Electron microscopes, which use beams of electrons instead of light, are designed for very high magnification usage. Electrons, which have a much smaller wavelength than visible light, allow a much higher resolution. The main limitation of the electron beam is that it must pass through a vacuum as air molecules would otherwise scatter the beam.Instead of relying on refraction, lenses for electron microscopes are specially designed electromagnets which generates magnetic fields that are approximately parallel to the direction that electrons travel. The electrons are typically detected by a phosphor screen, photographic film or a CCD.
Two major variants of electron microscopes exist:
• Scanning electron microscope: looks at the surface of bulk objects by scanning the surface with a fine electron beam and measuring reflection. May also be used for spectroscopy.
• Transmission electron microscope: passes electrons completely through the sample, analogous to basic optical microscopy. This requires careful sample preparation, since electrons are scattered so strongly by most materials. It can also obtain detailed information on the sample’s crystallography through selected area diffraction.

History.
The first electron microscope prototype was built in 1931 by the German engineers Ernst Ruska and Max Knoll. It was based on the ideas and discoveries of French physicist Louis de Broglie. Although it was primitive and not fit for practical use, the instrument was still capable of magnifying objects by four hundred times.
Reinhold Rudenberg, the research director of Siemens, had patented the electron microscope in 1931, although Siemens was doing no research on electron microscopes at that time. In 1937 Siemens began funding Ruska and Bodo von Borries to develop an electron microscope. Siemens also employed Ruska’s brother Helmut to work on applications, particularly with biological specimens. Siemens produced the first commercial TEM in 1939, but the first practical electron microscope had been built at the University of Toronto in 1938, by Eli Franklin Burton and students Cecil Hall, James Hillier and Albert Prebus. Although modern electron microscopes can magnify objects up to two million times, they are still based upon Ruska’s prototype. The electron microscope is an integral part of many laboratories. Researchers use it to examine biological materials such as microorganisms and cells, a variety of large molecules, medical biopsy samples, metals and crystalline structures, and the characteristics of various surfaces.

Types.

Transmission Electron Microscope-
The original form of electron microscopy, Transmission electron microscopy or TEM involves a high voltage electron beam emitted by a cathode and focused by electrostatic and electromagnetic lenses. The electron beam that has been transmitted through a specimen that is in part transparent to electrons carries information about the inner structure of the specimen in the electron beam that reaches the imaging system of the microscope. The spatial variation in this information (the “image”) is then magnified by a series of electromagnetic lenses until it is recorded by hitting a fluorescent screen, photographic plate, or light sensitive sensor such as a CCD or the charge-coupled device camera. The image detected by the CCD may be displayed in real time on a monitor or computer. Resolution of the TEM is limited primarily by spherical aberration, but a new generation of aberration correctors have been able to partially overcome spherical aberration to increase resolution. Software correction of spherical aberration for the High Resolution TEM HRTEM has allowed the production of images with sufficient resolution to show carbon atoms in diamond separated by only 0.89 ångström or 89 picometers and atoms in silicon at 0.78 ångström or 78 picometers at magnifications of 50 million times. The ability to determine the positions of atoms within materials has made the HRTEM an important tool for nano-technologies research and development.

Scanning Electron Microscope-
Unlike the TEM, where electrons of the high voltage beam form the image of the specimen, the Scanning Electron Microscope or SEM produces images by detecting low energy secondary electrons which are emitted from the surface of the specimen due to excitation by the primary electron beam. In the SEM, the electron beam is rastered across the sample, with detectors building up an image by mapping the detected signals with beam position. Generally, the TEM resolution is about an order of magnitude greater than the SEM resolution, however, because the SEM image relies on surface processes rather than transmission it is able to image bulk samples and has a much greater depth of view, and so can produce images that are a good representation of the 3D structure of the sample.

Reflection Electron Microscope-
In addition there is a Reflection Electron Microscope or REM. Like TEM, this technique involves electron beams incident on a surface, but instead of using the transmission (TEM) or secondary electrons (SEM), the reflected beam is detected. This technique is typically coupled with Reflection High Energy Electron Diffraction and Reflection high-energy loss spectrum or RHELS. Another variation is Spin-Polarized Low-Energy Electron Microscopy or SPLEEM, which is used for looking at the microstructure of magnetic domains.