When small molecules congregate on a clean metal surface, they can organize into large-scale ordered structures that are visually striking and often unexpected. (See, for example, Physics Today, May 2009, page 17.) They can even modify the surface itself, as exemplified by new work from Lars Diekhöner, at Aalborg University in Denmark, and his colleagues.
The researchers used scanning tunneling microscopy to study trimesic acid (TMA), a roughly triangular planar organic molecule, on the (111) surface of silver. Previous studies of TMA on Ag(111) found that the molecules form one structure at room temperature and another when annealed at 420 K. When Diekhöner and colleagues annealed their sample to 480 K, however, they found something different, as shown in the left panel of the figure: an irregular arrangement of flower-like motifs, some separate (as highlighted in green), and some interlocking. Raising the temperature to 510 K caused all the flowers to meld together, as shown on the right, into a large-scale repeating pattern whose unit-cell vectors are marked by the black arrows. The flowers’ triangular petals are TMA molecules, but their bright, round centers are not.
Density functional theory calculations revealed the identity of the flower centers: They’re hexagonal clusters of seven Ag atoms resting on top of the Ag(111) surface. The extra atoms are drawn, presumably, from the edges of surface step defects. Stable arrays of regularly spaced nanoclusters are potentially valuable for their catalytic properties. The TMA–Ag structure isn’t the first such array to be observed, but it’s one of only a few (and the only one so far on Ag) in which the clusters are made up of more than three atoms. The calculations also showed that the structure derives its stability from the strong bonds that form between Ag atoms and the TMA’s oxygen atoms. The analogous structure on gold, for example, is not energetically stable, even though Au’s lattice constant is almost the same as Ag’s. (K. L. Svane et al., J. Chem. Phys., in press.)
