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The non-Markov process exists widely in thermodymanic process, while it usually requires the packing of many transistors and memories with great system complexity in a traditional device structure to minic such functions. Two-dimensional (2D) material-based resistive random access memory (RRAM) devices have the potential for next-generation computing systems with much-reduced complexity. Here, we achieve a non-Markov chain in an individual RRAM device based on 2D mineral material mica with a vertical metal/mica/metal structure. We find that the potassium ions (K+) in 2D mica gradually move in the direction of the applied electric field, making the initially insulating mica conductive. The accumulation of K+ is changed by an electric field, and the 2D-mica RRAM has both single and double memory windows, a high on/off ratio, decent stability, and repeatability. This is the first time a non-Markov chain process has been established in a single RRAM, in which the movement of K+ is dependent on the stimulated voltage as well as their past states. This work not only uncovers an intrinsic inner ionic conductivity of 2D mica, but also opens the door for the production of such RRAM devices with numerous functions and applications.

Two-dimensional (2D) materials possess unique thickness- and lateral-size-dependent properties. Many efforts have been devoted to obtaining 2D materials with narrow structure heterogeneity while it is still challenging to independently control their thickness and lateral size, limiting their widespread applications. Here, we develop a three-step method which achieves independent thickness and lateral size sorting of 2D materials. Taking 2D h-BN flakes as an example, their thickness and lateral size are independently sorted to different fractions with thicknesses smaller than 6 nm. In addition, the 2D h-BN flakes possess narrow distributions of both thickness and lateral size. We further develop a force field extraction method and achieve scalable size sorting of 2D h-BN, which is universal for sorting other 2D materials including MoS2 and graphene oxide. This work reports an effective method to produce structure homogenous 2D materials and will help fundamental studies and applications of 2D materials where thickness and lateral size are of concern.

Magneto-optical (MO) effects, viz. magnetically induced changes in light intensity or polarization upon reflection from or transmission through a magnetic sample, were discovered over a century and a half ago. Initially they played a crucially relevant role in unveiling the fundamentals of electromagnetism and quantum mechanics. A more broad-based relevance and wide-spread use of MO methods, however, remained quite limited until the 1960s due to a lack of suitable, reliable and easy-to-operate light sources. The advent of Laser technology and the availability of other novel light sources led to an enormous expansion of MO measurement techniques and applications that continues to this day (see section 1). The here-assembled roadmap article is intended to provide a meaningful survey over many of the most relevant recent developments, advances, and emerging research directions in a rather condensed form, so that readers can easily access a significant overview about this very dynamic research field. While light source technology and other experimental developments were crucial in the establishment of today's magneto-optics, progress also relies on an ever-increasing theoretical understanding of MO effects from a quantum mechanical perspective (see section 2), as well as using electromagnetic theory and modelling approaches (see section 3) to enable quantitatively reliable predictions for ever more complex materials, metamaterials, and device geometries. The latest advances in established MO methodologies and especially the utilization of the MO Kerr effect (MOKE) are presented in sections 4 (MOKE spectroscopy), 5 (higher order MOKE effects), 6 (MOKE microscopy), 8 (high sensitivity MOKE), 9 (generalized MO ellipsometry), and 20 (Cotton-Mouton effect in two-dimensional materials). In addition, MO effects are now being investigated and utilized in spectral ranges, to which they originally seemed completely foreign, as those of synchrotron radiation x-rays (see section 14 on three-dimensional magnetic characterization and section 16 on light beams carrying orbital angular momentum) and, very recently, the terahertz (THz) regime (see section 18 on THz MOKE and section 19 on THz ellipsometry for electron paramagnetic resonance detection). Magneto-optics also demonstrates its strength in a unique way when combined with femtosecond laser pulses (see section 10 on ultrafast MOKE and section 15 on magneto-optics using x-ray free electron lasers), facilitating the very active field of time-resolved MO spectroscopy that enables investigations of phenomena like spin relaxation of non-equilibrium photoexcited carriers, transient modifications of ferromagnetic order, and photo-induced dynamic phase transitions, to name a few. Recent progress in nanoscience and nanotechnology, which is intimately linked to the achieved impressive ability to reliably fabricate materials and functional structures at the nanoscale, now enables the exploitation of strongly enhanced MO effects induced by light-matter interaction at the nanoscale (see section 12 on magnetoplasmonics and section 13 on MO metasurfaces). MO effects are also at the very heart of powerful magnetic characterization techniques like Brillouin light scattering and time-resolved pump-probe measurements for the study of spin waves (see section 7), their interactions with acoustic waves (see section 11), and ultra-sensitive magnetic field sensing applications based on nitrogen-vacancy centres in diamond (see section 17)., Despite our best attempt to represent the field of magneto-optics accurately and do justice to all its novel developments and its diversity, the research area is so extensive and active that there remains great latitude in deciding what to include in an article of this sort, which in turn means that some areas might not be adequately represented here. However, we feel that the 20 sections that form this 2022 magneto-optics roadmap article, each written by experts in the field and addressing a specific subject on only two pages, provide an accurate snapshot of where this research field stands today. Correspondingly, it should act as a valuable reference point and guideline for emerging research directions in modern magneto-optics, as well as illustrate the directions this research field might take in the foreseeable future., Journal of Physics D: Applied Physics, 55 (46), ISSN:0022-3727, ISSN:1361-6463

Artificial synapses are promising for dealing with large amounts of data computing. Great progress has been made recently in terms of improving the on/off current ratio, the number of states, and the energy efficiency of synapse devices. However, the nonlinear weight update behavior of a synapse caused by the uncertain direction of the conductive filament leads to complex weight modulation, which degrades the delivery accuracy of information. Here we propose a strategy to improve the weight update behavior of synapses using chemical-vapor-deposition-grown transition metal dichalcogenides (TMDCs) with a vertical composition gradient, where the sulfur concentration decreases gradually along the thickness direction of TMDCs and thus forms a certain direction of the conduction filament for synapse devices. It is worth noting that the devices show an excellent linear conductance of potentiation and depression with a high linearity of 0.994 (surpassing most state-of-the-art synapses), have a large number of states, and are able to fabricate synapse arrays with wafer-scale. Furthermore, the devices based on the TMDCs with the vertical composition gradient exhibit an asymmetric feature of potentiation and depression behaviors with high linearity and follow the simulated linear Leaky ReLU function, resulting in a high recognition accuracy of 94.73%, which overcomes the unreliability issue in the Sigmoid function due to the vanishing gradient phenomenon. This study not only provides a universal method to grow TMDCs with a vertical composition gradient but also contributes to exploring highly linear synapses toward neuromorphic computing.

2D material-based heterostructures are constructed by stacking or spicing individual 2D layers to create an interface between them, which have exotic properties. Here, a new strategy for the in situ growth of large numbers of 2D heterostructures on the centimeter-scale substrate is developed. In the method, large numbers of 2D MoS

Doping is an effective way to modify the electronic property of two-dimensional (2D) materials and endow them with additional functionalities. However, wide-range control of the doping concentrations in monolayer 2D materials with large-scale uniformity remains challenging. Here, we report

Two-dimensional (2D) materials are the frontier research direction of material sciences and condensed matter physics in recent years. Among different kinds of 2D materials, black phosphorus (BP) has received extensive attention due to its unique band gap and excellent optical and electrical anisotropy properties. Compared with graphene and transition metal dichalchogenides (TMDCs), BP stands out for its unique properties with widely tunable direct bandgaps ranging from 0.3–1.7 eV, covering the wavelength from ~600 to 4000 nm, which fill in the gap between graphene (0 eV) and TMDCs (~1.0–2.5 eV) and is considered the ideal material for optoelectronic applications, such as light detection and light modulation. For example, photodetectors are basic devices for optoelectronics, as they transform the information stored in light into electric signals. BP based photodetectors not only stand out for their wide detection range reaching far infrared (IR) region, but also for their high responsivity and high gain, demonstrating their excellent performance for mid-IR detection. In order to achieve above-mentioned applications, few or even monolayer BP flakes or continuous BP films are required. However, how to control the preparation of thin layer of BP is one of the biggest challenges in this field. In this paper, we carried out a systematic study on the growth of BP on different substrates and found that Au can guide and promote the growth of BP, thereby controlling its growth. Based on the above findings, the growth of BP nanosheets on the Au-Si substrate is achieved by using a Si substrate deposited with Au thin film on top, and by designing a strategy with gradient temperature and in confined space. We observed that the BP nanosheets grown by this system exhibited different shapes such as ribbons and flakes, and their thicknesses can be reduced to be about 51 nm with a lateral size of around 10 µm. In addition, by systematically tuning the growth parameters in the system, we found that the growth temperatures and the amount of red phosphorous loading are the key parameters for the growth of BP. With certain temperatures and precursor loadings, we achieve the growth of BP nanosheets, while by-products would be obtained on the substrate with other parameters. In our previous work, we found that the temperature gradients play a crucial role in the yield, purity, and crystal quality of bulk BP growth. Therefore, further optimization of temperature distribution is of great importance for the growth of BP nanosheets. In addition, we observed in this paper that Au would guide the nucleation and growth of BP. Therefore, it is important to regulate Au-P compounds during growth of BP, as on the one hand, it can control the by-products during the reaction and on the other hand, it can also improve the controllability of BP nucleation. Based on this, we believe that optimization of the temperature distribution and the growth regulation of Au-P compounds are important research directions, as to precisely control and reduce the thickness of BP nanosheets to grow few layer BP. The growth system proposed in this paper improves the chemical vapor transport (CVT) growth of traditional bulk BP from three aspects: Substrate design, confined space design, and temperature gradient control, which have paved the way for the future controllability growth of BP nanosheets and have deepened the understanding of BP growth mechanism.

Large-scale production of green hydrogen by electrochemical water splitting is considered as a promising technology to address critical energy challenges caused by the extensive use of fossil fuels. Although nonprecious nickel-based catalysts work well at low current densities, they need large overpotentials at high current densities, which hinders their wide applications in practical industry. Here we report a hydroxide-mediated nickel-based electrocatalyst for high-current-density hydrogen evolution, which delivers a current density of 1000 mA cm−2 at an ultralow overpotential of 97 mV. Combined X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) results show charge redistribution of Ni sites caused by Mo and surface Fe, which can stabilize the surface nickel hydroxide at high current densities for promoting the water dissociation step. Combined in situ XAS, quasi in situ XPS, and density functional theory calculations indicate that Fe plays an important role in the improved catalytic activity. Such a catalyst is synthesized at the metre-size scale and delivers a current density of 500 mA cm−2 at 1.56 V in overall water splitting, demonstrating its potential for practical use. This work highlights a charge engineering strategy to design efficient catalysts for high current density electrochemical applications.

CONSPECTUS: Two-dimensional (2D) compound materials are promising materials for use in electronics, optoelectronics, flexible devices, etc. because they are ultrathin and cover a wide range of properties. Among all methods to prepare 2D materials, chemical vapor deposition (CVD) is promising because it produces materials with a high quality and reasonable cost. So far, much efforts have been made to produce 2D compound materials with large domain size, controllable number of layers, fast-growth rate, and high quality features, etc. However, due to the complicated growth mechanism like sublimation and diffusion processes of multiple precursors, maintaining the controllability, repeatability, and high quality of CVD grown 2D binary and ternary materials is still a big challenge, which prevents their widespread use. Here, taking 2D transition metal dichalcogenides (TMDCs) as examples, we review current progress and highlight some promising growth strategies for the growth of 2D compound materials. The key technology issues which affect the CVD process, including non-metal precursor, metal precursor, substrate engineering, temperature, and gas flow, are discussed. Also, methods in improving the quality of CVD-grown 2D materials and current understanding on their growth mechanism are highlighted. Finally, challenges and opportunities in this field are proposed. We believe this review will guide the future design of controllable CVD systems for the growth of 2D compound materials with good controllability and high quality, laying the foundations for their potential applications.

Two-dimensional (2D) materials are promising candidates in wide applications including energy storage and conversion, sensors, flexible devices, etc. The low-cost production of 2D materials with large quantities and demanded quality is the precondition for their commercial uses. For graphene and its derivatives, relatively mature techniques have been established for their scalable preparation and industrial applications. Whereas the mass production of 2D materials beyond graphene is still in its early age and it lacks a summary on this topic. This review systematically describes the state-of-the-art approaches for high-yield preparation of 2D materials beyond graphene, including “top-down” exfoliation and “bottom-up” synthetic approaches. In particular, each method is discussed from the perspectives of its principle, optimization attempts, characteristics of the obtained 2D materials, and its scalability potential. The applications that require massively-produced 2D materials are highlighted, including electrocatalysis, batteries, supercapacitors, mechanical and chemical sensors, as well as electromagnetic interference shielding and microwave absorption devices. Finally, we propose the challenges and opportunities for scalable preparation and commercial applications of 2D materials.

Black phosphorus (BP) has recently attracted significant interest due to its unique electronic and optical properties. Doping is an effective strategy to tune a material's electronic structures, however, the direct and controllable growth of BP with a high yield and its doping remain a great challenge. Here we report an efficient short-distance transport (SDT) growth approach and achieve the controlled growth of high quality BP with the highest yield so far, where 98% of the red phosphorus is converted to BP. The doping of BP by As, Sb, Bi, Se and Te are also achieved by this SDT growth approach. Spectroscopic results show that doping systematically changes its electronic structures including band gap, work function, and energy band position. As a result, we have found that the air-stability of doped BP samples (Sb and Te-doped BP) improves compared with pristine BP, due to the downshift of the conduction band minimum with doping. This work develops a new method to grow BP and doped BP with tunable electronic structures and improved stability, and should extend the uses of these class of materials in various areas., 22 pages, 5 figures

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted great attention due to their physical and chemical properties that make them promising in electronics and optoelectronics. Because of the difficulties in controlling concentrations of solid precursors and spatially nonuniform growth dynamics, it is challenging to grow 2D TMDCs over large areas with good uniformity and reproducibility so far, which significantly hinders their practical use. Here we report a vertical chemical vapor deposition (VCVD) design with gaseous precursors to grow monolayer TMDCs with a uniform density and high quality over the whole substrate and with excellent reproducibility. Such a gaseous VCVD design can well control the three key parameters in TMDC growth, including precursor concentration, gas flow, and temperature, which cannot be done in a currently widely used horizontal CVD system with solid precursors. Statistical results show that VCVD-grown monolayer TMDCs including MoS

Constructing stable electrodes which function over long timescales at large current density is essential for the industrial realization and implementation of water electrolysis. However, rapid gas bubble detachment at large current density usually results in peeling-off of electrocatalysts and performance degradation, especially for long term operations. Here we construct a mechanically-stable, all-metal, and highly active CuMo

Electrochemical water splitting technology for producing "green hydrogen" is important for the global mission of carbon neutrality. Electrocatalysts with decent performance at high current densities play a central role in the industrial implementation of this technology. This field has advanced immensely in recent years, as witnessed by many types of catalysts designed and synthesized toward industrially-relevant current densities (> 200 mA cm-2 ). By discussing recent advances in this field, we summarize several key aspects that affect the catalytic performance for high-current-density electrocatalysis, including dimensionality of catalysts, surface chemistry, electron transport path, morphology, and catalyst-electrolyte interplay. We highlight the multiscale design strategy that considers these aspects comprehensively for developing high-current-density electrocatalysts. We also put forward our perspectives on the future directions in this emerging field. This article is protected by copyright. All rights reserved.

The use of highly-active and robust catalysts is crucial for producing green hydrogen by water electrolysis as we strive to achieve global carbon neutrality. Noble metals like platinum are currently used catalysts in industry for the hydrogen evolution, but suffer from scarcity, high price and unsatisfied performance and stability at large current density, restrict their large-scale implementations. Here we report the synthesis of a type of monolith catalyst consisting of a metal disulfide (e.g., tantalum sulfides) vertically bonded to a conductive substrate of the same metal tantalum by strong covalent bonds. These features give the monolith catalyst a mechanically-robust and electrically near-zero-resistance interface, leading to an excellent hydrogen evolution performance including rapid charge transfer and excellent durability, together with a low overpotential of 398 mV to achieve a current density of 2,000 mA cm−2 as required by industry. The monolith catalyst has a negligible performance decay after 200 h operation at large current densities. In light of its robust and metallic interface and the various choices of metals giving the same structure, such monolith materials would have broad uses besides catalysis., Water electrolysis is a promising hydrogen production technique but is restricted from large-scale application due to poor performance and high cost. Here, the authors report a mechanically stable monolith electrocatalyst that achieves superior hydrogen evolution at large current densities.

A spiking neural network consists of artificial synapses and neurons and may realize human-level intelligence. Unlike the widely reported artificial synapses, the fabrication of large-scale artificial neurons with good performance is still challenging due to the lack of a suitable material system and integration method. Here, we report an ultrathin (less than10 nm) and inch-size two-dimensional (2D) oxide-based artificial neuron system produced by a controllable assembly of solution-processed 2D monolayer TiOx nanosheets. Artificial neuron devices based on such 2D TiOx films show a high on/off ratio of 109 and a volatile resistance switching phenomenon. The devices can not only emulate the leaky integrate-and-fire activity but also self-recover without additional circuits for sensing and reset. Moreover, the artificial neuron arrays are fabricated and exhibited good uniformity, indicating their large-area integration potential. Our results offer a strategy for fabricating large-scale and ultrathin 2D material-based artificial neurons and 2D spiking neural networks.

Moiré superlattices of van der Waals heterostructures provide a powerful way to engineer electronic structures of two-dimensional materials. Many novel quantum phenomena have emerged in graphene and transition metal dichalcogenide moiré systems. Twisted phosphorene offers another attractive system to explore moiré physics because phosphorene features an anisotropic rectangular lattice, different from isotropic hexagonal lattices previously reported. Here we report emerging anisotropic moiré optical transitions in twisted monolayer/bilayer phosphorenes. The optical resonances in phosphorene moiré superlattice depend sensitively on twist angle and are completely different from those in the constitute monolayer and bilayer phosphorene even for a twist angle as large as 19°. Our calculations reveal that the Γ-point direct bandgap and the rectangular lattice of phosphorene give rise to the remarkably strong moiré physics in large-twist-angle phosphorene heterostructures. This work highlights fresh opportunities to explore moiré physics in phosphorene and other van der Waals heterostructures with different lattice configurations., Twisted phosphorene offers another attractive system to explore moiré physics. Here, the authors report emerging anisotropic moiré optical resonances in twisted monolayer/bilayer phosphorene, exhibiting strong twist dependence for angles as large as 19°.

The scalable and high-efficiency production of 2D materials is a prerequisite to their commercial use. Currently, only graphene and graphene oxide can be produced on a ton scale, and the inability to produce other 2D materials on such a large scale hinders their technological applications. Here we report a grinding exfoliation method that uses micro-particles as force intermediates to resolve applied compressive forces into a multitude of small shear forces, inducing the highly efficient exfoliation of layer materials. The method, referred to as intermediate-assisted grinding exfoliation (iMAGE), can be used for the large-scale production of many 2D materials. As an example, we have exfoliated bulk h-BN into 2D h-BN with large flake sizes, high quality and structural integrity, with a high exfoliation yield of 67%, a high production rate of 0.3 g h−1 and a low energy consumption of 3.01 × 106 J g−1. The production rate and energy consumption are one to two orders of magnitude better than previous results. Besides h-BN, this iMAGE technology has been used to exfoliate various layer materials such as graphite, black phosphorus, transition metal dichalcogenides, and metal oxides, proving its universality. Molybdenite concentrate, a natural low-cost and abundant mineral, was used as a demo for the large-scale exfoliation production of 2D MoS2 flakes. Our work indicates the huge potential of the iMAGE method to produce large amounts of various 2D materials, which paves the way for their commercial application.

Metallic transition metal dichalcogenides, such as tantalum disulfide (TaS2), have recently emerged as promising electrocatalysts for the hydrogen evolution reaction. This work reports an effective...

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have drawn intensive attention due to their ultrathin feature with excellent electrostatic gating capability, and unique thickness-dependent electronic and optical properties. Controlling the thickness and doping of 2D TMDCs are crucial toward their future applications. Here, we report an effective HAuCl4 treatment method and achieve simultaneous thinning and doping of various TMDCs in one step. We find that the HAuCl4 treatment not only thins thick MoS2 flakes into few layers or even monolayers, but also simultaneously tunes MoS2 into p-type. The effects of various parameters in the process have been studied systematically, and an Au intercalation assisted thinning and doping mechanism is proposed. Importantly, this method also works for other typical TMDCs, including WS2, MoSe2 and WSe2, showing good universality. Electrical transport measurements of field-effect transistors (FETs) based on MoS2 flakes show a big increase of On/Off current ratios (from 102 to 107) after the HAuCl4 treatment. Meanwhile, the subthreshold voltages of the MoS2 FETs shift from −60 to +27 V after the HAuCl4 treatment, with a p-type doping behavior. This study provides an effective and simple method to control the thickness and doping properties of 2D TMDCs, paving a way for their applications in high performance electronics and optoelectronics.

Black phosphorus (BP) has been shown as a promising two‐dimensional (2D) material for electronic devices owing to its high carrier mobility. To realize complementary electronic circuits with 2D materials, it is important to fabricate both n‐type and p‐type transistors with the same channel material. By engineering the contact region with copper (Cu)‐doped BP, here we demonstrate an n‐type carrier transport in BP field‐effect transistors (FETs), which usually exhibit strongly p‐type characteristics. Cu metal atoms are found to severely penetrate into the BP flakes, which forms interstitial Cu (Cuint)‐doped edge contact and facilitates the electron transport in BP. Our BP FETs in back‐gated configuration exhibit n‐type dominant characteristics with a high electron mobility of ~ 138 cm2 V−1 s−1 at room temperature. The Schottky barrier height for electrons is relatively low because of the edge contact between Cuint‐doped BP and pristine BP channel. The contact doping of BP by highly mobile Cu atoms gives rise to n‐type transport property of BP FETs. Furthermore, we demonstrate a p‐n junction on the same BP flake with asymmetric contact. This strategy on contact engineering can be further extended to other 2D materials.

Electrochemical energy conversion between electricity and chemicals through electrocatalysis is a promising strategy for the development of clean and sustainable energy sources. This is because efficient electrocatalysts can greatly reduce energy loss during the conversion process. However, poor catalytic performances and a shortage in catalyst material resources have greatly restricted the widespread applications of electrocatalysts in these energy conversion processes. To address this issue, earth-abundant two-dimensional (2D) materials with large specific surface areas and easily tunable electronic structures have emerged in recent years as promising high-performance electrocatalysts in various reactions, and because of this, this review will comprehensively discuss the engineering of these novel 2D material-based electrocatalysts and their associated heterostructures. In this review, the fundamental principles of electrocatalysis and important electrocatalytic reactions are introduced. Following this, the unique advantages of 2D material-based electrocatalysts are discussed and catalytic performance enhancement strategies are presented, including the tuning of electronic structures through various methods such as heteroatom doping, defect engineering, strain engineering, phase conversion and ion intercalation, as well as the construction of heterostructures based on 2D materials to capitalize on individual advantages. Finally, key challenges and opportunities for the future development of these electrocatalysts in practical energy conversion applications are presented.

Magnetically influenced light-matter interaction provides a contactless, noninvasive and power-free way for material characterization and light modulation. Shape anisotropy of active materials mainly determines the sensitivity of magneto-optic response, thereby making magnetic two-dimensional (2D) materials suitable in achieving the giant magneto-birefringence effect as discovered recently. Consequently, relationship between magneto-birefringence response and shape anisotropy of 2D materials is critical but has remained elusive, restricting its widespread applications. Here, we report the highly sensitive and largely tunable magneto-coloration via manipulating the shape-anisotropy of magnetic 2D materials. We reveal a quadratic increasing relationship between the magneto-optic Cotton-Mouton coefficient and the lateral size of 2D materials and achieve a more than one order of magnitude tunable response. This feature enables the engineerable transmissive magneto-coloration of 2D materials by tailoring their shape anisotropy. Our work deepens the understanding of the tunability of magneto-optic response by size effect of active materials, offering various opportunities for their applications in vast areas where color is concerned.

Modulating electronic structure of monolayer transition metal dichalcogenides (TMDCs) is important for many applications and doping is an effective way towards this goal, yet is challenging to control. Here we report the in-situ substitutional doping of niobium (Nb) into TMDCs with tunable concentrations during chemical vapour deposition. Taking monolayer WS2 as an example, doping Nb into its lattice leads to bandgap changes in the range 1.98 eV to 1.65 eV. Noteworthy, electrical transport measurements and density functional theory calculations show that the 4d electron orbitals of the Nb dopants contribute to the density of states of Nb-doped WS2 around the Fermi level, resulting in an n to p-type conversion. Nb-doping also reduces the energy barrier of hydrogen absorption in WS2, leading to an improved electrocatalytic hydrogen evolution performance. These results highlight the effectiveness of controlled doping in modulating the electronic structure of TMDCs and their use in electronic related applications., 20 pages, 5 figures

The six-membered ring (SMR) is a common structure unit for numerous material systems. These materials include, but are not limited to, the typical two-dimensional materials such as graphene, h-BN, and transition metal dichalcogenides, as well as three-dimensional materials such as beryllium, magnesium, MgB2 and Bi2Se3. Although many of these materials have already become ‘stars’ in materials science and condensed-matter physics, little attention has been paid to the roles of the SMR unit across a wide range of compositions and structures. In this article, we systematically analyze these materials with respect to their very basic SMR structural unit, which has been found to play a deterministic role in the occurrence of many intriguing properties and phenomena, such as Dirac electronic and phononic spectra, superconductivity and topology. As a result, we have defined this group of materials as SMR inorganic materials, opening up a new perspective on materials research and development. With their unique properties, SMR materials deserve wide attention and in-depth investigation from materials design, new physical discoveries to target-wizard applications. It is expected that SMR materials will find niche applications in next-generation information technology, renewable energy, space, etc., The authors defined SMR materials, which possess extensive six-membered-ring structural units and rich structural lattices, chemistry and physics, suitable for niche applications in next-generation information technology, renewable energy, space, etc.

One of the long sought-after goals in manipulation of light through light-matter interactions is the realization of magnetic-field-tuneable colouration, so-called magneto-chromatic effect, which holds great promise for optical, biochemical and medical applications due to its contactless and non-invasive nature. This goal can be achieved by magnetic-field controlled birefringence, where colours are produced by the interference between phase-retarded components of transmitted polarised light. Thus far birefringence-tuneable coloration has been demonstrated using electric field, material chirality and mechanical strain but magnetic field control remained elusive due to either weak magneto-optical response of transparent media or low transmittance to visible light of magnetically responsive media, such as ferrofluids. Here we demonstrate magnetically tuneable colouration of aqueous suspensions of two-dimensional cobalt-doped titanium oxide which exhibit an anomalously large magneto-birefringence effect. The colour of the suspensions can be tuned over more than two wavelength cycles in the visible range by moderate magnetic fields below 0.8 T. We show that such giant magneto-chromatic response is due to particularly large phase retardation (>3 pi) of the polarised light, which in its turn is a combined result of a large Cotton-Mouton coefficient (three orders of magnitude larger than for known liquid crystals), relatively high saturation birefringence (delta n = 2 x 10^-4) and high transparency of our suspensions to visible light. The work opens a new avenue to achieve tuneable colouration through engineered magnetic birefringence and can readily be extended to other magnetic 2D nanocrystals. The demonstrated effect can be used in a variety of magneto-optical applications, including magnetic field sensors, wavelength-tuneable optical filters and see-through printing., 10 pages, 4 figures. Nature Communications, 2020, Accepted

Large scale implementation of electrochemical water splitting for hydrogen evolution requires cheap and efficient catalysts to replace expensive platinum. Molybdenum disulfide is one of the most promising alternative catalysts but its intrinsic activity is still inferior to platinum. There is therefore a need to explore new active site origins in molybdenum disulfide with ultrafast reaction kinetics and to understand their mechanisms. Here, we report a universal cold hydrogen plasma reduction method for synthesizing different single atoms sitting on two-dimensional monolayers. In case of molybdenum disulfide, we design and identify a new type of active site, i.e., unsaturated Mo single atoms on cogenetic monolayer molybdenum disulfide. The catalyst shows exceptional intrinsic activity with a Tafel slope of 35.1 mV dec-1 and a turnover frequency of ~10^3 s-1 at 100 mV, based on single flake microcell measurements. Theoretical studies indicate that coordinately unsaturated Mo single atoms sitting on molybdenum disulfide increase the bond strength between adsorbed hydrogen atoms and the substrates through hybridization, leading to fast hydrogen adsorption/desorption kinetics and superior hydrogen evolution activity. This work shines fresh light on preparing highly-efficient electrocatalysts for water splitting and other electrochemical processes, as well as provides a general method to synthesize single atoms on two-dimensional monolayers., 25 pages, 4 figures

The high-throughput scalable production of cheap, efficient and durable electrocatalysts that work well at high current densities demanded by industry is a great challenge for the large-scale implementation of electrochemical technologies. Here we report the production of a two-dimensional molybdenum disulfide-based ink-type electrocatalyst by a scalable exfoliation technique followed by a thermal treatment. The catalyst delivers a high current density of 1000 mA cm−2 at an overpotential of 412 mV for the hydrogen evolution. Using the same method, we produce a cheap mineral-based catalyst possessing excellent performance for high-current-density hydrogen evolution. Noteworthy, production rate of this catalyst is one to two orders of magnitude higher than those previously reported, and price of the mineral is five orders of magnitude lower than commercial Pt electrocatalysts. These advantages indicate the huge potentials of this method and of mineral-based cheap and abundant natural resources as catalysts in the electrochemical industry., The large-scale implementation of electrochemical technologies will require the high-throughput production of high-performance, inexpensive catalysts. Here, authors demonstrate earth abundant molybdenite as raw materials to produce efficient MoS2 catalysts for high current density H2 evolution.

Nickel–iron (Ni–Fe) compounds have received ever-increasing interest because of their high performance for the oxygen evolution reaction (OER). However, the structure and composition of the active phase of Ni–Fe compounds are not yet understood. Rational design of Ni–Fe compounds with proper composition and exposed active sites is highly desirable to further improve their performance. Here we reported the synthesis of different Ni–Fe compounds by incorporating Fe into the Ni(OH)2 lattice with controlled concentrations and obtained several products including Ni–Fe layered double hydroxides (LDHs) with different amounts of Fe and NiFe2O4. In addition, β-Ni(OH)2 without Fe and α-Fe2O3 without Ni have also been synthesized for comparison. Among these four Ni/Fe based compounds, we found that the two-dimensional (2D) Ni0.8Fe0.2-LDH nanosheets with a hexagonal shape manifest a highest OER activity with a lowest overpotential of 235 mV at 10 mA cm−2, a smallest Tafel slope of 41 mV dec−1 and excellent stability over 24 h. In situ Raman, X-ray absorption spectroscopy and X-ray photoelectron spectroscopy show that the enhanced activity of Ni0.8Fe0.2-LDH is due to the increased oxidation state of Ni from Ni2+ to Ni3+. Our findings demonstrate that the composition of Ni–Fe compounds influences the oxidizing ability of Ni and consequently their catalytic performance. We also reveal a strong correlation between the structure and oxidizing power of the Ni in Ni–Fe compounds and their OER activity, indicating that the well-engineered structure and Ni-site electronics are the primary origins of their OER activity. This work not only provides a promising guideline for the design and synthesis of Ni/Fe based compounds, but also highlights the crucial role of each component in enhancing the activity of these electrocatalysts.

Two dimensional transition metal dichalcogenides (TMDCs) have attracted much interest and shown promise in many applications. However, it is challenging to obtain uniform TMDCs with clean surfaces, because of the difficulties in controlling the way the reactants are supplied to the reaction in the current chemical vapor deposition growth process. Here, we report a new growth approach called ‘dissolution-precipitation’ (DP) growth, where the metal sources are sealed inside glass substrates to control their feeding to the reaction. Noteworthy, the diffusion of metal source inside glass to its surface provides a uniform metal source on the glass surface, and restricts the TMDC growth to only a surface reaction while eliminating unwanted gas-phase reaction. This feature gives rise to highly uniform monolayer TMDCs with a clean surface on centimeter-scale substrates. The DP growth works well for a large variety of TMDCs and their alloys, providing a solid foundation for the controlled growth of clean TMDCs by the fine control of the metal source., 2D materials with good quality and ultraclean surface were obtained by a universal dissolution-precipitation growth method that precisely controls the metal source and restricts the growth of materials to surface reaction.

In recent years, 2D layered semiconductors have received much attention for their potential in next-generation electronics and optoelectronics. Wide-bandgap 2D semiconductors are especially important in the blue and ultraviolet wavelength region, although there are very few 2D materials in this region. Here, monolayer β-type zirconium nitride chloride (β-ZrNCl) is isolated for the first time, which is an air-stable layered material with a bandgap of ≈3.0 eV in bulk. Systematical investigation of layer-dependent Raman scattering of ZrNCl from monolayer, bilayer, to bulk reveals a blueshift of its out-of-plane A

Currently, massive preparation of boron nitride nanosheets (BNNSs) towards large-size and good structural integrity via ball milling remains a key challenge, limiting its extensive applications in thermal management and energy storage. Low-cost and recyclable BaTiO3 nanoparticles with good piezoelectric effect can concurrently act as collision buffers and strong reductants to suppress the in-plane crystal structure damage and surface oxidation triggered by high-energy mechanical impact. Herein, an environmentally friendly, simple to operate and easy to realize industrial production method, referred to as BaTiO3-assisted ball milling (BTABM), is demonstrated to prepare high quality few-layered BNNSs with an average lateral size of 0.56 μm and a high yield of 91.2%. This milling process involves the synergetic effects of mechanical shear and mechanochemical peeling provided by deformed BaTiO3. The incorporation of the prepared BNNSs into the P(VDF-CTFE) matrix by BTABM effectively improved simultaneously the dielectric properties and breakdown strength of the ternary P(VDF-CTFE)/BaTiO3/BNNS nanocomposites as well as their thermal management capability, due to the formation of 3D BNNS networks. This novel method provides a guidance for not only scalable exfoliation of other layered 2D materials but also massive fabrication of polymeric nanocomposites applied in high-temperature energy storage devices.

Recent success in isolating and growing various two-dimensional (2D) materials with intriguing properties has pushed forward the search for new 2D nanostructures with novel properties. Current experimental trial-and-error methods face the fundamental challenges of low efficiency and a lack of clear guidelines. In contrast, based on state-of-the-art first-principles calculations and well-developed structural prediction algorithms, computational simulations can not only predict an increasing number of new 2D materials with desirable properties but also suggest their possible synthesis routes. Among them, many predictions, such as the growth of monolayer boron sheets (borophene), piezoelectricity in molybdenum disulfide (MoS2), ferroelectricity in tin telluride (SnTe), topological defects in transition metal dichalcogenides, Dirac cones in borophene, and high carrier mobility and mobility anisotropy in black phosphorene, have been verified by experiments, showing the accuracy of computational approaches, as well as their power in facilitating experimental exploration in 2D flatland. To date, the rapid expansion in theoretical work has generated a large number of very important results, but the overall picture of recent progress, current challenges, and future opportunities is rarely discussed. Accordingly, this review aims at providing information about current trends and future perspectives for 2D materials research. To achieve this, the review is organized as follows: (1) discussion of structural predictions in 2D materials using borophene as an example; (2) predictions of the electronic, optical, mechanical, and magnetic properties in various 2D materials; (3) discussion of the influence of defects on the structures and properties of 2D materials; and (4) evaluation of current progress in computational simulations and perspectives for future development.

The development of abundant and cheap electrocatalysts for the hydrogen evolution reaction (HER) has attracted increasing attention over recent years. However, to achieve low-cost HER electrocatalysis, especially in alkaline media, is still a big challenge due to the sluggish water dissociation kinetics as well as the poor long-term stability of catalysts. In this paper we report the design and synthesis of a two-dimensional (2D) MoS2 confined Co(OH)2 nanoparticle electrocatalyst, which accelerates water dissociation and exhibits good durability in alkaline solutions, leading to significant improvement in HER performance. A two-step method was used to synthesize the electrocatalyst, starting with the lithium intercalation of exfoliated MoS2 nanosheets followed by Co2+ exchange in alkaline media to form MoS2 intercalated with Co(OH)2 nanoparticles (denoted Co-Ex-MoS2), which was fully characterized by spectroscopic studies. Electrochemical tests indicated that the electrocatalyst exhibits superior HER acti...

Layered black phosphorus (BP) has attracted wide attention for mid-infrared photonics and high-speed electronics, due to its moderate band gap and high carrier mobility. However, its intrinsic band gap of around 0.33 electronvolt limits the operational wavelength range of BP photonic devices based on direct interband transitions to around 3.7 μm. In this work, we demonstrate that black arsenic phosphorus alloy (b-AsxP1–x) formed by introducing arsenic into BP can significantly extend the operational wavelength range of photonic devices. The as-fabricated b-As0.83P0.17 photodetector sandwiched within hexagonal boron nitride (hBN) shows peak extrinsic responsivity of 190, 16, and 1.2 mA/W at 3.4, 5.0, and 7.7 μm at room temperature, respectively. Moreover, the intrinsic photoconductive effect dominates the photocurrent generation mechanism due to the preservation of pristine properties of b-As0.83P0.17 by complete hBN encapsulation, and these b-As0.83P0.17 photodetectors exhibit negligible transport hystere...

Summary With the advances of the electronics industry, the continuing trend of miniaturization and integration imposes challenges of efficient heat removal in nanoelectronic devices. Two-dimensional (2D) materials, especially graphene and hexagonal boron nitride (h-BN), are widely accepted as ideal candidates for thermal management materials due to their high intrinsic thermal conductivity and good mechanical flexibility. In this review, we introduce phonon dynamics of solid materials and thermal measurement methods at nanoscale, and highlight the unique thermal properties of 2D materials in relation to sample thickness, domain size, and interfaces. In addition, we discuss recent achievements of thermal management applications in which 2D materials act as heat spreader and thermal interface materials based on their controlled growth and self-assembly. Finally, critical consideration on the challenges and opportunities in thermal management applications of 2D materials is presented.

Hollow micro-/nanostructures of metal oxides have attracted tremendous research attention in energy storage due to their unique structural advantages. Although fabrication of spherical hollow architectures have been intensively reported, rational design and facile synthesis of anisotropic hollow structures are still quite challenging, especially for those complex mixed metal oxides with well-controlled interior structures. Herein, through facile citrate-assisted hydrothermal synthesis and subsequent controlled annealing, well-defined octahedral ZnCo2O4 solid, hollow and yolk-shell micro-/nanostructures were constructed for the first time. When used as anode materials for lithium ion batteries (LIBs), the hollow ZnCo2O4 octahedron exhibits the best lithium storage properties, delivering high discharge capacities of 880 mA h g−1 over 160 cycles at 0.2 A g−1, and 650 mA h g−1 over 300 cycles at 1 A g−1. Moreover, it shows a superior high-rate performance with 60% of the specific capacity maintained even at a high current density of 5 A g−1. These features render the hollow octahedral ZnCo2O4 micro-/nanostructure a promising anode for next generation LIBs.