Surface Raman scattering of alkanethiols at silver and gold.

Abstract

The overall goal of this research is a comprehensive characterization of self-assembled alkanethiol monolayer films at Ag and Au surfaces. A combination of surface Raman spectroscopy and electrochemistry is employed to study several important aspects of these films. Raman vibrational assignments for a series of n-alkanethiols (n = 4, 5, 8, 9, 10, 12, 16, and 18 for C(n)H(2n+1)SH) are determined in the spectral regions from 600 to 1300 cm⁻¹ and 2800 to 3000 cm⁻¹. Particular emphasis is given to the trans (T) and gauche (G) ν(C-S) and ν(C-C) bands and the ν(C-H) bands. Surface Raman spectra of n-alkanethiol films at electrochemically roughened and mechanically polished, polycrystalline Ag and Au surfaces are presented. Strong enhancements of surface Raman scattering are realized at roughened surfaces, while less enhancement is observed at the mechanically polished surfaces. The conformational order of these films at these surfaces is evaluated by the determination of T and G bands present in the spectra. The orientation of these films at Ag and Au is determined through the use of surface Raman selection rules. A method is developed for the determination of orientation of C-S and C-C bonds and methyl groups. The orientations deduced using the spectral results from each of these regions are found to be self-consistent. Different orientations are deduced for alkanethiol films at Ag and Au surfaces and are proposed to be influenced by metal-S bonding. The orientations can be altered in the electrochemical environment by control of the applied potential of these metal substrates. The potential-dependent behavior is correlated with the potential of zero-excess charge (PZC) of these metals. Defect structure of these films at Ag surfaces with various surface morphologies is evaluated with Pb deposition studies. Gross films defects are evaluated with Pb underpotential deposition and ion penetration is studied with bulk Pb deposition. Finally, surface Raman spectra of monolayer films at non-enhancing surfaces are presented. Spectra for butanethiol at single-crystal Ag surfaces and butanethiol and thiophenol at mechanically polished, polycrystalline Pt surfaces show the utility of Raman spectroscopy for studying films at a variety of surfaces.