Electron microscopes use electrons instead of photons for imaging, providing much greater magnification due to their smaller wavelength. There are four types, with the transmission and scanning electron microscopes being the most common. They offer high resolution and can observe individual atoms on a surface, but are expensive to purchase and maintain. Samples must be prepared and coated with conductive material.
An electron microscope is a type of microscope that uses electrons instead of photons, as a conventional light microscope does, for imaging. Since 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 dots. Yet light waves are much larger, for example with a wavelength of around 500 nanometers for the color green.
The best light microscopes only offer about 2000X magnification of a sample, while some electron microscopes can magnify a sample 50 million times; on the contrary, 2 million times is more typical. This works at a resolution limit of around 0.1 nanometers, allowing for the observation of individual atoms on a surface. This 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 achievements.
There are four types of these microscopes, the first two being the most common: the transmission electron microscope (TEM), the scanning electron microscope (SEM), the reflection electron microscope (REM), and the scanning transmission electron microscope ( STEM).
TEM is the electron microscope as it was originally invented. Using an electron-transparent sample, a beam of electrons is emitted directly through the sample. A receiver on the other end measures the density of electrons at each individual point and compiles them into a grayscale image. This is the sample image.
An SEM is slightly lower in resolution than a TEM, but is still the most popular type. As the name suggests, this microscope scans a beam of electrons through the sample. Instead of analyzing the original beam to obtain information about the composition of the sample, the sensors collect the secondary electrons released from the sample surface via excitation from the primary beam. This sacrifices some of the resolution for a 3D image of the sample. This is worth more than the exchange..
Most scanning electron microscopes are very expensive to purchase and maintain. They require a stable high voltage electricity source, a vacuum pump and cooling coils. Samples must be prepared, usually by coating them with a thin layer of conductive material such as gold.
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