Vacuum ultraviolet trimming of oxygenated functional groups from oxidized self-assembled hexadecyl monolayers in an evacuated environment

Highlights: • Decreasing the abundance of oxygenated groups through 172 nm VUV light irradiation. • Quantification the oxygenated groups verses different irradiation periods. • Study the influence of VUV light on the surface wettability and morphology. • Figuring out the plausible routes of the chemical conversions during light exposure. - Abstract: Vacuum ultraviolet light irradiation in dry air generates active oxygen species, which have powerful oxidation abilities. These active oxygen species (O) can oxidize the alkyl moieties of polymers, and generate new oxygenated groups such as OH, CHO and COOH groups. Reducing the oxygen content in the exposure environment decreases the rate of oxidation processes. In this study, we examined the influences of the 172 nm VUV irradiation in a high vacuum (HV, < 10⁻³ Pa) environment on the chemical constituents, surface properties and morphological structure of well-defined VUV/(O)-modified hexadecyl (HD-) self-assembled monolayer (SAM) prepared on hydrogen-terminated silicon (H-Si) substrate. After VUV light irradiation in a HV environment (HV-VUV), the chemical constituents and surface properties were changed in two distinct stages. At short irradiation time (the first stage), the C−O and COO groups decreased rapidly, while the C=O groups slightly changed. The dissociation of nonderivatizable groups (such as ether (C−O−C) and ester (C−COO−C) groups) compensated the dissociated OH, CHO, C−CO−C and COOH groups. With further irradiation (the second stage), the quantities of the oxygenated groups slightly decreased. The carbon skeleton (C−C) of SAM was scarcely dissociated during the HV-VUV treatment. These chemical changes affected the surface properties, such as wettability and morphology.