Vacuum Ultraviolet Treatment of Acid- and Ester-Terminated Self-Assembled Monolayers: Chemical Conversions and Friction Reduction.

We have prepared COOH- and COOCH3-terminated self-assembled monolayers (SAMs) from undec-10-enoic acid (UDA) and methyl undec-10-enoate (MUDO) molecules on hydrogen-terminated silicon (H-Si) substrates through ultraviolet (UV) irradiation. The as-prepared UDA- and MUDO-SAMs were exposed to 172 nm vacuum-UV (VUV) light in a high vacuum environment (HV, <10-3 Pa) for different periods. The presence of COO components at the surfaces of these SAMs without prior oxidation would simplify the understanding of the origin of the chemical conversions and the changes of surface properties, as the prior oxidation would change the surface properties and generate different oxygenated groups. After the HV-VUV treatment, the abundance of COOH and COOCH3 components of these SAMs decreased without significant dissociation of their C-C backbones. Degradation of these components occurred through dissociating their C-O bonds, resulting in different C═O components. Also, the occurrence of Norrish type pathways resulted in a slight decrease of carbon content and produced CH3 components. We have applied the HV-VUV lithography to control the abundance of COOH and COOCH3 components in well-defined areas and to investigate the friction differences between the irradiated and masked areas. The irradiated areas exhibited lower friction than the masked areas without observing significant height contrasts between these areas. The reduction in friction was attributed to the conversion of the COOH and COOCH3 components to less adhesive components such as C═O and CH3. These experiments suggest the HV-VUV treatments as an approach for low damage dry surface modifications and reductive lithographic techniques at surfaces terminated by acid and ester groups.