Characterization of Self-Assembled Alkanethiol Monolayers on Silver and Gold Using Surface Plasmon Spectroscopy

Abstract

We report the surface plasmon study on a variety of self-assembled monolayers (SAMs) of n-alkanethiols, HS(CH2)nCH3 (n = 11, 13, 15, 17, 19), absorbed to silver and gold surfaces, in terms of thicknesses, indices, and tilt-angles. We have found that the refractive indices for the alkanethiols attached to Ag increase gradually as alkyl chain length increases. Likewise, the refractive indices for Au increase gradually to an alkyl chain length of n = 17, at which point its refractive index decreases slightly. The indices vary from a low value of 1.33 to 1.44. This gradual increase in index is not unexpected since it suggests that differences in packing and consequent bulk polarizability, χ(1), is affected by alkyl chain length. A molecular modeling minimized n-alkanethiol length and its corresponding experimental surface plasmon thickness were used to determine the tilt-angle for each of the various alkanethiols via a projection analysis. We found that the tilt-angles of the n-alkanethiols attached to Ag begin to increase from 0° starting with n = 15; however, the tilts on Au have an average of ∼41°, seemingly independent of carbon chain length.