This article is reproduced with the permission of New Scientist for exclusive use by Nova users.

Atomic abacus
12 February 2000
From New Scientist Print Edition.
Jim Thomas

All hell breaks loose by the Temple of Marduk in ancient Babylon. A furious trader screams curses at the panicked herd of goats that has careered into his market stall. Lying on the ground in pieces is his pride and joy—a brand new stone abacus. It's Babylon's latest high-tech must-have gizmo, a counting frame made by slotting pebbles into grooves carved on a stone tablet.

Now fast-forward more than two millennia, to the dawn of the 21st century. Even in this age of electronic calculators and palm-top computers, the abacus is still a familiar sight in East Asia. In Japan, a wire-and-bead abacus is called a soroban, and when James Gimzewski saw one during a visit, it gave him a flash of inspiration. "I saw the ticket vendor at a railway station using a soroban," he recalls. "So I thought, why not make one ourselves?"

Gimzewski leads a group of researchers at the IBM Zurich Research Laboratory in Switzerland. Their stock in trade is shrinking technology to its tiniest limits, so the soroban he had in mind was just a few millionths of a millimetre across. Soon after the idea struck him, Gimzewski and his team had built their nano-sized abacus using football-shaped buckminsterfullerene molecules, or buckyballs, as the beads. And as the wires they used copper guide rails that are millions of times thinner than a human hair.

While the full-size abacus is great for doing sums quickly, its smaller cousin is a perfect way to store digital information. "If each molecule carries a bit, the memory density of our device is roughly ten thousand times that of a conventional device," explains Makoto Komiyama who is based in the department of chemistry and biotechnology at Tokyo University. Komiyama has built a molecular abacus of his own. It promises to increase computer memory capacities to undreamed-of levels and even lead to molecular computers. "There is no reason why technologies which work with picometre precision couldn't be produced," says Gimzewski. "This would enable us to do data processing with molecules."

Information recorded on the hard disc inside your computer is stored as trillions of individual bits of data. Each bit is like a tiny magnet, formed from a small region of the magnetic material that coats the disc. Data is recorded by adjusting the alignment of these tiny magnets to represent digital ones and zeros.

Shrinking memory

Over the years, engineers have been able to make these magnetic regions smaller and smaller—which is why hard discs now hold many times as much data as those of a decade ago. But now the engineers are really up against it. Shrink the size of each bit much further, and they will become unstable: electrical noise or a small rise in temperature, for instance, would be enough to realign these tiny magnets, wiping out the valuable data on your hard disc. What is needed is a way to store information reliably in the space occupied by single molecules—which is where the abacus comes in.

Building and operating molecular-scale devices requires an extremely delicate touch. One of the best tools for this kind of nano-construction is the scanning tunnelling microscope (STM), a device pioneered by researchers at IBM as a way of viewing molecules or even individual atoms. With care, the tip of the STM—which resembles a miniature gramophone needle—can also move these molecules around on a surface.

But as Gimzewski and his team set to work with the STM, they hit a snag. They could only position atoms and molecules on a surface if they kept it under a vacuum and cooled it to within 3 degrees of absolute zero. At normal temperatures and pressures, the molecules jig around, propelled by thermal energy and collisions with molecules of gas in the atmosphere around them.

The researchers needed a way to move individual molecules and anchor them in place. To do this, they made a ring-shaped porphyrin molecule with "sticky" legs that weakly grip the copper surface they are placed on. The idea was that the legs would hold the bead molecules in place until they were nudged to a new position by the tip of the STM. But when the researchers tried it out they found that they still had problems positioning the porphyrin molecules accurately. Although it was easy enough to move the molecules about, they could not accurately control the way they interacted with the STM tip. The result was that when the researchers pushed a molecular bead, they could not predict where it would come to a stop. They needed some way to guide the direction in which beads moved—and luckily, work they were already doing with buckyballs provided the answer.

Buckyballs bind weakly to copper, so Gimzewski and his colleagues made a copper surface with tiny steps just one atom high. When buckyballs are placed on this surface, they line up along the step edges. Now the researchers found that as they pushed individual buckyballs with the tip of the STM, the steps acted as guide rails. "A guide rail restricts the motion to one dimension, and that makes life easier," says Gimzewski.

They quickly found that they could line up 10 buckyball beads along step guide rails, and then use the STM to move and view the beads, sliding them across as they counted from one to 10. However, this buckyball abacus is not as sophisticated as its full-scale counterpart. At the moment, it has only one row of beads, although it is theoretically possible to make a version with several rows. What's more, the researchers have discovered that they can only move one bead at a time.

Molecular machines

Despite these limitations, Gimzewski is happy that the tiny device is providing important technological insights. "The long-term objective is to manipulate molecules on an individual basis, to learn about fabricating devices and machines where every atom is in the right place," he says. Gimzewski is also confident that molecular data storage is on its way. "The debate is not on feasibility, but how and when we can engineer and mass-produce on this scale," he says.

Meanwhile, Komiyama is working with Hidemi Shigekawa at the Institute of Applied Physics at the University of Tsukuba and researchers at several other Japanese universities to build an abacus in which the beads are threaded onto a molecular rail. Their abacus developed from research carried out by Akira Harada at Osaka University. The beads are cyclodextrins, bucket-shaped molecules with a hole at both ends, which are commonly used as an additive to improve the solubility, taste or stability of foods. The wires are made from long, thin strands of the polymer polyethylene glycol (PEG), another additive, which is used to thicken foods. Harada found that he could thread over 20 cyclodextrin beads onto individual strands of PEG.

Komiyama and his colleagues have discovered that if they lay the beads onto the right surface, they will not only stick but also line up into a regular pattern. "We can even arrange several necklaces in parallel to produce something that looks like a real abacus," says Komiyama. Like the IBM group, Komiyama and Shigekawa use an STM to view and manipulate their abacus. But since the beads are bound to a surface and threaded to a guide rail, they can push beads around in an air atmosphere. "In the IBM case, the abacus manipulations were done under ultra-high vacuum. In our case, we can move molecules reversibly under completely ambient conditions," says Komiyama.

There are other attractions to their approach too. It is possible to slide two beads at a time, and also bend the PEG rail so that the beads move at right-angles to their usual direction. Combining these modes of transformation will make it easier to store complex data, the Japanese researchers say.

Komiyama readily admits that practical applications are still some way off. Ironically, the device that opened up the world of nanotechnology is the problem. An STM takes too long to store and read data on an abacus, as it can only move along it one bead at a time. "It will play a role in the development of nanotechnology," Gimzewski says, "but not necessarily mass production and the operation of future devices."

The answer, Komiyama believes, is to replace the sluggish STM with beams of light. He plans to find a way to move the cyclodextrin beads back and forth with tiny molecular switches that respond to light. It should even be possible to use light to read the positions of each bead.

Komiyama has already taken the first steps towards this goal. The switch he has chosen is a derivative of a molecule called azobenzene. When the molecule absorbs light, it isomerises, flicking from a straight form into one with a twist at its centre (New Scientist, 13 September 1997, p 20). Thread a cyclodextrin bead on to the centre of the straight form of the molecule, shine light onto it, and as the azobenzene changes shape, the bead is forced to move away from the twisted segment.

By spacing azobenzene molecules regularly along the PEG chain, and threading a cyclodextrin bead over each one, Komiyama hopes to build a prototype light-activated abacus. At present, however, there are two main problems. Firstly, a flash of light won't return the beads to their original position—Komiyama has to use the STM to flick the beads back. But he believes he will be able to design a way to move the beads without needing the STM. "The light-driven device is possible in the future," he says.

Komiyama also needs to develop finer control of his abacus so that he can select which beads are flicked back and forth, as well as a way to read out the data. One solution, he suggests, could be to attach a coloured dye molecule to each cyclodextrin bead. Light shining onto the beads will be absorbed by the dye. With a little chemical tinkering, it should be possible to design a dye molecule that will transfer this light energy to the azobenzene switch to trigger it. Equip each cyclodextrin bead with a dye that absorbs light of a specific colour, and you can control which beads move, simply by changing the colour of the light you beam onto the abacus (see Diagram). Komiyama estimates that his molecular abacus could store up to about 100 terabits per square centimetre—one thousand times as much data as the ultimate limit for today's magnetic technology.

Whatever the difficulties at present, both scientists agree that in the future, nano-machines will be a reality. "This work is a milestone towards molecular devices," says Komiyama. Gimzewski goes further, suggesting that data storage at the molecular level may just be the start of a nanotechnology revolution. If you can store data on single molecules, why not use those molecules to build a mechanical information processor on an atomic scale?

Komiyama also sees other applications. He has found that his arrays of cyclodextrin beads recognise and bind to large molecules. This binding is highly selective: change the position of the beads and you can select which molecule will bind. "We can prepare receptors for various guest molecules in a tailor-made fashion," he says. Use the cyclodextrin beads on your light-activated abacus as receptors, and you could end up with a highly selective sensor. One moment it can be set up to look for one kind of pollutant, for example, the next it could be reconfigured—at the flick of a switch—to recognise a completely different chemical.

Despite its age and its humble beginnings, the heyday of the abacus may be yet to come. The Babylonian trader might have been cheered to know, as he tramped gloomily home along the banks of the Euphrates, that one day his ill-fated abacus might cut the supercomputers of the 21st century down to size.

Jim A. Thomas is a Royal Society research fellow in the chemistry department at Sheffield University

From issue 2225 of New Scientist magazine, 12 February 2000, page 40

For the latest from New Scientiist visit www.newscientist.com