The Dust in the Machine
If the materials studied by these researchers seem baffling and unpredictable, the machinery they use is even more so. Progress in molecular electronics is often at the mercy of unpredictable glitches in the experimental equipment. This is, after all, laboratory science and not engineering.Tan Ha, a native of Vietnam, is in charge of the equipment used in the lab’s clean room. Two or three times a day he dons a clean-room suit and goes into the room to test, adjust, and modify equipment for what are in many cases first-of-a-kind experiments. We suit up. “Now we’re ready for chemical warfare,” he says. The mask over his face makes it difficult to judge whether he is joking.
Once inside we make a beeline for a machine called a chemical vapor deposition reactor. It looks like a big steel cylinder on its side, encased in glass. “I have a special relationship with this machine,” he says, and touches the glass with a gloved hand.
This type of reactor is standard fare in semiconductor fabrication facilities, but Ha has modified the machine to perform the ultraprecise experiments required by Ted Kamins, a member of Williams’s group since 1995. Kamins has worked for years on the ultimate dream of nano research: making devices “grow” in desired structures rather than building them piece by piece. His goal is to grow the nanowires required by molecular electronics, as an alternative to using nano imprint lithography. So far, Kamins has synthesized wires as small as 10 nanometers in diameter by exposing “nanoparticles” of various materials to a mixture of gases in the deposition reactor. In the ensuing reaction, long chains of silicon grow up around the particles, producing what looks under the electron microscope like a forest of needles.
Growing the wires required for molecular electronics is exciting stuff, but Kamins’s particular experiments almost didn’t happen. Ha tells me that he spent over a year of his life trying to make the machine work. “Every time we ran an experiment, contamination would destroy the process,” he says. It wasn’t that the machine was broken; it’s just that no one had ever needed to do the experiments that Kamins wanted to do. “It got to be a spiritual agenda for me,” says Ha. “Ted was frustrated. So was I. I’d be in here on my knees all day long, modifying things screw by screw. I’d go to bed at night and close my eyes and see the plumbing diagram on my eyelids. It turned out to be a problem in the exhaust system. I went home and told my wife, That’s it; I am a proven equipment engineer.’ That’s how happy I was.”
Picking a Winner
Much to Duncan Stewart’s disappointment, Williams asked him to publish his results with the hydrocarbon molecule after six months and concentrate on other work. Yet Williams encouraged Ha to keep working on his knees and dreaming about plumbing diagrams for a year, for experiments that Williams estimates are at least six years from fruition and may never yield a practical result. In a sea of competing theories and possibilities, and with the budget pressures he complains about with some regularity, how does he decide?
“It’s a matter of experience,” Williams says. “I’ve been down many blind alleys many times in my career. They’re so enticing. You can get into these things and think, okay, just one more step, just one more step. Other things feel like they are in the right direction, and I can see where we’re going.” In other words, he has learned to trust his intuition, because it’s all he has. “I’ve been through the cycle many times.”
Williams’s longest commitment to any idea in molecular electronics is to the crossbar architecture. But he admits that even this idea might be a blind alley. Will it ever be possible, for example, to cleanly trap molecules at the junction of two wires with complete confidence in their orientation? Then there’s the practical problem of gain, or turning a weak electrical input into a strong output; this is a critical capability needed both to carry out logic operations and to amplify the tiny currents crossing the molecular switches so that conventional silicon systems can detect them. And it’s a problem with no demonstrated solution.
“Stan is a smart guy, God bless him, and if anyone can solve these things, it’s going to be his team,” says James Tour, a Rice University chemist who is working on a competing approach to molecular computing. “But he’s got a tough problem. At every crosspoint the molecules need to be stable. Then they need to interface with all the wires coming out. There’s an enormous cost to that. They have a steep hill to climb.”
“It’s certainly possible that we are wrong,” admits Williams. Then he shakes his head and stops being humble for a brief moment.
“I don’t think so,” he says. “I think we’ve picked the winner, something that will allow this thing we call Moore’s Law to continue on for another 50 years. I used to think it was impossible. Now I think it’s inevitable.”
|Alternatives to Silicon|
|Technology ||computing elements||Leading Institutions|
|DNA computing||DNA and RNA strands in solution||University of Southern |
Institute of Science
|Molecular electronic devices||Molecules such as rotaxane||University Hewlett-Packard, Yale|
|Nanocells||Gold nanoparticles deposited in random arrays||Rice University|
|Nanotube electronic components||Carbon nanotubes acting as transistors, memory, and wires||IBM, Harvard University, NASA |
Ames Research Center
|Quantum computing||Quantum properties of electrons and molecules||MIT, IBM, Hewlett-Packard, National|
Institute of Standards