Biomedical Engineering Reference
In-Depth Information
Parallel Beam
Sample
Objective Lens
Objective Aperture
Conventional
Microscope
Column
Image Plane
Omega Energy
Filter
Image Plane
Inelastic
Electrons
Viewing Screen
Magnified-Image Plane
Viewing Screen
Magnified-Image Plane
Transmitted
Electrons
a
b
Analyzer
EELS or PEELS
Detector
Fig. 3.14
Different types of energy filters: (
a
) an Omega filter located in the column and (
b
)a
PEELS filter located outside the TEM column
type is an Omega filter situated in the column. PEELS imaging can be made in
parallel beam, fixed convergent beam, or scanning mode. The important difference
between these two types of detection is the sample surface analyzed. The first mode
is often used to analyze interfaces up to nanometric scales; the image observed on
the screen is very small (1 cm
2
). On the other hand, EFTEM imaging with an Omega
filter is used to observe a large sample area (i.e., the whole surface of the viewing
screen), and because of this feature, this tool is frequently used in biology.
5.5 EDS Chemical Analysis Methods in TEM and TEM/STEM
Chemical analysis can be performed using an EDS (energy-dispersive spectroscom-
etry) detector, which uses a fixed beam or a scanning beam as in SEM or STEM
mode with a probe size down to 1 or 2 nm, depending on the microscope. The
focused beam can induce sample contamination and cause radiation damage that
may result in material decomposition or destruction. To prevent or minimize these
problems, which vary depending on the chemical nature of the material, it is
necessary to use a cooling stage with a liquid-nitrogen-cooled sample holder.
An EDS detector is used to simultaneously acquire the analytical spectrum of
all the chemical elements of a material, starting with the light elements, on a
multichannel screen.
Quantitative or semi-quantitative element analysis requires the use of a known
external standard or an internal standard within the sample. The problems related to
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