12.3 Electron Microscopy 297
12.3.4
Some Examples of Transmission Electron Microscopy
As mentioned above, only the elastically scattered electrons carry information
about the image. Elastic electron scattering depends on the energy of the electrons V and the number of electrons of the atoms in the specimen, described by
the atomic number Z, is essentially proportional to
Z
V
2
. This has important
consequences on the contrast within an image. Assuming zirconia, ZrO 2 as specimen; for zirconium Z = 40 and therefore Z
2
= 1600; for oxygen Z = 8 and Z
2
= 64
one sees that the scattering power of zirconium is 25 times higher than that of
oxygen; hence, there is no chance to get a picture showing the positions of the
zirconium and the oxygen atoms. The probability for inelastic scattering is, to
a first approximation, proportional to the atomic number. Therefore, methods
of analysis measuring the energy loss of inelastic scattered electrons (such as
EELS = electron energy loss spectrometry) work better with elements of high Z.
However, as there is a linear proportionality, one has a chance to see, for example,
the oxygen signal besides that of a metal of higher atomic number. Lastly, the
same considerations are valid with respect to the emitted X-rays. This means,
X-ray analysis works better with heavy elements as compared to light ones.
Image formation in optical microscopy depends primarily on contrast due to
absorption. In high-resolution electron microscopy, differences in absorption play
a negligible role. High-resolution images are formed by interference of the elastic
scattered electrons leading to a distribution of intensities depending on the orientation of the lattice planes in a crystal relative to the electron beam (diffraction
contrast). Therefore, at certain angles the electron beam is diffracted strongly off
the axis of the incoming beam, while at others, the beam is nearly completely
transmitted, allowing deductions as to the arrangement of atoms within a crystal
lattice. Certainly, in the case of a noncrystallized specimen, absorption plays a
distinctive role in image formation.
Figure 12.16 displays a high-resolution electron micrograph of a zirconia specimen coated with alumina. This figure displays all the advantages and problems
connected to electron microscopy. The core of this composite particle is crystallized. One sees the regular arrangement of the columns of zirconium ions. Because
of its low atomic number, the oxygen ions are not visible. The zirconia core is
surrounded by amorphous alumina. As the atomic numbers of aluminum (Z = 13)
is very low, the visibility of the coating layer is poor. Furthermore, as the coating
is amorphous, this layer does not show any structure.
Figure 12.16 displays a high-resolution electron micrograph of crystallized zirconia, shows a “picture” of the lattice. Micrographs like this are obtained if the
electron beam is exactly in the direction of a crystallographic orientation. The
observers “sees” columns of atoms in their crystallographic exact neighborhood.
(As mentioned above, these high-resolution images are due to the diffraction
contrast. Changing the focus adjustment may change the contrast. In particular,
this point makes the interpretation of high-resolution electron micrographs very
12.3.4
Some Examples of Transmission Electron Microscopy
As mentioned above, only the elastically scattered electrons carry information
about the image. Elastic electron scattering depends on the energy of the electrons V and the number of electrons of the atoms in the specimen, described by
the atomic number Z, is essentially proportional to
Z
V
2
. This has important
consequences on the contrast within an image. Assuming zirconia, ZrO 2 as specimen; for zirconium Z = 40 and therefore Z
2
= 1600; for oxygen Z = 8 and Z
2
= 64
one sees that the scattering power of zirconium is 25 times higher than that of
oxygen; hence, there is no chance to get a picture showing the positions of the
zirconium and the oxygen atoms. The probability for inelastic scattering is, to
a first approximation, proportional to the atomic number. Therefore, methods
of analysis measuring the energy loss of inelastic scattered electrons (such as
EELS = electron energy loss spectrometry) work better with elements of high Z.
However, as there is a linear proportionality, one has a chance to see, for example,
the oxygen signal besides that of a metal of higher atomic number. Lastly, the
same considerations are valid with respect to the emitted X-rays. This means,
X-ray analysis works better with heavy elements as compared to light ones.
Image formation in optical microscopy depends primarily on contrast due to
absorption. In high-resolution electron microscopy, differences in absorption play
a negligible role. High-resolution images are formed by interference of the elastic
scattered electrons leading to a distribution of intensities depending on the orientation of the lattice planes in a crystal relative to the electron beam (diffraction
contrast). Therefore, at certain angles the electron beam is diffracted strongly off
the axis of the incoming beam, while at others, the beam is nearly completely
transmitted, allowing deductions as to the arrangement of atoms within a crystal
lattice. Certainly, in the case of a noncrystallized specimen, absorption plays a
distinctive role in image formation.
Figure 12.16 displays a high-resolution electron micrograph of a zirconia specimen coated with alumina. This figure displays all the advantages and problems
connected to electron microscopy. The core of this composite particle is crystallized. One sees the regular arrangement of the columns of zirconium ions. Because
of its low atomic number, the oxygen ions are not visible. The zirconia core is
surrounded by amorphous alumina. As the atomic numbers of aluminum (Z = 13)
is very low, the visibility of the coating layer is poor. Furthermore, as the coating
is amorphous, this layer does not show any structure.
Figure 12.16 displays a high-resolution electron micrograph of crystallized zirconia, shows a “picture” of the lattice. Micrographs like this are obtained if the
electron beam is exactly in the direction of a crystallographic orientation. The
observers “sees” columns of atoms in their crystallographic exact neighborhood.
(As mentioned above, these high-resolution images are due to the diffraction
contrast. Changing the focus adjustment may change the contrast. In particular,
this point makes the interpretation of high-resolution electron micrographs very