Optical characterization of electronic transitions arising from the Au/S interface of self-assembled n-alkanethiolate monolayers

Analyses of optical frequency ellipsometric spectra obtained in situ upon completion of the self-assembly of n-alkanethiolate monolayers on gold substrates immersed in methanol reveal the presence of a 1.2 ± 0.4 Å thick S/Au interface layer. This layer exhibits unique optical transitions, modeled as two Lorentzian oscillators with imaginary parts peaked at ≈ 570 nm (≈ 2.2 eV) and ≈ 290 nm (≈ 4.3 eV), indicating a perturbation of the near-surface gold electronic structure upon thiol chemisorption. A further, more rigorous, spectral analysis involving an anisotropic, uniaxially oriented chain assembly reveals the presence of significant microscopic void defects in the otherwise densely packed monolayers.