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.

Electronic and optical properties of magnesium phthalocyanine (MgPc) solid films studied by soft X-ray excited optical luminescence and X-ray absorption spectroscopies.

In a study to link the optical and structural properties of solid films of magnesium Phthalocyanine (MgPc), a range of synchrotron based spectroscopic methods have been used. These include X-ray excited optical luminescence (XEOL) together with X-ray absorption spectroscopy (XAS) measured both by total electron yield methods (TEY) and by using the optically detected photoluminescence yield method (PLY). XEOL spectra below K shell threshold show a broad emission peak at approximately 860 nm which can be attributed to the optical Q-band of these organic systems, which is then suppressed above the threshold. The shift to higher wavelength compared to optical emission spectra from MgPc in solution is consistent with intermolecular coupling of the excited states in the loosely intermolecular bonded phthalocyanine crystal structure. Zero order total PLY spectra at both C and N K edges are compared to TEY spectra where at the C K edge an inversion of intensity ratios between features is observed. Wavelength-specific PLY absorption spectra taken at 860 nm at the N K edge show a role for sigma* states participating in the luminescence process possibly through the sigma-like lone pair of bridging nitrogen atom, denoted the n --> pi* transition.

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.

Electronic Band Structure of Monolayer Bi on GaP(110).

The two-dimensional electronic band structure of monolayer Bi on GaP(110) has been mapped using angle-resolved UV photoelectron spectroscopy (ARUPS) with synchrotron radiation. Surface photovoltage effects are corrected for by simultaneous second-order core spectroscopy. From valence-band spectra along the four symmetry directions of the surface Brillouin zone at three photon energies it is possible to distinguish three states as surface related. The topmost band is found to be inside the fundamental band gap, at ca 0.75 eV above the bulk valence-band maximum. Comparison with other V/III-V(110) systems shows that this system is not significantly different, despite the relatively large size of the Bi atoms with respect to the GaP lattice; in a selective comparison with InP and GaAs it would appear that Bi-substrate anion bonding is a more important factor than strain.