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ATOMIC FORCE MICROSCOPY
Visualization at the nanoscale is often performed using atomic force microscopy. As the name indicates, this operates by measuring force, not by using a stream of particles such as photons (as in light microscopy) or electrons (as in electron microscopy).
Physicists sometimes compare the operation of an AFM to an old-fashioned record player, which uses a needle to scrape the surface of a record. Perhaps to a biologist, the difference between a light microscope and AFM is like the difference between reading text with the eyes and feeling Braille.
The atomic force microscope was invented in 1985 by Gerd Binnig, Calvin Quate, and Christof Gerber. The AFM uses a sharp probe that moves over the surface of the sample and which bends in response to the force between the tip and the sample. The movement of the probe performs a raster scan and the resulting topographical image is displayed onscreen.
During scanning, the movement of the tip or sample is performed by an extremely precise positioning device and is made from piezoelectric ceramics. (These are materials that change shape in response to an applied voltage.) It usually takes the form of a tube scanner that is capable of sub-Ångstrom resolution in all three directions.
The AFM probe is a tip on the end of a cantilever. As the cantilever bends because of the force on the tip, its displacement is monitored by a laser, as shown in Fig. 7.5. The beam from the laser is reflected onto a split photodiode. The difference between the A and B signals measures the changes in the bending of the cantilever. For small displacements, the displacement is proportional to the force applied. Hence the force between the tip and the sample can be derived.
The distance between tip and sample is adjusted so that it lies in the repulsive region of the intermolecular force curve; that is, the AFM probe is repelled by its molecular interaction with the surface. The repulsion gives a measure of surface topography, and this is what is generally displayed, with color coding indicating relative height. It is possible to scan a surface for topography and then raise the AFM probe and rescan to detect electrostatic or magnetic forces. These can then be plotted for comparison with the topography.
As with STM, it is possible to use AFM to move single atoms, although this was only achieved in 2003. Researchers at Osaka University in Japan removed a single silicon atom from a surface and then replaced it.
Using AFM, it is possible to visualize polymeric biological molecules such as DNA or cellulose and even to see the individual monomers and, at high resolution, even the atoms of which they are composed.
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