STEM imaging with a thin annular detector

In scanning transmission electron microscopy, the use of a thin annular detector (TAD), with a ratio of outer to inner radii of about 1.1, allows the selective imaging of various components of a specimen so that different crystalline or amorphous phases may be distinguished. The range of scattering angles to be selected can be varied by using post-specimen lenses to magnify the diffraction pattern of the object. Marginal image modes, giving differentiated images, and ultra-high resolution bright-field images may be obtained when the central spot of the diffraction pattern is magnified so that its radius approximates the inner radius of the TAD. Consideration of the relationship of the TAD imaging to the form of the nanodiffraction pattern obtainable from any small area of the specimen shows that TAD imaging is equivalent to 'variable coherence imaging' and may be used in the determination of medium-range ordering in amorphous or disordered materials.