Response of biphenyl-substituted alkanethiol self-assembled monolayers to electron irradiation: Damage suppression and odd- even effects

The low-energy electron-induced damage in self-assembled monolayers (SAMs) formed from omega-(4'-methylbiphenyl-4-yl)alkanethiols CH3(C6H4)(2)(CH2)(n)SH (BPn,n = 0, 1, 4, 5, and 12) on gold substrates was studied. The pristine and heavily (8000 muC/cm(2)) irradiated films were characterized in detail by X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, infrared reflection absorption spectroscopy, and advancing contact angle measurements. In contrast to SAMs of conventional alkanethiols but similar to pure aromatic thiol-derived systems, only minor damage is observed for the aliphatic-aromatic BPn films. In particular, the orientational order and anchoring to the substrate are retained upon the irradiation. At the same time, C-H bond scissions in the aromatic part occur, leading to a cross-linking between the neighboring biphenyl moieties. Whereas the general behavior of the BPn SAMs with respect to electron irradiation is qualitatively similar, the extent of the irradiation-induced changes depends on the packing of these systems. The densely packed BP1 and BP5 SAMs are much more stable with respect to electron bombardment than the less densely packed BP4 films. The relation between the packing density and the extent of the irradiation-induced changes seems to be a general phenomenon in monomolecular films, which provides a tool to tailor the reaction of these systems toward ionizing radiation for lithographic applications.