Maps surface structure & molecules
Does not require a vacuum
Invented in Switzerland around 1980, the atomic force microscope (AFM) has undergone little change itself in the intervening time, but its interface with computer control and offload processing has involved considerable advancement. Essentially, the atomic force microscope is not an optical projection system as in the compound light microscope or the transmission electron microscope, but a mapping mechanism that reveals surface topography at the macromolecular level. It has achieved its primary usefulness in applications with microcircuits and in materials science, but has long held the promise of revealing great structural knowledge of macromolecules in biological systems. In fact, it has shown some images of nucleic acids and protein complexes, but has yet to reach the level of useful information as generated by cryo-electron microscopy and X-ray crystallography.
The heart of the AFM is a small integrated circuit that generates a very weak piezo-electron current. Attached to this chip is a very thin, electrically conductive filament with a very fine and very small tungsten crystal attached at its distal end. The crystal, shaped like a sub-miniature phonographic needle, carries the piezo electric charge. On the backside of the filament is a mirror-like surface that is made by the deposition of an evaporated layer of gold or similar high atomic number metal. Elsewhere in the system, a very fine laser beam is focused at a low angle onto the mirrored surface so that a very small movement of the end of the filament in a vertical axis will produce a large deflection of the reflected laser beam. That reflected laser beam is cast onto an electronic recording surface, which takes changes in the deflection angle and transposes them as a measure of the vertical elevation of the needle.
Beneath the filament containing the charged tungsten crystal is a specimen. The specimen must not have vertical elevations greater than a few microns (typically much less), and is mounted onto the surface of a fine piezo-electric driven stage. The motors that drive the stage operate in two axes under the control of a computer. In essence, the stage moves in a series of horizontal lines, which comprise a raster. The area covered is very small, usually much less than a micrometer in each axis. The specimen is elevated in the vertical axis until it is in near contact (only a few nm distance) with the tungsten crystal at the end of the filament. At close distances, the electrostatic charges on molecular complexes interact with the piezo charge on the crystal to force the filament up, or to make it drop. This vertical movement as the crystal moves over the specimen deflects the laser beam at a greater angle, which in turn becomes recorded as a true measure of vertical distance in a computer system. Thus, the instrument records the vertical elevation of a specimen in a given small area, and therefore acts as a microscopic mapping system. The AFM does not require a vacuum as does the transmission electron microscope. A schematic representation of the AFM design is shown in the top illustration.
The instrument is obviously subject to any slight external vibrational patterns which can physically deflect the extremely small and fragile filament that supports the tungsten crystal. As a consequence, the instrument must be mounted on an extremely stable surface, and must also be in an environment that is free of stray electromagnetic fields. A variety of mounting systems can be employed to achieve this level of stability, including shock-absorbing tables or suspension systems. The lower figure illustrates an installed AFM with its computer control and image capture system.
A number of variations on the basic principle of the AFM have been developed to record differences in surface magnetic potentials (of interest to the metallurgist), differences in pH, atomic number, etc. All, however, share in common the basic theoretical principle of the atomic force microscope.
