Electron tomography

It is the nature of ‘transmission’ that we can never overlook the finite thickness of our specimens. We will always lose information about the specimen in the beam direction; this can be both concealing and misleading. However, in some cases in order to successfully characterize the structure of nanoscale materials this information is invaluable. So how do we return the missing information, and resolve three-dimensional(3D) information from what is essentially a two dimensional (2D) technique? The problem of determining higher dimensionality information from lower dimensionality data arises in many technical disciplines; from radio astronomy to medical imaging to geophysics. Tomography,from the Greek ‘tomos’ – to slice, describes a broad class of techniques to solve this problem. Electron tomography, specifically,can be used to restore the 3D structure of a specimen from a series of 2D micrographs, usually acquired at a range of tilts. Electron tomography was briefly mentioned in Sect. 1.3.B and 29.1 of W&C and specifically in relation to energy loss tomography in Sect. 40.9; this section seeks to expand on this brief introduction. While electron tomography was developed primarily as a tool for the examination of macromolecular assemblies and cellular structure, in this chapter we will explore how this technique can be applied in the physical sciences. This chapter will address some of the most important considerations when applying this technique for materials science. We will cover in some detail the choice of imaging technique, introduce the experimental limitations of the microscopy, cover the processing of the tomography data from tilt series to volume, and discuss the care that you must take in interpreting the results.