Phase Contrast in the Scanning Transmission Electron Microscope

Abstract

It has already been shown 1,2,3 that the phase contrast in both the scanning transmission electron microscope and the conventional electron microscope is given by the same expression provided that the angular correspondences in Fig. 1 are maintained. This provides much useful information to the scanning microscope user as it suggests many phase contrast and interference techniques which may be used. However, the scanning microscope is a more flexible instrument, particularly with regard to the choice of the aperture angle βo and the use of a spectrometer between object and detector. In order to observe these phase effects it is necessary for the illuminating scanning spot to possess transverse coherence. This implies both that the chromatic aberration and the paraxial image of the electron source be small compared with the diffraction aberration.