Effect of underpotentially deposited adlayers on sulfur bonding schemes of organothiols self-assembled on polycrystalline gold:: sp or sp3 hybridization

Improvement in stability of self-assembled monolayers (SAMs) has been demonstrated by using an underpotentially deposited (upd) interlayer between the SAMs and gold substrate. In this study, we show that the upd adlayers can be applied to improve the packing density of thiophenol SAMs due to controlling the binding scheme of the sulfur headgroup from an sp(3) to an sp hybridization configuration. Examined here are the structures of SAMs on gold substrate pre-modified by four types of upd adatoms that are electrodeposited at several coverages representing the finish of distinct upd processes. The SAM structures of a homologous series of n-alkanethiols (CH3(CH2)(n)SH, n = 4-11 and 13-15) are characterized by contact angle measurements and infrared reflectance absorption spectroscopy. The results show the odd-even effect that the orientation of the terminal methyl group is alternatively associated with the number of methylene units, indicating that sulfur headgroup adopts an sp hybridization on Au/Bi(upd) and an sp hybridization on Au/Ag(upd) and Au/Cu(upd). For SAMs on Au/Hg(upd) at a low upd coverage, an sp(3) hybridization of the sulfur headgroup is in favor of short chain alkanethiols. As the number of methylene unit increases, the molecular axis appears to align toward the surface normal, suggesting that intermolecular attractions become more influential than the sulfur bonding scheme on the SAM structures on Au/Hg(upd). In contrast, at a high upd coverage of Au/Hg(upd), the sulfur headgroup adopts an sp hybridization as indicated by the odd-even characteristics. In all cases, the increase in the coverage of upd adatoms results in a smaller tilt angle of the molecular axis with respect to the surface normal.