An electron microscope is a microscope that uses a beam of accelerated electrons, rather than visible light, to examine specimens. Electron lenses control the beam, and detectors record signals produced by its interaction with matter. The short wavelength of accelerated electrons permits much finer spatial resolution than conventional light microscopy. Different instruments reveal surface features, internal structures, crystal arrangements, or chemical composition; suitably equipped systems can resolve individual atoms under appropriate conditions. (jeol.com)
Physical principles and instrumentation
Electron microscopy depends on the wave behavior of particles described by quantum mechanics. The de Broglie wavelength is inversely proportional to electron momentum. Increasing the accelerating voltage therefore shortens the wavelength: at 200 kilovolts, it is approximately 0.0025 nanometers, far shorter than visible wavelengths. This wavelength does not itself determine the instrument’s practical resolving power. Lens imperfections, specimen properties, and imaging conditions also matter. (jeol.com)
An electron gun produces electrons by heating an emitter or applying a strong electric field to a sharp tip. Electrostatic lenses and lenses using a magnetic field focus and steer the beam. Condenser lenses regulate illumination, while objective and projection systems control image formation. The electron column normally operates under vacuum to minimize collisions with gas molecules. Images can be recorded using fluorescent screens or electronic detectors. (jeol.com)
Principal types
Transmission electron microscope (TEM). A TEM directs electrons through an electron-transparent specimen. Transmitted and scattered electrons form an image of its internal structure, projected through the specimen’s thickness. Contrast may arise from differences in thickness, composition, diffraction, or electron-wave phase. TEM can also record diffraction patterns that provide information about crystalline structure. Specimen thickness must suit the material and accelerating voltage; many preparations are below approximately 100 nanometers. (jeolusa.com)
Scanning electron microscope (SEM). An SEM scans a focused probe across the specimen and assigns measured signal intensities to successive image positions. Secondary electrons are especially useful for displaying surface topography. Backscattered electrons provide contrast related to composition, although geometry and other factors also affect their intensity. SEM images often appear three-dimensional because of their shading and large depth of field, but a single image is not a complete three-dimensional measurement. (jeol.com)
Scanning transmission electron microscope (STEM). STEM combines a scanning probe with a thin, transmitting specimen. Detectors collect electrons passing through or scattered by each illuminated position. Annular dark-field imaging can reveal atomic columns, while spectroscopic detectors provide localized chemical measurements. TEM and STEM modes may be available in the same instrument. (nist.gov)
Specimen preparation and cryogenic methods
Preparation is integral to the measurement. Conventional biological TEM commonly involves chemical fixation, dehydration, resin embedding, and ultrathin sectioning. Heavy-metal staining increases contrast by enhancing electron scattering. These procedures make the fine internal organization of a cell visible, but the resulting image represents a processed specimen rather than an untreated living system. (histology.leeds.ac.uk)
SEM preparations often require drying and, for insulating specimens, a thin conductive coating to reduce charge accumulation. Variable-pressure and environmental instruments allow some specimens to be examined with less extensive preparation, although operating conditions affect the available signals and resolution. (jeol.com)
Cryo-electron microscopy preserves specimens at low temperature. In many biological applications, rapid cooling converts water into vitreous ice without forming large ice crystals, retaining hydrated structures for observation in vacuum. Combining many particle images through image processing can produce three-dimensional models of a protein or molecular assembly. Jacques Dubochet, Joachim Frank, and Richard Henderson received the 2017 Nobel Prize in Chemistry for developing this approach to high-resolution biomolecular structure determination. (nobelprize.org)
Analytical capabilities and applications
Electron microscopes can combine imaging with spectroscopy. Energy-dispersive X-ray spectroscopy detects characteristic X-rays emitted after electron irradiation, identifying elements and mapping their distributions. Electron energy-loss spectroscopy measures energy lost by transmitted electrons and can supply information about elemental composition and bonding. Electron diffraction complements images by probing crystal order and orientation. (jeol.com)
Applications include examining semiconductor structures, interfaces, particles, and crystal defects, as well as cellular organization and biological macromolecules. Combining structural and chemical measurements makes it possible to relate a feature’s appearance to its composition rather than interpreting image contrast alone. (nist.gov)
Resolution, limitations, and historical development
Magnification and resolution are distinct: enlarging an image cannot recover details that were never recorded. Aberration-corrected electron optics reduce lens-induced blurring; specialized STEM instruments can achieve resolution below 0.1 nanometers and resolve individual atoms. Such performance is conditional on the specimen and measurement configuration, not a capability shared by every electron microscope. (nist.gov)
Electron irradiation can damage sensitive material, limiting the usable exposure and obtainable detail. Conventional biological preparation is destructive, while freezing preserves structure but does not eliminate beam damage. Consequently, specimen preparation and electron dose are central constraints, particularly in biological microscopy. (histology.leeds.ac.uk)
Ernst Ruska and Max Knoll developed an early electron-microscope prototype in 1931; Ruska’s subsequent work produced an instrument exceeding light-microscope resolution in 1933. Commercial SEM instruments followed in 1965. Ruska received half of the 1986 Nobel Prize in Physics for his work in electron optics and the design of the first electron microscope. (nobelprize.org)