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Methods for Structural and Chemical Characterization of Nanomaterials 123

Figure 4.12 A. An FESEM secondary electron image of Ag nanoparticles where the
particle size is determined using the appropriate scale (courtesy of Vladimir Tarabara).
B. A TEM image of imogolite (single-walled aluminosilicate nanotube) (courtesy of
Clément Levard).

electron beam is passed through a series of lenses to determine the
image resolution and obtain the magnified image (Figure 4.12B). The
highest structural resolution possible (point resolution) is achieved
upon use of high-voltage instruments (acceleration voltages higher
than 0.5 MeV). Enhanced radiation damage, which may have stronger
effects for nanostructured materials, must however be considered in
these cases. With corrections it is possible to achieve sub-angstrom res-
olution with microscopes operating at lower voltages (typically, 200
keV), allowing the oxygen atoms to be resolved in oxides materials. On
the other hand, as high resolution is achieved in TEM as the result of
electron wave interference among diffracted peaks and not only to the
transmitted beam in the absence of deflection, a limitation to struc-
tural resolution can arise from nanoparticles with a very low number
of atoms. Nevertheless conventional TEM is the most common tool used
to investigate the crystal structure of materials at the sub-nanometer
scale. There exist a number of different TEM techniques that may be
used to obtain structural images with atomic level resolution; two of
these techniques are detailed below: high-resolution TEM (HRTEM)
and high angle annular dark field (HAADF) scanning transmission elec-
tron microscopy (STEM).
HRTEM images are formed by the interference of coherent electron
waves. The object transmits the (nearly) planar incident electron wave,
at the exit plane
interacts with it, and the resulting electron wave e
of the object carries information about the atom arrangement in the
object. The corresponds to a set of “diffracted” coherent plane waves.
e
The electron optics transfers these waves to the image plane, and the