Effect of the phase shift due to dynamical scattering on the contrast of crystal structure images

The contrast of through-focus images of a niobium tungsten bronze, 2Nb₂O₅·7WO₃, taken by a 1 MV high resolution electron microscope, is discussed in terms of the transfer function, in which, in addition to the conventional phase aberration, the phase shift due to the dynamical scattering is taken into account. It is found that the phase shift may be advantagous in the formation of structure images, as long as the crystal very thin, i.e., a few nanometers in thickness. It is also made clear why the best defocus for optimum structure images differs from the Scherzer defocus, depending on the crystal thickness. An almost exact contrast reversal occurs at overfocus only in the images of extremely thin crystals, because the positive range of the transfer function is made narrower due to the above phase shift. The discussion is supported by the computer simulation of the image contrast.