50
3 Principle and Practice of Three-Dimensional Transmission …
winning due to his living. To be alive is the first condition to be elected, and hence
his case may not be a rare one at all.
Electron microscopes are designed to use high-speed electron of a smaller wavelength than that of visible light. Accordingly, they use electromagnetic lenses in
place of optical glass lenses. In other words, electron beam is used in TEM together
with the electromagnetic lens, which enables us to enjoy much higher resolution
than that of the optical counterpart. By using a higher accelerated electron beam,
TEM has afforded the point resolution of the atomic scale, i.e., 0.3–0.1 nm range
[2, 3]. One factor determining the resolution is spherical aberration of the lens. It
causes the poor convergence of the electron beams after their passing through the
lens. Recently, spherical aberration-corrected TEM is put to the practical use and a
resolution of 0.1 nm at the acceleration voltage of 200 kV has been reported [4].
Two contrasts are defined in TEM; amplitude contrast and phase contrast. When
using higher voltage than 100 kV for imaging on a very thin sample, it is assumed
that not the amplitude of electron wave, but the phase contrast is explaining the
TEM contrast. In this case, the electron wave, the phase of which has been altered,
is focused on the image plane of the objective lens. The contrast of image thus
obtained may not be enough for clear imaging, and a certain amount of defocusing
of the objective lens can give a beneficial phase difference between the scattered and
un-scattered electron waves to produce a necessary contrast on the screen, which is
the phase contrast. This procedure is in accordance with the Scherzer’s treatment [2,
5]. The following is a brief description of imaging by means of high-resolution TEM
(HRTEM).
Figure 3.1 shows (a) imaging mode and (b) selected area electron diffraction
(SAED) mode by HRTEM [2]. The incidental electron beam is irradiated perpendicularly onto the thin sample. In the case of SAED mode, the electron beam scattered by
the sample is converged at a point shifted away from the optic axis in the back-focal
plane of the objective lens. When this plane is imaged in focus onto the screen or photographic film by controlling the combined lens system made up of the intermediate
and projector lenses, a magnified electron diffraction pattern is observed. Selecting
a partial area by adjusting the aperture, the corresponding SAED is obtained (b).
When the imaging plane of the objective lens is imaged in focus by controlling the
lens system, a magnified image of the irradiated specimen is visualized on the screen
(a).
If a crystalline sample is oriented suitably to give a certain Bragg reflection, lattice
fringes with the spacing corresponding to the reflection in question will be obtained
in the image when the objective aperture has a large enough opening to permit both
the diffracted and un-diffracted waves to pass through. This is a rough scheme for
making one-dimensional interference fringe pattern. This pattern affords shape, size,
and direction of crystallites, and crystal defects if any. Also, under the conditions of
the optimal defocusing, namely at the Scherzer focus of the TEM, crystal structure
images, which directly indicate atomic or molecular arrangement in the unit cell, are
elucidated.
The sample for HRTEM measurement is as thin as possible (less than 100 nm)
since the electron beam has to pass through it. Still, diffracted and scattered beams
3 Principle and Practice of Three-Dimensional Transmission …
winning due to his living. To be alive is the first condition to be elected, and hence
his case may not be a rare one at all.
Electron microscopes are designed to use high-speed electron of a smaller wavelength than that of visible light. Accordingly, they use electromagnetic lenses in
place of optical glass lenses. In other words, electron beam is used in TEM together
with the electromagnetic lens, which enables us to enjoy much higher resolution
than that of the optical counterpart. By using a higher accelerated electron beam,
TEM has afforded the point resolution of the atomic scale, i.e., 0.3–0.1 nm range
[2, 3]. One factor determining the resolution is spherical aberration of the lens. It
causes the poor convergence of the electron beams after their passing through the
lens. Recently, spherical aberration-corrected TEM is put to the practical use and a
resolution of 0.1 nm at the acceleration voltage of 200 kV has been reported [4].
Two contrasts are defined in TEM; amplitude contrast and phase contrast. When
using higher voltage than 100 kV for imaging on a very thin sample, it is assumed
that not the amplitude of electron wave, but the phase contrast is explaining the
TEM contrast. In this case, the electron wave, the phase of which has been altered,
is focused on the image plane of the objective lens. The contrast of image thus
obtained may not be enough for clear imaging, and a certain amount of defocusing
of the objective lens can give a beneficial phase difference between the scattered and
un-scattered electron waves to produce a necessary contrast on the screen, which is
the phase contrast. This procedure is in accordance with the Scherzer’s treatment [2,
5]. The following is a brief description of imaging by means of high-resolution TEM
(HRTEM).
Figure 3.1 shows (a) imaging mode and (b) selected area electron diffraction
(SAED) mode by HRTEM [2]. The incidental electron beam is irradiated perpendicularly onto the thin sample. In the case of SAED mode, the electron beam scattered by
the sample is converged at a point shifted away from the optic axis in the back-focal
plane of the objective lens. When this plane is imaged in focus onto the screen or photographic film by controlling the combined lens system made up of the intermediate
and projector lenses, a magnified electron diffraction pattern is observed. Selecting
a partial area by adjusting the aperture, the corresponding SAED is obtained (b).
When the imaging plane of the objective lens is imaged in focus by controlling the
lens system, a magnified image of the irradiated specimen is visualized on the screen
(a).
If a crystalline sample is oriented suitably to give a certain Bragg reflection, lattice
fringes with the spacing corresponding to the reflection in question will be obtained
in the image when the objective aperture has a large enough opening to permit both
the diffracted and un-diffracted waves to pass through. This is a rough scheme for
making one-dimensional interference fringe pattern. This pattern affords shape, size,
and direction of crystallites, and crystal defects if any. Also, under the conditions of
the optimal defocusing, namely at the Scherzer focus of the TEM, crystal structure
images, which directly indicate atomic or molecular arrangement in the unit cell, are
elucidated.
The sample for HRTEM measurement is as thin as possible (less than 100 nm)
since the electron beam has to pass through it. Still, diffracted and scattered beams