Physical Principles of Electron Microscopy: An Introduction to TEM ...

Recent Developments 179
Figure 7-2. (a) ADF-STEM image of a catalyst specimen, showing individual Pt atoms on the
surface of an alumina support; the circles indicate Pt3 trimers. Courtesy of R.D. Adams and
M. Amiridis, University of South Carolina, A. Borisevich and S. Pennycook, Oak Ridge
National Laboratory. (b) Image of a silicon crystal recorded with the electron beam parallel to
(112), showing pairs of atomic columns 0.078 nm apart. Courtesy of Matthew F. Chisholm,
ORNL, reprinted with permission from Science 305 (2004) 1741; copyright 2004 AAAS.
Both of these images were recorded using the aberration-corrected VG603 STEM at ORNL.
Because the dedicated STEM has a field-emission source and a strongly
excited objective lens (low Cs), it can form a probe of diameter � 0.2 nm,
offering the possibility of single-atom resolution. One area of application has
been the study of the structure of catalysts, which typically consist of very
small
metal particles on an oxide support; see Fig. 7-2a.
The STEM can also produce atomic-resolution images of crystalline
specimens. If the specimen is oriented with a low-index crystal axis parallel
to the incident beam, columns of atoms can be resolved (Fig. 7-2b), as in the
case of phase-contrast TEM images. However, the ADF detector provides an
amplitude (scattering-contrast) signal that is relatively insensitive to the
objective-lens focus; there are no contrast reversals (Section 4.9) and the
image
is more directly interpretable in terms of atomic coordinates.
As shown in Fig. 7-1, the STEM is usually fitted with an energy-loss
spectrometer that forms a spectrum from electrons (with scattering angle up
to �min) that pass through a central hole in the ADF detector. This
arrangement allows dark-field and energy-filtered bright-field images to be
recorded simultaneously. Also, after recording an atomic-resolution ADF
image, the electron probe can be set to a precisely known location on the
specimen and the electron spectrometer used to provide elemental or finestructure
analysis with a spatial resolution down to the atomic level
(Browning et al., 1997).