Optical Properties of Silver Nanoshells from Time-Dependent Density Functional Theory Calculations

The absorption spectra of Ag monatomic nanoshells of size between 12 and 272 atoms with different shapes (icosahedra, cuboctahedra, and truncated octahedra) are theoretically studied via a time-dependent density functional theory approach and compared with previous results on compact nanoparticles of similar size and shape. Three main findings are drawn from this study: (a) Ag monatomic nanoshells possess absorption spectra exhibiting clear plasmonic features starting already with the 92-atom system; (b) the position of the absorption peaks moves toward lower energy as the radius of the nanoshells increases, with a blue shift of ≈0.2 eV for icosahedral with respect to cuboctahedral shells (also, the peak substantially gains in intensity with increasing size); (c) the absorption maxima are strongly red-shifted by 0.8−1.0 eV with respect to homologous compact arrangements. The red-shift phenomenon (c) is shown to be due not to a combination mode of internal and external surface plasmon resonances, as in thicker nanoshells, but rather to the relief of the charge compression by the underlying metallic layers on the resonating electrons. An analysis of the character of the electronic transitions mostly contributing to the absorption peaks finally provides information on the atomistic character of the corresponding modes.