The Working Principle of a TEM

>

A light source at the top of the microscope emits the electrons that travel through vacuum in the column of the microscope. To focus the electrons into a very thin beam, the TEM uses electromagnetic lenses. The electron beam then travels through the specimen you want to study. Depending on the density of the material present, some of the electrons are scattered and disappear from the beam. At the bottom of the microscope the unscattered electrons hit a fluorescent screen, which gives rise to a 'shadow image' of the specimen with its different parts displayed in varied darkness according to their density.

Step by step it works as the following:

1 - The "Virtual Source" at the top represents the electron gun, producing a stream of monochromatic electrons.

2 - This stream is focused to a small, thin, coherent beam by the use of two condenser lenses. The first lens(usually controlled by the "spot size knob") largely determines the "spot size"; the general size range of the final spot that strikes the sample. The second lens(usually controlled by the "intensity or brightness knob" actually changes the size of the spot on the sample; changing it from a wide dispersed spot to a pinpoint beam.

3 - The beam is restricted by the condenser aperture (usually user selectable), knocking out high angle electrons (those far from the optic axis, the dotted line down the center)

4 - The beam strikes the specimen and parts of it are transmitted

5 - This transmitted portion is focused by the objective lens into an image

6 - Optional Objective and Selected Area metal apertures can restrict the beam; the Objective aperture enhancing contrast by blocking out high-angle diffracted electrons, the Selected Area aperture enabling the user to examine the periodic diffraction of electrons by ordered arrangements of atoms in the sample

7 - The image is passed down the column through the intermediate and projector lenses, being enlarged all the way

8 - The image strikes the phosphor image screen and light is generated, allowing the user to see the image. The darker areas of the image represent those areas of the sample that fewer electrons were transmitted through (they are thicker or denser). The lighter areas of the image represent those areas of the sample that more electrons were transmitted through (they are thinner or less dense)