Transmission Electron Microscopy (TEM):
Principle: In this technique, a beam of high-energy electrons (typically 100 - 400keV) is
collimated by magnetic lenses and allowed to pass through a specimen under high vacuum.
The transmitted beam and a number of diffracted beams can form a resultant diffraction
pattern, which is imaged on a fluorescent screen kept below the specimen. The diffraction
pattern gives the information regarding lattice spacing and symmetry of the structure under
consideration. Alternatively, either the transmitted beam or one of the diffracted beams can
be made to form a magnified image of the sample on the viewing screen as bright-and dark-
field imaging modes respectively, which give information about the size and shape of the
micro-structural constituents of the material. High-resolution image, that contains
information about the atomic structure of the material, can be obtained by recombining the
transmitted beam and diffracted beams together.
Instrumentation: It comprises of a tungsten filament or LaB6 or a field emission gun as
source of electron beam, objective lens, imaging lens, CCD camera, monitor, etc.
Applications: Transmission electron microscopy is used to study the local structures,
morphology, and dispersion of multi-component polymers, cross sections & crystallization of
metallic alloys and semiconductors, microstructure of composite materials, etc. The
instrument can be extended to include other detectors like Energy Dispersive Spectrometer
(EDS) or Energy Loss Spectrometer (ELS) to study about the local chemistry of the material
similar to SEM technique.
Disadvantages: The instrumentation is complicated and needs high vacuum. Sample
preparation is very time consuming. Some materials, especially polymers, are sensitive to
electron beam irradiation which results in the loss of crystallinity and/or mass.
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