If you want your iPhone to run faster, you've got to shrink it. The bigger the device, the longer it takes electricity to move around the circuits. But making electronic circuits tiny poses problems. The laws of physics get weird at really small scales, so scaled-down versions of bigger devices may not work as well. Now, scientists may have taken one step toward surmounting that problem by creating the smallest conceivable mechanical switch—one involving just two atoms.
Over the years, the resistors, capacitors, and transistors on silicon chips have gotten smaller and smaller, but with each subsequent generation, it becomes more difficult to find ways to shrink them further. Nanotechnology might one day allow researchers to make atomic-scale devices that perform the same functions as their large-scale counterparts. Some experts, for example, have suggested that swarms of nanorobots could be sent into the body to seek out and destroy cancer cells. But to find out whether such grandiose ideas will ever be possible—let alone how to realize them—physicists need to get a lot better at manipulating individual atoms and molecules.
As a first step, nanophysicist Adam Sweetman and colleagues at the University of Nottingham in the United Kingdom set out to make an atomic-scale toggle switch, the miniature equivalent of a light switch. The researchers employed a dynamic force microscope, which uses a needle whose tip is just one atom thick, to deposit, remove, or slide around individual atoms on the surface of a sample—and to measure how those atoms respond. They tinkered with a particular type of silicon whose surface is covered with dumbbell-shaped pairs of atoms called dimers. These dimers sit at an angle to the surface, with one atom protruding slightly farther than the other. The electrical conductivity of the silicon atom differs slightly if the dimer tilts one way instead of another way. At room temperature, the dimers constantly rock back and forth like tiny seesaws, but if they are cooled to within 120°C of absolute zero, they settle in either position and stay there unless prodded. Other researchers have found that a tiny electric current can flip the dimer, so Sweetman and colleagues decided to create their switch by running the process backward, mechanically flipping the dimer to control an electric current.
In research published online this week in Physical Review Letters, the researchers report that they positioned the needle of the dynamic force microscope over the lower atom of a single dimer in a sample of silicon chilled to -268°C. The attractive force between the dimer and the needle drew the lower atom of the dimer upward, causing the dimer to shift from one position to the other. In other words, they flipped a switch, which allowed them to change the electrical conductivity of the silicon without making or breaking a single chemical bond within the surface. And because the two states are symmetrical, the dimer could be flipped back by grabbing the opposite atom of the dimer. In theory, this could one day provide a key component of any nanoscale electrical circuit—a way to mechanically turn the current on and off or change its direction.
In practice, Sweetman says, actually using the switch to divert an electric current would be hugely complex and difficult, partly because the extreme delicacy of the mechanical switching process would be very difficult to industrialize and partly because an applied electric current might flip the dimer back again. If the cancer-fighting nanorobots ever happen, it won't be any time soon.
Nevertheless, the ability to adjust the orientation of a single chemical bond within a molecule is a new benchmark in the precise mechanical control of matter. Nanosystems scientist Paul Weiss of the University of California, Los Angeles, says the technique has potential to provide further useful insights into how individual atoms respond to subtle manipulations. As for the switches themselves, he cautions, "There is no current way to wire these up that has any advantage over existing technologies, so the excitement here is that this work is really a test of the fundamental limits of miniaturization rather than a technological advance."