Surface Raman Scattering of Self-Assembled Monolayers Formed from 1-Alkanethiols: Behavior of Films at Au and Comparison to Films at Ag

Surface Raman scattering is used to study self-assembled monolayers formed from a series of 1-alkanethiols, CH₃(CH₂)_nSH, where n = 3–5, 7, 8,11, and 17, at mechanically polished and electrochemically roughened Au surfaces. Defect structure in these films is investigated by use of the relative intensities of peaks due to trans and gauche conformations in the v(C–S) and v(C–C) frequency regions. Surface selection rules for Raman spectroscopy are used to estimate orientation of the alkanethiol layers at Au. The orientation proposed on the basis of the Raman spectral data is consistent with those previously reported on the basis of other measurements at Au surfaces. This orientation is compared to that previously determined for these films at Ag. Alkanethiols at Ag are found to have a chain tilt from the surface normal less than the 30° previously reported for Au. The C–S bond is found to be perpendicular to the Ag surface while it is largely parallel to the surface at Au. Differences in the spectra of short-chain alkanethiols from smooth and rough surfaces are attributed to disordering of the film at the roughened Au surface which occurs predominantly near the S end of the molecule on rough Au surfaces.