Introduction: The optical nature of a charged particle beam 15
The ultimate resolution of the TEM and STEM is limited by
spherical aberration and diffraction. The spherical aberration can
be substantially corrected in a modern TEM and STEM, making
both instruments capable of resolution in the range of 0.05 nm.
This is more than sufficient to form an image of a single atom.
An example of a corrected STEM image is shown in Figure 1.8.
The specimen is graphene, which consists of one or more atomic
Figure 1.8: Aberration-corrected STEM images of graphene.
layers of graphite. A single layer of graphene is one atomic layer
of carbon in its hexagonal crystalline form. The image on the left
is a single scan recorded at 60 KV accelerating voltage in a Nion
aberration-corrected STEM. The bright spots are single carbon
atoms with nearest-neighbor spacing of 0.14 nm. The image on
the right is derived by digitally superimposing 350 different areas
of the larger image, with each area consisting of 128 × 128 pixels. This averages out the noise in the individual scans, without
having to resort to smoothing algorithms. (The individual pixels
are visible in the two images). This annular dark field image is
remarkable in several respects. First, single atoms of carbon are
clearly resolved with resolution better than 0.1 nm. Second, the
atomic number of carbon is six, which is low relative to most solid
materials. The specimen is therefore weakly scattering everywhere,
thus limiting the available contrast. The fact that the contrast is
The ultimate resolution of the TEM and STEM is limited by
spherical aberration and diffraction. The spherical aberration can
be substantially corrected in a modern TEM and STEM, making
both instruments capable of resolution in the range of 0.05 nm.
This is more than sufficient to form an image of a single atom.
An example of a corrected STEM image is shown in Figure 1.8.
The specimen is graphene, which consists of one or more atomic
Figure 1.8: Aberration-corrected STEM images of graphene.
layers of graphite. A single layer of graphene is one atomic layer
of carbon in its hexagonal crystalline form. The image on the left
is a single scan recorded at 60 KV accelerating voltage in a Nion
aberration-corrected STEM. The bright spots are single carbon
atoms with nearest-neighbor spacing of 0.14 nm. The image on
the right is derived by digitally superimposing 350 different areas
of the larger image, with each area consisting of 128 × 128 pixels. This averages out the noise in the individual scans, without
having to resort to smoothing algorithms. (The individual pixels
are visible in the two images). This annular dark field image is
remarkable in several respects. First, single atoms of carbon are
clearly resolved with resolution better than 0.1 nm. Second, the
atomic number of carbon is six, which is low relative to most solid
materials. The specimen is therefore weakly scattering everywhere,
thus limiting the available contrast. The fact that the contrast is