individual atoms to move slowly and to stop on demand, permitting measurements of exquisite precision.

THE SCANNING TUNNELING MICROSCOPE

How is it possible to manipulate atoms on a surface, as if they were a child's toy blocks? The beginning of an answer lies in appreciating that, when seen at close quarters, an atom is not a hard-shelled object like a billiard ball. It lacks a distinct surface; instead, it surface is fuzzy. The fuzziness, in turn, results because the properties of an atom's perimeter derive from the behavior of its outer electrons. If those electrons could move outward only to a distinct radius and no farther, the atom indeed would resemble a billiard ball. In fact, these electrons have some likelihood of being found at greater distances from the nucleus. This likelihood or probability drops off rapidly as the radius increases, but that merely means that the electron density falls off quickly, rather than dropping off abruptly, as one proceeds to greater radii. The region of this falloff in electron density contains the fuzzy periphery.

The same is true of a surface formed of atoms. It does not resemble a cobblestoned pavement, with individual atoms as the stones. It, too, is fuzzy, as if the cobblestones had an overlay of cotton. Again, the fuzziness represents a region where the electron density is falling off sharply but is not terminating abruptly. In manipulating atoms on a surface, then, physicists work with the fuzziness, surrounding both the atoms and the surface.

This fuzziness has a thickness on the order of an angstrom (symbol Å), or 10−8 centimeters. This is a very small thickness. 10,000 times finer than the smallest details that appear in an electronic microchip. Indeed, an angstrom is as much smaller than a postage stamp as the stamp is smaller than the state of Texas. Manipulations made with atomic precision nevertheless must achieve such accuracy, and this immediately rules out any type of mechanical system with moving parts. Even if built with the care of a watchmaker, such a mechanism would amount to carrying out delicate surgery on the eye using the clanking machinery of a Mississippi steamboat. Hence, it would appear at the outset that one must accomplish the impossible: To build a movable mechanism that has no moving parts.



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