Optical response of quantum-sized Ag and Au clusters--cage vs. compact structures and the remarkable insensitivity to compression.

ABSTRACT

Quantum-sized silver and gold clusters show very different spectral characteristics. While silver exhibits a strong localized surface-plasmon resonance (LSPR) band down to very small sizes, the resonance is broadened beyond recognition in Au clusters below about 2 nm. In the present work, we study icosahedral hollow-shell structures, or cages, of about 1.8 nm diameter in comparison with compact clusters and show that the qualitative difference between Ag and Au remains but is reduced, as a significant increase of absorption is found for the Au cage structures. The silver shell Ag92 exhibits a resonance that is red-shifted compared to the compact Ag147, coinciding with the general result found in much larger shells that are amenable to the classical description by Mie theory. However, the electronic structure in particular of the d band is strongly changed. The spectrum of the empty Ag shell is remarkably similar to the spectrum of the respective Au55Ag92 core-shell structure. Inspection of the time-dependent electronic density does not explain this similarity. However, it shows that the overall classical picture of a collective charge oscillation remains valid, although clearly with modifications. We further show a remarkable insensitivity of the absorption spectra of both Ag and Au clusters to even rather extreme values of compression or dilatation.