Small Wonders

Hongjie Dai grows self-assembling nanotubes from the bottom up. That's one reason why the China-born physical chemist was recently awarded a $625,000 research fellowship by the David and Lucile Packard Foundation. Paul McEuen says that Dai, an assistant professor of chemistry at Stanford, is one of the world's three top people in nanotech.

Dai, age 34, is certainly a new breed of scientist. His research group works simultaneously in chemistry, physics, engineering, and biology. Yet in some ways Dai is a farmer. His fields are laboratories full of vacuum pumps and super-hot ovens. He grows crops of carbon nanotubes. He fertilizes his crops with methane and other hydrocarbons.

It all started with the buckyball, invented in 1985 by a Nobel Prize-winning team led by Richard E. Smalley of Rice University. Smalley's buckyballs -- short for buckminsterfullerenes, a new element Smalley discovered in his lab -- are incredibly strong molecules made of carbon atoms. Hongjie Dai, and other nanotubeologists, learned how to transform the balls into elongated tubes. At first, the long, thin tubes of strongly bonded carbon atoms grew, noodlelike, in a carbon soup, all hopelessly entangled with each other.

Dai improved on this manufacturing method by learning how to grow the carbon nanotubes symmetrically. He heats up his methane feedstock, dashes in a bit of iron oxide catalyst, and sits back for an hour. Soon arrays of tubes sprout up in compact, orderly bundles, looking for all the world like cities of little world trade centers. This achievement is something on the order of growing millions of soda straws straight upward into outer space.

Courtesy of Hongjie Dai Multiple magnifications of nanotubes. (Lengths are given in microns) click to view larger image

Dai's tubes are, in a sense, the first self-assembling nanomaterial. In the futurist world of K. Eric Drexler, self-assembly means that armies of tiny robots build greater armies of tinier robots, ad infinitum. In the real world, Dai's self-assembly makes use of the same physical processes of attraction and repulsion that make the rainwater on a car windshield bead up in an orderly fashion.

The atom-thin nanostructures that Dai grows have several revolutionary applications, depending on which way the carbon atoms link to each other. In one form, the nanotubes are a metal. In another form, the tubes are a semiconductor. Either way, says Dai, the tubes are 100 times stronger than steel. Used in composite materials, they may one day be capable of making everything from tennis rackets to automobiles and airplane frames.

Hongjie Dai's semiconducting nanotubes can also function as transistors, which means a single tube can be used as a switch to turn flows of electricity on or off. Or, in a quantum sense, the tubes can function as controllable gates through which discrete packages of energy enter and exit. This important function of on-off control lies at the heart of electronics, classical and quantum.

In another atomic pattern, the crystalline tubes become metallic wires -- possibly "ballistic" wires, through which electricity travels almost without losing energy. These extraordinary wires could enable the production of atom-sized transistors and electronic circuits powered by single electrons. These wires are so fine that they can be connected to atom-sized electrodes in electronic circuits measured in angstroms. (There are 10 angstroms in a nanometer.) This interconnectivity means that it may one day be possible to construct the most mind-boggling machine yet imagined by the human brain: the quantum computer.

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Michael F. Crommie moves individual atoms around like some people move poker chips: He slides them one by one into piles. Only he does it with a scanning tunneling microscope. That's why the Physics Department at the University of California at Berkeley recently hired Crommie -- and the rather incredible microscope he put together piece by piece -- away from Boston University.

Crommie, age 37, was born in Southern California, where his father, an aerospace engineer, designed heat shields for the Apollo moon program. Crommie says he "grew up wanting to build spaceships, like Dad." Instead, the younger Crommie ended up going about as far inside space as one can get. Using his scanning tunneling microscope, Crommie finds lone atoms, and then pushes them into geometric structures called quantum corrals.

In Crommie's wild and woolly frontier world, quantum mechanics calls the shots. His microscope doesn't magnify -- it "tunnels." What does that mean? It means that electrons sitting at the tip of the microscope's thin probe do the impossible: They shoot through barriers that the rules of classical physics absolutely forbid an electron to pass.

Imagine a golf ball rolling down a slight slope until it hits a brick wall. Classical physics says that the ball does not have enough energy to pass through the brick wall. But quantum mechanics says that there is an extremely small probability that the golf ball will jump through the wall and continue to roll on the other side. (Although possible, this event is so improbable that it would take several ages of our universe for it to occur.)