By Eric Smalley, Technology Research News

Because carbon nanotubes are so small -- the size of molecules -- if each could store one bit of information, devices made from nanotubes could potentially hold colossal amounts of information. With this in mind, many researchers are working to build memory devices from the tiny tubes.

Researchers from Texas A&M University and Rensselaer Polytechnic Institute have designed a type of nanotube flash memory that has a potential capacity of 40 gigabits per square centimeter and 1,000 terabits per cubic centimeter.

A terabyte is 1,000 gigabits, or the contents of about 26 DVDs.

Each memory element is made from a pair of crossed carbon nanotubes filled with iron or embedded in a ferromagnetic medium. Carbon nanotubes are rolled up sheets of carbon atoms that have useful electrical properties and can be narrower than a nanometer.

The researchers' design calls for storing information using magnetization to control electrical current -- the same principle behind magnetic disk drive read heads.

In an array of crossed nanotubes, each junction could store a bit of information -- a 1 or a 0. "If you crossed the same number of nanotubes as the number of transistors in today's microprocessors, then you get about 1015 bits... that is one million gigabytes," said Laszlo Kish, an associate professor of electrical engineering at Texas A&M University.

The amount of current that flows through a pair of magnetic layers sandwiching a non-magnetic layer depends on the magnetic orientations of the layers. Each electron has a magnetic orientation. Layers that are magnetically parallel will allow electrons that are magnetically aligned with the layers to pass through while layers with opposite magnetic orientations will block all electrons.

A pair of magnetic nanotubes crossed at other than a right angle acts like the magnetic layers in disk read heads. Writing a bit to the crossed pair involves sending a positive or negative electric pulse to set the magnetic orientation of the two sides of the junction. Reading the bit is accomplished by sending a weaker pulse, which is always either positive or negative. If the readout pulse has the opposite clarity from the write pulse, the current in the circuit is diminished. The high current and low current states represent the 1s and 0s of computer information.

Because nanotubes are so small, a storage device based on the researchers' design would also be extraordinarily fast, according to Kish. Data rates of 1,000 gigabytes per second are conceivable, he said. Such devices would also use relatively little power, he said.

The design would also yield more durable devices than other schemes using crossed nanotubes to build storage systems because they require the nanotubes to bend into and out of proximity with each other, said Pulickel Ajayan, a professor of materials science and engineering at Rensselaer Polytechnic Institute and Kish's colleague.

The researchers' next step is to build a single memory element and test its properties in various conditions to find the optimal composition and electrical properties for this type of memory. Then they must figure out how to integrate the simple memory junctions in a three-dimensional structure in order to make memory device from them.

A prototype device could be developed in five years, said Ajayan.

The crossed-nanotube memory elements could eventually be used to build magnetic memory devices with "extraordinary storage capacity," but it will take time to make practical memory elements from nanotubes, said Kish.

It took 15 years of development after Nobel laureate Jack Kilby came up with the idea of integrating transistors into computer chips for the idea to become practical, said Kish. Researchers working to make electronics from nanotubes are only at the stage of imagining the first transistor, he said.

The work appeared in the February 28, 2005 issue of Applied Physics Letters.