Unraveling the atomic structure, ripening behavior, and electronic structure of supported Au₂₀ clusters

The free-standing Au₂₀ 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" Au₂₀ 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 Au₂₀ clusters are obtained. We demonstrate that individual size-selected Au₂₀ 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 Au₂₀ agglomerates. The evolution of the electron density of states deduced from the spectra reveals gap reduction with increasing agglomerate size.