New imaging modes in STEM

The most often quoted advantage of STEM over conventional TEM is the ability to produce multiple simultaneous images by the use of multiple detector systems. In practice, this postulated advantage has seldom been fully utilised, mainly because of the practical difficulties in designing such detector systems.Most STEMs to date have been constructed as two-channel instruments combining annular dark-field imaging with either filtered bright-freld or inelastic imaging. More complex forms of bright-field detector have been employed1, as have parallel-readout systems for energy-loss spectra but the ability of the spectrometer to produce multiple simultaneous images has not been fully utilised.The basis of the problem lies in the fact that the objective lens and the detector system(s) have in most cases been designed by the manufacturers as separate entities in order to simplify the later addition of user-specific detectors. Since the acceptance angle of even the best spectrometers is relatively small, additional post-specimen lenses [with their attendant aberrations] had to be added in order to make full use of the spectrometer.