Diffraction 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. 

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. 

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. 

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... 

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. 

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. 

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. 

J. Spence and J. M. Zuo, Electron Microdiffraction , Plemun Publishing, 1992. An advanced text on quantitative analysis of CBED with useful examples, tables and computer simulation programs for electron diffraction analysis. 

Microdiffraction.—Perhaps more important than SAD techniques, particularly in the context of catalyst research, microdiffraction allows the user to benefit from the small probe size generated in STEM in the structural analysis of small particles and localized areas in thin foils. If the small probe is stopped on a particle, then clearly a transmission diffraction pattern will be observable after the beam has traversed the sample, provided we have the means available for its display. In CTEM such a pattern will, of course, be formed by the imaging system in a manner identical to SAD, but in STEM the pattern must be scanned across the detector. This is accomplished by means of a set of post-specimen scan coils which once more scan the diffracted beams across the axial bright-field detector. Such a pattern is shown in Figure 13 where a beam of approximately 10 A FWHM was stopped on a small second-phase particle during the omega-phase transformation in a Zr-Nb alloy. The relatively poor definition of the reflection is a consequence of both the convergent nature in the probe (necessary in order to obtain the smallest probe sizes) and a S/N limited by the available current in the probe. Nevertheless, weak reflections with half-order indices are clearly visible between the main alloy reflections and it is therefore possible to attempt structural... 

Whereas microscopy observations have revealed the existence of metallic icosahedral clusters, diffraction techniques alone may provide information about their structure. Powder and microdiffraction patterns have been obtained from gold icosahedral clusters 100 to 200 A in diameter in order to determine whether their structure was rhombohedral or nondeformed fcc. The somewhat surprising result is that they always show the nondeformed fee structure. This proves that above some given size, stresses inherent in the icosahedral... 

The atomic structure of the films was studied by transmission electron microscopy (TEM) using a JEM lOOC electron microscope in the microdiffraction mode. The diffraction patterns were obtained at a low electron beam intensity using the CCD high-sensitive registration system to prevent the films from radiation damage by the electron beam. 

The preparation and structure of magnesium polyphosphide, MgP4, have been described. The compound was prepared by the reaction of gaseous phosphorus with the phosphide MgaP2 at 600 °C in a sealed silica tube. Evidence for a primitive monoclinic cell was obtained from electron microdiffraction. Refinement of X-ray powder diffraction data showed that the compound is isostructural with... 

M. Tanaka, M. Terauchi, K. Tsudaand K. Saitoh, Convergent Beam Electron Diffraction IV , JEOE Etd, Tokyo and earlier volumes. An outstanding collection of CBED patterns and application examples, including detailed description of the techniques used for analysis. J. Spence and J. M. Zuo, Electron Microdiffraction , Plenum Publishing, 1992. An advanced text on quantitative analysis of CBED with useful examples, tables and computer simulation programs for electron diffraction analysis. 

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