Perfecting Imperfect “Monolayers”: Removal of Siloxane Multilayers by CO2 Snow Treatment

Abstract

Self-assembled monolayers (SAMs) of N-(3-triethoxysilylpropyl)-4-hydroxybutyramide were prepared on silicon oxide on silicon (Si/SiO(2)). Initial silane adsorption and high-temperature annealing led to a stable base monolayer with many large over-lying islands of disordered multilayers as a result of the non-self-limited growth process. The disordered multilayers were hydrolyzed and subsequently removed by CO(2) snow treatment. The resulting films were one monolayer thick as measured by ellipsometry. Atomic force microscopy, attenuated total reflection Fourier transform infrared spectroscopy, and contact angle analysis showed that the films were composed of monolayers with full and uniform surface coverage rather than nonuniform coverage by islands or patches of multilayers. Monolayers of octadecyltrichlorosilane were also prepared by multilayer removal via CO(2) treatment, showing the general applicability of the technique toward siloxane SAMs. We believe that CO(2) is an excellent solvent for weakly bound and hydrolyzed molecules that compose multilayers, and this ability to prepare near-perfect monolayer films from imperfect ones allows for less stringent formation conditions.