Z-contrast images

It is clear that since supported heterogeneous catalysts frequently comprise heavy atom clusters, such as Pt and Pd, distributed over light supports such as charcoal, silica and alumina, Z contrast imaging could be very useful in catalyst studies, particularly for detecting the smaller clusters (< 10A) which are frequently missed by the conventional imaging methods. 

Blom, D.A., Bradley, S.A., Sinkler, W., and Allard, LF. (2006) Observation of Pt atoms, clusters and rafts on oxide supports, by sub-Angstrom Z-contrast imaging in an aberration-corrected STEM/TEM. Proc. Microsc. Microanal, 12, 50-51. 

Fig. 9 Z -contrast imaging and EELS. The electron probe is scanned across the sample. For each scan position, the HAADF detector collects the high-angle scattering intensity. The intensity of one scan-position is represented as the corresponding pixel intensity in the STEM image. The forward scattered beam is not affected by the detector and can be used for EELS. (View this art in color at www.dekker. com.)...

Fig. 10 High-resolution Z-contrast imaging. Z-contrast image of a grain boundary in SrTi03 (perovskite structure) recorded in 0 0 1 direction. One of the unit cells framed in the micrograph is illustrated on the left, the Sr columns (bright) are at the corner of the unit cell, in the center there is a TiO column. The pure oxygen columns, black in the model, are not observable in the Z-contrast image. The atomic number (Z) contrast is apparent with increasing atomic number (Z) of the elements, the intensity increases. (View this art in color at WWW. dekker. com.)...

Nellist, P.D. Pennycook, S.J. The principles and interpretation of annular dark-field Z-contrast imaging. Adv. Imag. Electr. Phys. 2000, 113, 147-203. 

Fig. 11.2. Z-contrast image of an asymmetric section of a YBCO 30° [001] tilt boundary. For this boundary, grown on a similarly oriented SrXi03 substrate, an asymmetric boundary plane was the predominant feature.

Fig. 11.3. The Z-contrast image of the YBCO asymmetric 30° [001] tilt boundary shown in Fig. 11.2 after maximum entropy processing.

Crucial in the use of EELS to study carrier concentrations at grain boundaries in high materials is the ability to position the probe with atomic precision a precision only afforded by the Z-contrast image. In order to achieve with EELS the same atomic resolution as the image, the range over... 

However, if we remember that the Z-contrast image, like any transmission image, is simply a two-dimensional projection of the 3-dimensional crystal structure, a solution to this problem is for only one of the two sites in each... 

Fig. 11.7. Z-contrast image of two [100] dislocation cores in YBCO. The two dislocation cores have the same basic structure with the difference being that one is centered on the copper sub-lattice and the other is centered on the Y/Ba sub-lattice.

The combination of Z-contrast imaging and EELS allows the hole concentration occurring at grain boundaries to be correlated with defined structural features. Using bond-valence sum analysis to interpret the results has highlighted the differences between structural units containing reconstructed atomic columns on different sub-lattice sites. In particular, it has been found that for... 

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