At the moment scanning transmission electron microscopy (stem) instruments are not competetive with conventionaltem instruments for high resolution bright field imaging. For studies of the structure and defects of crystalline materials, their special virtues lie in the application of dark field imaging modes combined with observations of microdiffraction patterns from regions of diameter comparable with the microscope resolution limit (currently about 5 Å). They also offer capabilities for microanalysis by use of energy dispersive x-ray spectroscopy (eds) or electron energy loss spectroscopy (els). In principle the spatial resolution of these microanalysis methods is comparable to that of the imaging modes but in practice it is limited by poor signal-to-noise ratios or by the nonlocalized nature of the inelastic scattering process.
The capabilities for microdiffraction are illustrated by sequences of diffraction patterns obtained as the incident beam is moved within the unit cell of a crystal of large (20 Å) periodicity. Applications of more immediate practical significance include diffraction studies of small crystallites of gold 20 to 50 Å in diameter and of the near-amorphous, thin oxide layers formed on chromium and iron films at room temperature.
Microdiffraction, combined with reflection electron microscopy andels analysis, provides a powerful new approach to the study of the surface structure of crystals, including bulk samples, and the investigation of surface reactions. In particular, if a beam of small diameter (10–20 Å) is made to run along the face of a small crystal, the diffraction pattern andels curves are very sensitive to the form of the potential distribution at the surface and the excitations of the surface states of the crystal.