Chemistry Reference
In-Depth Information
Fig. 3 STEM-HAADF
image showing Z-contrast
between the segregated Au
core and Pd shells.
Reproduced from [
51
].
Reproduced by permission
of the Royal Society of
Chemistry. [For the
electronic version:
http://dx.
The high-angle annular dark-field (HAADF) detector collects electrons that are
scattered to wider angles through interaction with the sample. These scattered elec-
trons can be regarded as incoherent. The cross section of the incoherently scattered
electrons depends on mainly sample thickness and the atomic numbers of the ele-
ments, which makes interpretation of HAADF image contrast relatively straightfor-
ward [
50
]. Where the sample is of uniform thickness, HAADF image contrast
variation can be directly related to sample elemental variation. Figure
3
shows a
typical HAADF image of segregated AuPd nanorods [
51
], demonstrating a clear
difference in image intensity between the Au core (
Z
46).
In addition to the HAADF detector, ac-STEM also has the capability for bright-
field (BF) imaging through the detection of electrons scattered to small forward
angles by the sample, and, as such, assumed to retain their coherence. BF image
contrast can be related to phase changes caused by interactions between the beam
electrons and the sample. They are sensitive to extremely fine structural detail, such
as the presence of lattice strain, and are also more affected by lighter elements, such
as the amorphous carbon substrate of TEM grids, than DF images. Figure
4
shows
simultaneously acquired STEM-HAADF and STEM-BF “as taken” images from
the edge of an Au nanorod and illustrates the difference between DF amplitude
contrast images and BF phase contrast images. Atomic resolution is apparent in
both images; however, the greater sensitivity of BF imaging to lighter atoms means
the grain of the underlying amorphous carbon support is apparent in the BF image
but not in the HAADF image.
Imaging information can be maximised by using the two detectors simulta-
neously. This allows the simplicity of HAADF interpretation to aid interpretation
of more complex BF images, while the finer structural detail available in BF images
can complement the superior atomic resolution of HAADF images. Recently, Goris
et al. further demonstrated the potential of ac-STEM in the characterisation of core-
shell interfaces by employing tomography techniques to examine the core-shell
AuAg nanorods from several different imaging angles [
52
]. Although tomography
¼
79) and the Pd shell (
Z
¼
