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Scanning Electron Microscope
Scanning Electron Microscope (SEM)
When the scanning electron microscope was invented in the 1940's, it revolutionized the field of electron microscopy. Though it did not fully replace the transmission electron microscope (TEM) that was already in widespread use since the 1930's, the SEM brought a number of attractive qualities to the field of high-power microscopy. Though not as powerful in magnification as modern Scanning Tunneling Microscopes and Atomic Force Microscopes, the Scanning Electron Microscope is a jack-of-all-trades device that can perform a number of useful functions.
SEMs are modeled after light microscopes. Unlike a light microscope or a TEM, the SEM does not require electrons to pass through a material. Hence, thin samples are not required and bulk investigations are made possible by 'scanning' the surface with a moving beam.
An SEM works as follows: at the top of the column, an electron source (usually thermally activated) emits electrons of a specific energy (usually specified as a voltage for the accelerating field or as electron-volt units). The electrons head down the column at a terrific speed, but they're still fairly spread out just after launch. The stream of electrons must pass through two condenser lenses and two apertures, which tighten the beam down to a few nanometers in width. This is the maximum resolution that an electron microscope can achieve due to imperfections in the magnetic condenser lenses. The final stage is the objective lens that scans the beam horizontally and vertically over the sample area. Depending on the results you want, different measurements then take place. Measuring the current that is absorbed for a specific location on the sample can relay simple 'visual' information. Chemical information can be relayed through photoluminescence spectra. Surface information can be examined using auger electrons. Lastly, the crystallographic structure of a sample can be analyzed using electron diffraction measurements of an adequately large sample.
With so many different qualitative and quantitative options, the SEM is truly a versatile piece of equipment. No nanotechnology research lab is complete without its own SEM.
The resolution of a scanning electron microscope depends on several factors. The highest resolution possible, however, is on the order of a few nanometers. For chemical composition, the resolution is worse on the several hundred nanometers. Crystallographic diffraction measurements require crystals at least a few microns in diameter.