Characteristics of alkanethiol self assembled monolayers prepared on sputtered gold films: a surface enhanced Raman spectroscopic investigation

Abstract

Self assembled monolayers (SAMs) of CH₃(CH₂)_n SH (n = 2, 5, 7, 8, 9, 11 and 17) on polycrystalline gold films have been investigated using FT-Raman spectroscopy. All the thiols chemisorb dissociatively in the thiolate form and the adsorbate-surface interaction is the same in all the cases. Initial chemisorption occurs with fast kinetics and it is complete in the first few hours of exposure of the thiol to the surface. The formation of self assembly occurs rather slowly and the evolution of spectral features continues till 12-15 h of exposure. The change is pronounced particularly on the (C---C)_T stretching mode. Initial adsorption results in the (C---S)_G form and equilibration between (C---S)_T and (C---S)_G occurs rather quickly. In lower member thiols, equilibration occurs within about 30 min of exposure. Upon heating, the monolayers begin to desorb at about 348 K followed by melting of the close packed assembly. Spectrum upon melting resembles that of the liquid.