The resolution of crystal lattice planes is a useful test of the performance of an electron microscope, particularly mechanical and electrical stability. The crystal spacings are known accurately from X-ray data, so the high magnification used can be calibrated with accuracy.
Crystal specimens are mounted on perforated carbon films.
Selection and Placement
Wherever possible, thin crystals that cross one of the holes should be selected so that support film structure does not interfere. Good crystals can usually be located by checking the selected area diffraction pattern - unless a clear single crystal pattern is obtained, the lattice planes will not be observed.
When a suitable crystal has been selected
- a very high electron optical magnification should be used, so that the lattice planes can clearly be resolved on the fluorescent screen under the viewing telescope.
- The objective lens focus has to be adjusted carefully to optimise the contrast.
The phase contrast of a given crystal lattice spacing is critically dependent on the amount of objective lens defocusing. If the contrast is inadequate, a significant improvement may be obtained by defocusing the condenser lens (reduction of illumination semi-angle).
Further improvement is possible by tilting the illuminating beam so that the central beam and first order diffraction spot are symmetrical about the instrument axis (take care that the objective aperture is large enough to accept the diffracted beams). calibration