Visualizing The Future
Electron Microscopes
Under an electron microscope, the infinitesimal begins to look like sweeping geographical landscapes. Blood clots look like UFO's caught in an extraterrestrial traffic jam. Micro-minerals give the appearance of vast landscapes dotted with buttes and canyons. Synthetic kidney stone crystals look like falling snowflakes. The shells of microscopic plants stand out like Christmas tree ornaments. Nylon looks like a plate of spaghetti. Bugs look like monsters.
The world of a grain of sand was once as far as the human eye could go. Now, using electron microscopes, a grain of sand is like the universe, filled with untold galaxies, planetary systems and maybe even a few black holes. At the organic level, humans are learning how to Mother Nature builds life, one atom at a time.
Conventional microscopes use particles of light, or photons, to look directly at small objects, employing glass lenses to magnify things several thousand times. The SEM opens the door to an even tinier level by using electrons, which are much smaller than photons.
The process is the same for all electron microscopes, where a stream of electrons is formed (by the Electron Source) and accelerated toward the specimen using a positive electrical potential. This stream is confined and focused using metal apertures and magnetic lenses into a thin, focused, monochromatic beam. This beam is focused onto the sample using a magnetic lens. Interactions occur inside the irradiated sample, affecting the electron beam. These interactions and effects are detected and transformed into an image.
Electron microscopes provide morphological, compositional and crystallographic information at the atomic level (nanometers). Topography is the surface features of an object or "how it looks." Texture is the direct relation between these features and materials properties (hardness, reflectivity). Morphology is the shape, size and relationship of the particles making up the object (ductility, strength, reactivity).
Composition explains the relative amounts of elements and compounds that the object is composed of (melting point, reactivity, hardness). Crystallographic Information determines how the atoms are arranged in the object and their relationships with other properties (conductivity, electrical properties, strength).
To create the images, a filament inside an electron "gun" shoots a stream of electrons through a stack of electromagnetic lenses, which focus the electrons into a beam. The beam is directed to a fine point on the specimen, and scans across it rapidly. The sample responds by emitting electrons that are picked up by a detector inside the sample chamber, beginning an electronic process that results in an image that can be displayed on a TV screen.
The Transmission Electron Microscope (TEM), developed by Max Knoll and Ernst Ruska in Germany in 1931, was the first type of Electron Microscope and is patterned exactly on the Light Transmission Microscope except that a focused beam of electrons is used instead of light to "see through" the specimen.
The first Scanning Electron Microscope (SEM) appeared in 1942 with the first commercial instruments around 1965. A TEM works much like a slide projector. A projector shines a beam of light through (transmits) the slide, as the light passes through it is affected by the structures and objects on the slide. These effects result in only certain parts of the light beam being transmitted through certain parts of the slide. This transmitted beam is then projected onto the viewing screen, forming an enlarged image of the slide.
Scanning Electron Microscopes (SEM) are patterned after Reflecting Light Microscopes and yield similar information as TEMs. Unlike the TEM, where electrons are detected by beam transmission, the SEM produces images by detecting secondary electrons which are emitted from the surface 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.
Scientists have used the SEM to identify micro-plankton in ocean sediments, fossilized remains found in underwater canyons, the structure of earthquake-induced micro-fractures in rocks and micro-minerals, the microstructure of wires, dental implants, cells damaged from infectious diseases, and even the teeth of microscopic prehistoric creatures.
There are other types of electron microscopes. A Scanning Transmission Electron Microscope (STEM) is a specific sort of TEM, where the electrons still pass through the specimen, but, as in SEM, the sample is scanned in a raster fashion. A Reflection Electron Microscope (REM), like the TEM, uses a technique involving electron beams incident on a surface, but instead of using the transmission (TEM) or secondary electrons (SEM), the reflected beam is detected.
Near-field scanning optical microscopy (NSOM) is a type of microscopy where a sub-wavelength light source is used as a scanning probe. The probe is scanned over a surface at a height above the surface of a few nanometers.
A Scanning Tunneling Microscope (STM) can be considered a type of electron microscope, but it is a type of Scanning probe microscopy and it is non-optical. The STM employs principles of quantum mechanics to determine the height of a surface. An atomically sharp probe (the tip) is moved over the surface of the material under study, and a voltage is applied between probe and the surface.
Depending on the voltage electrons will tunnel or jump from the tip to the surface (or vice-versa depending on the polarity), resulting in a weak electric current. The size of this current is exponentially dependent on the distance between probe and the surface. The STM was invented by scientists at IBM's Zurich Research Laboratory. The STM could image some types of individual atoms on electrically conducting surfaces. For this, the inventors won a Nobel Prize.
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