Molecular Memory: Carbon-nanotube device stores data in molecules

If all goes according to some researchers’ plans, organic molecules will replace silicon as the workhorses in electronic devices. Edging toward that goal, chemists at the University of California, Los Angeles have fabricated a memory device in which data are stored in organic molecules connected to a carbon nanotube.

INFO JUNCTION. Sandwiched between a metal electrode (silver) and a perpendicular carbon nanotube (black roll), catenane molecules (blue and red) change configuration when a voltage is applied. Other molecules (orange) protect catenanes from the top electrode. Stoddart et al.

Storing data in such tiny amounts of material could enable chip manufacturers to dramatically boost the storage capacity of memory devices, such as the dynamic random access memory in personal computers and the flash-memory chips in digital cameras. And UCLA team member Hsian-Rong Tseng estimates that 1 gram of these molecules could supply enough memory for all new computers worldwide for several years.

Rather than encode 1s and 0s on the basis of the amount of charge stored in a memory cell, as conventional memory chips do, the UCLA approach encodes data in catenane molecules, each of which has two interlocked rings.

The researchers, led by Fraser Stoddart, sandwich the catenanes between two electrodes. The top electrode is made of metal and the bottom one is a carbon nanotube that resembles rolled-up chicken wire and measures just 1 nanometer in diameter. An applied voltage strips electrons from one ring of each catenane.

This causes the electron-depleted ring to rotate 180 relative to the other ring, placing the molecule in the “on” state. An opposite voltage replenishes the lost electrons, causing the ring to rotate back to its original configuration, the “off” state. When the catenane is “on,” more electrical current flows across the molecule than when the catenane is “off.”

Labs around the world are striving to find ways of using individual molecules for storing bits of data. To succeed in this quest, researchers also need to integrate such molecules with ultrathin electrodes, only a few nanometers thick, for writing and reading data to and from the molecules.

That’s not an easy task and it’s expensive, but James Tour of Rice University in Houston, says that carbon nanotubes could simplify the process and make it less expensive. Carbon nanotubes are about as wide as a catenane molecule. With electrodes this thin, says Tour, researchers could squeeze billions of catenane-based molecular switches onto a single chip.

An added advantage, Stoddart notes, is that carbon nanotubes are much more chemically compatible with organic switching molecules, such as catenanes, than more conventional metallic electrodes are.

“It’s a very nice marriage of two materials,” says Stoddart, who described his device on Sept. 10 at the American Chemical Society meeting in New York.

“This is truly novel,” says Tour. The next big hurdle, he predicts, is to make memory devices in which both the bottom and top electrodes are carbon nanotubes. “Right now, the [UCLA group] has half of that structure . . . a big step in the right direction,” says Tour.


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