Interfacial organic layers: Tailored surface chemistry for nucleation and growth

Abstract

The interfaces between inorganic and organic materials are important to a wide variety of technologies. A significant challenge concerns the formation of these interfaces when the inorganic layer must be grown on a pre-existing organic layer. In this review the authors focus on fundamental aspects of inorganic-organic interface formation using transition metal coordination complexes and atomic layer deposition. First, the authors discuss aspects of the synthesis and characterization of ultrathin interfacial organic layers, formed mostly on SiO2 and possessing a variety of functional groups, including layers with a branched microstructure. The authors go on to discuss the reactions of transition metal coordination complexes with these layers. A number of factors control the uptake of the transition metal complex and the composition of the adsorbed species that are formed. These include the identity, density, and dimensionality or spatial distribution of the functional groups. At room temperature, adsorption on layers that lack functional groups results in the penetration of the organic layer by the transition metal complex and the reaction with residual OH at the organic/SiO2 interface. Adsorption on layers with a mostly two-dimensional arrangement of reactive functional groups results in the formation of molecular “bipods,” where the surface bound functional groups react with the complex via two ligand exchange reactions. In contrast, for layers that possess a high density of functional groups arranged three dimensionally, the transition metal complex can be virtually stripped of its ligands. Atomic layer deposition on interfacial organic layers also depends strongly on the density and accessibility of reactive functional groups. On surfaces that possess a high density of functional groups, deployed two dimensionally, growth via atomic layer deposition is initially weakly attenuated, mostly uniform and smooth, and eventually evolves to growth characteristic of unmodified SiO2. Growth on layers that lack sufficient densities of functional groups is initially strongly attenuated, in contrast, and the resulting films are rough, severely islanded and three dimensional. As a consequence, there is a correlation between the strength of the initial attenuation in the rate of growth and the thin film morphology. Correlations between the initial uptake of the transition metal complex by the organic layer and the initial rate of thin film growth are less direct, however, as the composition and structure of the chemisorbed species must also be considered.