SAD

Selected-area diffraction (SAD)

A diffraction pattern is made under broad, parallel electron illumination. An aperture in the image plane is used to select the diffracted region of the specimen, giving site-selective diffraction analysis. SAD patterns are a projection of the reciprocal lattice, with lattice reflections showing as sharp diffraction spots. By tilting a crystalline sample to low-index zone axes, SAD patterns can be used to identify crystal structures and measure lattice parameters. SAD is essential for setting up DF imaging conditions. Other uses of SAD include analysis of: lattice matching; interfaces; twinning and certain crystalline defects. SAD of nanocrystals gives ring patterns analogous to those from X-ray powder diffraction, and can be used to identify texture and discriminate nanocrystalline from amorphous phases. While SAD can obtain diffraction information from relatively small volumes, CBED (see below) is necessary for diffraction on the nanometre scale.