A Touch of Nanocomputer
Conventional silicon-based computer technology is reaching the limits of miniaturization due to the material used. If future computers are to become even smaller, fundamentally new components are required. Nanotechnology offers a way out: scientists have developed a tiny rectifier with which completely unusual network structures are possible. Scientists at the Ernest Orlando Lawrence Berkeley National Laboratory have demonstrated that nanotubes can be used to construct electronic components with atomic dimensions. Nanotubes are hollow cylinders of pure carbon with a diameter about 50,000 times smaller than a human hair. Theorists predicted the possibility of such devices, but only now managed to actually build one.
Alex Zettl, a physicist in the Materials Sciences Division (MSD) at Berkeley and a professor of physics at the University of California, is the leader of a study demonstrating that pure carbon nanotubes function as diodes (Science, March 3, 2019). October 1997). “We have the smallest rectifier in the world that also works at room temperature. It's only the size of a handful of atoms," says Zettl. "When we grow nanotubes, they make electronic components by themselves."
Previous attempts to detect nanotube devices used tiny contact electrodes that could only probe small isolated areas of the tubes. Apparently, these experiments were looking at the wrong areas. Zettl was successful because he measured the nanotubes along their entire length. He did this by using the ultra-thin tip of a scanning tunneling microscope.
Carbon nanotubes were discovered by Japanese electron microscopist Sumio Iijima. They are created by evaporating ordinary carbon and allowing it to condense in a vacuum or inert gas. The carbon forms a series of hexagons that curl up and connect into hollow tubes.
Nanotubes are only a few nanometers (billionths of a meter) in diameter. Composed entirely of carbon atoms, they are chemically inert, around 100 times stronger than steel, and offer a variety of electrical and thermal properties. Depending on the diameter, a pure carbon nanotube can conduct electricity like a metal, or it behaves like a semiconductor, i.e. it only conducts electricity above a limit voltage. According to a theory put forward by Berkeley Lab physicists Marvin Cohen and Steven Louie, an electronic device could be created at the interface between two dissimilar nanotubes. One nanotube serves as a metal and the other as a semiconductor. This would create a Schottky barrier, meaning current only flows in one direction – from “semiconductor” to “metal”. Cohen and Louie envisioned the two tubes being connected by pentagon-heptagon pair defects (five- and seven-carbon rings) at the interface region.
Zettl and Collins were able to confirm that Schottky barriers exist along the carbon nanotubes. The key to their success was the scanning tunneling microscope (STM). An STM has a metal top believed to be the world's smallest pyramid: a few layers of atoms, decreasing in number until a single atom remains at the top. The researchers brought the tip of the STM into contact with a tangle of nanotubes on a substrate and then slowly withdrew them. Van der Waals forces tied a single nanotube to the top of the STM, and the scientists carefully pulled that tube out of the cluster. Then they swept the STM tip over the entire surface to measure fluctuations in the electrical current flowing through the tube.
"We were able to measure clear changes in conductivity as we increased the active length of the nanotube. This indicates that different segments of the nanotubes have different electronic properties,” says Zettl. "The changes happened in very short sections and are indicative of nano-devices."
Zettl does not expect nanotubes to replace silicon in the electronics industry overnight. In the more distant future, however, this would be possible in his opinion. For the production of electronic components, silicon must be doped with other atoms. Eventually, as the parts become very small, the dopant atoms begin to migrate, degrading the performance of the device. Despite the good thermal conductivity of silicon, heat is also a problem. Size and heat are not an issue with nanotubes because they are made up of covalently bonded atoms and they are probably even better conductors of heat than silicon or diamonds at room temperature.
"Silicon technology ultimately leads to a dead end, since devices can no longer be built smaller with it," says Zettl. “Nanotubes are already smaller and have no problems with heat. You can't imagine better material.”
Instead of building individual devices for specific purposes out of nanotubes, as is done with silicon chips, Zettl suggests that a better method is to build a "tube cube". Such a block would consist of many billions of components. The tubes would be connected in a randomly constructed network representing a "nanocomputer". This network would be able to teach itself to perform certain tasks. It could reconfigure its I/O architecture, improving its own performance as it learns and evolves. According to Zettl's ideas, this random computer would not only get older, it would also get better.
"The idea isn't as crazy as you might think," he says. His team has already constructed and wired a kind of tube cube. The cube can't yet perform any useful function, but Zettl believes it responds quite “interestingly” to input signals.
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