SILVER METALIZATION OF OCTADECANETHIOL MONOLAYERS SELF-ASSEMBLED ON GOLD

The interaction between evaporated silver and self-assembled monolayers of octadecanethiol (ODT) [CH3(CH2)17SH] on gold was studied at temperatures of 300 and 90 using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and ion scattering spectroscopy (ISS). Equivalent Ag coverages ranging from submonolayers to multilayers were examined. Ag deposited at 300 K penetrates the ODT monolayer and resides at the ODT/gold interface. The attenuation behavior of the XPS C 1s and Au 4f signals reveals that Ag nucleates as clusters underneath the ODT monolayer. UPS and ISS data are in accord with XPS results and, furthermore, indicate that the deposited Ag does not significantly alter the ODT monolayer film structure. In addition, an unusual shift to higher binding energy is observed for both the C 1s core level and the hydrocarbon valence band with increasing Ag coverage. It is suggested that the shift is due to a work function induced change whereby the photoemission of ODT molecules is referenced to the local vacuum level, not the metal Fermi level. In contrast to the behavior observed at 300 K, results from Ag deposition at 90 K indicate that Ag forms clusters on the ODT monolayer surface. In this case the Ag 3d core level exhibits a shift to higher binding energy relative to that of metallic silver that is attributable to a final state Coulombic effect characteristic of metal clusters on relatively poorly conducting substrates. There is apparently little chemical interaction between Ag and the hydrocarbon chains at both 90 and 300 K, although at 300 K Ag 3d XPS data suggest that Ag bonds to sulfur head groups. The low-temperature Ag/ODT/Au structure is apparently a metastable phase because warming of the sample to room temperature results in the migration of the Ag to the ODT/Au interface and possibly further clustering. ODT monolayer defects may play an important role in passage of Ag through the film.