One sentence summary:We describe a general liquid-phase method to exfoliate layered compounds to give monoand few-layer flakes in large quantities. TMDs consist of hexagonal layers of metal atoms, M, sandwiched between two layers of chalcogen atoms, X, with stoichiometry MX 2 . While the bonding within these tri-layer sheets is covalent, adjacent sheets stack via van der Waals interactions to form a 3D crystal. TMDs occur in more than 40 different types (2, 3) depending on the combination of chalcogen (S, Se or Te) and transition metal(3). Depending on the co-ordination and oxidation state of the metal atoms, TMDs can be metallic, semi-metallic or semiconducting(2, 3), e.g. WS 2 is a semiconductor while NbSe 2 is a metal(3). In addition, superconductivity(4) and charge density wave effects(5) have been observed in some TMDs. This versatility makes them potentially useful in many areas of electronics.However, like graphene(6), layered materials must be exfoliated to fulfil their full potential. For example, films of exfoliated Bi 2 Te 3 should display enhanced thermoelectric efficiency by suppression of thermal conductivity(7). Exfoliation of 2D topological insulators such as Bi 2 Te 3 and Bi 2 Se 3 would reduce residual bulk conductance, 4 highlighting surface effects. In addition, we can expect changes in electronic properties as the number of layers is reduced e.g. the indirect bandgap of bulk MoS 2 becomes direct in few-layer flakes(8). Although exfoliation can be achieved mechanically on a small scale(9, 10), liquid phase exfoliation methods are required for many applications(11).Critically, a simple liquid exfoliation method would allow the formation of novel hybrid and composite materials. While TMDs can be chemically exfoliated in liquids(12-14), this method is time consuming, extremely sensitive to the environment and incompatible with most solvents.We demonstrate exfoliation of bulk TMD crystals in common solvents to give mono-and few layer nano-sheets. This method is insensitive to air and water and can potentially be scaled up to give large quantities of exfoliated material. In addition, we show that this procedure allows the formation of hybrid films with enhanced properties.We initially sonicated commercial MoS 2 , WS 2 and BN (15, 16) powders in a number of solvents with varying surface tensions. The resultant dispersions were centrifuged and the supernatant decanted (Section S3). Optical absorption spectroscopy showed that the amount of material retained (characterised by / A l C α = , where A/l is the absorbance per length, α is the extinction coefficient and C is the concentration) was maximised for solvents with surface tension close to 40 mJ/m 2 (17, 18) ( Fig. 1A-C). Detailed analysis, within the framework of Hansen solubility parameter theory(19), shows successful solvents to be those with dispersive, polar and H-bonding components of the cohesive energy density within certain well-defined ranges (Section S4, Figs. S2-S3). This can be interpreted to mean that successful solvents are those w...

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Abstract:In order to progress from the lab to commercial applications it will be necessary to develop industrially scalable methods to produce large quantities of defect-free graphene.Here we show that high-shear mixing of graphite in suitable, stabilizing liquids results in large-scale exfoliation to give dispersions of graphene nanosheets. XPS and Raman spectroscopy show the exfoliated flakes to be unoxidised and free of basal plane defects. We have developed a simple model which shows exfoliation to occur once the local shear rate exceeds 10 4 s -1 . By fully characterizing the scaling behaviour of the graphene production rate, we show that exfoliation can be achieved in liquid volumes from 100s of ml up to 100s of litres and beyond. The graphene produced by this method performs well in applications from composites to conductive coatings. This method can be applied to exfoliate BN, MoS2 and a range of other layered crystals. Main Text:Due to its ultra-thin, 2-dimensional nature and its unprecedented combination of physical properties, graphene has become the most studied of all nano-materials. In the next decade graphene is likely to find commercial applications in many areas from high-frequency electronics to smart coatings.

A method is demonstrated to prepare graphene dispersions at high concentrations, up to 1.2 mg mL(-1), with yields of up to 4 wt% monolayers. This process relies on low-power sonication for long times, up to 460 h. Transmission electron microscopy shows the sonication to reduce the flake size, with flake dimensions scaling as t(-1/2). However, the mean flake length remains above 1 microm for all sonication times studied. Raman spectroscopy shows defects are introduced by the sonication process. However, detailed analysis suggests that predominantly edge, rather than basal-plane, defects are introduced. These dispersions are used to prepare high-quality free-standing graphene films. The dispersions can be heavily diluted by water without sedimentation or aggregation. This method facilitates graphene processing for a range of applications.

Owing to their unique mechanical properties, carbon nanotubes are considered to be ideal candidates for polymer reinforcement. However, a large amount of work must be done in order to realize their full potential. Effective processing of nanotubes and polymers to fabricate new ultra‐strong composite materials is still a great challenge. This Review explores the progress that has already been made in the area of mechanical reinforcement of polymers using carbon nanotubes. First, the mechanical properties of carbon nanotubes and the system requirements to maximize reinforcement are discussed. Then, main methods described in the literature to produce and process polymer–nanotube composites are considered and analyzed. After that, mechanical properties of various nanotube–polymer composites prepared by different techniques are critically analyzed and compared. Finally, remaining problems, the achievements so far, and the research that needs to be done in the future are discussed.

We have developed methods to exfoliate MoS 2 in large quantities in surfactant-water solutions. This method can be extended to a range of other layered compounds. The layered material tends to be exfoliated as relatively defect free flakes with lateral sizes of 100s of nm. 2With high surface area and novel properties, two-dimensional (2D) materials are potentially useful for a range of applications. In addition to graphene, many 2D compounds exist with BN, MoS 2 and Bi 2 Te 3 generating renewed interest. Such materials are found stacked in layered crystals and can be metals, semiconductors or insulators.[ tend to bond via van der Waals interactions, stacking to form 3D crystals. These materials span the whole gamut of electronic structures from insulator to metal [1] and display interesting properties [6] such as superconductivity, [3] thermoelectricity [2] and topological insulator effects.[4]While micro-mechanically exfoliated [7] single flakes of materials such as MoS 2 are ideal for electronic devices, [8] large scale liquid-phase exfoliation methods will lead to a range of thin film applications such as nano-scale hybrids for use in thermoelectrics, [9] supercapacitors [10] or Li-ion batteries [11] . One advantage of such applications is that, as the electronic properties of TMDs vary relatively slowly with layer number, [12,13] full exfoliation to monolayers is not necessary; dispersed few-layer flakes are sufficient.While a number of layered compounds can be exfoliated by ion intercalation, [14][15][16][17] this method is time consuming, extremely sensitive to environmental conditions and results in structural deformations in some TMDs.[18] Furthermore, removal of the ions results in re-aggregation of the layers.[19] More promisingly, it has recently been shown that both TMDs [20] and BN can be exfoliated in organic solvents. [21][22][23][24] However, for large-scale applications, exfoliation in an aqueous environment would be hugely advantageous. While BN can be dispersed in water due to sonication-assisted hydrolysis, this method cannot be extended to other layered compounds.[25] The discovery of a facile, scalable method to exfoliate a range of layered materials in water would assist the production and 3 characterisation of a range of new materials and greatly facilitate the potential transfer of such technology to industry. In this work we show that a number of layered crystals can be exfoliated in water, resulting in thin flakes stabilised by a surfactant coating. This method is robust, can be carried out in ambient conditions, is scalable and allows the preparation of films, hybrids and composites.One possible reason why ion intercalation has been prevalent for TMDs rather than other liquid based dispersion methods is the relatively high exfoliation (surface) energy of TMDs. Computational studies have estimated this as greater than 250 mJ/m 2 for both MoS 2 and WS 2 ; [26,27] many times greater than that of graphene [28] or BN [29] . We suggest that sonication can be used to exfoliate TMDs in water,...

A method is presented to produce graphene dispersions, stabilized in water by the surfactant sodium cholate, at concentrations up to 0.3 mg/mL. The process uses low power sonication for long times (up to 400 h) followed by centrifugation to yield stable dispersions. The dispersed concentration increases with sonication time while the best quality dispersions are obtained for centrifugation rates between 500 and 2000 rpm. Detailed TEM analysis shows the flakes to consist of 1-10 stacked monolayers with up to 20% of flakes containing just one layer. The average flake consists of approximately 4 stacked monolayers and has length and width of approximately 1 mum and approximately 400 nm, respectively. These dimensions are surprisingly stable under prolonged sonication. However, the mean flake length falls from approximately 1 mum to approximately 500 nm as the centrifugation rate is increased from 500 to 5000 rpm. Raman spectroscopy shows the flake bodies to be relatively defect-free for centrifugation rates below 2000 rpm. The dispersions can be easily cast into high-quality, free-standing films. The method extends the scope for scalable liquid-phase processing of graphene for a wide range of applications.

Despite its widespread use in nanocomposites, the effect of embedding graphene in highly viscoelastic polymer matrices is not well-understood. We add graphene to a lightly crosslinked polysilicone, often encountered as Silly Putty, changing its electro-mechanical properties significantly. The resulting nanocomposites display unusual electromechanical behavior such as post-deformation temporal relaxation of electrical resistance and nonmonotonic changes in resistivity with strain. These phenomena are associated with the mobility of the nanosheets in the low-viscosity polymer matrix. By considering both the connectivity and mobility of the nanosheets, we develop a quantitative model that completely describes the electromechanical properties. These nanocomposites are sensitive electromechanical sensors with gauge factors >500 which can measure pulse, blood pressure and even the impact associated with the footsteps of a small spider.