Self-assembly of n-alkanethiol monolayers: a study by IR-vis sum frequency spectroscopy (SFG)

Adsorption of docosanethiol (CH3(CH2)21SH) onto polycrystalline gold from ethanol solution was investigated by nonlinear vibrational spectroscopy. The formation of the self-assembled monolayer (SAM) was monitored by recording of the temporal evolution of the C−H stretching modes. Three different steps with significantly different time scales were identified. The fastest step in the adsorption process is related to the Au−S bond formation. Progressing with a rate constant of 2500−3000 L mol-1 s-1, it is characterized by hydrocarbon chains with a high fraction of gauche kinks. The second step proceeds 3−4 times more slowly and comprises the straightening of the alkane chains. During the first two steps a pronounced band around 2813 cm-1 appears, which is interpreted as mode softening of the C−H vibrations due to the interaction with the substrate. The time scale of the final step is a factor of 35−70 slower than the rate of chain stretching and encompasses the reorientation of the terminal methyl end groups. The methylene group adjacent to the methyl group exhibits the same temporal behavior and thus behaves oppositely to the other methylene groups. The SFG data imply that the last few percent of the thiol molecules adsorbing produce the complete monolayer by inducing the transition from a high number of gauche defects to an all-trans conformation. The sequence of changes in the vibrational spectra suggests an ordering and annealing of the film from the interface to the film surface