We're already at the point where the features we etch into processors are about the same size as some molecules—hemoglobin, for example, is five nanometers across. Too much smaller, and the behavior of electrons will become dominated by quantum effects, potentially causing some unpredictable behavior.

One potential solution to this is to actually use individual molecules to create the features on chips. Carbon nanotubes are promising candidates, as they naturally come in both semiconducting and metallic forms. But getting the right kind of nanotubes into a structure where they perform consistently has been a challenge.

Now, a team has shown that the properties of nanotubes can be manipulated and preserved in a way that could make them useful for electronics. While the work was done with populations of nanotubes rather than single molecules, it did manage to turn the nanotubes into functional RAM.

Current semiconductor technology relies on two different types of material: one that favors the transport of electrons, another that carries the positively charged "holes" they leave behind. (Technically, these are called p and n type semiconductors.) These are both typically made of silicon that has had small amounts of different materials added to change its properties.

It's possible to do the same thing with carbon nanotubes. Oxygen, for example, gets adsorbed onto the surface of the nanotubes, converting them into a p-type semiconductor. Other chemical treatments can make an n-type.

To make circuitry, the authors started out with collections of p-type nanotubes, which were then sealed under a material that could be etched off with light—the same procedure that's used to make chips. This material was then etched off in specific locations, and a chemical added that converted the nanotubes to n-type semiconductors. The whole surface was then coated with aluminum oxide, sealing everything in place.

The formation of this seal is critical, since oxygen in the atmosphere is able to alter the behavior of the nanotubes. In nanotubes that weren't sealed, performance degraded by a factor of 1,000 over a short period of use. The sealed ones performed well for at least two days.

Rather than simply stop there, the authors decided to build some actual circuitry using nanotubes. So, they put together a configuration that functioned as static RAM and showed that its performance remained stable for thousands of reads and writes, all done at room temperature.

It's important to emphasize that this work didn't involve creating single nanotube features; each component was a mesh of nanotubes, all oriented randomly. Charges moved across the features by hopping from tube to tube.

But as a proof-of-principle, the work is pretty intriguing. The authors show they can control the behavior of nanotubes and, once it's set, keep it stable for as long as they decided to run tests. The challenge will be to figure out how to translate the general approach to an implementation that provides much finer control over setting the nanotubes' properties.

Nature Nanotechnology, 2015. DOI: 10.1038/NNANO.2015.197 (About DOIs).

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