The Role of Low-Frequency Plasmons in Molecular Adsorption: A Theoretical and Spectroscopic Study of Gold and Titanium Compounds

Abstract

We explore the question of whether surface plasmons arising from metallic nanostructures can be used advantageously to affect binding interactions of adsorbed molecules, specifically, the adsorption of dihydrogen leading to enhanced binding energies under ambient conditions. To begin to address this question, we report a systematic, theoretical survey of the plasmon spectra of gold (AuLi, AuNa2, AuB2, AuAl2, AuGa2, and AuIn2) and titanium (TiB2) compounds, examining closely the low-frequency characteristics of these spectra in relation to crystal structure. First-principle calculations of the real and imaginary components of the dielectric function were performed, and the energy-loss function was derived. These calculations show that most of the compounds give rise to low-intensity, plasmon-like modes at low frequencies and can be attributed to interband transitions between states lying close to the Fermi level. We further determined experimentally whether these plasmon-like modes are excited at low frequencies from clusters of two gold compounds (AuLi and AuAl2) by measuring surface-enhancement effects in the near-infrared Raman spectrum. Strong surface-enhancement effects were indeed observed, and we report the first observation of near-infrared, surface-enhanced Raman from compounds of this kind.