Resolution-Limiting Effects in Electron Microscope Images

The resolution of a high-resolution lattice image is usually regarded as being set by the size of the objective aperture used in obtaining the image. However, experimental structure images (i.e. images obtained at optimum defocus) do not usually obtain the resolution one might expect merely from a consideration of the spatial frequencies of the beams contributing to the image. Structure images obtained by Iijima (1) (e.g. Fig. 4a), with an objective aperture admitting beams corresponding to a resolution of 3. 8Å at lOOkeV, show instead a resolution of 3. 8Å. This loss of resolution may be viewed as occurring due to the imposition of a virtual aperture by the resolution- limiting factors of chromatic aberration and incident electron beam convergence.