Where θ B is the Bragg angle of the specimen, a is the width of the diffraction disks and b is the distance between the 000 disk and the hkl disks (see Figure). The size of the diffraction disks depends on α: the beam semi-convergence angle α - is controlled by the C2 aperture.When a CBED pattern is formed, there are some variables that need to be controlled: For small semi-convergence angle, the disks do not overlap with each-other, whereas for larger semi-convergence angles, the disks overlap. The beam semi-convergence angle, α, is of the order of milliradians, ranging from 0.1˚ - 1˚. Consequently, instead of diffraction peaks, diffraction disks are seen. This small diameter of the beam in real space, opens up a greater angular range in reciprocal space. In CBED, the diameter of the electron beam is relatively small, usually 1–100 nm. crystal symmetry information - by looking at the symmetries that appear in the disks, point group and space group determination is performed.defects such as stacking faults, dislocations, grain boundaries, 3-dimensional deformations, lattice displacements.crystal structural parameters such as lattice parameters, sample thickness.Applications īy using CBED, different types of information can be obtained: by using different techniques of CBED which covered different applications. In the seventies, CBED was being used for the determination of the point group and space group symmetries by Goodman and Lehmpfuh, Steeds, Buxton and starting on 1985, by Tanaka et al. The development of the Field Emission Gun (FEG) in the 1970s, the Scanning Transmission Electron Microscopy (STEM), energy filtering devices and so on, made possible smaller probe diameters and larger convergence angles, and all this made CBED more popular. This technique was first introduced in 1939 by Kossel and Möllenstedt, who worked with large (~40 μm) probes and small convergence angles.
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