Microdiffraction, electron

Figure 3. Microdiffraction patterns obtained with an electron beam of diameter about 1o8 from particles of Ru and Au on a MgO support, (a) MgO crystal, (b) Ru crystal, is2 in diameter, on MgO. (c) Au crystal, 20A in diameter, on MgO.

The new TEM techniques can provide a full characterization of small particles. The combination of weak beam images and microdiffraction information can render a very complete picture of the particle structure. In addition, refracted electron images can be... 

Electron Diffraction (SAED), Microdiffraction, Convergent-Beam Eleetron Diffraetion (CBED), Large-Angle Convergent-Beam Eleetron Diffraetion (LACBED) and electron precession. They produee spot, ring, disk or line patterns at microseopie or nanoseopie seales in eorrelation with the image of the diffracted area. An overview of the main applieations is given. 

Convergent-Beam eleetron Diffraction and Microdiffraction) become available on analytical transmission electron microscopes. Most of the electron diffraction techniques use a stationary incident beam, but some specific methods like the precession method take advantage of a moving incident beam. 

Various electron diffraction techniques are available on modem transmission electron microscopes. Selected-Area Electron Diffraction (SAED) and Microdiffraction are performed with a parallel or nearly parallel incident beam and give spot patterns. Convergent-Beam Electron Diffraction (CBED) and Large-Angle Convergent-Beam Electron Diffraction (LACBED) are performed with a focused and defocused convergent beam... 

TEM and associated techniques such as EELS are powerful tools in investigations of heterogeneous catalysts. Electron diffraction can provide structural information in phase constitutions by electron crystallographic analysing methods. Microdiffraction offers the possibility in studying small particles down to several nanometers. In combination with electron... 

A comparative study has been made by optical and electron microscopy of the anisotropic texture of several cokes from caking coals and pitches carbonized near their resolidification temperature. A simple technique made it possible to examine, by both methods, the same area of each sample and to identify the corresponding zones of the two very similar images. The anisotropy observed in polarized light appears in electron microscopy as differences in contrast resulting not from inequalities in electron absorption, but, as revealed by microdiffraction and dark Reid examinations, from diffraction phenomena depending on the general orientation of the carbon layers within each anisotropic area. 

We shall first examine the microscopic techniques which allowed us to study these transformations and to show the striking analogy between the images obtained by optical microscopy in polarized light and by electron microscopy with ultrathin sections, despite the difference of the absorption mechanisms of light and electrons. Once this analogy was established, we sought to use electron microscopy and electron microdiffraction to learn more about the texture and structure of the anisotropic areas. 

Microscopes. There are two basic modes of operation for X-ray analysis in a modern-day AEMs with a static (or flood) beam and with a rastered beam. This instrument is essentially a conventional TEM with either (a) scanning coils to raster and focus the beam or (b) an extra NminiN (or objective pre-field) condenser lens to provide a small (nm-sized) cross-over of a static beam at the objective plane. Some AEM configurations contain both scanning coils and a third condenser lens whilst others may have only one of these. In either condition, a small-sized electron probe can be obtained as a static or a rastered beam. The basic electron-optical principles which provide nanometer-sized beams for microanalysis are similar to those for electron microdiffraction which are well described by Spence and Carpenter [19]. 

Transmission Electron Microscopy. An electron microdiffraction technique was employed to identify crystal structures developed in two Pt-Sn-alumina catalysts (47). One catalyst was prepared by co-precipitating Sn and A1 oxides and then impregnating the calcined material with chloroplatinic acid to give... 

Si MASNMR spectrum of a sample of synthetic Na-Y which had crystallized slowly (over a period of 17 years) at room temperature. Using the equation given in the text we calculate the (Si/Al) framework to be 2.28. This does not correspond to a unique composition, so that the framework must consist of patches or domains of different local composition (e.g. of Si/Al = 16/8, 17/7, 18/6 etc.) Detailed electron microscopic analyses (using microdiffraction and microanalysis) may clarify the nature of the Si,Al disorder. It is clear, however, from results obtained to date that there is no strong tendency for Si,A1 ordering otherwise a ratio of 2.28 would not have been obtained. 

As discussed in Section X, irradiation effects are the main enemy in the case of electron microdiffraction. Granier et al. [31,190] found that not only does the irradiation rapidly destroy the sample, but long before, satellite reflections vanish (Fig. 23). In fact, the coherence between charge density waves is much more sensitive than the material itself. 

Figure 23 Electron microdiffraction study of TMA.TCNQ.I. Changes in superstructure reflections at about 40 K as a function of the irradiation dose. (From Ref. 190.)...

This demonstrates that electron microdiffraction may compete with other diffraction techniques but is not easy to use. Nowadays synchrotron radiation is probably more convenient, in most cases, for the observation of very weak sublattice reflections even from small crystallites however not as small as with electron microdiffraction. 

Figure 25 Electron microdiffraction diagram of plane [40T].i> of TMA.TCNQ.I taken at 2000 kV and T 105 K. The satellites (solid arrows) are visible at )fc and are accompanied by a second series of reflections at b (fine hollow arrows) with the same component in the direction [40T], (From Ref. 31.)...

The sensitivity of these small crystallites to the electron beam makes obtaining microdiffraction patterns very difficult. Usually apparatus which involves specialized optics and electronic monitoring devices such as those developed by Cowley (15) can reduce beam exposure time and thus provide some meaningful data which might be useful in studying the support-metal interaction or structure-reactivity relationship. 

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