Unraveling the Origin of Localized Surface Plasmon Resonance in Noble Metals and Its Integration with Surface Plasmon Polaritons

We investigated two types of surface plasmons―localized surface plasmon resonance (LSPR) and surface plasmon polaritons (SPPs)―and their interplay using size-selected Ag, Au, and Cu nanoclusters (NCs) and ultrafast spectroscopy. Atomically precise NCs were prepared by magnetron sputtering with mass selection and deposited on C60-modified substrates. Two-photon photoemission spectroscopy (2PPE) revealed threshold sizes for plasmon emergence (≥9 atoms for Ag, ≥21 for Au) and clarified the influence of d-band structures. Dual-color 2P-PEEM visualized SPP propagation at metal–organic interfaces, showing that LSPR enhancement and molecular modifications can tune SPP characteristics, even at buried interfaces using NC sensitizers. These results provide key insights into size- and structure-dependent plasmonic behavior and inform the design of advanced photoelectronic and nanophotonic devices.