XPS measurements on L-cysteine and 1-octadecanethiol self-assembled films: a comparative study

The adsorption of l-cysteine on Au(111) surfaces from the solution phase has been investigated by X-ray photoelectron spectroscopy (XPS). To check the reliability of the XPS analysis, we performed measurements on l-cysteine/Au(111) and alkanethiol/Au(111) interfaces paying particular attention to spectra time evolution during X-ray exposure. In both systems, X-ray irradiation increases the Au4f intensity, but while in l-cysteine the line shape of the S2p spectra does not change, in 1-octadecanethiol it does. The analysis of the Au4f and S2p core level regions indicates the occurrence of a dissociative molecular chemisorption on the metal surface for both systems. In the case of l-cysteine, the S2p spectra indicate the presence of a minor sulphur component attributed to the presence of unbound cysteine molecules physisorbed on the first chemisorbed layer.