Transmission electron microscopy (TEM) – diffraction

About this technique

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Crystalline materials scatter electrons by Bragg diffraction. This results in the formation of patterns wherein the symmetries and intensities provide information on the crystalline structure of the material. The most frequently used diffraction mode on a transmission electron microscope (TEM) is selected area diffraction. Here an aperture is used to select the region of interest. This may be as small as 0.5 µm in diameter. Putting the microscope into diffraction mode results in a pattern being displayed on the screen. Single crystal regions of specimen, where the electron beam lies parallel to a low index crystallographic direction will result in simple spot patterns. The separation of the spots can be used to determine the d-spacing of the crystal (with an accuracy of about 1 %) and the angles between, and the relative spacings of, the spots can be used to determine the zone axis of the crystal. Where two phases are selected by the aperture (e.g. a precipitate within a matrix), then the composite pattern arising from the two lattices can be used to determine the orientation relationship between the phases. Where polycrystalline materials are examined, ring diffraction patterns are obtained. Here the ring radii may be converted into d-spacings.

Electron diffraction is a powerful adjunct to the imaging and spectroscopy capabilities of a TEM. Generally, one already knows a great deal about the material before carrying out diffraction. The limited accuracy and precision of electron diffraction when compared with X-ray diffraction for example, makes it difficult to identify phases with diffraction alone, in the absence of supporting information like composition.

There are a number of more specialised electron diffraction techniques. Nanobeam diffraction uses a very fine parallel probe of just a few namometres to generate diffraction information. This is useful for investigating discrete nanoparticles or localised regions such as interfacial layers. Convergent beam diffraction uses a highly convergent beam focused to a very small spot. This technique can be used for space group determination, precise orientation, foil thickness and a number of other highly specialised techniques.