Selected Area Electron Diffraction (SAED) and Convergent Beam Electron Diffraction (CBED)

Selected Area Electron Diffraction (SAED) and Convergent Beam Electron Diffraction (CBED)

Here are some of the characteristics of SAED and CBED. These are what I've heard from my lecture in school.

1. SAED

• Function:

• identifying crystal structure and lattice structure of a sample
• examine crystal defects
• uses parallel beam

Why use SAED?

• sample, comes in foils, easily distorted because of the small size. This causes orientation change from its original crystal structure. So, SAED allows experimenter to SELECT the AREA on the sample with constant orientation
• SELECT the AREA to study a single crystal in a polycrystalline sample
• study the crystallographic orientation between 2 crystalline structure

How to tell the difference between defects on a sample vs bending of the foil on a sample?

• tilt the sample slightly

• if contours move very fast, then it's bending of the foil
• if contours move slow, then it's the defects

CBED

• Function:

• same as SAEB but used when crystal defects and second phase ppt at much smaller resolution (nm to Angstrom range)
• uses convergent beam

Why use CBED?

• improved spatial resolution due to smaller probe beam

• Specimen thickness
• unit cell and precise lattice parameters
• crystal system and 3D crystal symmetry

Drawbacks of CBED:

• high current density damages sample
• heat might change microstructure of sample
• local contamination of specimen

4 variables to control CBED

• convergent semiangle (higher angle higher...) - bigger aperture bigger convergent angle
• higher camera length higher magnification but lower angle of view of pattern
• focus of pattern (use 1st intermediate lens to focus)
• size of beam (smaller size beam higher intensity and resolution)