In SAED
Since, Single Crystalline material have only single orientation at equal spacing, so we get EQUAL SPACED DOTS PATTERN
But for POLYCRYSTALLINE, since grains have varying orientation so we will not get EQUAL SPACED DOT PATTERN.
But then why only CIRCLE pattern is formed? Why not other shapes?
Dear Rahul Prasad ,
that is to my opinion due to the cylindrical symmetry of the set up and the diffraction process in the TEM.
The SAED diffraction pattern arise from diffractions cones, the axis of which coincide with the primary electron beam. This situation is identical to the Debye-Scherrer set up in x-ray diffraction applications.
The image plane is perpendicular to the incoming beam. Thus you will get circular rings for polycrystalline and amorphous material.
A tilt of this plane with respect to the primary electron beam will transform these diffraction circles into ellipsis...
Bragg's law gives you the possible diffraction angle for a certain (h k l)-plane and this makes a cone when all possible incident angles are present. You can compare with the Debye_Scherrer method, for instance.
It has been my experience that dot patterns often degrade to ring patterns when viewing with TEM. This results from a form of degradation when the sample is sensitive to the electron probe. If possible, one can reduce TEM KV, or one can reduce aperture sizes at all points within your TEM where that is a variable.
Another option would be to use SEM defraction methods if that option is available to you, since SEMs use lower accelerations energies.
Too, some materials do have less prominent crystalline structure, which tends to produce ring structure rather than the more desirable dot patterns.
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