Expunging diffraction contrast: EELS and HAADF in STEM

Abstract

In 1975 the Scanning Transmission Electron Microscope promised to deliver point-by-point information, both structural and chemical, relating to problems of grain boundary segregation, nanostructured phases, glassy materials, and heterogeneous catalysts. Diffraction contrast, so important in TEM, for a while continued to play an important role. But the full utilisation of Crewe's ADF technique, developed for single atoms, but now applied to crystals, required the elimination so far as possible of diffraction contrast. Archie Howie recognised that elimination of Bragg scattered beams, whilst retaining incoherently scattered electrons at larger angles, could be achieved by modifying Crewe's annular detector. High Angle Annular Dark Field, HAADF, is now the most widely used imaging mode in STEM. 'HAADF' might with justification stand for 'Howie's Adaptation of Annular Dark Field'! © Published under licence by IOP Publishing Ltd.