Localized Surface Plasmon Resonances of Size-Selected Gold and Copper Nanoclusters Soft-Landed on a C60 Organic Substrate

This study investigates the size-dependent localized surface plasmon resonance (LSPR) properties of gold (Au), copper (Cu), and palladium (Pd) nanoclusters (NCs) deposited on a C₆₀ substrate by using two-photon photoemission spectroscopy (2PPE). The LSPR responses of these NCs are characterized by their photoemission enhancement under polarized light, focusing on the influence of cluster size and electronic structure. The results reveal that Au_n NCs exhibit distinct LSPR responses to visible light when their size exceeds approximately 21 atoms (Au₂₁), with significantly enhanced photoelectron intensities under p-polarized light. In contrast, Cu_n and Pd_n NCs, even at sizes of around 70–90 atoms, show minimal or no LSPR responses. These results indicate that LSPR characteristics are strongly influenced by the electronic structure, particularly the arrangement of d-electrons near the Fermi level. For Au_n NCs, relativistic effects of 5d-electrons facilitate LSPR in the visible spectrum, whereas narrow 3d-bands and the absence of free 5s electrons hinder plasmonic activity for Cu_n and Pd_n NCs, respectively. This study highlights the critical role of d-electron configurations in determining the plasmonic properties of NCs, providing insights for designing advanced plasmonic nanomaterials with tailored electronic and structural characteristics.