Deciphering cooperative effects in plexciton formation between Ag nanocluster and fullerene

In this work, we use optical reflectance spectroscopy to study the plexciton formation between Ag nanoclusters and C₆₀ molecules. The Ag clusters are fabricated on high-quality graphene under ultrahigh vacuum (UHV) and exhibit a strong absorption band at 3.4--3.6 eV due to the localized surface plasmons (LSPs). The plexciton formation is studied by depositing C₆₀ on the Ag clusters in the same chamber under UHV. The deposition of C₆₀ molecules leads to a splitting of the LSP band into multiple peaks with a systematic peak energy shift as a function of C₆₀ coverage θ_C₆₀. Notably, the details of the energy shifts and intensity variation sensitively depend on the LSP energy. Model calculations in which the plexciton coupling is approximated by a point-dipole interaction predict the evolution of the plexciton eigenenergy and oscillator strength with θ_C₆₀ that reasonably explains the experiments. We demonstrate that spectral changes under the deposition of C₆₀ cannot be explained by considering only one electronic transition in C₆₀, as is commonly assumed. Instead, it is necessary to take into account three electronic transitions of C₆₀ and their intermixing with the LSP to fully comprehend the spectral evolution. The plexciton coupling energy is estimated to exceed 0.6 eV, indicating that the system reaches a strong coupling regime. Our findings suggest the importance of simultaneously considering multiple electronic excitations of molecules to understand strongly coupled plexcitons.