Modification of self-assembled monolayers of alkanethiols on gold by ionizing radiation

Self-assembled films of octadecanethiols on polycrystalline Au have been studied with monochromated X-ray photo-electron spectroscopy (XPS). The focus has been on the SAu interface. It is demonstrated that ionizing radiation causes breaking of SAu bonds and formation of disulfides. The modification of the alkane chains and the S headgroups has been investigated as a function of X-ray exposure. Long time X-ray exposed films have also been characterized by polarized reflection infrared spectroscopy, single-wavelength ellipsometry and water and hexadecane contact angle measurements. All these data show that in addition to the disulfide formation at the gold interface, the originally well ordered hydrophobic film is strongly modified by the ionizing radiation, involving CH and CC bond breaking, interchain cross linking and a general disordering of the film.