High resolution XPS of the S 2p core level region of the L-cysteine/gold interface

L-cysteine self-assembled monolayers (SAMs) have been deposited on gold from the liquid and vapour phase. Synchrotron based high resolution x-ray photoemission spectroscopy has been used to characterize the sulfur chemical states at the SAM/gold interface. Results obtained from pristine and x-ray irradiated samples, prepared with both as-received and purified L-cysteine, are reported. Pristine samples prepared with purified cysteine are characterized by an intense, largely dominant S 2p state at a binding energy around 162 eV (2p3/2 level) assigned to thiolates. A second doublet around 161 eV develops during irradiation. By comparison with the literature, this doublet is assigned to atomic sulfur present either as impurity or generated by S-C bond scission. Comparative measurements performed, under similar experimental conditions, on pristine 3-mercaptopropionic acid [HS(CH2 )2 COOH] layers deposited from the liquid phase are also presented and discussed.