The first transmission electron microscope (TEM) was built by Dr. Ernst Ruska at the University of Berlin in 1931. For that, and other related work, he was awarded the Nobel Prize for Physics in 1986. A TEM can be compared to the more familiar light microscope. Instead of light, a TEM uses high-energy electrons because their wavelength is about 100,000 times smaller than light and thus can be used to image objects much smaller than can be seen with light. Instead of glass lenses, a TEM uses electromagnetic lenses, which must be quite large to affect the path of the high-velocity electrons. Instead of using our eyes to see the image, a TEM requires either a fluorescent screen, which emits light when struck by electrons, or a CCD camera with a fluorescent coating on top. Another major difference between a TEM and a light microscope is that the electron source, the column the electrons travel through and the viewing chamber or camera must all be under a high vacuum.
Modern TEMs use a field emission (FE) source for their electrons. We use a Schottkey type Zr/W(100) emitter which is heated to 1,800 K. The tip of the single-crystal tungsten emitter has been sharpened to a needle with only a few nanometer radius of curvature. These sources produce two to three orders of magnitude more brightness than the old LaB6 or hairpin-type tungsten filaments. The FE source allows modern TEMs to produce electron beams smaller than 1 nm with about 0.5 nA of current that can be scanned over the specimen, turning the microscope into a scanning transmission electron microscope (STEM), which is a powerful analytical instrument.