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The 1T metallic phase of MoS 2 shows high volumetric capacitance and electrochemical properties that are attractive for supercapacitor applications. Efficient intercalation of ions in layered materials forms the basis of electrochemical energy storage devices such as batteries and capacitors 1 , 2 , 3 , 4 , 5 , 6 . Recent research has focused on the exfoliation of layered materials and then restacking the two-dimensional exfoliated nanosheets to form electrodes with enhanced electrochemical response 7 , 8 , 9 , 10 , 11 . Here, we show that chemically exfoliated nanosheets of MoS 2 containing a high concentration of the metallic 1T phase can electrochemically intercalate ions such as H + , Li + , Na + and K + with extraordinary efficiency and achieve capacitance values ranging from ∼400 to ∼700 F cm −3 in a variety of aqueous electrolytes. We also demonstrate that this material is suitable for high-voltage (3.5 V) operation in non-aqueous organic electrolytes, showing prime volumetric energy and power density values, coulombic efficiencies in excess of 95%, and stability over 5,000 cycles. As we show by X-ray diffraction analysis, these favourable electrochemical properties of 1T MoS 2 layers are mainly a result of their hydrophilicity and high electrical conductivity, as well as the ability of the exfoliated layers to dynamically expand and intercalate the various ions.

Low-dimensional materials and their hybrids have emerged as promising candidates for electrocatalytic and photocatalytic hydrogen evolution and CO 2 conversion into useful molecules. Progress in synthetic methods for the production of catalysts coupled with a better understanding of the fundamental catalytic mechanisms has enabled the rational design of catalytic nanomaterials with improved performance and selectivity. In this Review, we analyse the state of the art in the implementation of low-dimensional nanomaterials and their van der Waals heterostructures for hydrogen evolution and CO 2 reduction by electrocatalysis and photocatalysis. We explore the mechanisms involved in both reactions and the different strategies to further optimize the activity, efficiency and selectivity of low-dimensional catalysts. The electrochemical oxidation and reduction of water and carbon dioxide are associated with the release or storage of energy. This Review reports the latest developments in the design and use of low-dimensional materials and their van der Waals heterostructures for electrocatalytic and photocatalytic hydrogen evolution and CO 2 conversion.

Owing to their large absorption cross-sections and high photoluminescence quantum yields, lead halide perovskite quantum dots (PQDs) are regarded as a promising candidate for various optoelectronics applications. However, easy degradation of PQDs in water and in a humid environment is a critical hindrance for applications. Here we develop a Pb-S bonding approach to synthesize water-resistant perovskite@silica nanodots keeping their emission in water for over six weeks. A two-photon whispering-gallery mode laser device made of these ultra-stable nanodots retain 80% of its initial emission quantum yield when immersed in water for 13 h, and a two-photon random laser based on the perovskite@silica nanodots powder could still operate after the nanodots were dispersed in water for up to 15 days. Our synthetic approach opens up an entirely new avenue for utilizing PQDs in aqueous environment, which will significantly broaden their applications not only in optoelectronics but also in bioimaging and biosensing. Lead halide perovskite quantum dots (PQDs) promise applications in optoelectronics but are limited by sensitivity to wet environments. Here the authors develop a Pb-S bonding approach to synthesize PQDs@silica nanodots that are capable of emitting and lasing in aqueous environments for long periods.

Organic–inorganic hybrid perovskites have been intensively researched in the past decade for their optoelectronic properties. The emergence of Ruddlesden–Popper perovskites, which have mixed dimensionality, has heralded new opportunities for tailor-made semiconductors that combine enhanced stability with useful properties between those of 2D and 3D systems. Inspired by advances in 2D materials research, there is growing interest in molecularly thin versions of these hybrid perovskites, owing to their ease of incorporation into electronic devices. There is, thus, a need to understand thickness-dependent electrical, excitonic and phononic properties that go beyond quantum-confinement effects. Recent studies have shown that, apart from tuning the dimensionality of the system, fine-tuning its thickness also helps to optimize performance in different applications, ranging from third-harmonic generation to photodetectors and spintronics. Owing to their layered structure, the properties of 2D perovskites can be controlled by tuning their thickness. This Review surveys how fine-tuning the thickness of 2D perovskites from the sub-micrometre to the molecularly thin regime helps to optimize their electrical and optical properties for use in different applications.

The integration of novel materials such as single-walled carbon nanotubes and nanowires into devices has been challenging, but developments in transfer printing and solution-based methods now allow these materials to be incorporated into large-area electronics 1 , 2 , 3 , 4 , 5 , 6 . Similar efforts are now being devoted to making the integration of graphene into devices technologically feasible 7 , 8 , 9 , 10 . Here, we report a solution-based method that allows uniform and controllable deposition of reduced graphene oxide thin films with thicknesses ranging from a single monolayer to several layers over large areas. The opto-electronic properties can thus be tuned over several orders of magnitude, making them potentially useful for flexible and transparent semiconductors or semi-metals. The thinnest films exhibit graphene-like ambipolar transistor characteristics, whereas thicker films behave as graphite-like semi-metals. Collectively, our deposition method could represent a route for translating the interesting fundamental properties of graphene into technologically viable devices.

Chemically derived graphene oxide (GO) has recently moved on from simply being a graphene precursor to attracting interest for its own properties. This Review discusses how the presence of oxygenated groups and domains of sp 2 - and sp 3 -hybridized carbons makes GO tunable and promising for various physical and biological applications. Chemically derived graphene oxide (GO) is an atomically thin sheet of graphite that has traditionally served as a precursor for graphene, but is increasingly attracting chemists for its own characteristics. It is covalently decorated with oxygen-containing functional groups — either on the basal plane or at the edges — so that it contains a mixture of sp 2 - and sp 3 -hybridized carbon atoms. In particular, manipulation of the size, shape and relative fraction of the sp 2 -hybridized domains of GO by reduction chemistry provides opportunities for tailoring its optoelectronic properties. For example, as-synthesized GO is insulating but controlled deoxidation leads to an electrically and optically active material that is transparent and conducting. Furthermore, in contrast to pure graphene, GO is fluorescent over a broad range of wavelengths, owing to its heterogeneous electronic structure. In this Review, we highlight the recent advances in optical properties of chemically derived GO, as well as new physical and biological applications.

In addition to graphene, a wide range of layered materials, including oxides, chalcogenides, and clays are of interest because of their optical, electrical, and mechanical properties. While many methods can be used to cleave layered sheets from the bulk material, they are difficult to scale up. Liquid exfoliation routes may hold the best promise for making materials in large quantities. Nicolosi et al. (p. 1226419 ) review progress in developing exfoliation routes, both aqueous and nonaqueous for a wide range of starting materials. Not all crystals form atomic bonds in three dimensions. Layered crystals, for instance, are those that form strong chemical bonds in-plane but display weak out-of-plane bonding. This allows them to be exfoliated into so-called nanosheets, which can be micrometers wide but less than a nanometer thick. Such exfoliation leads to materials with extraordinary values of crystal surface area, in excess of 1000 square meters per gram. This can result in dramatically enhanced surface activity, leading to important applications, such as electrodes in supercapacitors or batteries. Another result of exfoliation is quantum confinement of electrons in two dimensions, transforming the electron band structure to yield new types of electronic and magnetic materials. Exfoliated materials also have a range of applications in composites as molecularly thin barriers or as reinforcing or conductive fillers. Here, we review exfoliation—especially in the liquid phase—as a transformative process in material science, yielding new and exotic materials, which are radically different from their bulk, layered counterparts.

As the dimensions of the semiconducting channels in field-effect transistors decrease, the contact resistance of the metal–semiconductor interface at the source and drain electrodes increases, dominating the performance of devices 1 – 3 . Two-dimensional (2D) transition-metal dichalcogenides such as molybdenum disulfide (MoS 2 ) have been demonstrated to be excellent semiconductors for ultrathin field-effect transistors 4 , 5 . However, unusually high contact resistance has been observed across the interface between the metal and the 2D transition-metal dichalcogenide 3 , 5 – 9 . Recent studies have shown that van der Waals contacts formed by transferred graphene 10 , 11 and metals 12 on few-layered transition-metal dichalcogenides produce good contact properties. However, van der Waals contacts between a three-dimensional metal and a monolayer 2D transition-metal dichalcogenide have yet to be demonstrated. Here we report the realization of ultraclean van der Waals contacts between 10-nanometre-thick indium metal capped with 100-nanometre-thick gold electrodes and monolayer MoS 2 . Using scanning transmission electron microscopy imaging, we show that the indium and gold layers form a solid solution after annealing at 200 degrees Celsius and that the interface between the gold-capped indium and the MoS 2 is atomically sharp with no detectable chemical interaction between the metal and the 2D transition-metal dichalcogenide, suggesting van-der-Waals-type bonding between the gold-capped indium and monolayer MoS 2 . The contact resistance of the indium/gold electrodes is 3,000 ± 300 ohm micrometres for monolayer MoS 2 and 800 ± 200 ohm micrometres for few-layered MoS 2 . These values are among the lowest observed for three-dimensional metal electrodes evaporated onto MoS 2 , enabling high-performance field-effect transistors with a mobility of 167 ± 20 square centimetres per volt per second. We also demonstrate a low contact resistance of 220 ± 50 ohm micrometres on ultrathin niobium disulfide (NbS 2 ) and near-ideal band offsets, indicative of defect-free interfaces, in tungsten disulfide (WS 2 ) and tungsten diselenide (WSe 2 ) contacted with indium alloy. Our work provides a simple method of making ultraclean van der Waals contacts using standard laboratory technology on monolayer 2D semiconductors. Ultraclean van der Waals bonds between gold-capped indium and a monolayer of the two-dimensional transition-metal dichalcogenide molybdenum disulfide show desirably low contact resistance at the interface, enabling high-performance field-effect transistors.

Efficient exfoliation of graphite in solutions to obtain high-quality graphene flakes is desirable for printable electronics, catalysis, energy storage, and composites. Graphite oxide with large lateral dimensions has an exfoliation yield of ~100%, but it has not been possible to completely remove the oxygen functional groups so that the reduced form of graphene oxide (GO; reduced form: rGO) remains a highly disordered material. Here we report a simple, rapid method to reduce GO into pristine graphene using 1- to 2-second pulses of microwaves. The desirable structural properties are translated into mobility values of >1000 square centimeters per volt per second in field-effect transistors with microwave-reduced GO (MW-rGO) as the channel material and into particularly high activity for MW-rGO catalyst support toward oxygen evolution reactions.

A hexagonal boron nitride aerogel has high resistance to thermal and mechanical shock Materials that operate in extreme environments, such as aerospace applications that require operation at high temperatures and in reactive atmospheres, must be ultralight, very mechanically strong, and thermally insulating. Achieving such disparate functionalities requires rational design not only of the material itself but also of hierarchical structures at multiple length scales that can respond in the desired way to extreme environmental factors in real time. On page 723 of this issue, Xu et al. ( 1 ) report the synthesis of a multifunctional structure with hyperbolic surfaces (saddle shapes with negative curvature) in the form of an aerogel where the solid medium is a network of atomically thin sheets of hexagonal boron nitride (hBN). By careful mechanical design of the microstructure, the authors report that their aerogels exhibit extraordinary mechanical and thermal resistance properties far superior to those of current aerogels. Their discovery opens new pathways for the integration of rationally designed ultralightweight materials with the correct combination of mechanical and thermal properties for a variety of extreme environments.