Freestanding clusters of twenty gold atoms take the shape of a pyramid, researchers discovered. This is in contrast with most elements, which organize themselves by forming shells around one central atom. The team of researchers led by KU Leuven published their findings in Science Advances.
Clusters composed of a few atoms tend to be spherical. They are usually organized in shells of atoms around a central atom. This is the case for many elements, but not for gold! Experiments and advanced computations have shown that freestanding clusters of 20 gold atoms take on a pyramidal shape. They have a triangular ground plane made up of 10 neatly arranged atoms, with additional triangles of 6 and 3 atoms, topped by a single atom (see the figure below, in which a model of twintig oranges is compared with the theoretical and experimental structure).
Researchers used intense plasmas in a complex vacuum chamber setup to sputter gold atoms from a macroscopic piece of gold. "Part of the sputtered atoms grow together to small particles of a few up to a few tens of atoms, due to a process comparable with condensation of water molecules to droplets," says Zhe Li, the main author of the paper and currently at the Harbin Institute of Technology. "We selected a beam of clusters consisting of exactly twenty gold atoms. We landed these species with one of the triangular facets onto a substrate covered with a very thin layer of kitchen salt (NaCl), precisely three atom layers thick."
The study also revealed the peculiar electronic structure of the small triangular pyramid of gold. Very similar to the noble gas atoms or aromatic molecules, it only has completely filled electron orbitals, which makes them much less reactive than clusters sizes with one or a few atoms more or less.
Gold clusters in the size range of a few to some tens of atoms are known to possess remarkable properties. The reported detailed information on these small gold clusters are important to evaluate their catalytic and optical performances, which is highly relevant for designing cluster-based catalyst and optical devices. Recent applications of clusters include utilization in fuel cells and carbon capture.
The study "Unraveling the atomic structure, ripening behavior, and electronic structure of supported Au20 clusters" was published in the journal ’Science Advances’.