Virtual Nanotech: Modeling Materials One Atom at a Time

Article excerpt

It's hard enough to thread a needle. Imagine trying to manipulate threads and needles miniaturized to one-millionth the normal size. Now, you're thinking like the emerging group of nanotechnologists whose growing dexterity at fashioning new materials and devices may eventually improve every arena of technology, from aerospace to drug development. While many researchers focus on developing tools for working on nanoscale materials, others are pursuing a virtual pathway toward nanotechnology applications. As ever-more powerful computers have become ever more affordable, computational nanoscientists can readily simulate materials atom by atom.

"Now, we can do many marvelous things," says Sidney Yip, a computational materials scientist at the Massachusetts Institute of Technology.

By running such precise computer models of the chemical and physical properties of materials, researchers can examine tiny constructions more thoroughly than a bench scientist ever could.

Models can also simulate materials that have been envisioned but not yet created. For instance, in the late 1990s, Deepak Srivastava of the computational nanotechnology group at NASA Ames Research Center in Moffett Field, Calif., used computer models to study carbon nanotubes--sheets of graphite rolled into a tube. The models indicated that a tube created by rolling the graphite in one particular way would result in a nanotube that behaves like a metal. When the scientists roll the graphite sheet in a slightly different way, however, the resulting tube behaves like a semiconductor. The model also suggested that connecting tubes with these subtly different structures would generate a nanodevice capable of functioning like a transistor.

"At the time, people thought it was a little far out because no one knew how to build these devices," says Srivastava. Several years later, researchers motivated by the computer models proved the NASA Ames team right by fabricating such a nanotube-based switch in the lab.

ENERGIZED MATERIALS One arena in which simulations are moving nanotechnology forward is energy. Academia and industry have been investing millions of dollars in research preparing for a hydrogen economy, where fossil fuels would be replaced by cleaner, more-abundant, and more-efficient hydrogen fuel. That goal, however, requires new materials for storing and extracting usable energy from hydrogen sources.

Along those lines, computational physicist Kyeongjae Cho of Stanford University is using models to design fuel cell materials. In a fuel cell, a catalyst strips electrons from hydrogen atoms to generate electricity. Typically, the catalyst consists of nanoparticles of platinum. "However, platinum is not a cheap metal, and it's a finite resource," says Cho. "There will be lots of problems if we try to launch a hydrogen economy based on fuel cells, because we don't have enough platinum to do the job."

To tackle this problem, the Stanford researchers have been using computer models of atomic-scale structures to learn what makes platinum a good catalyst. At the nanometer scale, a material's properties are dictated by the arrangement of its atoms. For instance, if you assemble carbon atoms so that each one bonds to four others in a tetrahedral pattern, you end up with diamond. But if you change that arrangement to a planar stucture with only three atoms neighboring each carbon atom, it becomes graphite.

With metals such as platinum, the number of possible configurations is huge. To find the ones likely to represent those of catalytic nanoparticles, the Stanford team used computer models to simulate many different atomic configurations and calculate their stabilities. The researchers identified the most stable candidate--a configuration of 611 atoms that measures 3.1 nanometers in diameter, as it turned out--and calculated that it would be efficient at stripping electrons from hydrogen atoms in a fuel cell. …