Comparison of Self-Assembled Monolayers ofn-Alkanethiols and Phenylthioureas on the Surface of Gold

Abstract

Long-chain alkanethiols [R(CH2)nSH] and substituted phenylthioureas (RPhNHCSNH2) adsorb from solutions on the surface of gold. Analysis by XPS indicates that the binding energies of the S 2p and Au 4d5 peaks in alkanethiol monolayers are 162.3 and 335.4 eV, which are 1.4 and 0.4 eV higher than those in phenyl-thiourea monolayers, respectively; the widths (FWHM) of the S 2p XPS peaks in alkanethiols are less than those in phenylthioureas. The alkyl chain in alkanethiol monolayers on gold is canted 30° from the normal to the surface, while the longitudinal dimension of the phenylthiourea molecule is perpendicular to the surface. It is suggested that the covalent bond between sulphur and gold dominates the conversion of alkanethiols to gold alkane-thiolates, whereas the σ–π delocalized coordinate bond between sulphur and gold controls the adsorption of phenylthioureas on the surface of gold because of the existence of the conjugated π4U6 bond in thioureas.