In the central section of the microscope the electron beam interacts with the specimen, and the transmitted 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 tranferred to the sample and dissipated as heat). By analysing the changes to the electrons transmitted through the specimen we can gather information about the material, and in particular we can study its morphology, crystal structure, and composition.
The sample itself is inserted into the path of the electrons, and for the best resolution must be extremely thin; a few nanometers. This is to maximise the number of transmitted electrons, and minimise multiple scattering events which make it more difficult to deduce information about the material.
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 good imaging. Common specimen holders allow rotation about two horizontal axes, along with lateral movement. Other holders might include heating or cooling elements, or nano-indenters to deform the specimen as it is imaged.
Objective/Intermediate Lens System
The objective lens takes electrons transmitted through the specimen and forms a diffraction pattern (in the back focal plane) and an image of the specimen (in the image plane).
In the conventional TEM we have the option of magnifying either the image or the diffraction pattern formed by the objective lens. This is achieved by changing the settings of the intermediate lens from the imaging mode to the diffraction mode. The ease with which the microscopist can move between the two modes is one of the things which makes the TEM such a useful and versatile instrument.
In imaging mode, the microscopist focuses the intermediate lens onto the image plane of the objective lens to produce a magnified version of the image further along the optic axis and on the viewing screen. To view a diffraction pattern, the intermediate lens is adjusted so that its object plane coincides with the back focal plane of the objective lens, where the first diffraction pattern is formed. The diffraction pattern is then displayed on the viewing screen.