ELECTRON TOMOGRAPHY FOR GEOLOGICAL SCIENCES

During the past decade progress in the study of materials ordered at the nanometer scale has increased the demand for techniques capable of imaging nanoparticles in 3D. Electron tomography is a promising technique that can be used to reconstruct 3D structures using a series of 2D electron micrographs. Our interest in this technique is to determine the 3D shapes of nanometer-sized particles in order to derive information about their origins and to measure their volumes and surface areas.

We investigated biogenic and synthetic magnetite crystals, carbonaceous aerosol particles (soot), and grains of a chrysotile-like phase (Mighei meteorite) using a Tecnai F20 transmission electron microscope (TEM), in combination with bright-field (BF) TEM and high-angle annular dark-field (HAADF) STEM tomography. The 3D structures were reconstructed using interactive data language (IDL®) scripts based on the simultaneous iterative reconstruction technique (SIRT).

The 3D reconstructions of magnetite show that crystals produced by magnetotactic bacteria strain MV-1 have a distinctly different shape compared with crystals produced by a simple chemical process. The 3D structure of the meteoritic chrysotile-like phase suggests that it has a tube-like (rather than spherical) morphology, a result that could not be determined from conventional electron micrographs. The 3D reconstructions obtained for soot particles show their highly complex 3D structures and allow for measurement of volumes and surface areas. These are the first such measurements of these properties of single carbon particles. It is apparent that electron tomography is highly useful for determining the 3D morphology of nanometer-sized particles from which questions like origin, volume or surface area can be derived.