Environmental Engineering Reference
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spectrometry (MALDI-TOF), ultraviolet-visible spectroscopy, dual polarization
interferometry, nuclear magnetic resonance (NMR).
13.2.1 Transmission Electron Microscope (TEM)
and Scanning Electron Microscopy (SEM)
The electron microscope is a microscope that uses a beam of electrons to create an
image of the specimen. It is capable of much higher magnifications and has a
greater resolving power than a light microscope, allowing it to see much smaller
objects in finer detail. They are large, expensive pieces of equipment, generally
standing alone in a small, specially designed room and requiring trained personnel
to operate them.
The original form of electron microscopy, Transmission electron microscopy
( TEM ) involves a high voltage electron beam emitted by a cathode and formed by
magnetic lenses. The electron beam that has been partially transmitted through the
very thin (and semitransparent for electrons) specimen carries information about
the structure of the specimen. The spatial variation in this information (the “image”)
is then magnified by a series of magnetic lenses until it is recorded by hitting a
fluorescent screen, photographic plate, or light sensitive sensor such as a CCD
(charge-coupled device) camera. The image detected by the CCD may be displayed
in real time on a monitor or computer. Transmission electron microscopes produce
two-dimensional, black and white images.
Resolution of the TEM is also limited by spherical and chromatic aberration, but
a new generation of aberration correctors has been able to overcome or limit these
aberrations. Software correction of spherical aberration has allowed the production
of images with sufficient resolution to show carbon atoms in diamond separated by
only 0.089 nm and atoms in silicon at 0.078 nm at magnifications of 50 million
times. The ability to determine the positions of atoms within materials has made the
TEM an indispensable tool for nanotechnologies research and development in
many fields, including heterogeneous catalysis and the development of semicon-
ductor devices for electronics and photonics. In the life sciences, it is still mainly
the specimen preparation which limits the resolution of what we can see in the
electron microscope, rather than the microscope itself.
The Scanning electron microscope ( SEM ) is a powerful and frequently used
instrument, in both academia and industry, to study, for example, surface topogra-
phy, composition, crystallography, and properties on a local scale. The spatial
resolution is better than that of the optical microscope although not quite as good
as for the transmission electron microscope (TEM). The SEM has an extremely
large depth of focus and is therefore well suited for topographic imaging. Besides
surface topographic studies the SEM can also be used for determining the chemical
composition of a material, its fluorescent properties, and the formation of magnetic
domains and so on. The scanning electron microscope (SEM) uses a focused beam
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