Atomic oxygen reactions with semifluorinated and n-alkanethiolate self-assembled monolayers

Abstract

The interaction of atomic oxygen (O(³P)) with semifluorinated self-assembled monolayers (CF-SAMs), two different n-alkanethiolate self-assembled monolayers, and a carbonaceous overlayer derived from an x-ray modified n-alkanethiolate SAM have been studied using in situ x-ray photoelectron spectroscopy. For short atomic oxygen exposures, CF-SAMs remain intact, an effect ascribed to the inertness of C–F and C–C bonds toward atomic oxygen and the well-ordered structure of the CF-SAMs. Following this initial induction period, atomic oxygen permeates through the CF₃(CF₂)₇ overlayer and initiates reactions at the film/substrate interface, evidenced by the formation of sulfonate (RSO₃) species and Au₂O₃. These reactions lead to the desorption of intact adsorbate chains, evidenced by the loss of carbon and fluorine from the film while the C(1s) spectral envelope and the C(1s)/F(1s) ratio remain virtually constant. In contrast, the reactivity of atomic oxygen with alkanethiolate SAMs is initiated at the vacuum/film interface, producing oxygen-containing carbon functional groups. Subsequent reactions of these new species with atomic oxygen lead to erosion of the hydrocarbon film. Experiments on the different hydrocarbon-based films reveal that the atomic oxygen-induced kinetics are influenced by the thickness as well as the structural and chemical characteristics of the hydrocarbon overlayer. Results from this investigation are also discussed in the context of material erosion by AO in low Earth orbit.