Modification of Self-Assembled Mono layers of Perfluoroterphenyl-Substituted Alkanethiols by Low-Energy Electrons

Using self-assembled monolayers (SAMS) of perfluoroterphenyl-substituted alkanethiols (FTPn) on Au-(111) as test systems, we studied the effect of low-energy (10 eV) electron irradiation on fluorinated aromatic SAMs. FTPn films were found to mimic the typical behavior of aromatic hydrocarbon SAMs under ionizing radiation. The dominant process is the cleavage of C-F bonds in the FTP moieties followed by desorption of the released fluorine atoms, rapid conformational and orientational disordering, and cross-linking between the residual skeletons of the FTP backbones. The stability of these skeletons and the development of a cross-linked network hinder other typical irradiation-induced processes such as desorption of molecular fragments and damage of the headgroup-substrate interface. Along with these findings, FTPn SAMs gave a unique possibility to monitor the destiny of the fluorine atoms, which are analogues of the hydrogen atoms in the respective hydrocarbon systems. The extent of fluorine release upon irradiation was estimated and the cleavage of C-F bonds was found to be the main channel of this release, which is in striking contrast to fluorocarbon aliphatic monolayers for which such a release is dominated by the desorption of fluorocarbon fragments. Finally, irradiation-induced reorientation of FTPn films at very small irradiation doses was observed suggesting that their structure is kinetically trapped and can be relaxed toward the thermodynamic minimum by physical means.