Liquid phase production of graphene by exfoliation of graphite in surfactant/water solutions.

We have demonstrated a method to disperse and exfoliate graphite to give graphene suspended in water-surfactant solutions. Optical characterization of these suspensions allowed the partial optimization of the dispersion process. Transmission electron microscopy showed the dispersed phase to consist of small graphitic flakes. More than 40% of these flakes had <5 layers with approximately 3% of flakes consisting of monolayers. Atomic resolution transmission electron microscopy shows the monolayers to be generally free of defects. The dispersed graphitic flakes are stabilized against reaggregation by Coulomb repulsion due to the adsorbed surfactant. We use DLVO and Hamaker theory to describe this stabilization. However, the larger flakes tend to sediment out over approximately 6 weeks, leaving only small flakes dispersed. It is possible to form thin films by vacuum filtration of these dispersions. Raman and IR spectroscopic analysis of these films suggests the flakes to be largely free of defects and oxides, although X-ray photoelectron spectroscopy shows evidence of a small oxide population. Individual graphene flakes can be deposited onto mica by spray coating, allowing statistical analysis of flake size and thickness. Vacuum filtered films are reasonably conductive and are semitransparent. Further improvements may result in the development of cheap transparent conductors.

Preparation of buckypaper-copper composites and investigation of their conductivity and mechanical properties.

Buckypaper-metal composites are prepared by substrate-enhanced electroless deposition of copper onto strips of buckypaper (see picture). Electrical conductivity and mechanical properties of these buckypaper-copper composites at different copper content are investigated. The conductivity is shown to increases by up to 350 %, and the Young's modulus and the ultimate tensile strength of the composites by 282 % and 290 % respectively over pristine buckypaper.

Strong dependence of mechanical properties on fiber diameter for polymer-nanotube composite fibers: differentiating defect from orientation effects.

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.

Towards tough, yet stiff, composites by filling an elastomer with single-walled nanotubes at very high loading levels.

We have used recent advances in nanotube dispersion technology to prepare composites based on polyurethane, with mass fractions of up to 80% polyethylene glycol functionalized nanotubes. Mechanical testing shows increases in Young's modulus compared to polyurethane films by up to 800 ×. While the composite strength did not vary significantly with nanotube content, the ductility and so the toughness fell by a factor of 240 × on addition of ∼40 wt% nanotubes. Depending on the nanotube content we can produce films ranging from the stiff and brittle at high nanotube loading to the compliant and ductile at low nanotube volume fraction.

High-yield production of graphene by liquid-phase exfoliation of graphite.

Fully exploiting the properties of graphene will require a method for the mass production of this remarkable material. Two main routes are possible: large-scale growth or large-scale exfoliation. Here, we demonstrate graphene dispersions with concentrations up to approximately 0.01 mg ml(-1), produced by dispersion and exfoliation of graphite in organic solvents such as N-methyl-pyrrolidone. This is possible because the energy required to exfoliate graphene is balanced by the solvent-graphene interaction for solvents whose surface energies match that of graphene. We confirm the presence of individual graphene sheets by Raman spectroscopy, transmission electron microscopy and electron diffraction. Our method results in a monolayer yield of approximately 1 wt%, which could potentially be improved to 7-12 wt% with further processing. The absence of defects or oxides is confirmed by X-ray photoelectron, infrared and Raman spectroscopies. We are able to produce semi-transparent conducting films and conducting composites. Solution processing of graphene opens up a range of potential large-area applications, from device and sensor fabrication to liquid-phase chemistry.