Spatial resolution in transmission electron microscopy.

We review the practical factors that determine the spatial resolution of transmission electron microscopy (TEM) and scanning-transmission electron microscopy (STEM), then enumerate the advantages of representing resolution in terms of a point-spread function. PSFs are given for the major resolution-limiting factors: aperture diffraction, spherical and chromatic aberration, beam divergence, beam broadening, Coulomb delocalization, radiolysis damage and secondary-electron generation from adatoms or atoms in a matrix. We note various definitions of beam broadening, complications of describing this effect in very thin specimens, and ways of optimizing the resolution in bright-field STEM of thick samples. Beam spreading in amorphous and crystalline materials is compared by means of simulations. For beam-sensitive specimens, we emphasize the importance of dose-limited resolution (DLR) and briefly recognize efforts to overcome the fundamental resolution limits set by the wave and particle properties of electrons.