Nonlocal substrate effect on supported silver clusters revealed by lifted plasmon degeneracy

Plasmon resonances of supported metal nanostructures are affected by the substrate in an anisotropic way, which gives rise to mode splitting and inhomogeneous energy shifts. We provide experimental evidence that the commonly observed and intuitive case of a weaker influence at larger distances to the substrate does not apply to the plasmon mode structure of small, quasispherical particles. Instead, for the latter, we observe that the energy shift induced by a carbon substrate is strongest if the plasmon is excited farthest from the support. This nonlocal behavior results from the plasmon mode geometry in conjunction with carbon-induced symmetry breaking, which is revealed by electron energy loss spectroscopy (EELS) of silver clusters deposited at the rim of a carbon film. Decomposing the experimental maps based on spectra obtained from the boundary element method (BEM) uncovers the full mode structure. These results strongly suggest that some of the persisting literature controversy on plasmon energies of small, spherical nanoparticles is due to unconsidered spectral effects of an anisotropic local environment.