STEM imaging with a thin annular detector

In scanning transmission electron microscopy, the use of a thin annular det ector (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 cry stalline or amorphous phases may be distinguished. The range of scattering angles to be selected can be varied by using post-specimen lenses to magnif y the diffraction pattern of the object. Marginal image modes, giving diffe rentiated images, and ultra-high resolution bright-field images may be obta ined when the central spot of the diffraction pattern is magnified so that its radius approximates the inner radius of the TAD. Consideration of the r elationship of the TAD imaging to the form of the nanodiffraction pattern o btainable from any small area of the specimen shows that TAD imaging is equ ivalent to 'variable coherence imaging' and may be used in the determinatio n of medium-range ordering in amorphous or disordered materials.