Plasmon Dispersion of the Au(111) Surface with and without Self-Assembled Monolayers

The surface plasmon dispersion of gold films with and without chemisorbed, alkane thiol self-assembled monolayers (SAMs) has been investigated using high resolution electron energy loss spectroscopy (HREELS). For a bare Au(111) film, the surface plasmon energy (2.49 eV at the zone center) shows a positive dispersion. After adsorption of ethylbenzenethiol or dodecanethiol SAMs, the plasmon energy at the zone center blueshifts and the dispersion switches sign to become negative, thus mimicking the behavior of a free-electron system. This striking behavior represents a benchmark for models of the electronic structure of the gold-sulfur interface, as manifest both in SAMs and in monolayer-protected nanoparticles.