Adsorption characteristics of p-xylene-α,α′-dithol on gold and silver surfaces:: Surface-enhanced Raman scattering and ellipsometry study

The adsorption behavior of p-xylene-alpha, alpha'-dithiol (p-XDT) on colloidal gold and silver surfaces has been investigated by means of surface-enhanced Raman scattering (SERS). p-XDT chemisorbed dissociatively on both the gold and silver surfaces but as mono- and dithiolate, respectively. Regardless of the bulk concentration of p-XDT, only a monolayer was assembled on the silver surface with a flat orientation by forming two Ag-S bonds. On the gold surface, the monothiolate species, p-XDT1-, appeared to assume a rather flat orientation at a very low surface coverage, but as the surface coverage was increased, the adsorbate took a perpendicular orientation. Furthermore, when the bulk concentration of p-XDT was close to that required for a full-monolayer coverage limit, a band assignable to the S-S stretching vibration appeared at similar to 509 cm(-1) in the gold sol SER spectra. A separate ellipsometry measurement performed with vacuum-evaporated gold substrates revealed that up to trilayers could be assembled on gold in 1 mM n-hexane solution of p-XDT while only a monolayer formed in either methanol or ethanol solution. Although the exact causes were uncertain, such a difference in behavior seemed to have nothing to do with the oxygen species present in ethanol or methanol solution.