Physics Colloquium-February 23, 2012

Graphene Cores in Graphitic Stardust

Abstract: Presolar graphite spherules are a subset of graphitic stardust exhibiting an intriguing micron-sized nanocrystalline core that is surrounded by concentric graphitic layers, similar to those of a carbon onion. These grains are presolar as indicated by isotopic measurements being significantly different from solar values (i.e. C12/C13 < solar = 89). The r and s type nuclear processes required to explain these aberrant isotopic ratios suggest grain-forming regions of red giant (AGB) atmospheres as a likely point of origin for this subset of meteoritic dust. Electron diffraction data has indicated the cores are comprised primarily of unlayered graphene sheets, approximately 2-4 nm in breadth. Previous diffraction modeling and simulation has focused on examining the differences between the experimental data and a flat, hexagonal graphene diffraction model. Here, improvements in fitting experimental diffraction profiles are realized when altering the shape of the graphene sheet. In addition, curvature of atom-thick sheets or regular relationships between neighboring sheets can introduce coherence effects which manifest in diffraction. Analysis of diffraction in conjunction with HRTEM imaging, EFTEM imaging, EELS, HAADF imaging, and comparisons to simulations, begins to elucidate the structure of this material and opens up the unexpected possibility that these presolar cores may have been formed by the dendritic crystallization of liquid carbon droplets in an AGB atmosphere.

by
Eric Mandell
Department of Physics and Astronomy
Bowling Green State University