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...

We have prepared polyvinylalcohol-SWNT fibers with diameters from ∼1 to 15 μm by coagulation spinning. When normalized to nanotube volume fraction, V(f), both fiber modulus, Y, and strength, σ(B), scale strongly with fiber diameter, D: Y/V(f) ∝ D(-1.55) and σ(B)/V(f) ∝ D(-1.75). We show that much of this dependence is attributable to correlation between V(f) and D due to details of the spinning process: V(f) ∝ D(0.93). However, by carrying out Weibull failure analysis and measuring the orientation distribution of the nanotubes, we show that the rest of the diameter dependence is due to a combination of defect and orientation effects. For a given nanotube volume fraction, the fiber strength scales as σ(B) ∝ D(-0.29)D(-0.64), with the first and second terms representing the defect and orientation contributions, respectively. The orientation term is present and dominates for fibers of diameter between 4 and 50 μm. By preparing fibers with low diameter (1-2 μm), we have obtained mean mechanical properties as high as Y = 244 GPa and σ(B) = 2.9 GPa.

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Electrospinning has been used to produce porous, low density, polymer–nanotube composite membranes. The membrane mechanical properties can be enhanced by tuning the nanotube content, aligning the fibers during spinning, and by post production drawing. The mechanical properties are maximized for membranes with a nanotube content of 0.43 vol %. Aligned composites at this volume fraction have been prepared by spinning onto a rotating drum collector electrode. This method results in significant increases in modulus, strength, and toughness. The best composites, produced at the maximum drum rotation rate, were post treated by a drawing step to result in further increases in modulus and strength. These methods allows the production of membranes with densities as low as ∼340 kg m−3 but with values of stiffness, strengths and toughness's more typically found in bulk thermoplastics; 1.2 GPa, 40 MPa, and 13 J g−1.

We have prepared composite fibres based on the polyester, polyethylene terephthalate (PET), filled with both single walled nanotubes and graphene by a combination of solution and melt processing. On addition of #2 wt% filler we observe increases in both modulus and strength by factors of between Â2 and Â4 for both fillers. For the nanotube-based fibres, the mechanical properties depend strongly on fibre diameter due to a combination of defect and nanotube orientation effects. For the graphene filled fibres, the modulus is approximately invariant with diameter while the strength is defect limited, scaling weakly with diameter. Using this production method, the best fibre we prepared had modulus and strength of 42 GPa and 1.2 GPa respectively (2 wt% SWNT). We attribute this reinforcement predominately to the dispersion quality resulting from the solvent exfoliation of both nanotubes and graphene. In general, marginally better reinforcement was observed for the nanotube filled fibres. However, because of the low cost of graphite, we suggest graphene to be the superior reinforcement material for polymer fibres.