Here, we introduce how to obtain various CBED patterns under different experimental conditions. CBED (convergent-beam electron diffraction) patterns are obtained by producing an electron
probe of 5–20 nm in diameter as a demagnified image of the electron source (~10 nm - 40 µm). A strong upper-objective polepiece is needed to create a convergent beam to form a CBED pattern. Four microscope variables are needed to be adjusted to provide different information, for instance, about the crystal symmetry:
i) The beam convergence semiangle (The convergence angle can be defined by an aperture).
ii) The camera length (L) (i.e., the magnification).
iii) The focus of the pattern.
iv) The size of the beam.
Although CBED operation procedure can vary from microscope to microscope, the basic steps are essentially the same for all TEMs:
i) After aligning the microscope, start with your specimen in the eucentric plane as usual and then, form a focused image on the TEM screen with the area of interest onto the crosshairs.
ii) Select a large C2 aperture about 100 - 200 µm in diameter (to get nonoverlapping disks - Kossel-Möllenstedt (K-M) pattern, an appropriate C2 aperture should be selected; typically, the Bragg angle is a few milliradians and C2 apertures in the 10-50 µm range usually ensures the K-M conditions.), center it on the crosshairs, and adjust the C2 lens to form a focused spot on the area of interest.
iii) Keep C1 weakly excited to give a relatively large spot, about 100-200 FWTH, containing sufficient current to give high intensity in the pattern.
iv) Select a small camera length, < 500 mm, to give a wide-angle view of the pattern.
v) To observe the CBED pattern by pressing CBED or DIFF mode ( make sure the objective and SAD apertures are retracted).
vi) Focus the resulting CBED pattern using the diffraction spot knob. Different camera lengths are available by turning the magnification knob but this does not affect the imaging magnification when you return to image mode.
vii) Tilting to a zone axis can easily be accomplished by following Kikuchi lines to a point where two or more Kikuchi lines intersect (this intersect is a zone axis).
viii) The size of the diffraction disks depends on convergence semiangle α, which can be changed by changing the C2 aperture.
In principle, for CBED the largest obtainable convergence angle is used to observe as much structure as possible within the CBED disks.