Self-Assembled Monolayers of Small Aromatic Disulfide and Diselenide Molecules on Polycrystalline Gold Films: A Comparative Study of the Geometrical Constraint Using Temperature-Dependent Surface-Enhanced Raman Spectroscopy, X-ray Photoelectron Spectroscopy, and Electrochemistry

A detailed investigation of the self-assembled monolayers of diphenyl disulfide (DDS), diphenyl diselenide (DDSe), and naphthalene disulfide (NDS) on polycrystalline gold films using surface-enhanced Raman spectroscopy (SERS), X-ray photoelectron spectroscopy (XPS), and electrochemistry is presented. Whereas DDS dissociatively chemisorbs on Au, in both DDSe and NDS, the Se-Se and S-S bonds, respectively, are preserved upon adsorption. All of the molecules adsorb with the molecular plane perpendicular to the surface. Temperature-dependent SERS studies suggest that the DDS monolayer is by far the most stable one and is stable up to a temperature of 423 K. Both DDSe and NDS desorb without breaking the diselenide and disulfide bonds. None of the monolayers show any structural change upon heating. XPS investigations show the presence of beam-induced damage upon X-ray exposure to DDS and NDS monolayers, and the damage is greater in the latter. Electrochemical investigations support the SERS and XPS data. Number of pinholes and defects are much less in the DDS monolayer than in NDS and DDSe. The impedance parameters such as double-layer capacitance, charge-transfer resistance, and diffusion coefficients measured at different frequencies support the above conclusion. It is suggested that the geometric constraint imposed by the rigid naphthalene ring inhibits the cleavage of the S-S bond, and consequently, the adsorption sites for sulfurs are not strongly bonded. For DDSe, it appears that the Se-Se distance is such that appropriate binding sites are available, thus leading to a more ordered monolayer. For DDS, the facile cleavage of the S-S bond leads to strong binding of the adsorbate molecules at the preferred surface sites, resulting in a rather well-ordered self-assembled structure.