STRUCTURE, DYNAMICS, AND REACTIVITY OF MOLECULAR ASSEMBLIES AT INTERFACES

The central theme of my research is to understand self-assembly of molecules at interfaces and to develop strategies that control the arrangement of atoms and molecules at the nanometer length scale. We have sought molecular level insight into the mechanism of photoreactivity of octadecylsiloxane (ODS) self-assembled monolayers (SAMs), which has implications in high resolution nanolithography. We have improved the reproducibility of preparation of ODS SAMs by controlling the water content in the reaction solution and in the ambient environment. We have demonstrated that atomic oxygen is the primary agent for the UV photoreactivity of ODS SAMs. UV degradation proceeds via oxidation of carbon chains instead of the siloxane headgroups. We found that degradation introduces microscopic roughness in SAMs. Using a novel technique, Fluorescence Labeling of Surface Species, we identified low-concentration surface functional groups produced as intermediates in UV degradation. We proposed a reaction mechanism based on hydrogen abstraction. To understand the rules governing self-assembly at electrochemical interfaces and how they can be exploited to control the arrangement of molecules, we performed in-situ STM investigations. To probe the effect of hydrophobic interactions, we investigated how the electrode potential transforms the microscopic structure of a completely hydrophobic molecule, hexadecane. Molecular level evidence suggests that the competition between hexadecane and the electrolyte plays a pivotal role in the self-assembled structures at the charged interface. We studied how the electrode potential can affect the dynamics of self-assembly of 5,10,15,20-Tetra(4-Pyridyl)-21H,23H-Porphine (TPyP), which can not displaced by the electrolyte due to much stronger adsorption. Our results suggest that while TPyP can be kinetically trapped in a disordered structure, its ordering process can be facilitated at electrode potentials where molecule substrate interaction is strong enough to confine molecules in ordered arrays but also weak enough to allow facile lateral reorganization, a key requirement for self-assembly. Our investigation also suggests that electrode potential can direct the formation of multilayers due to the Ĉ stacking interaction of TPyP. We observed distinct electrochemical reactivity of adsorbed TPyP, due to the interaction between TPyP and the Au substrate. Finally, we have demonstrated that the intrinsic length scales of molecular assemblies may be exploited to grow metal nanostructures of controlled spacing. Our study of metal electrodeposition on surface micelles affords insight into the interaction between metal nanostructures. In particular, how the interaction can be controlled to grow metal nanoscale structures, are relevant to electrocatalysis and nanoscale electronics