Page 136 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

122 Principles and Methods

TEM SEM
Cathode

Anode source of electrons
Filament
Condensor Diaphragm Anode Image
Lens
Lens
Condenser Scan
generator
Specimen
Condensor aperture Diaphragm Beam Magnification
EDS
Lens
Intermediary
lens control Cathode
objective
Photo multiplier
Observation screen
Specimen collector
TEM detector
Figure 4.11 Schematics illustrating the operating principles of SEM and TEM
microscopes.

of the sample. Since the scattering angle is strongly dependent on the
atomic number of the nucleus involved, the primary electrons arriving
at a given detector position can be used to yield images containing both
topological and compositional information. The backscattering mode is
generally used on a polished section to minimize the effects of local topol-
ogy and therefore obtain information on the composition of the sample.
The high-energy incident electrons can also interact with loosely
bound conduction band electrons in a sample. The amount of energy
given to these secondary electrons as a result of these interactions is
small, and so they have a very limited range in the sample (a few
nanometers). Because of this, only secondary electrons that are emitted
within a very short distance of the surface are able to escape from the
sample. This means that the detection mode boasts high-resolution top-
ographical images, making this the most widely used of the SEM modes.
SEM can provide both morphological information at the submicron scale
and elemental information at the micron scale. Recent developments in
terms of electron source (field emission) have led to the development of
high-resolution SEM. Using a secondary or backscattering electron
image one can look at particles as small as 10–20 nm (Figure 4.12A).
Chemical information using EDX, however, is obtained at the micron
scale and not for individual particles.
In contrast with SEM, transmission electron microscopy (TEM)
analyzes the transmitted or forward-scattered electron beam. Here the