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

104 Chapter 4
replica
crystallites
metal substrate
(a) (b)
Figure 4-7. Plastic or carbon replica coating the surface of (a) a metal containing grainboundary
grooves and (b) a flat substrate containing raised crystallites. Vertical arrows
indicate electrons that travel through a greater thickness of the replica material and therefore
have a greater probability of being scattered and intercepted by an objective diaphragm.
Figure 4-8. Bright-field TEM image of carbon replica of lead selenide (PbSe) crystallites
(each about 0.1 �m across) deposited in vacuum onto a flat mica substrate.
In many cases, thickness gradient contrast is obtained: through diffusion,
the carbon acquires the same thickness (measured perpendicular to the local
surface), but its projected thickness (in the direction of the electron beam) is
greater at the edges of protruding features (Fig. 4-7b). These features
therefore appear dark in outline in the scattering-contrast image of the
replica;
see Fig. 4-8.
Because most surface replicas consist mainly of carbon, which has a
relatively low scattering cross section, they tend to provide low image
contrast in the TEM. Therefore, the contrast is often increased using a
process known as shadowing. A heavy metal such as platinum is evaporated
(in vacuum) at an oblique angle of incidence onto the replica, as shown in
Fig. 4-9. After landing on the replica, platinum atoms are (to a first
approximation) immobile, therefore raised features present in the replica cast
sharp “shadows” within which platinum is absent. When viewed in the
TEM, the shadowed replica shows strong atomic-number contrast. Relative
to their surroundings, the shadowed areas appear bright on the TEM screen,