High resolution X-ray photoelectron spectroscopy of l-cysteine self-assembled films

The increasing demand for heterogeneous materials prompts the interest for the physics and chemistry of organic–inorganic interfaces. Within this field of interest we have investigated the electronic states of L-cysteine (Cys) adsorbed on Au(111) from aqueous solution with synchrotron-based, high resolution X-ray photoemission spectroscopy. The analysis of the S 2p core level region in pristine samples prepared with purified Cys indicates that Cys adsorbs on gold as a thiolate. The absence of a signal related to physisorbed sulfur indicates that Cys forms a monomolecular layer. The high resolution allowed us to resolve two components in the C 1s spectral region related to carboxyl. The comparative analysis of C 1s, N 1s and O 1s core level regions indicates the presence of the neutral and ionic form for both the carboxyl and the amine groups. From the quantitative analysis of the C 1s and N 1s data we can infer that the zwitterionic form is the prevalent one, with a ratio of zwitterionic to neutral molecules of 3∶1. This value is quite different from that in the deposition solution. Prolonged exposure to the X-ray beam and thermal annealing induce the formation of atomic sulfur through S–C bond breaking and molecular fragment desorption. This response to X-ray irradiation differs from the results recently reported on long chain alkanethiols.