Structure Determination Through Z-Contrast Microscopy

The technique of Z-contrast scanning transmission electron microscopy (STEM) provides an incoherent image of crystals at atomic resolution. There are no phases in an incoherent image, therefore, no phase problem for structure determination. Location of atom column positions in an image is greatly simplified. In addition, the resolution is a factor of two higher than in a coherent image, the information is more highly localized, the intensity of atom columns directly reflects their mean square atomic number (Z), and there are no contrast reversals with crystal thickness. It is also the only means to achieve spectroscopy from individual atomic columns. Here we give a simple but quantum mechanically correct description of this imaging technique. The value of the method lies in providing an approximate starting model which can then be refined by other techniques such as X-ray or electron crystallography or density-functional calculations.