STEM, a tiny, convergent electron beam is scanned over a defined
area of the sample. At each spot, the generated signal is simultaneously
recorded by selected detectors, building up an image. Furthermore,
such a convergent beam is used to gain a highly localized signal
from the specimen in analytical TEM (e.g., EDXS, EELS), and thus the combination of STEM
with analytical methods is a main application in practical work. As the resolution of a STEM is limited by the diameter of the electron beam, the correction of the spherical aberration of the probe-forming lens leads to substantial improvements of the imaging properties and gives access to the sub Angstrom regime (see here).
signals: elastically scattered
electrons - direct and diffracted beams (BF/DF) and incoherent
scattering (Z contrast)
of crystalline areas, defects and grain boundaries, phase
analysis, particle size
of small clusters or even single atoms of a heavy metal
in a matrix of light elements (Z-contrast), direct
visualisation of structures and defects
introduction into theory and practice of STEM is given by Rodenburg.