7 - Specialised EM- and other microscopical and analytical techniques

Summary

Dark-field electron microscopy

In a transmission microscope the bright-field image is formed by electrons which have traversed the specimen without significant change in direction. Darker areas show where electrons have been deflected out of the beam and intercepted by the objective aperture, by atomic scattering if the specimen is amorphous or by Bragg reflections from a crystal.

It is often more informative to form the image using the deflected electrons rather than the transmitted beam, since one is then using only electrons which have actually interacted with the specimen. Examination modes based on this principle are known as dark-field modes, since the final screen is dark unless there is a specimen present to deflect electrons into the image (see, for example, Figure 7.1). The principle is the same as that used in dark-ground light microscopy and is similarly useful in showing up low-contrast detail more strongly. There are several ways in which dark-field observations can be made.

Displaced objective aperture

In normal bright-field microscopy the objective or contrast diaphragm is placed symmetrically about the axis of the objective lens and scattered electrons are absorbed by its periphery. If the diaphragm is displaced sideways it will absorb the direct beam and transmit electrons scattered or diffracted in its direction, so that the final image is formed only from these electrons (Figure 7.2(a)). If the specimen is crystalline the diaphragm may be moved so as to transmit only the electrons from a specific diffraction ring or spot so that areas of specimen with a particular crystallographic orientation will be highlighted in the image.