Transmission Electron Microscope (TEM)

As the grandfather of all electron microscopes, the Transmission Electron Microscope has the honor of being the first man-made device to delve into the world of the nanometer.  In the early 20^th century, scientists were starting to feel the crunch as light microscopes began reaching their fundamental limit (also the same limit that will ultimately spell doom for shrinking microchips).  Enough was known about the microscopic world that people were beginning to ask questions about what lied beyond it.  A curious researcher by the name of Ernst Ruska developed the world's first electron microscope in response to the growing need for more magnification.

His reasoning was straightforward: since visible light has wavelengths on the order of hundreds of nanometers, there was no way that such a source could probe features smaller than that.  But electrons, on the other hand, were extremely small, could be accelerated, and could interact with matter through electrical current.  He began building prototypes in the 1920's, and finally finished his first working model in 1931.  Ruska would later receive the Nobel Prize in 1986 in conjunction with the inventors of the Scanning Tunneling Microscope.

Like a light microscope, the transmission electron microscope requires a thin sample that the source may pass through.  In this case, the source at the top of the column is an electron gun firing monochromatic streams of electrons (all of the same speed and energy).  The higher the energy, the smaller the wavelength, and the higher the spatial resolution—It's that simple!  The electron beam must be focused with several condensing lenses, like any light microscope.  At that point, they'll hit the sample and some electrons will be absorbed.  These differences in absorption are recorded on the other side of the microscope either by a cathode ray tube (TV screen) or a photographic plate.  The result is a highly detailed visual image of the sample with a resolution on the order of Angstroms (the atomic diameter), or roughly equivalent to 500,000x magnification!  It is therefore an order of magnitude better than its successor, the SEM.  However, the SEM allows for analysis of thick samples which more than makes up for its weaker magnification.

The one major drawback with electron microscopes in general is the need for a good vacuum within the specimen chamber and electron column.  This complicates a lot of the research and consumes a lot of time.  Furthermore, samples must be prepared with great caution otherwise they will be marred with artificial artifacts.

Like the SEM, the TEM offers additional features like diffraction measurements and chemical analysis.  It is considered a simpler microscope despite having a better resolution, but is still valued in a laboratory if it can be afforded.