Session: Basic Research in Colloids, Surfactants and Interfaces:

A combined experimental and computational study was performed to investigate the role of water for binding trifunctional organosilanes on silicon, which is a widely used industrial material for manufacturing semiconductors. The thickness and structure of interfacial water is dictated by interaction forces between the surface and the adsorbed water molecules. Experiments with atomic force microscopy (AFM) were designed to compare the differences in wettability for two substrates, silicon and mica. Nanopatterning protocols with colloidal lithography combined with vapor deposition of octadecyltrichlorosilane (OTS) enabled AFM studies of the thickness and the distribution of water deposited by condensation within sealed reaction vessels. The spreading of water on hydrophilic mica(0001) generated a continuous thin film of OTS covering areas throughout the sample that were not masked. However, the experimental studies with hydrophobic silicon substrates reveal that OTS binding is localized to the confined regions of a water meniscus, which surrounds the spheres of the surface mask. Further, reactive molecular dynamics simulations using the ReaxFF force-field performed on a model silicon surface suggests that the distribution of water molecules on a silicon surface is not uniform. The surface of silicon is oxidized to different degrees by the initial water vapor added to the reaction vessel. Electronic structure calculations performed on representative clusters of the oxygenated silicon surface were used to derive a mechanism for how the OTS molecules bind to a silicon surface. The amount of hydroxyl groups present on the surface was shown to direct the molecular self-assembly with the subsequent deposition of OTS from a heated vapor, for surfaces wetted with successive increments of water. At very low concentrations of water vapor there will be few hydroxyl groups present on silicon for OTS molecules to interact. Local differences in surface wettability produced discontinuous surface coverage with OTS binding to silicon.

Olajumoke Olubowale

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