CBED/LACBED

Convergent beam electron diffraction (CBED)/Large angle CBED (LACBED)

Convergent beam electron diffraction is a diffraction technique in which the electron beam is converged in to a small probe on the specimen, rather than using parallel illumination as for SAD. CBED is a good method for navigating reciprocal space while tilting the specimen and aligning on zone axes, by way of the strong Kikuchi lines present in the off-axis CBED pattern. The on-axis CBED pattern consists of diffraction disks analogous to the spots in a SAD pattern (with the size of those discs determined by the convergence angle of the beam). However, compared to SAD, CBED can be obtained from much smaller volumes of material (as determined by the size of the electron probe), thus enabling very localized crystallographic analysis. Further, unlike SAD, CBED gives information on: specimen thickness; unit cell and precise lattice parameters; crystal system and true 3D crystal symmetry (inversion centres, point group and space group);and enantiomorphism. Large angle CBED is a variant of the technique, in which the beam is converged below the specimen plane and so illuminates a larger area of the specimen. It provides an alternative to conventional TEM contrast methods for analysing defects such as twins and dislocations, and can be used for strain measurements and thickness determination.