Illumination: Electron Source
At the top of the TEM column is the electron gun, which is the source of electrons. The electrons are accelerated to high energies (typically 100-400 keV) and then focussed towards the sample by a set of condenser lenses and apertures.
The source is chosen so that the emitted current density per solid angle (brightness) is maximised. This is so that the maximum amount of information can be extracted from each feature of the sample.
There are two major types of electron source, thermionic emitters and electron field emitter. Electron guns based on thermionic emission are cheaper and more robust, hence often found on older instruments. If enough thermal energy is added to a material its electrons may overcome the energy barrier of the work function and escape. To avoid the source melting, the material used must either have a very high melting point (such as W) or an exceptionally low work function (certain rare-earth boride crystals such as LaB6 are widely used).
Another way of extracting electrons from a material is by applying a very large electric field. By drawing tungsten wire to a very fine point (<0.1 μm), application of a potential of 1 kV gives an electric field of 1010 V m-1 which is large enough to allow electrons to tunnel out of the sample. This is called electron field emission.
Field emission guns are more expensive than thermionic electron guns, and must be used under ultra-high vacuum conditions. They are favourable for applications in which a high brightness and low energy-spread of incident electrons is needed (eg. high resolution TEM, electron energy loss spectroscopy)