Oxidizing octadecylphosphonic acid molecules without disrupting their self-assembled monolayers

In examination of oxidation of octadecylphosphonic acid (OPA) self-assembled monolayers (SAMs) prepared on the native oxide of a Si wafer upon exposure to UV/ozone (UVO) for up to 5 min, atomic force microscopy studies of the surface elucidated that there were no changes in morphology of the SAMs. Surprisingly, investigations on the same samples of OPA SAMs using time-of-flight secondary ion mass spectrometry revealed that the intensity of the deprotonated molecular ion fragment decreased exponentially to approximately one fourth of that of the control. We clarify that UVO-induced oxidation of alkyl chains of the OPA molecules in their SAMs precedes the disruption of the monolayer structure, which is the basis for controlling surface chemistry of SAMs with brief UVO exposure.