The research studied superlattice structures composed of clusters with cores of 44 silver atoms each. The silver clusters are protected by 30 ligand molecules of an organic material mercaptobenzoic acid (p-MBA) that include an acid group. The organic molecules are attached to the silver by sulfur atoms.
"It's not the individual atoms that form the superlattice," explained Landman. "You actually make the larger structure from clusters that are already crystallized. You can make an ordered array from those."
In solution, the clusters assemble themselves into the larger superlattice, guided by the hydrogen bonds, which can only form between the p-MBA molecules at certain angles.
"The self-assembly process is guided by the desire to form hydrogen bonds," Landman explained. "These bonds are directional and cannot vary significantly, which restricts the orientation that the molecules can have."
The superlattice was studied first using quantum-mechanical molecular dynamics simulations conducted in Landman's lab. The system was also studied experimentally by a research group headed by Terry Bigioni, an associate professor in the Department of Chemistry and Biochemistry at the University of Toledo.
The unusual behavior occurred as the superlattice was being compressed using hydrostatic techniques. After the structure had been compressed by about six percent of its volume, the pressure required for additional compression suddenly dropped significantly. The researchers discovered that the drop occurred when the nanocrystal components rotated, layer-by-layer, in opposite directions.
Just as the hydrogen bonds direct how the superlattice structure is formed, so also do they guide how the structure m
|Contact: John Toon|
Georgia Institute of Technology