Features:

QUANTUM PLAYGROUNDS
by Katherine A. Caponi, NCSA Science Writer

Playground equipment used to be simpler. Where posh lumber ladders and soft- sided plastic slides now top the woodchip and grass fields of our children's playgrounds, skeletal Catskills of welded metal once towered over dusty gravel. An insurance liability, to be sure, but they had the stark beauty of metal joints and poles forced by fire to form rudimentary castles for schoolyard crowds.

Chemistry and quantum mechanics were once simpler, too. Models built of Styrofoam balls and pipe cleaners--those other staples of elementary school-- used to rule the field. Today, computer programs run simulations predicting the behavior of molecules at the quantum level.

The molecules themselves are also more complex. "The last decade has witnessed explosive growth in basic and applied research on inorganic particles," says Steven Lewis, assistant professor of physics at the University of Georgia. "Advances in both theoretical and experimental techniques have resulted in the discovery of a wealth of new stable nanoparticles exhibiting a wide range of novel behaviors."

But there is much to learn about the behaviors and properties of the transition-metal carbide nanoparticles that Lewis and his team study before scientists can begin to turn them into useable materials for industry. Transition metals are the chemical elements occupying the skinny middle section of the Periodic Table. They are very chemically active and exhibit a wide range of physical properties, features that makes them and their compounds appealing to industries such as chemical processing and magnetic data storage.

A particularly interesting feature of the transition metals is that they form a host of different compounds when paired with other elements. One such family of compounds is transition-metal carbides (TMCs), which have many industrially important uses. When researchers combine energetic transition-metal and carbon atoms in a on-to-one ratio in the gas phase, they form very stable crystalline formations called nanocrystals. TMC nanocrystals are structurally similar to rock salt--a cubic-shaped crystal forming a 3D checkerboard of atoms. What is so unusual is that they retain this crystalline for even for particles containing only two-dozen or so atoms. Using Alliance supercomputers, the team analysed the electronic structure of titanium carbide nanocrystals to provide theoretical understanding for recent puzzling experimental findings.

Schoolyard bonds The most abundant titanium carbide nanocrystal contains a mere twenty-seven atoms in nearly a perfect one-to-one ratio of titanium atoms to carbon atoms. At this tiny scale, the twnety-seven atoms line up in orderly fashion to form a three-by-three-by-three cube of atoms. Atoms in that cube alternate between titanium and carbon. Each "bar" in between an atom pair is a nearest neighbor chemical bond.

Because the symmetry of titanium carbide nanocrystals is so perfect, only two arrangements are possible. In the first arrangement, the nanocrystal contains 14 titanium atoms and 13 carbon atoms, with the titanium atoms occurring at all eight corners. The second possibility is for the nanocrystal to have carbon atoms at all eight corners. However, only arrangements of the first type are observed in experimental settings--the other possibility is completely absent. "For some reason, there is a disfavor of corner sites for carbon-atom occupation," states Lewis. "The question is: Why?"

Lewis and his team members found out. Using supercomputers, Lewis and his team were able to simulate titanium carbon nanocrystals at the 27-atom size. They looked at molecules representative of both physical arrangements and computed the energies of the bonds between each atom in both configurations. They also studied how the average bond energy changed when the corner atoms were stolen from each molecule.

The team found that in the atomic configuration with titanium at the corners, the average bond energy stayed the same when all the corner atoms were removed. However, for the atomic configuration with carbon atoms at the corners, the calculated average energy per bond shot up considerably. To a quantum physicist, this red flag signals that bonds to carbon atoms at the corners are naturally weaker than bonds to corner titanium atoms. This is the reason that titanium carbide molecules do not ever appear with carbon atoms in the corner positions--those carbons are very easy for other molecules passing by to steal.

The team also discovered that when they look at natural vibration patterns for the two types of nanocrystals they studied, one particular vibration is highly localized, involving only the bonds to corner atoms. "If you think of chemical bonds as springs, then weaker bonds mean softer springs, which in turn means lower vibration," said Lewis. He and his team found that the vibration of the nanocrystal with corner carbon atoms was significantly lower in frequency than that of the titanium-cornered nanocrystal. This also shows the bond weakening that would make it easier for other molecules to steal carbon atoms from the nanocrystal's corners.

There's a new swing set in town To run the simulations, Lewis and his team used Density Functional Theory (DFT), a method of doing quantum mechanics that is very efficient, and 120,000 combined hours on NCSA's Titan Linux cluster and SGI Origin2000. This approach was ideally suited for the team's investigations because the approximations that DFT uses are found to produce insignificant errors for the TMC nanocrystals. What was important was finding a way to study the molecules that demonstrated the interactions they undergo with other molecules seeking to steal carbons.

"The only way to do that is computationally. You can't just collect them on a plate and analyze them," says Lewis. "We needed to construct a theoretical explanation of why experimenters just don't see the carbon atoms at the corner positions."

The old models made of Styrofoam balls and pipe cleaners could easily show you the possible arrangements for molecules. Creating molecules in the lab will visibly demonstrate that they occur in one particular formation, but not another. But it takes a new kind of science--computational science--to explain why.

Funding This project is supported by the National Science Foundation and the American Chemical Society Petroleum Research Fund.

For further information: http://www.physast.uga.edu/~lewis/, http://www.physast.uga.edu/~lewis/Pubs.html