ELECTRON MICROSCOPE

An electron microscope is a type of microscope that uses electrons rather than photons — as a conventional light microscope does — for imaging. Because electrons have a much smaller wavelength than photons, they provide much greater magnification. Electrons are the tiny “satellites” that orbit the atomic nucleus and carry electric charge – these particles are so small that in physics they are often modeled as points. Yet light waves are much larger, with a wavelength of around 500 nanometers for the color green, for instance.

The best optical microscopes only offer about 2000X magnification of a sample, whereas some electron microscopes can magnify a sample by 50 million times; in contrast, 2 million times is more typical. This works out to a resolution limit of about 0.1 nanometers, allowing the observation of individual atoms on a surface. The electron microscope was invented in 1931, when the first working prototype was built by Ernst Ruska and Max Knoll. Ruska was eventually awarded the Nobel Prize in Physics (1986) for his achievement.

There are four types of electron microscopes, the first two being the most common: the transmission electron microscope (TEM), scanning electron microscope (SEM), reflection electron microscope (REM), and the scanning transmission electron microscope (STEM).

The transmission electron microscope is the electron microscope as it was originally invented. Using a sample that is semi-transparent to electrons, an electron beam is fired directly through the sample. A receiver on the other side measures the density of electrons at each individual point and compiles them into a grayscale image. This is the image of the sample.

A scanning electron microscope has somewhat less resolution than a TEM, but is still the most popular variety of electron microscope. Like its name implies, a scanning electron microscope scans an electron beam across the sample. Instead of analyzing the original beam for information about the makeup of the sample, sensors pick up secondary electrons released from the surface of the sample via excitation from the primary beam. This sacrifices some resolution for a 3D image of the sample. This is more than worth the trade off, and SEMs are accordingly the most popular electron microscopes.

Most scanning electron microscopes are very costly to buy and maintain. They require a stable high-voltage electricity source, a vacuum pump, and cooling coils. Samples have to be prepared, usually by coating them with a thin layer of conductive material such as gold.