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

TEM Specimens and Images 109
(discussed in Section 4.3). Instead of a continuous angular distribution, as
depicted in Fig. 4-5, the scattering is concentrate`d into sharp peaks (known
as Bragg peaks; see Fig. 4-12d) that occur at scattering angles that are twice
the Bragg angle �B for atomic planes of a particular orientation.
Figure 4-12. (a) Bright-field TEM image of a polycrystalline thin film of bismuth, including
bend contours that appear dark. (b) Dark-field image of the same area; bend contours appear
bright. (c) Ring diffraction pattern obtained from many crystallites; for recording purposes,
the central (0,0,0) diffraction spot has been masked by a wire. (d) Spot diffraction pattern
recorded from a single Bi crystallite, whose trigonal crystal axis was parallel to the incident
beam. Courtesy of Marek Malac, National Institute of Nanotechnology, Canada.
This angular distribution of scattering can be displayed on the TEM
screen by weakening the intermediate lens, so that the intermediate and
projector lenses magnify the small diffraction pattern initially formed at the
back-focal plane of the objective lens. Figure 4-12c is typical of the pattern
obtained from a polycrystalline material. It consists of concentric rings,
centered on a bright central spot that represents the undiffracted electrons.
Each ring corresponds to atomic planes of different orientation and different
interplanar spacing d, as indicated in Fig. 4-11b.