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The latest and most sophisticated
instrument of CWRU's "Swagelok Center for
Surface Analysis of Materials" (SCSAM) is
the Tecnai F30 ST, an energy-filtering
field-emission gun high-resolution
analytical transmission electron
microscope. The small electron wavelength,
provided by the high accelerating voltage
of 300 kV, the coherent electron
source, provided by the Schottky
field-emission electron gun (FEG), and the
high mechanical and electrical stability of
this instrument allow for high-resolution
imaging with an information resolution
limit of 0.14 nm. Equally important,
the small spherical aberration of the
"super twin" objective lens
(Cs = 1.2 mm) and the high
brightness of FEG allow for microanalysis
at both high spatial resolution and high
probe current (>0.6 nA in a
1 nm spot), which is important for
obtaining good signal-to-noise ratios. A
scanning unit enables the instrument to
acquire images and analytical data not only
in the stationary mode, but also by
scanning a fine electron probe with a
diameter as small as 0.17 nm across
the specimen. Owing to these basic
features, this instrument is ideally suited
for studying the structure and the local
chemical composition of materials on the
nanoscale. The synergy of various powerful
methods of analytical and high-resolution
TEM techniques in the same instrument
greatly enhance the capability of SCSAM,
particularly for nanotechnology
research.
Under coherent imaging conditions, the
Tecnai F30 in conjunction with adequate
digital image processing of images recorded
at different focus settings of the
objective lens enables quantitative HRTEM
with a resolution of 0.14 nm. The STEM
unit also enables "Z-contrast" imaging by
detection of the electrons scattered to a
high-angle annular dark-field (HAADF)
detector, which constitutes a powerful
technique for high-resolution imaging under
conditions that reduce the interpretation
problems associated with conventional HRTEM
imaging.
The Tecnai F30 is equipped with a
state-of-the-art XEDS system by EDAX. The
heart of this system is a Li-drifted Si
detector, which has been specially selected
to provide an outstanding energy
resolution, which was measured to
130 eV. Accordingly, this system is
particularly sensitive to light elements
(carbon, for example).
The basic capabilities of the Tecnai F30
are strongly enhanced by a post-column,
imaging energy filter (GIF 2002 by Gatan).
This component forces the electrons on an
energy-dispersive path, enabling a powerful
variety of advanced methods of
microanalysis.
Among these advanced methods, electron
energy-loss spectroscopy (EELS) is of
particular importance. The
energy-dispersive plane of the filter, when
imaged onto the slow-scan CCD camera of the
GIF, reveals an "electron energy-loss
spectrum" of the illuminated area. Electron
energy-loss spectra contain absorption
edges that are specific to the elements in
the specimen. By recording the electron
intensity in the energy-dispersive plane
with a CCD camera, the local chemical
composition of the specimen can be
analyzed. This method of high spatial
resolution chemical microanalysis is
particularly powerful for light
elements.
Apart from this application, electron
energy-loss spectra can provide information
on the local electronic structure and atom
coordination. Information on the electronic
structure becomes available by analyzing
the fine structure of energy-loss spectra
near absorption edges, i. e. by
analyzing the energy-loss near-edge
structure (ELNES). Usually, this type of
analysis is carried out on spectra obtained
with a focused electron probe and an EELS
spectrometer that allows for parallel data
processing.
The second important application of the
imaging energy filter is "zero-loss
imaging." Given the energy-dispersive ray
path and the energy-dispersive plane of the
GIF, energy filtering of the transmitted
electrons is achieved by placing a slit
aperture in the energy-dispersive plane. In
this way, it is possible to admit only
electrons with a particular energy (or,
equivalently, a particular energy-loss) to
the image or diffraction pattern. One
important mode of operation of the filter
is known as "zero-loss filtering." In this
case, the slit aperture is positioned such
that only those electrons that suffered no
energy loss in the specimen can pass. This
means that only elastically scattered
electrons (electrons that have not suffered
any energy loss) arrive at the electron
detector (viewing screen, photographic
plate, or CCD camera). Zero-loss filtering
has two major applications: imaging of
thick specimens and quantitative electron
diffraction (ED), particularly with a
highly convergent primary electron beam
(CBED).
Finally, the imaging energy filter (GIF)
enables elemental mapping via
"electron-spectroscopic imaging" (ESI). In
this technique, the energy filter is
employed for recording images with
electrons that have lost a well-defined,
element-specific amount of kinetic energy
in the specimen. For elemental mapping, it
often suffices to record ESI images with
three different settings of the slit
aperture in the energy-dispersive plane,
which makes ESI a much more efficient
technique than the competing technique of
scanning the specimen with a focused
electron beam and recording entire EELS
spectra at every scan point.
This material is based upon
work supported by the National Science
Foundation under Grant No. DMR0114134.
Any opinions, findings, and conclusions or
recommendations expressed in this material
are those of the author(s) and do not
necessarily reflect the views of the
National Science Foundation.
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