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Viewing images

After the electrons have passed through the specimen and been scattered to varying degrees, the information from the system is converted into a macroscopic image. The simplest way of doing this is by simply magnifying the diffraction pattern or image formed of the sample until it is of the required size for analysis. This is the basis of conventional TEM (CTEM).

Alternatively, if a very fine beam of electrons is rastered across the sample, the amount of scattering from each point may be measured separately and successively, and an image gradually built up. This technique, requiring no lenses after the specimen, is called scanning TEM (STEM).

CTEM

Projector System

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The projection system magnifies the images or diffraction patterns formed from the specimen and focuses images in the plane of the screen, where the electron density is converted into light-optical images for the microscopist to see.

It is here that the effects of spherical and chromatic aberrations in the lenses are most significant. The chromatic aberrations are much more pronounced since the interactions of the electrons with the sample often absorb energy, so the beam of electrons passing through the projector lenses contains electrons of a much wider range of energies.

Screen

Beneath all the lenses is a phosphorescent screen that glows when it is struck by electrons, displaying the image or diffraction pattern. The screen is viewed through a lead-glass window.

In the transmission optical microscope, we may think of colour images being formed as light of different colours is absorbed at each point of the sample. The degree of absorption leads to the contrast in the image.

In the electron microscope, we cannot usually see the effect of the wavelength of the electrons (the "colour") in each image, so we only have "black and white" images. Hence, the information contained in an electron micrograph is solely due to the difference in the flux of electrons through each point in the image - the contrast. The electron microscopist must understand the reasons for contrast in order to gather information from the sample. We shall deal briefly with the main sources of contrast in the following:

  • Mass absorption contrast
    • On passing through matter, a beam of electrons is gradually attenuated. The degree of attenuation increases with the thickness of the specimen and its mass, so variations of mass and thickness across the sample give rise to contrast in the image.
  • Diffraction contrast
    • Diffraction of electrons from Bragg planes causes a change in their direction of travel. Hence, contrast can arise between adjacent grains or between different regions near the core of a dislocation.
  • Phase contrast
    • Scattering mechanisms often cause a change in the phase of the scattered electrons, as well as a change in direction. Interference between electrons of different phase which are incident on the same part of the image will cause a change in intensity and give rise to contrast.

STEM

Instead of recording the image from a sample all at once, we can illuminate a very small segment of the sample at one time and record the magnitude of electron scattering from the point. This can by done rapidly and an image built up in the same way as on a television screen by scanning the beam across the sample. This technique is called scanning transmission electron microscopy (STEM).

Since the whole image is not collected and focussed at the same moment, no lenses are needed after the sample. Instead, a set of annular detectors is used. An advantage in image formation is that electrons scattered through large angles (Rutherford scattering) may be detected using a high-angle annular dark-field (HAADF) detector and a fourth mechanism of contrast exploited. At large angles the intensity of scattering,

I  ∝ Z x .

This allows structure to be imaged, as contrast will appear between areas of different elemental composition.

Finally, the absence of the projection lenses means that there is a lot of left over space in the chamber of the microscope, and this can be filled with analytical detectors, which may measure the energies of the scattered electrons. STEM is used for high resolution chemical analysis of specimens.

Note: This animation requires Adobe Flash Player 8 and later, which can be downloaded here.


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