Although these early applications are worlds away from the billion-transistor molecular computers that enthusiasts envision, they could show the value of organic molecules as an electronic material. “They are a camel’s nose under the tent,” says Reed, adding that “these hybrid devices are already very realistic. They’re the first step down the road to more complex [molecular] circuits.”
It’s likely, however, to be a long road. Even a simple computer made of molecular components is at least a decade away-and then “only if we get really clever,” acknowledges Williams. But the HP chemist says his group is already on its way. In their initial prototypes, the California researchers have fabricated the top and bottom metal wires as perpendicular grids, creating a “crossbar” structure with the molecules sitting at the junctions of the wires. So far, the group has made devices with metal contacts that are thousands of nanometers in diameter; there are millions of molecules at each junction. But Williams says that by later this year the group expects to have wires measuring a few nanometers across. “It didn’t make sense to do everything hard right away. So we used much larger wires. Now we’re doing the experiments to switch to smaller wires and make the measurements.”
The nearly perfect candidates for such tiny wires are structures known as carbon nanotubes. These regularly shaped pipes, only a few nanometers in diameter, could be excellent conduits for electrons speeding through a molecular circuit. The problem is that nanotubes tend to form as a tangled mess-far from the neatly ordered arrays needed to fabricate complex circuits. Building any structures with nanotubes “is now an art form,” says physicist Paul McEuen of the University of California, Berkeley. “We basically throw them down on the ground and look for [the structure] we want.”
The HP/UCLA group is confident they’ll solve the wiring problem. “Eventually nanotubes will be used. Their electronic and physical properties are so desirable,” says Williams. For now, he says, the group is also working on silicon nanowires. And, promises Williams, with or without carbon nanotubes, by late summer the scientists will scale down the junctions of devices to smaller than 10 nanometers. The near-term targets are a 16-bit memory that is 100 nanometers on a side, and soon after that a similarly sized logic device. These rudimentary circuits may not threaten the reign of silicon, but they could be a milestone in helping prove that molecular electronics is feasible.
But then comes the truly daunting part: turning these simple devices into complex logic circuits, and integrating them into an actual computer. One of the penalties you pay for making microelectronics based on chemistry is that, unlike silicon chips made in high-tech fabrication plants, molecular devices synthesized in vats of chemicals will inherently be full of defects. At the scale of individual molecules, chemistry is given to statistical fluctuations-sometimes it works and sometimes it doesn’t. But it’s here that the HP/UCLA scientists contend they have made their most important breakthrough.
Their answer: software that overcomes the defects. Several years ago, computer scientists at HP built a supercomputer called Teramac, using defective silicon chips so flawed they were considered worthless. The HP scientists cobbled these rejected chips into a computer by developing a “crossbar” architecture that makes it possible to connect any input with any output. Once the hardware was built, the computer was programmed to identify and route around any defects. The system worked-and its massive parallelism provided an archetype that the California scientists plan to use for their molecular computer.
“A chemist working on a computer is a bizarre thing. You can’t go to a chemist and ask them to build a computer,” says Heath, one of the UCLA scientists who is helping to synthesize the necessary components. But, he says, the Teramac architecture has provided the HP/UCLA group a clearly defined target. “The software will turn it into a machine,” says Heath. That molecular computer “may be a long way off,” he acknowledges. “But there’s no reason why it won’t work.”