Atom-Scale Resolution of Plasmon Point of Gold Nanoclusters

The emergence of plasmonic characteristics in gold nanoclusters is fundamentally influenced by the size of the clusters. Previous theoretical and experimental studies have found that the transition of gold nanoclusters from the molecular to the metallic state typically occurs at some point within the size regime of 2–3 nm. In this work, we aim to determine the point of transition from molecular to metallic state at the atomic scale, identified as the “plasmon point”, based on the atomistics and the topology of the gold nanoclusters. The clusters with increasing size have been synthesized using the wet chemical technique, and the emergence of the plasmon band of these specific clusters has been correlated with their atomic arrangement. Theoretical investigations have been carried out for Au 13, Au 55 and Au 147 clusters of icosahedral and cuboctahedral geometries which could be associated with the thermodynamically stable amorphous and crystalline states, respectively. The stability and the electronic properties of the clusters have been analyzed with density functional calculations, which elucidate the appearance of metallic characteristics in the cuboctahedral Au 147 although the icosahedral analogue remains molecular in nature. The overall plasmon point has been determined to be at 309 atoms, although the phase-specific plasmon point for the cuboctahedral phase is attained, in particular, at the composition of 147 atoms. Comparing the experimental and theoretical results, it can be concluded that the cuboctahedral or crystalline clusters are more susceptible to show metallic characteristics than the amorphous clusters of similar composition. It has been revealed that the aurophilic interactions play a significant role in determining the emergence of metal-like properties in the nanoclusters.