Electron diffraction example patterns

Due to aperture restrictions, selected area methods are only applicable for examining areas of perhaps 0.5 Xm or more in diameter. It is also possible to obtain spot patterns from much smaller areas—down to as little as a nanometer or so in diameter—by using a focused incident electron beam, rather than an aperture, to limit the sample area under examination. Such microdiffraction patterns are somewhat more complex due to the nonparallel illumination, but they may also contain more information. For example, so-called convergent-beam electron diffraction (CBED) patterns provide three-dimensional information about the compounds crystal chemistry and can be used as a sensitive fingerprint for a given atomic structure. ... 

Selected Area Electron Diffraction (SAED) Pattern The selected area electron diffraction (SAED) pattern is a diffraction technique that is usually coupled with TEM to identify crystal structures and examine crystal defects. The technique is similar to XRD, but unique in that areas as small as several hundred square nanometers in size can be examined. An example is shown in figure 9.31, in which ammonium salt-stabilized Pd nanoparticles that exhibit (111), (200), (220) and (311) ring patterns are measured and indicate the face-centered cubic (fee) crystal structure of Pd [73]. Today, SAED is recognized as being a routine and important method for characterizing Pd nanostructures. 

Another example of symmetry breaking is provided by the crystal structure of form III of iP4MP. Electron diffraction patterns of the square single crystals of form III have indicated that chains in fourfold helical conformation... 

Analytical electron microscopy of individual catalyst particles provides much more information than just particle size and shape. The scanning transmission electron microscope (STEM) with analytical facilities allows chemical analysis and electron diffraction patterns to be obtained from areas on the order of lOnm in diameter. In this paper, examples of high spatial resolution chemical analysis by x-ray emission spectroscopy are drawn from supported Pd, bismuth and ferric molybdates, and ZSM-5 zeolite. 

Figure 1. Electron diffraction pattern examples a - spot pattern from a single crystal, b -oblique texture pattern tilted by 60°, c - powder ring pattern.

EXAMPLE 9.7 Comparison Between Bulk and Surface Structures Using Low-Energy Electron Diffraction Patterns. If we accept that an LEED pattern has the same symmetry as the net of surface atoms responsible for its formation, what additional information is needed to complete the comparison between bulk and surface structures for the tungsten surface shown in Figure 9.16a ... 

Five samples with the nominal compositions shown in Table 2.8 were prepared by heating weighed mixtures of V2O3 and TiO2 in evacuated silica tubes at 1200 °C for 1-10 days. Electron diffraction patterns were taken for these samples, and the patterns on the [111] zone could be classified as Dj, D2, and Dj, as shown in Fig. 2.110. D, and D3 patterns come from the structures derived by the shear operations of (121) [0il] and (132) [O11], respectively. D2 patterns, on the other hand, show the arrays rotating between (121) and (132), which is very similar to the Ti O2 i system. Some of the examples with D2 patterns on the [111] zone axis are shown in Fig. 2.111. 

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