Electron Beam Characterization of Carbon Nanostructures

by Mandell, Eric Samuel

Atom-thick carbon nanostructures represent a class of novel materials that are of interest to those studying carbon’s role in fossil fuel, hydrogen storage, scaled-down electronics, and other nanotechnology. Electron microscope images of “edge-on” graphene sheets show linear image features due to the projected potential of the sheets. Here, intensity profiles along these linear features can measure the curvature of the sheet, as well as the shape of the sheet (i.e. hexagonal, triangular). Also, electron diffraction powder profiles calculated for triangular graphene sheet shapes show a broadening of the lowfrequency edge of diffraction rings, in comparison to those calculated for hexagonal sheets with a similar number of atoms. Calculated powder profiles further indicate that curvature of a sheet will broaden the tailing edge of the diffraction peaks. These simulation results are applied to the characterization of nanocrystalline carbon cores found in a subset of graphitic presolar stardust. Electron diffraction data from these cores indicates they are comprised primarily of unlayered graphene sheets. Comparison to simulations indicates that these sheets are more triangular than equant, and thus likely the result of some anisotropic growth process. This assertion is separately supported by intensity profiles of linear features in HRTEM images. The density of the cores is further shown to be less than 90% of the density of graphitic rims surrounding these cores. This structural data constrains proposed grain formation mechanisms in AGB atmospheres, and opens up the unexpected possibility that these presolar cores may have been formed by the dendritic crystallization of liquid carbon droplets. Mandell, Eric, 2007, UMSL, p. iv