Energy
filtering is a valuable tool for materials characterization.
It can be applied for various problems:
1. Zero-loss filtering
By using only the zero-loss beam, all inelastically
scattered electron are omitted (reduction of noise). That leads
to increased contrast in TEM images (important for specimens
with
low contrast,
e.g.
biological specimens) or in diffraction patterns.
2. Electron spectroscopic imaging
By
using electrons with a well-defined energy loss (ionization
edge), elemental distribution maps can be generated.
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The
GIF (Gatan Imaging Filter) is mounted below the column of
a TEM. It consists of two main parts:
1. Magnetic prism
In the curved magnetic field, electrons with different energies
are dispersed, and consequently an EEL spectrum is
obtained at the end of this part. In fact, the magnetic prism
is the
same as in a parallel electron energy loss spectrometer, and,
of course, an EEL spectrum can be measured with the GIF as
well.
2. Optical column
An energy range is selected by a slit located at the dispersion
plane. In imaging
mode, electrons of the selected energy pass through the lenses
of the optical system, and an energy-filtered
image occurs at the
end of the GIF, where a slow-scan CCD camera is installed for
image recording.
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