure, which makes them interesting for basic studies and applications in high-end electronics, spintronics, optoelectronics, energy storage, flexible electronics, DNA sequencing and personalized medicine to this day. Many of these properties and effects can only be investigated on chemically and crystallographically pure samples - usually on single crystals. The vast majority of these compounds can be crystallized using chemical vapour transport. However crystallization from the melt is also possible in a considerable number of compounds, including the frequently used self-flux technique. For several compounds the crystallization from different solvents or solvent mixtures by means of solvothermal or hydrothermal synthesis is described.     

Mechanistic considerations on catalytic H/D exchange mediated by organometallic transition metal complexes

The purpose of this review is to analyze the different reaction mechanisms of the H/D exchange on organic substrates catalyzed by transition metal complexes in homogeneous phase. The metal-catalyzed H/D exchange is a multifaceted reaction whose mechanism depends strongly on the reaction conditions and on the metal complex used as a catalyst. It is possible to group the different mechanisms into three main families depending on the “role” and behavior of the catalyst: (i) Lewis acid–base catalysis; (ii) CH activation (iii) insertion/β-elimination. For each macro-group, several representative examples are discussed and critically evaluated in order to provide the reader with keys to the understanding of how the different catalytic systems act and how their modification may affect their performance in terms of activity and selectivity. This knowledge is fundamental for designing improved organometallic H/D catalysts for labeling organic products in greener conditions with more cost-effective processes.     

Structural and electronic properties of defective 2D transition metal dichalcogenide heterostructures

We present a first-principles study on the structure-property relationships in MoS₂ and WS₂ monolayers and their vertically stacked hetero-bilayer, with and without Sulfur vacancies, in order to dissect the electronic features behind their photocatalytic water splitting capabilities. We also benchmark the accuracy of three different exchange-correlation density functionals for both minimum-energy geometries and electronic structure. The best compromise between computational cost and qualitative accuracy is achieved with the HSE06 density functional on top of Perdew–Burke–Ernzerhof minima, including dispersion with Grimme's D3 scheme. This computational approach predicts the presence of mid-gap states for defective monolayers, in accordance with the present literature. For the heterojunction, we find unexpected vacancy-position dependent electronic features: the location of the defects leads either to mid-gap trap states, detrimental for photocatalyst or to a modification of characteristic type II band alignment behavior, responsible for interlayer charge separation and low recombination rates.     

MoTe2-based low energy consumption artificial synapse for neuromorphic behavior and decimal arithmetic

Two-dimensional material-based memristors have shown attractive research prospects as brain-like devices for neuromorphic computing. Among them, transition metal dichalcogenides–based memristors have proved to be one of the most promising competitors. In this work, a two-dimensional memristor based on MoTe₂ nanosheets was fabricated and demonstrated. The experimental results illustrate that the two-terminal synaptic based on the Ag/MoTe₂/ITO structure exhibits stable bipolar and non-volatile resistive switching characteristics attributed to the controllable formation and rupturing of silver conductive filaments. The device can be successively modulated by a pulse train with a minimum pulse width of 40 ns. More interestingly, the energy consumption of the device to complete one write event is only 74.2 pJ. In addition, biological synaptic behaviors, such as excitatory postsynaptic current gain properties, long-term potentiation (LTP)/long-term depression, spike-timing-dependent- plasticity, short-term plasticity, long-term potentiation (LTP), paired-pulse facilitation, post-tetanic potentiation, and learning-experimental behaviors were mimicked faithfully. Finally, the decimal arithmetic application was introduced to the device, and it is confirmed that addition and multiplication functions can be performed. Therefore, the artificial synapse based on MoTe₂ nanosheets not only exhibits the stable non-volatile resistive switching behavior but also facilitates the development of low-energy consumption neuromorphic computing chips based on transition metal dichalcogenides.     

Towards Scalable Fabrications and Applications of 2D Layered Material-based Vertical and Lateral Heterostructures

Among 108423 unique, experimentally known 3D compounds, there exist 1825 ones that are either easily or potentially exfoliable. This increasingly broad library of 2D layered materials(2DLMs) with variable physical properties as well as the unique ability to vertical stacking or lateral stitching 2DLMs into complex heterostructures enables a new dimension for materials engineering and device design, offering novel functional electronics and optoelectronics for flexible industry. In this review, we present a comprehensive summary of the state-of-the-art scalable fabrication technologies, the unique properties as well as the potential device applications of the emerging 2D heterostructures. Firstly, we depict an overall picture of the 2D vertical van der Waals heterostructures. Secondly, we focus on the 2D lateral heterostructures by CVD technique. For a quick access and full coverage, both the vertical and lateral 2D heterostructures are classified into several types according to their chemical compounds with different dimensions. In the end, both the challenges and potential applications of these 2D heterostructures are discussed.     

CNDO-S2—a semiempirical SCF MO method for transition metal organometallics

A new semiempirical SCF MO procedure available for prediction of the transition metal compounds energy and geometry is developed. The procedure takes an explicit account of the orthogonality of the basis set in the calculation of the core-Hamiltonian elements. A new formula for the resonance integral used in CNDO-S² gives a physically correct treatment of diffuse orbital-localized orbital interaction. The parametrization for atoms H, C, N, O and Ni is presented, with one-center empirical parameters only used. The results of CNDO-S² energy and geometry calculations performed for a number of organic compounds and some nickelorganics are compared with the experimental data. The average absolute errors for the binding energies of organic compounds and nickel complexes are 6.6 kcal/mol and 9.3 kcal/mol respectively.