Electron microscope

An electron microscope is a powerful scientific instrument used to observe the microscopic world.

While standard optical microscopes use visible light to illuminate samples, electron microscopes use a beam of electrons. This shift in technology allows for much higher resolution. The wavelength of an electron is more than 100,000 times smaller than that of visible light. Because of this, an electron microscope can achieve a resolution of about 0.1 nm. In comparison, a light microscope is limited to about 200 nm.

This ability to see at such a small scale is vital for modern science.

The most fundamental type is the transmission electron microscope, or TEM. In a TEM, an electron gun produces a high-voltage electron beam. These electrons typically have energies between 20 and 400 keV. Electromagnetic lenses then focus this beam onto a very thin specimen. As the electrons pass through the sample, they carry structural information about it.

This information is magnified by lenses and projected onto a detector. Operators might view the image on a fluorescent screen coated with zinc sulfide. More often, a digital camera captures the image through a lens system. Modern high-resolution TEMs can reach magnifications exceeding 50 million times. They can even determine the exact positions of atoms within a material.

Another major type is the scanning electron microscope, or SEM. Unlike the TEM, the SEM uses electrons with much lower energy, generally below 20 keV.

Instead of passing through a sample, the SEM probes the surface with a focused beam. This beam performs a process called raster scanning, where it moves across the specimen. When the beam hits the sample, it causes various interactions. These interactions result in the emission of secondary electrons, backscattered electrons, or X-rays.

By detecting these signals, the microscope builds an image of the surface topography and composition. This creates a three-dimensional appearance, much like a reflected light image.

A third specialized type is the scanning transmission electron microscope, or STEM. The STEM combines features from both the TEM and the SEM. It uses a focused probe that is rastered across a specimen, similar to an SEM. However, it primarily detects the electrons that are transmitted through the sample, like a TEM.

This allows for high spatial resolution and makes certain analytical techniques easier to perform. One drawback of the STEM is that it acquires data in a serial fashion rather than in parallel. Other variations include the electron microprobe for chemical analysis and the low-energy electron microscope (LEEM) for imaging surfaces.

The history of this technology is a series of important discoveries. In 1883, Heinrich Hertz demonstrated how to manipulate electron beams using a cathode-ray tube. In 1899, Emil Wiechert showed how to focus electrons using an axial magnetic field. Arthur Wehnelt improved the production of electrons in 1905 with oxide-coated cathodes. Hans Busch made a major breakthrough in 1926 by developing the electromagnetic lens. In 1931, Max Knoll and Ernst Ruska used two magnetic lenses to create the first magnified images of mesh grids.

This achievement marked the creation of the first electron microscope.

There is significant historical debate regarding the true inventor of the device. While Knoll and Ruska are famous for their 1931 work, Reinhold Rüdenberg at Siemens-Schuckert also worked on the technology. Rüdenberg filed patents in 1932 and claimed his work was parallel to the university research. In 1933, Ruska and Knoll built a microscope that surpassed the resolution of light microscopes. By 1937, Ernst Ruska and Bodo von Borries were working with Siemens to develop applications for biological specimens. Siemens eventually produced the first commercial electron microscope in 1938.

Advancements in the late 20th century further transformed the field. In 1965, Albert Crewe introduced the STEM using a field emission source. By the early 1980s, higher accelerating voltages and better mechanical stability allowed for imaging at the atomic scale. The field emission gun became a common standard in the 1980s, which improved image quality by reducing chromatic aberrations. In the 2000s, researchers focused on aberration-corrected microscopy to improve clarity. These tools allow scientists to study the interaction volume of electrons with matter in great detail.

This progress connects electron microscopy to the broader study of chemistry, biology, and material science.