In the central section of the microscope the electron beam interacts with the specimen, and the resulting electrons are gathered and focussed ready for further magnification of the desired images.
As the electrons are incident on the sample they may be scattered by several mechanisms.
These scattering mechanisms will change the angle the electrons are moving at relative to the optic axis, and may be elastic (conserving energy) or inelastic (with energy dissipated as heat). It is by measuring the changes to the electrons on passing through the sample, either by measuring the angle that they have been scattered through (such as by studying diffraction patterns or images) or by measuring the amount of energy that they have lost, that we can gather information about the sample.
The specimen itself is inserted into the path of the electrons, and for the best resolution must be extremely thin; a few nanometers. This is to minimise multiple scattering of the electrons which decreases the number of electrons detected, and makes it more difficult to deduce information about the sample.
Once inside the microscope, the specimen sits right inside the objective lens and must therefore be small - typically less than 3 mm in diameter. It is necessary to align the specimen very accurately with the electron beam to achieve the required images. Common specimen holders allow rotation about two horizontal axes, along with lateral movement. Other holders might include heating elements or nano-indenters to deform the specimen as it is imaged.
Objective/Intermediate Lens System
This lens takes the electrons leaving the specimen and forms a diffraction pattern in the back focal plane of the lens, and an image of the specimen in the image plane.
In the conventional TEM we have the option of magnifying the image of the sample formed by the objective lens, or the diffraction pattern. The ease with which the microscopist can move between the two modes (imaging mode and diffraction mode) is one of the things which makes the TEM such a useful and versatile instrument.
To view an image, the microscopist focuses the intermediate lens onto the image plane of the objective lens. To view a diffraction pattern, the lens is refocused onto the back focal plane of the lens. In this plane the diffraction pattern of the sample forms, and may be magnified for viewing on the screen.
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