Dark field (DF)
An objective aperture is used, and the sample is tilted, so that only crystal(s) fulfilling a particular diffraction condition contribute to the TEM image. Aberrations from the perfect crystal, such as dislocations or second phases, do not fufil the diffraction condition, and so are discriminated as dark objects on the image. Commonly, the operator sets up a 2-beam condition, in which one lattice place diffracts very strongly, in order to obtain maximum image intensity. DF imaging is used, for example, for imaging crystalline defects, twinning, and second phase precipitates, and for determining dislocation types and Burger’s vectors. It is also useful for imaging size, shape and distribution of nanocrystals by selecting only a proportion of the nanocrystals to contribute to the image.