Characterization of the molecular orientation of self-assembled monolayers of alkanethiols on obliquely deposited gold films by using infrared-visible sum-frequency spectroscopy

This paper reports the use of infrared (IR)-visible sum-frequency spectroscopy to characterize the surface of gold films deposited by physical vapor deposition at an oblique angle of incidence and the orientation of the methyl end groups of self-assembled monolayers (SAMs) of n-alkanethiols formed on these gold films. The oblique deposition of the gold breaks the macroscopic azimuthal degeneracy that is characteristic of gold films grown without a preferred direction of deposition. By tuning the frequency of the incident IR light to either the symmetric or the asymmetric methyl stretch resonance and by recording the azimuthal dependence of the sum-frequency signal, the orientation of the methyl groups of the n-alkanethiols on the obliquely deposited gold films were shown to be distributed around a preferred in-plane direction coincident with the direction of the gold deposition. The angles between the normal of the sample and the methyl groups were determined to be 85 +/- 2degrees and 44 +/- 10degrees for the SAMs formed from pentadecanethiol and hexadecanethiol, respectively. In contrast, when using gold films that were not obliquely deposited, no preferred in-plane orientation of the methyl groups could be observed.