Schoolyard bonds

The most abundant titanium carbide nanocrystal contains a mere 27 atoms in nearly a perfect one-to-one ratio of titanium atoms to carbon atoms. At this tiny scale, the 27 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.

go to page 3