Alloying Effects on Plasmonic Responses at the Single-AtomLimit in Ag13 Nanoclusters on a C60 OrganicSubstrate

Nanostructures composed of group-11 metals are promising plasmonic materials due to their localized surface plasmon resonance (LSPR) in the visible to ultraviolet (UV) region. However, the optical responses of ultrasmall alloy nanoclusters (NCs) remain poorly understood because of the difficulty in experimentally probing size- and composition-specific excitations. 13-atom silver (Ag13) NCs represent a cluster size capable of supporting collective valence-electron excitations associated with LSPR-like behavior. In this study, atomically precise Ag-based Ag–gold (Au) and Ag–copper (Cu) alloy NCs─Ag13, Ag12Au1 and Ag12Cu1─were synthesized using magnetron sputtering and deposited on fullerene (C60) substrates, enabling a systematic examination of alloying effects down to the single-atom limit. Their optical responses were investigated by wavelength-tunable ultrafast two-photon photoemission spectroscopy. All three NCs exhibit a pronounced increase in photoelectron yield toward the UV region, attributed to LSPR-related excitation. Notably, Ag12Au1 exhibits an additional distinct peak at around 3.25 eV, absent in Ag13 and Ag12Cu1. This characteristic response is attributed to Au-atom-induced electronic resonance between occupied and additional unoccupied states in Ag12Au1, arising from the geometric and electronic effects of exterior Au doping, in contrast to interior Cu-atom substitution. These assignments are in good agreement with time-dependent density functional theory calculations.