Microdiffraction

Microdiffraction. By concentrating the incident x-ray beam on a small portion of a sample it is possible to get a complete diffraction pattern of very small regions of a sample. Of course, the intensity from such small regions is weak and an area detector that can coUect a large portion of the diffraction pattern at one time makes this appHcation practical. A typical region size is about 50 p.m in diameter. 

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.

Figure 4. STEM images of Au particles on a MgO support, (a) Image taken with the small objective aperture used for microdiffraction (b) Image obtained with larger objective aperture showing better resolution.

The first BioCD took its inspiration from the compact disc. The compact disc was invented in 1970 by Claus Campaan of Phillips Laboratory. The concept is purely digital and uses null interferometers that are far from quadrature, as appropriate for the readout of two binary intensity states. The interferometers were common-path and stable, as required for the mechanical environment of portable compact disc readers. The original BioCD used the same physics as the compact disc, but modified the on-disc microstructures to change from the digital readout to an analog readout that operated in quadrature for sensitive detection of surface-bound proteins7,8. Because the quadrature condition is established by diffraction off of microstructures on the disc, this is called the microdiffraction-class (MD-Class) of BioCD. 

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... 

Dillmann, P., Populus, P., Fluzin, P., et al. (1997). Microdiffraction of synchrotron radiation - identification of non-metallic phases in ancient iron products. Revue De Metallurgie-Cahiers d Informations Techniques 94 267-268. 

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. 

Nevertheless, this technique has a main disadvantage the minimum size of the diffracted area, which is selected by means of the selected-area aperture, is about 500 nm. It becomes difficult to prevent some thickness variations and/or some orientation variations in the diffracted area. The SAED patterns are, in fact, average patterns and the diffracted intensities can be strongly affected. For that reason, it is recommended to use Microdiffraction or CBED because the diffracted area is directly defined by the incident beam and can reach a few nanometers with recent microscopes. 

Microdiffraction is the pertinent method to identify the crystal system, the Bravais lattices and the presence of glide planes [4] (see the chapter on symmetry determination). For the point and space group identifications, CBED and LACBED are the best methods [5]. 

Abstract Symmetry determinations performed from Microdiffraction, Convergent-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... 

The Bravais lattice can be identified, on some specific Zone-Axis Patterns, from the observation of the shift between the reflection net located in the ZOLZ and the one located in the FOLZ. This shift is easily observed by considering the presence or the absence of reflections on the mirrors. Thus, in the example given on figure 1, some reflections from the ZOLZ are present on the four mi, m2, m3 and mirrors. This is not the case in the FOLZ where reflections are present on the m3 and m4 mirrors but not on the mi and m2 mirrors. Simulations given in reference [2] allow to infer the Bravais lattice from such a pattern. It is pointed out that Microdiffraction is very well adapted to this determination due to its good angular resolution (the disks look like spots). 

Figure 1. Example of a Zone-Axis Microdiffraction Pattern.

Glides plane can also be identified from Microdiffraction patterns. Their presence is characterized by a periodicity difference between the reflections present in the ZOLZ with respect to the ones in the FOLZ. This effect is visible on figure 1 where the ZOLZ reflections are placed at the nodes of a centered square net while those of the FOLZ are at the nodes of a smaller non-center square net. The corresponding glide plane is identified from simulations given in reference [2]. 

The SAED patterns consist of an intense base set of a - PbO subcell reflections and weak superstructure reflections hV4, kV3, T/2 referring to the a - PbO cell due to a modulation of the structure. To determine the Pb and Mo positions in PbsMoOg on the base of the relative intensities of the SAED reflections a multislice dynamic calculation [12] was performed. The calculations were performed separately for each of the 6 experimental microdiffraction patterns. The thickness of the specimen was determined independently for each of the patterns. The performed procedure is close to the published idea by Bing - Dong et al. [13]. 

Microdiffraction patterns are taken from individual particles after the reduction treatment and are shown in Figs. 3a-d. Most particles with platelet shape and straight edges produce similar microdiffraction patterns, one of which is shown in Fig. 3a. It is indexed as PtsSi with CusAu structure on [100] zone axis. Figs. 3b and 3c show the diffraction patterns from the not reacted Pt on [100] and [310] zone axes, respectively. Particles with irregular forms show various diffractions and a considerable amount of them can be attributed to Pt Sis. One such pattern is shown in Fig. 3d, exhibiting Pt Sis on [152] zone axis. 

Figure 3. Microdiffraction patterns from four single particles after the reduction, indexed as (a) [100] PtsSi, (b) [100] Pt, (c) [310] Pt and (d) [152] Pt Sis.

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... 

Rautureau, C., C. Tchoubar, and J. Mering (1972). Analyse structurale de la se-pioUte par microdiffraction electronique. R. Seanc. Acad. Sci., Paris 274C 25-28. 

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