The Importance of the Detector Angle in STEM Imaging of Crystals
The image obtained in a scanning transmission electron microscope (STEM) is related to that obtained, under otherwise identical conditions, in the conventional transmission electron microscope (CTEM) by the principle of reciprocity. This principle is only applicable under rigorously defined mathematical conditions which are difficult to achieve in practice, nevertheless its general validity has been demonstrated experimentally for both amorphous and crystalline materials. The principle of reciprocity states, in part, that if equivalent beam cone angles are utilized then the STEM and CTEM images should be similar. Conversely, for a given set of electron-optical conditions in the STEM mode, the expected form of the images can be deduced from a knowledge of the CTEM image under reciprocally related conditions.