one key to the advances in molecular electronics could be an exotic molecule called the carbon nanotube. This remarkable carbon structure-discovered by researchers at Japan’s NEC in 1991-is a close chemical cousin of the buckyball, a new form of carbon discovered by Smalley in 1985. But while the buckyball is a soccerball-shaped molecule of 60 carbon atoms, nanotubes are long pipes of a rolled-up sheet of graphite. They’re electrically conducting and have been made into wires only a few nanometers in diameter.
Nanotubes are, both literally and metaphorically, a tunnel between the nano and macroscopic worlds. These structures make possible a long fiber that is only a few atoms wide. On a practical level, says Smalley, batteries might use nanotubes both to shuttle electrons between atoms and to carry a charge centimeters away. “Their great virtue is that they are molecular,” says Smalley. Each nanotube, he says, is “an entity that has its own behavior and integrity.” That means you can push the individual carbon molecules around, like tiny nanologs.
Actually, a nanotube acts a bit more like cooked spaghetti, says Phaedon Avouris, manager of IBM Research’s nanometer-scale science and technology group in Yorktown Heights, N.Y. Each nanotube will stick to a surface and this adhesion is strong enough to maintain any shape you push it into. The adhesion also provides good electrical contact between the nanotube and metal electrodes.
Most recently, Avouris and his coworkers have maneuvered one of these “nanonoodles” to bridge a pair of electrodes and prodded the molecules into rings and letters. The IBM scientists have also made a functional field-effect transistor-a basic electronic device-at room temperature out of a single nanotube.
The successful development of molecular electronics would mean a single chip could hold billions of nanoscale transistors, making a computer orders of magnitude more powerful than today’s machines. It also could mean building tiny and cheap computers that house millions of nano-transistors; such salt-grain-sized computers could be easily and cheaply incorporated into scores of other products-even into “smart” materials.
Nanotechnology could also make possible information-storage devices with immense capacity. Investigators at IBM Research in Zurich, led by physicists Gerd Binnig and Peter Vettiger, are building a micromechanical prototype that uses tiny silicon tips to read and write data bits that are less than 50 nanometers wide. That would translate into hard disks with storage capacities of close to a trillion bytes (terabytes)-a couple of orders of magnitude larger than the hard drives on today’s top-of-the-line PCs. It could also mean small products, the size of a wristwatch, say, that have immense storage capacity.
In their experiments, Binnig and his co-workers use the AFM tip to read nanobits of information on a polymer surface. Using a single tip, however, would mean a process that’s far too slow to be practical. Binnig has therefore wired arrays of more than 1,000 AFM tips that act in parallel. The arrays can rapidly write information by punching tiny divots in the substrate and read the nanobits by detecting the depressions.
Meanwhile, Binnig’s colleagues at IBM Zurich have used the STM to turn out even smaller nanoobjects with clockwork precision. James Gimzewski, an IBM chemist, has built an exquisitely small abacus. Gimzewski used the STM tip as the “finger” to move the abacus beads, which are buckyballs with diameters of less than 1 nanometer.
Gimzewski’s latest invention is a wheel constructed from a propeller-shaped molecule that spins on a tiny, bearing-like structure. Gimzewski says that while the rotating molecule suggests possible future nanomachines, the research remains embryonic. At this point, he says, “if you can get anything to work in the nanoworld, you don’t worry about its practicality. We’re just starting. It’s like children playing with Legos.”
The Zurich work reflects a deeply entrenched-and strongly Swiss-belief in mechanics. Physicist Binnig says, “Mechanics have been overlooked because electronics is so successful. It’s considered old-fashioned.” His device for information storage, however, works more or less like a tiny phonograph needle.
As you explore the nanoworld, he says, mechanical devices become an attractive alternative to electronics.
Binnig says the mechanical approach can be extended well beyond data storage, and that “everything you can do electronically, you can do mechanically.” Electronics are particularly good at guiding energy along precise paths to a well-defined place. But, he says, nanomechanics has an advantage of working with very low power consumption. While a 3-D nanoelectronics device would melt immediately from its own heat, Binnig says, you “could imagine” a 3-D nanomechanical device that would run cool. What’s more, mechanical devices may prove easier than electronics to integrate with biological, optical and chemical systems.