Surface chemical conversion of organosilane self-assembled monolayers with active oxygen species generated by vacuum ultraviolet irradiation of atmospheric oxygen molecules

The chemical conversion of the top surface of n-octadecyltrimethoxy silane self-assembled monolayers (ODS-SAMs) on oxide-covered Si substrates using active oxygen species generated from atmospheric oxygen molecules irradiated with vacuum ultraviolet (VUV) light at 172 nm in wavelength has been studied on the basis of water contact angle measurements, ellipsometry, X-ray photoelectron spectroscopy, and atomic force microscopy. An ODS-SAM whose water contact angle was 104 degrees on average was prepared using chemical vapor deposition with substrate and vapor temperatures of 150 degrees C. The VUV treatment of an ODS-SAM sample was carried out by placing the sample in air and then irradiating the sample surface with a Xe-excimer lamp. The distance between the lamp and the sample was regulated so that the VUV light emitted from the lamp was almost entirely absorbed by atmospheric oxygen molecules to generate active oxygen species, such as ozone and atomic oxygen before reaching the sample surface. Hence, the surface chemical conversion of the ODS-SAM was primarily promoted through chemical reactions with the active oxygen species. Photochemical changes in the ODS-SAM were found to be the generation of polar functional groups, such as -COOH, -CHO, and -OH, on the surface and the subsequent etching of the monolayer. Irradiation parameters, such as irradiation time, were optimized to achieve a better functionalization of the SAM top surface while minimizing the etching depth of the ODS-SAM. The ability to graft another SAM onto the modified ODS-SAM bearing polar functional groups was demonstrated by the formation of alkylsilane bilayers.