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August 26, 2013

Birth of the Electron Microscope Part 3: The Scan

The creation of the transmission electron microscope (TEM) was a revolution in the field of microscopy; for the first time, it allowed humans to see things that were too small for traditional light-based microscopes to resolve, such as individual cells and large atomic molecules, by exposing samples to a beam of electrons instead of a beam of light. However, the TEM had limitations of its own; it could only resolve an image if the sample was thin enough for electrons to pass through, so biological samples had to be preserved and sliced up, destroying any potential for viewing the minute changes in a living organism and making it impossible to view a complete image of the specimen. TEM also suffered from diffraction issues, as the electron beam could only resolve to a certain magnification level before the electrons scattered too much to form a definite image.

Shortly after the TEM’s 1931 debut, a Russian scientist named Manfred von Ardenne invented a true electron-based microscope that worked on a slightly different principle, and patented the Scanning Electron Microscope (SEM) in 1937. This machine finally enabled scientists to see complete specimens in high detail, and resolve three-dimensional shapes. Instead of relying on a beam of electrons to carry the image away from the specimen, the scanning electron microscope works by scanning the beam across the specimen in a series of rectangular areas. This technique is known as raster scanning, and it is common in computer graphics; it’s how printers create images on paper, and how older CRT televisions created their images. When an SEM scans a specimen, the electron beam loses energy; this energy is converted into heat, scattered electrons, X-rays, and light emission. The SEM’s lenses can detect this energy, and it maps these signals into an image based on where the electron beam was located when it lost that particular amount of energy. By scanning in this manner, an SEM can resolve specimens as three-dimensional shapes.

The specimens in an SEM must be electrically conductive, in order to attract the electrons in the first place. While metals require very little preparation, non-conductive specimens must be coated with a very thin layer of gold, platinum, or tungsten. The SEM uses an electron gun much like the TEM, and uses a tiny cathode of tungsten at its tip. The SEM also requires the specimen to sit in a vacuum, in order to prevent interference from artifically disrupting the electron beam.

There are other types of electron microscopes, but the SEM was a major breakthrough because it allowed researchers to capture minute details of things like a house fly’s eye, a snowflake, or an ant’s head. Special environmental SEMs can observe samples that are in low-pressure environments (rather than complete vacuums) and do not require biological materials to be coated in gold. It is highly useful for seeing biological specimens, even scanning still-living insects.