Exchange of methyl- and azobenzene-terminated alkanethiols on polycrystalline gold studied by tip-enhanced Raman mapping

Mixed thiol self-assembled monolayers (SAMs) presenting methyl and azobenzene head groups were prepared by chemical substitution from the original single-component n-decanethiol or [4-(phenylazo)phenoxy]hexane-1-thiol SAMs on polycrystalline gold substrates. Static contact-angle measurements were carried out to confirm a change in the hydrophobicity of the functionalized surfaces following the exchange reaction. The mixed SAMs presented contact-angle values between those of the more hydrophobic n-decanethiol and the more hydrophilic [4-(phenylazo)phenoxy]hexane-1-thiol single-component SAMs. By means of tip-enhanced Raman spectroscopy (TERS) mapping experiments, it was possible to highlight that molecular replacement takes place easily and first at grain boundaries: for two different mixed SAM compositions, TERS point-by-point maps with <50 nm step sizes showed different spectral signatures in correspondence to the grain boundaries. An example of the substitution extending beyond grain boundaries and affecting flat areas of the gold surface is also shown. Sub-micrometer Raman imaging: Distribution of azobenzene thiol and decanethiol molecules in a self-assembled monolayer formed by subsequent adsorption on polycrystalline gold substrates is evidenced by tip-enhanced Raman spectroscopic (TERS) imaging with <50 nm pixel size. The images show that molecular replacement takes place first at grain boundaries.