290 12 Characterization of Nanomaterials
12.3.2
Setup of Electron Microscopes
Lenses for electron microscopes nowadays apply magnetic fields. These magnetic
lenses show rotational symmetry. When the first electron microscopes were built,
instruments using electrostatic lenses were also on the market.
To understand the design principles of an electron microscope, one has to
understand the interaction of the electrons with the specimen. Electrons passing
a specimen are scattered
• elastically – the energy of the electrons is retained, or
• inelastically – electrons lose energy.
Electron optical systems are adjusted for exactly one energy of the electrons; therefore, in contrast to light optical systems; the electron lenses must be readjusted
when the energy of the electrons is changed. As a consequence, the illumination
(condensor) systems must deliver electrons of only one energy. A modern field
emission system for the electrons has an inherent energy spread of typically 0.7 eV.
Even this is too much to obtain maximum resolution. Therefore, this energy
spread must be reduced to values smaller than 0.2 eV applying an electron monochromator in the illumination system. This is depicted in Figure 12.9.
As mentioned above, a certain number of the electrons are scattered inelastically. To avoid image blurring, these electrons, scattered in a larger angle, are
caught by a diaphragm. Inelastically scattered electrons going in the direction of
the axis of the electron microscope are removed by a second electron monochromator. (Even when the inelastic scattered electrons blur the image, using special
devices, they are a precious tool for detailed analysis of the specimen.) Besides
chromatic aberration, spherical aberration, caused by the different focal length of
rays close to the axis of the microscope as compared to those with larger distance
from the optical axis, is the most important limitation for image resolution. Minimizing the numerical aperture reduces image blurring by spherical aberration.
Limited by chromatic and spherical aberration the resolution of electron microscopes is in the range of 0.15 to 0.2 nm.
Correction of the spherical aberration is impossible in electron optical systems
with rotational symmetry. The problem of spherical aberration was overcome by
the electron microscope is 5 × 10
−3 , a reasonable value for electron microscopes,
an electron energy of at least 10
5 eV is needed. To compensate for other problems, electron microscopes for materials science apply voltages in the range
from 150 to 300 kV. For very special purposes, instruments with acceleration
voltages up to 1 MV have been built. However, it must be noted that the resolution power of these instruments is not significantly better.
More recent instruments working with systems to obtain monochromatic
electrons (monoenergetic) and correction of the spherical aberration have a
larger aperture, hence, they no longer need such high electron energies, as they
have a larger numerical aperture.
12.3.2
Setup of Electron Microscopes
Lenses for electron microscopes nowadays apply magnetic fields. These magnetic
lenses show rotational symmetry. When the first electron microscopes were built,
instruments using electrostatic lenses were also on the market.
To understand the design principles of an electron microscope, one has to
understand the interaction of the electrons with the specimen. Electrons passing
a specimen are scattered
• elastically – the energy of the electrons is retained, or
• inelastically – electrons lose energy.
Electron optical systems are adjusted for exactly one energy of the electrons; therefore, in contrast to light optical systems; the electron lenses must be readjusted
when the energy of the electrons is changed. As a consequence, the illumination
(condensor) systems must deliver electrons of only one energy. A modern field
emission system for the electrons has an inherent energy spread of typically 0.7 eV.
Even this is too much to obtain maximum resolution. Therefore, this energy
spread must be reduced to values smaller than 0.2 eV applying an electron monochromator in the illumination system. This is depicted in Figure 12.9.
As mentioned above, a certain number of the electrons are scattered inelastically. To avoid image blurring, these electrons, scattered in a larger angle, are
caught by a diaphragm. Inelastically scattered electrons going in the direction of
the axis of the electron microscope are removed by a second electron monochromator. (Even when the inelastic scattered electrons blur the image, using special
devices, they are a precious tool for detailed analysis of the specimen.) Besides
chromatic aberration, spherical aberration, caused by the different focal length of
rays close to the axis of the microscope as compared to those with larger distance
from the optical axis, is the most important limitation for image resolution. Minimizing the numerical aperture reduces image blurring by spherical aberration.
Limited by chromatic and spherical aberration the resolution of electron microscopes is in the range of 0.15 to 0.2 nm.
Correction of the spherical aberration is impossible in electron optical systems
with rotational symmetry. The problem of spherical aberration was overcome by
the electron microscope is 5 × 10
−3 , a reasonable value for electron microscopes,
an electron energy of at least 10
5 eV is needed. To compensate for other problems, electron microscopes for materials science apply voltages in the range
from 150 to 300 kV. For very special purposes, instruments with acceleration
voltages up to 1 MV have been built. However, it must be noted that the resolution power of these instruments is not significantly better.
More recent instruments working with systems to obtain monochromatic
electrons (monoenergetic) and correction of the spherical aberration have a
larger aperture, hence, they no longer need such high electron energies, as they
have a larger numerical aperture.