Accelerating the kinetics of thiol self-assembly on gold - A spatial confinement effect

The adsorption of alkanethiols onto gold surfaces to form self-assembled monolayers (SAMs) occurs more than 10 times faster in a spatially confined environment than on unconfined bare substrates, and the adsorbed layers exhibit higher coverage and two-dimensional-crystallinity. The spatially constrained reaction environment is prepared with use of an atomic force microscope tip to displace thiols within a previously formed SAM. During the displacement, the thiol molecules present in the solution above the SAM rapidly assemble onto the exposed nanometer-size gold area that is confined-by the scanning tip and surrounding SAM. The accelerated rate is attributed to a change in the pathway for the self-assembly process as the spatial confinement makes it geometrically more probable and energetically more favorable for the initially adsorbed thiols to adopt a standing-up configuration directly in this microenvironment. In contrast, thiols that self-assemble onto gold surfaces in an unconstrained environment initially form a lying-down phase, which subsequently degrades and forms a standing-up phase. Our observations suggest that spatial confinement can provide an effective means to change the mechanism and kinetics of certain surface reactions by sterically preventing alternative reaction pathways and stabilizing particular transition states or reaction intermediates. In addition, the results underlie the development of a new method ("nanografting") for patterning SAMs laterally with nanometer-level precision.