CBED convergent beam electron

For a long time, Selected-Area Electron Diffraction (SAED) performed with a parallel incident beam and a selected-area aperture was the only experimental method available. During the three last decades, new diffraction techniques based on a convergent electron incident beam (CBED Convergent-Beam Electron Diffraction, LACBED Large-Angle... 

CBED convergent beam electron diffraction (patterns)... 

J. Mansfield. Convergent Beam Electron Dijfraction of Alloy Phases, (by the Bristol Group, under the direction of J. Steeds, and compiled by J. Mansfield) Hilger, Bristol, 1984. This book is an atlas of CBED patterns that may be used to identify phases by comparing published patterns with experimental patterns. 

The diffraction pattern obtained in the detector plane when the beam scan in a STEM instrument is stopped at a chosen point of the image comes from the illuminated area of the specimen which may be as small as 3X in diameter. In order to form a probe of this diameter it is necessary to illuminate the specimen with a convergent beam. The pattern obtained is then a convergent beam electron diffraction (CBED) pattern in which the central spot and all diffraction spots from a thin crystal are large discs rather than sharp maxima. Such patterns can normally be interpreted only by comparison with patterns calculated for particular postulated distributions of atoms. This has been attempted, as yet, for only a few cases such as on the diffraction study of the planar, nitrogen-rich defects in diamonds (21). 

Several other techniques, such as electron holography (Lichte, 1986) and convergent beam electron diffraction (CBED) have also been developed for structure analysis. CBED can provide information not only on the lattice parameters and the S5mimetry of crystals, but also accurate structure-factor amplitudes and phases (Hoier et al, 1993). Accurate structure factor determination by CBED can provide information on the location of valence electrons. However, it is more favourable for thick crystals (> 500 A) with small unit cells (< 10 A). Structure analysis by CBED has been summarized in two review articles (Spence, 1993 Tanaka, 1994). 

Electron Diffraction (CBED) and Large-Angle Convergent-Beam Electron Diffraction (LACBED) allow the identification of the crystal system, the Bravais lattice and the point and space groups. These crystallographic features are obtained at microscopic and nanoscopic scales from the observation of symmetry elements present on electron diffraction patterns. 

Figure 6. A 3D rendering that reveals the details of chemical bonding and dz2 orbital-like holes in Cu20. The amount of charge redistribution is very small and its detection requires a high degree of experimental accuracy. In this picture, the small charge differences between the measured crystal charge density derived from convergent-beam electron diffraction (CBED) and that derived from superimposed spherical 02- and Cu+ ions are shown. The red and blue colors represent excess electrons and holes, respectively.

There is another diffraction mode, called convergent beam electron diffraction (CBED), in which the incident electron beam is focused to a fine spot on the specimen. If the convergence angle is appropriately chosen, the diffraction pattern consists of an array of nonoverlapping disks. For thin specimens ( SO nm) the CBED disks are featureless, but... 

Figure 3.17. A convergent beam electron diffraction pattern (CBED) from a specimen of TUHrOn. Note that the ZOLZ suggests six-fold symmetry, but the HOLZ s indicate that the symmetry is only three-fold.

Figure 10. (a) TEM bright-field image, (b) selected area diffraction (SAD) pattern, and (c) convergent beam electron diffraction (CBED) pattern of the optimally reannealed (r, 83 K) Tl-2201 thin film. 

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