Physical Principles of Electron Microscopy: An Introduction to TEM ...

108 Chapter 4
remain undiffracted (undeviated) and will pass through the objective
aperture. The grain structure of a polycrystalline material can therefore be
seen in a TEM image as an intensity variation between different regions; see
Fig. 4-12a or Fig. 4-21. A TEM image is said to display diffraction contrast
in any situation in which the intensity variations arise from differences in
diffracting conditions between different regions of the specimen.
4.6 Dark-Field Images
Instead of selecting the undiffracted beam of electrons to form the image, we
could horizontally displace the objective aperture so that it admits diffracted
electrons. Strongly diffracting regions of specimen would then appear bright
relative to their surroundings, resulting in a dark-field image because any
part
of the field of view that contains no specimen would be dark.
Figure 4-12b is an example of dark-field image and illustrates the fact that
the contrast is inverted relative to a bright-field image of the same region of
specimen. In fact, if it were possible to collect all of the scattered electrons
to form the dark-field image, the two images would be complementary: the
sum
of their intensities would be constant (equivalent to zero contrast).
Unless stated otherwise, TEM micrographs are usually bright-field images
created with an objective aperture centered about the optic axis. However,
one advantage of a dark-field image is that, if the displacement of the
objective aperture has been calibrated, the atomic-plane spacing (d) of a
bright image feature is known. This information can sometimes be used to
identify crystalline precipitates in an alloy, for example.
With the procedure just described, the image has inferior resolution
compared to a bright-field image because electrons forming the image pass
though the imaging lenses at larger angles relative to the optic axis, giving
rise to increased spherical and chromatic aberration. However, such loss of
resolution can be avoided by keeping the objective aperture on-axis and reorienting
the electron beam arriving at the specimen, using the illuminationtilt
deflection coils that are built into the illumination system of a materialsscience
TEM.
4.7 Electron-Diffraction Patterns
Diffraction is the elastic scattering of electrons (deflection by the Coulomb
field of atomic nuclei) in a crystalline material. The regularity of the spacing
of the atomic nuclei results in a redistribution of the angular dependence of
scattering probability, in comparison with that obtained from a single atom