High resolution X-ray photoelectron spectroscopy measurements of octadecanethiol self-assembled monolayers on Au(111)

We have measured high-resolution X-ray photoelectron spectroscopy (XPS) spectra of 1-octadecanethiol (ODT) self-assembled monolayers (SAMs) on a Au(111) surface in order to investigate the Au-S binding properties at the initial stage of SAM growth and after the desorption of the ODT from the Au surface. For the SAM prepared by the 1 min immersion into the ODT solution, we found new sulfur species around 161 eV and assigned it to isolated sulfur without C-S cleavage. We also confirmed the presence of a similar 161 eV peak in the S(2p) spectra after desorption of the ODT molecules. Furthermore, we observed a peak shift in the carbon 1s (C(1s)) peak, depending on the surface coverage of the ODT. In addition to the C(1s) peak of 285.1 eV which might correspond to alkyl chains of densely packed ODT molecules, another lower binding energy peak at 284.3 eV appeared after annealing. This lower C(1s) binding energy peak formation suggested that some of the alkyl chains for the remaining ODT molecules might be oriented parallel to the Au surface after annealing.