Biomedical Engineering Reference
In-Depth Information
Source
Condensor Diaphragm
Sample
Diffraction
Plane
Objective Diaphragm
Image Plane
Area Selection
Diaphragm
Intermediate
Lens
2nd Image
Projector
Lens
Screen
(a)
(b)
Bright Field Image
Diffraction
Fig. 3.6
Diagram of the path of electrons through the system of lenses in the TEM column
illustrating (
a
)imageand(
b
) diffraction modes
selected area electron diffraction (SAED) and convergent beam electron diffrac-
tion (CBED). Figure
3.6
shows a comparison of BF imaging mode with SAD
mode.
In image observation (MAG mode), the magnified images are formed at different
levels in the column, while in selected area mode, both the image provided by the
objective lens (which is in the Gaussian plane) and the image of the selected area
aperture coincide in the same plane. In selected area electron diffraction mode, the
area selected by the aperture in the image plane will provide the diffraction pattern
at the back focal plane of the objective lens.
In CTEM mode, the parallel electron beam is broad and illuminates the entire
sample at the same time. This allows for a number of imaging modes. Sample
illumination in parallel mode is used to make bright-field contrast images (bright-
field mode), crystallographic dark-field images (dark-field and weak-beam modes),
selected-area electron diffraction (SAED), micro- and nano-diffraction, and high-
resolution images (HRTEM). It is also used to perform point chemical analysis
using X-rays (EDS) or inelastic electron energy loss (EELS). This illumina-
tion mode provides the best observation conditions for high-resolution (HRTEM)
or low-dose radiation illumination, as well as for dark-field and weak-beam
images.
In TEM mode, a convergent beam (Fig.
3.7)
is used to perform convergent beam
electron diffraction (CBED), microdiffraction, and large-angle convergent beam
electron diffraction (LACBED). It is also used for annular dark-field mode, either
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