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New Computer Memory Material Goes Easy on the Juice

24 August 2012 5:55 pm
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A. S. Tayi et. al., Nature, 488 (23 August)

Assembly required. Electron-rich compounds (blue) nestle very close to electron-hungry compounds (red) but maintain weak links to neighboring pairs, creating long chains that may form the basis for novel computer memory devices.

Multitasking has a price: Your computer is sucking up a lot of electricity keeping track of work you haven't yet saved to the hard drive. Americans spend $6 billion a year on electricity to keep that data stored in a computer's memory during operation. But that figure could drop sharply, scientists report this week, thanks to a new type of material than can permanently store such data—without needing a continuous trickle of electricity to do it.

Standard desktop computers rely on two types of memory technology to store streams of 1s and 0s that make up binary data. The computer’s hard disk stores data as strips of magnetic orientation recorded on a magnetic disk: Imagine billions of patches of compass needles pointing either north or south, each representing a 1 or a 0. Because this magnetic orientation endures until it's deliberately switched, this type of memory is stable—it doesn’t require any added electricity to maintain it.

The second type of memory, however, does. This is Random Access Memory (RAM), or working memory, which the computer uses to perform tasks. Conventional RAM is made by linking several transistors together in a circuit; this type of memory is "volatile," meaning that it needs to be fed electricity continually to retain each bit of information. Turn off your computer without saving your data to your hard disk and you’ve lost that information forever.

Computers suck up billions of dollars worth of power every year to ensure that doesn’t happen. Alternatives to conventional RAM do exist, some of which are nonvolatile memories. But these have drawbacks: They may be more expensive, heavier, or simply take up too much computer-chip real estate.

Now, researchers in the United States and South Korea report in this week’s issue of Nature that they've created a new material that may overcome all of those problems. The material, a crystalline organic compound made from cheap building blocks, came about as a "happy accident," says Samuel Stupp, a chemist at Northwestern University in Evanston, Illinois, who headed the new study along with fellow Northwestern chemist Fraser Stoddart. Stupp explains that Alexander Shveyd, a former member of Stoddart's lab and now a postdoctoral fellow at the University of Rochester in New York, was experimenting with ring-shaped organic molecules, working to create pairs of these molecules that would assemble together. But he wasn’t having much luck. He enlisted the help of Alok Tayi, a friend in Stupp's lab, who is now a postdoctoral fellow at Harvard University. Together, the pair tweaked the design of the two molecules and got them to spontaneously stack together in alternating positions, like beads on a string.

That design turned out to have another benefit. The new material is ferroelectric, which means that one side is negatively charged while the other is positively charged. By applying an electric field to a ferroelectric material, engineers can flip those charges—and once flipped, the material keeps that charge orientation unless it’s hit with another burst of juice. That relatively permanent orientation makes ferroelectrics enticing materials for nonvolatile memories.

Ferroelectrics aren't a new concept—in fact, ferroelectric materials made out of inorganic compounds have been around for decades and are even used in some computers today. But inorganic ferroelectric materials require expensive processing, so researchers have long hoped that organic materials would be cheaper—it's typically easier to tinker with the molecular and electronic structure of these compounds. Previous organic ferroelectrics, however, didn’t hold onto their crystalline molecular order as well as the inorganics, and, therefore, were only ferroelectric at temperatures well below freezing where the ordering was stable.

The new materials do work at room temperature because of the way the building block molecules spontaneously assemble into well-ordered crystals. One molecule, called a donor, has an extra electron. The other, called the acceptor, is missing one electron. The donor and acceptor molecules nestle close to one another to share that extra electron—but both types of molecules can also form weaker links to other neighbors. The result was that while the donor in each pair nestled close to its partner acceptor on one side, on its other side it was a handshake away from the acceptor in the next donor-acceptor pair. So the donors and acceptors formed long stacks, alternating donors and acceptors. The way these stacks assembled made the material ferroelectric, with an overall electrical orientation in one direction.

The coup de grâce was that when the researchers applied an electric field with an opposite orientation to that in their crystals, the building blocks acted like square dancers: Donors pulled acceptors close that were previously held at arm's length, letting the former partners drift slightly away. As a result, the overall electrical orientation flipped direction, just like a bit of data being switched from a 1 to a 0.

"This finding is seminal," says Takuzo Aida, a chemist at the University of Tokyo. Aida notes that the new material may still need some tweaking to make it a viable data storage technology. Stupp agrees. But, he says, the building blocks are easy to change. "Now that we know the design rules, we can change the molecules and put them together in different ways."

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