Information on the emergence of plasmons in noble metal nanoparticles is limited. This is primarily due to the challenge of preparing a series of molecularly precise clusters spanning a size range from small clusters to colloidal nanoparticles, but it is also due to the complex physical and chemical interactions between the metal core and surrounding media on optical excitations. Here, we overcome both of these challenges by recording a systematic series of spectra for discrete Ag magic-number clusters with different ligand shells. Spectra of the smallest clusters contained discrete superatom transitions that depended on the ligand but they evolved toward a single plasmonic peak, independent of ligand, for the largest particles. Surface-enhanced Raman scattering from Ag44(p-MBA)30 clusters also showed that dipole transitions in superatom clusters are fundamentally similar to plasmonic excitations. A superatom description therefore emerges, describing plasmons as highly-degenerate single-electron excitations with very large transition dipole moments.

Birth of a plasmon

245th National Meeting (2013)

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