Structural evolution of larger gold clusters

The preferred structures of larger gold clusters comprised of 100 to 1000 atoms (1.4-3.0 nm equivalent diameter) have been determined theoretically via exhaustive search and energy-minimization methods and experimentally by synchrotron x-ray diffraction analysis of purified powder samples of small gold nanocrystals passivated by alkylthiol(ate) self-assembled monolayers. Theory predicts a persistent, close competition, across the entire size-range, among three structure-types: Marks-type decahedral (Dh) structures, monocrystals of a particular (TO+) truncated-octahedral (or 'Wulff') morphology, and symmetrically twin-faulted variants (t-TO+) of the second; all other forms are much less stable. Quantitative comparison of the experimental diffraction patterns with patterns calculated from the structures provides clear evidence for a high abundance of the Dh and t-TO+ forms, but also reveals a definite transition from the former to the latter structures in the 1.7 to 2.0 nm range (similar to 200 atoms), Further, the observed (mean) lattice contraction is only about half that predicted, suggesting that the surfactant monolayer acts to reduce the surface energy of the clusters. Taken together, these results suggest that the surfactant monolayer may play a small but important role in differentially stabilizing the higher energy {100}-type facets present to a greater extent in the TO-type structures.