TEM / STEM Tomography
Transmission electron and scanning transmission electron tomography, techniques for the three-dimensional characterization of microstructural features at a nanometer length scale, make use of the fact that, despite their limited thickness, thin foils do have a third dimension. This limited thickness has traditionally caused difficulties in quantifying features observed in TEM micrographs which are projections of the three-dimensions. Such difficulties, including well-known phenomena such as the Holmes effect (i.e., overprojection) and particle truncation [1-3], make traditional 2-D stereological procedures such as volume fraction, difficult or impossible. However, the same third dimension (i.e., foil thickness) which renders traditional stereological methods impractical makes electron tomography possible.
Tomography is a technique where images are recorded from a specimen in as many viewing directions as possible. Thus, tomography is the integration of data collected from a series of images in which the orientation of the specimen relative to the incident beam is progressively varied, and the series then reconstructed. Traditionally in the TEM this has meant recording a series of TEM images at different tilt-angles, typically with a CCD camera and over the range of -70° to +70° at regular tilt intervals of approximately 1° or 2°. This approach has been applied successfully in life science for already more than decade [4-7]. In many cases it is possible to record data series by hand – though it is a very tedious process involving precise tilting, image shifting (one of the most important aspects of tomography to minimize information loss through variation in imaging area), focusing, imaging, and tilting. Researchers performing this manually can only process a very limited number of samples in a given timeframe, resulting in an increase in cost, both operationally (e.g., microscope time) and from an opportunity cost perspective.