Here we demonstrate sub- 2Å resolution, in compositionally sensitive Z-contrast imaging, for a 200 kV field-emission transmission electron microscope (FEGTEM). This method has shown great promise for determining the atomic structure of interfaces in such materials as ceramics, semiconductors and high-Tc superconductors.
The Z-contrast technique involves focusing an electron probe at a specimen surface (which is oriented with a zone-axis along the beam direction). The transmitted signal, at an annular detector, is displayed on a VDU, rastered synchronously with the probe. Coherent, contrast reversal effects are suppressed in this geometry; images approximate to a convolution of the probe intensity profile with the square of the projected specimen potential. The Z2 dependence means that images can, with care, be interpreted as chemically sensitive maps of the projected structure. Furthermore, electron energy loss spectroscopy (EELS) can be performed simultaneously with Z-contrast imaging to yield additional compositional information.