Spectroscopic Characterization and Modeling of Methyl- and Hydrogen-Terminated Oligo(ethylene glycol) Self-Assembled Monolayers

Abstract

Two series of oligo(ethylene glycol) (OEG) thiol compounds HS-(CH2CH2O)nR, with R = CH3, H and n = 5, 6, 7, have been synthesized and used to form self-assembled monolayers (SAMs) on gold. The data from null ellipsometry, infrared reflection–absorption spectroscopy, and ab initio calculations of this type of OH- and CH3-terminated OEG SAMs are used to examine the rarely addressed in-SAM orientation of oligo(ethylene glycols) and to provide detailed assignments of infrared bands in the fingerprint and CH-stretching regions. On the basis of these results, a new spectral band has been observed at 2947 cm–1 and identified by the first-principle calculations as localized vibrations that are specific for hydrogen-terminated OEG thiolate SAMs. This band can be used as an indicator of a high crystalline-like ordering. It is furthermore stressed that theory agrees with the experimentally obtained CH-stretching spectra remarkably well if, and only if, the OEG helix axis within studied SAMs is tilted by about 20° with respect to the surface normal.