that electrons are ejected from its surface. These ejected electrons are
condensed and then focused by electromagnetic lenses, which are analogous to optical lenses, but function on different principles. The focused
beam of electrons is shone on the sample, and those electrons that have
been transmitted are gathered by another lens and then shone onto a
fluorescent screen or other detector to produce an image of the sample.
The setup of a TEM can be modified so that it is able to scan an entire
sample, and this technique is called scanning transmission electron
microscopy (STEM). As with AFM and STM, STEM typically scans a given
sample in a raster pattern to produce an image.
In order to prevent deflection of the beam of electrons due to interactions
with gas molecules inside the machine, the interior of a TEM must be
operated under high vacuum conditions. This requirement is one of the
principal disadvantages of TEM, although recently there have been rapid
developments in the field of environmental TEM that allow operation
under lesser vacuums. We will focus on the more common, high vacuum
TEM. For this technique, the sample being examined must be able to
withstand the high vacuum; otherwise, the image won’t be an accurate
representation of the sample under normal conditions. For nonsolid
samples, two principal methods of sample preparation have emerged.
These methods allow for the imaging of biological samples or other soft
nanomaterials that otherwise could not be imaged using TEM. The first
method is to dehydrate the sample, then stain the sample or coat it with
metal to produce the necessary contrast. The second is to cryogenically
freeze the sample and image the frozen specimen. One requirement of
this second method is that the sample must be frozen so quickly that it
does not have time to rearrange into a crystalline state. Otherwise, the
ordered ice crystals produce a diffraction pattern of the electron beam
that obscures the image of the sample. If the sample is frozen quickly
enough, amorphous ice is produced and one can obtain an accurate
image of the sample in its “natural” state.
The second major difficulty with TEM is that it is limited to observing very
thin samples. In order to be effectively imaged, the sample must be thin
enough to be transparent to the incoming beam of electrons. As a general
rule, the sample should be less than 100 nm thick, although the exact
suitable thickness depends on the material being examined and the
energy of the electrons used. Thicker samples can be examined if electrons with higher energies are employed, but at higher energies the
electrons may begin to destroy the sample. In terms of sample mounting,
the sample can sometimes be placed on a sample holder that is very thin
CHAPTER 8: Surface Characterization and Imaging Methods
322
condensed and then focused by electromagnetic lenses, which are analogous to optical lenses, but function on different principles. The focused
beam of electrons is shone on the sample, and those electrons that have
been transmitted are gathered by another lens and then shone onto a
fluorescent screen or other detector to produce an image of the sample.
The setup of a TEM can be modified so that it is able to scan an entire
sample, and this technique is called scanning transmission electron
microscopy (STEM). As with AFM and STM, STEM typically scans a given
sample in a raster pattern to produce an image.
In order to prevent deflection of the beam of electrons due to interactions
with gas molecules inside the machine, the interior of a TEM must be
operated under high vacuum conditions. This requirement is one of the
principal disadvantages of TEM, although recently there have been rapid
developments in the field of environmental TEM that allow operation
under lesser vacuums. We will focus on the more common, high vacuum
TEM. For this technique, the sample being examined must be able to
withstand the high vacuum; otherwise, the image won’t be an accurate
representation of the sample under normal conditions. For nonsolid
samples, two principal methods of sample preparation have emerged.
These methods allow for the imaging of biological samples or other soft
nanomaterials that otherwise could not be imaged using TEM. The first
method is to dehydrate the sample, then stain the sample or coat it with
metal to produce the necessary contrast. The second is to cryogenically
freeze the sample and image the frozen specimen. One requirement of
this second method is that the sample must be frozen so quickly that it
does not have time to rearrange into a crystalline state. Otherwise, the
ordered ice crystals produce a diffraction pattern of the electron beam
that obscures the image of the sample. If the sample is frozen quickly
enough, amorphous ice is produced and one can obtain an accurate
image of the sample in its “natural” state.
The second major difficulty with TEM is that it is limited to observing very
thin samples. In order to be effectively imaged, the sample must be thin
enough to be transparent to the incoming beam of electrons. As a general
rule, the sample should be less than 100 nm thick, although the exact
suitable thickness depends on the material being examined and the
energy of the electrons used. Thicker samples can be examined if electrons with higher energies are employed, but at higher energies the
electrons may begin to destroy the sample. In terms of sample mounting,
the sample can sometimes be placed on a sample holder that is very thin
CHAPTER 8: Surface Characterization and Imaging Methods
322