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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)
"Brave New Nano-World Lies Ahead:
One atom at a time, scientists are
building a future of the fantastic"
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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.
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.)
But if the golf ball were an electron riding a conductive wire, and the brick wall a piece of atom-scale insulation, the seemingly impossible would become probable. Quantum mechanics says there is a definite probability that the electron will jump, or "tunnel," through the insulation-barrier and appear on the other side to continue its journey. The reason that this apparent magic can happen: The barrier is only a few atoms thick, and the mathematics of quantum mechanics says that at the scale of a few atoms, electrons will jump through the insulation a quantifiable percentage of the time. Above a certain thickness, the probability of tunneling falls off dramatically.
And this is why quantum effects can play havoc on electronics at the small scales: If electrons jump willy-nilly through insulating barriers in electronic circuits, the circuits short out. Learning how to control the flight of electrons is one of the principal focuses of nanoscience. Crommie wants his electrons to tunnel only upon command.
At the tip of Crommie's scanning tunneling microscope electrons jump off through space, to atoms resting on a surface. This creates a measurable electrical current. Slight fluctuations in the current are transmitted to Crommie's computer, which turns electrical variations into pictures of atoms.
These atoms do not look anything at all like the classical models of atoms we learned to draw in grade school (that is, tiny solar systems with electrons whizzing in orbits about a nucleus). Crommie's atoms-on-a-surface resemble ball bearings nestled in corrugated egg cartons. What the microscope sees is the electron cloud that surrounds the nucleus of the atom and interacts with other atoms. Pictures of atoms can be used to study their essential properties: how they sit and move, and how they repel and bind to one another.