Unraveling the atomic structure, ripening behavior, and electronic structure of supported Au 20 clusters

The free-standing Au cluster has a unique tetrahedral shape and a large HOMO-LUMO (highest occupied molecular orbital-lowest unoccupied molecular orbital) gap of around 1.8 electron volts. The "magic" Au has been intensively used as a model system for understanding the catalytic and optical properties of gold nanoclusters. However, direct real-space ground-state characterization at the atomic scale is still lacking, and obtaining fundamental information about the corresponding structural, electronic, and dynamical properties, is challenging. Here, using cluster-beam deposition and low-temperature scanning tunneling microscopy, atom-resolved topographic images and electronic spectra of supported Au clusters are obtained. We demonstrate that individual size-selected Au on ultrathin NaCl films maintains its pyramidal structure and large HOMO-LUMO gap. At higher cluster coverages, we find sintering of the clusters via Smoluchowski ripening to Au agglomerates. The evolution of the electron density of states deduced from the spectra reveals gap reduction with increasing agglomerate size.