Convergent beam electron diffraction

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Convergent beam electron diffraction (CBED) is an electron diffraction technique where a convergent or divergent beam (conical electron beam) of electrons is used to study materials.

Contents

CBED scheme, adapted from W. Kossel and G. Mollenstedt, Elektroneninterferenzen im konvergenten Bundel, Annalen der Physik 36, 113 (1939). CBED scheme.png
CBED scheme, adapted from W. Kossel and G. Möllenstedt, Elektroneninterferenzen im konvergenten Bündel, Annalen der Physik 36, 113 (1939).

History

CBED was first introduced in 1939 by Kossel and Möllenstedt. [1] The development of the Field Emission Gun (FEG) in the 1970s, [2] 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. In the seventies, CBED was being used for the determination of the point group and space group symmetries by Goodman and Lehmpfuh, [3] and Buxton, [4] and starting in 1985, CBED was used by Tanaka et al. for studying crystals structure. [5] [6] [7] [8] [9]

Applications

By using CBED, the following information can be obtained:

Parameters

where is the distance between the crystallographic planes , is the Bragg angle, is an integer, and is the wavelength of the probing electrons.

Advantages and disadvantages of CBED

Since the diameter of the probing convergent beam is smaller than in the case of a parallel beam, most of the information in the CBED pattern is obtained from very small regions, which other methods cannot reach. For example, in Selected Area Electron Diffraction (SAED), where a parallel beam illumination is used, the smallest area that can be selected is 0.5 µm at 100 kV, whereas in CBED, it is possible to go to areas smaller than 100 nm. [40] Also, the amount of information that is obtained from a CBED pattern is larger than that from a SAED pattern. Nonetheless, CBED also has its disadvantages. The focused probe may generate contamination, which can cause localized stresses. But this was more of a problem in the past, and now, with the high vacuum conditions, one should be able to probe a clean region of the specimen in minutes to hours. Another disadvantage is that the convergent beam may heat or damage the chosen region of the specimen. [41] Since 1939, CBED has been mainly used to study thicker materials.

CBED on 2D crystals

Recently, CBED was applied to study graphene [42] and other 2D monolayer crystals and van der Waals structures. For 2D crystals, the analysis of CBED patterns is simplified, because the intensity distribution in a CBED disk is directly related to the atomic arrangement in the crystal. The deformations at a nanometer resolution have been retrieved, the interlayer distance of a bilayer crystal has been reconstructed, and so on, by using CBED. [43]

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